Development
Hull shape is the largest contributor to success or failure in yacht design. Appendages can be replaced, rigs modified, but a bad hull is usually where the buck stops. It is just as true for motor vessels.
The design of the Nordkyn hull was underpinned by two key objectives: exceptional seakeeping properties and excellent light wind performance
In order to achieve these goals, the hull design had to be matched with rig and appendages in order to assess parameters such as stability and forces acting on the boat.
The considerations offered below relate to how the Nordkyn hull was developed, starting from a blank canvas and bringing together the physics of sailing and seakeeping with personal sea experience. Some concepts in yacht design today are quantifiable and were able to be modelled as such, others remain more qualitative and were taken into account differently.
Quantified yacht design theory is the same for everyone, it is widely accessible and broadly used, but it is the appraisal of those qualitative aspects and the fundamental understanding of what matters, how and why based on years at sea that was going to differentiate the project.
At times, this involved taking some design risks and departing from common trends.
Seakeeping
Trying to answer the question of what constitutes good seakeeping properties is somewhat like trying to define what is a good sea boat. The challenge is developing an objective definition and focus on the desirable properties, rather than design particulars. Deciding on what determines a good sea boat is one aspect I had plenty of time to ponder over at sea for years all around the world, and to me, seaworthiness in a yacht – in no particular order – comes down to:
- Course stability. It is the ability of the boat to track straight in spite of the influence of speed or sea state. Without course stability, control is eventually lost and one can’t sail any more.
- Manoeuvrability. It is the ability to alter course at will, independently of wind angle and speed. It is important to include it here, because some yachts are quite stable on course for the simple reason that they don’t manoeuvre. This is obviously undesirable.
- Low hull resistance. Low hull resistance is very important, not so much in order to “go fast” all the time, but rather not to “get caught”. A yacht with a high resistance hull cannot accelerate before steep seas and ends up further up the wave slope and ultimately in the breaking crest. High hull resistance also compromises course stability, because as speed increases, the pressure build-up occurs in the bow area and has a long list of detrimental effects that will be evoked further.
- Upwind ability. Being able to point and progress upwind is extremely important for safety reasons as well as being able to go anywhere independently of wind direction. Low resistance is one component of it again. Any excessive increase in resistance as the sea state deteriorates will compromise the ability of the boat to point and beat into the weather. Hydrodynamic lift (appendages) and aerodynamic driving force (sail plan) are the other elements to be considered for upwind sailing ability.
- Dynamic trim. Keeping the bow up at speed is highly desirable in most circumstances as burying it increases resistance forward and compromises course stability. A linkage also exists with hull resistance, in the sense that a hull with a large drag tends to trim down by the bow much more under the influence of sail forces. Hull shape can significantly impact trim at speed by creating lift forward – or not.
- Transverse stability. Besides opposing the obvious and annoying prospect of ending up upside down, good transverse stability helps with limiting heel and narrowing the “operating window” of the hull. The further a hull heels over, the more sail forces up high become offset laterally in relation with the hydrodynamic forces at work on the immersed body. This does not promote good behaviour in the sea.
Following my own experience, on fixed keelers in particular, I like to see stability still close to its maximum as the keel becomes “horizontal enough” in the water to start “disappearing behind the hull”, permitting it to skid sideways. At that point, I do not want to see the side deck significantly immersed either, which ties with the next and last item below.
Inverted stability is an aspect that should not be overlooked either for an offshore yacht. At this point, the hull shape is less relevant, it comes down to the maximum beam at deck level, hull sheer to some extent and roof shape. Exaggerated flare in the topsides amidships is one great way of achieving outstanding inverted stability. - Reserve buoyancy. Low freeboard can be quite pretty, seas climbing on deck and side-decks under water a lot less so. A yacht needs to have sufficient volume above its waterline not to get buried in the sea in heavy weather. It should not have excessive freeboard either as it makes it a bigger target for wave action – besides the obvious windage aspect.
All aspects above are equally important to me. There is however one critical assumption to remember: the yacht is sailing when seakeeping is an important matter. This is deliberate, this is the way I sail: I don’t stop. There are very good reasons for that, but this is a different subject.
Designing the Nordkyn Hull
The gradual change in the characteristics of the hull through heel received an extraordinary amount of attention, prompting repeatedly redrawing new sets of lines over a period of nearly 4 years
Eight different series of hulls were successively drawn, each one based on a fresh set of lines. Most of those parent models led to subsequent versions through changes to improve their characteristics up to the point where no more progress could realistically be made and a new starting point was required.
The objective was achieving good shape stability without excessive wetted surface with a volume distribution optimised for low hydrodynamic resistance not only upright, but also throughout a range of heel angles. In the same time, a great deal of attention was paid to the variations in yaw moments as the boat moves in the sea. A moment is a force acting at the end of a lever arm, trying to cause rotation. Yaw moments influence course stability, either by keeping the boat on track or throwing it off. A constant yaw moment can be offset with rudder angle and course stability will still be preserved, but if yaw moments vary wildly as the boat pitches, rolls and accelerates in the sea, the vessel becomes completely unmanageable.
This brought hull, rig and appendages all together into the picture and some of the candidate hulls were further developed towards a complete package and later discarded as further improvement opportunities were identified.
A complete discussion of how the requirements exposed earlier were achieved and how design decisions were made to meet seakeeping targets would be very technical and lengthy. Some specialised software tools were developed to carry out simulations and analysis along the way. Still, a summary discussion may be of interest to some.
At a glance, the Nordkyn hull has a fine entry and curved sections progressively evolving into a long flat run aft. While the upright hull is relatively flat and shallow with good form stability, when heeled the underwater body becomes deeper, much narrower and exhibits a much reduced wetted surface. This allowed combining a very efficient upwind boat in a heeled attitude with a good downwind boat upright.
Attention was placed in the design of the forebody to combat the tendency to slam upwind while heeled and provide lift at more upright angles. The simplistic view that “flat hulls pound upwind” is not truly warranted. At the risk of stating the obvious, flat hulls sailing upwind heel and don’t sail “on the flat”, but rather on the turn of the bilge and if pounding is to occur, this is where it is likely to take place and should be addressed.
I also wanted good reserve buoyancy in the topsides, especially in the forward sections. This prompted departing from the commonly found flat hull sides forward above the waterline. Cruising yachts carry weight on board, the ground tackle is substantial, they also require stronger rigs that are heavier and the outcome is that they achieve higher pitching inertia than an empty shell racer. Without sufficient volume forward, they can engage the bow in situations where a light racer wouldn’t. A typical scenario where things go wrong very quickly is when hitting a secondary wave at speed at the bottom of a through in a huge sea. A forebody that is too skinny will tend to nose-dive into it with extremely unpleasant consequences on board.
Reducing inverted stability was another concern that impacted shape in the topsides. Excessive beam at the deck was not desirable. At its main beam, the Nordkyn hull is near-vertical at the gunnel. More stability and power can arguably be found by flaring the hull out in this area, but this also leads to increased inverted stability. Main beam and the shape of the deck plane were designed to mitigate it. Capsizing the Nordkyn 13 design requires nine times more energy than what is needed to bring it back up from a fully inverted position.
Light Wind Ability Considerations
Performance in light winds largely comes down to sail area versus hull wetted surface. The latter is reduced in designs featuring rounded sections and a narrow stern; flatter hulls are at a definite disadvantage for a comparable displacement. Offsetting this penalty requires increasing sail area. More sail area prompts for more stability, but there the flatter hull can come to the rescue with greater form stability. It is a more difficult balancing act however. Going down this path leads to yachts that are powerful, with more sail area for the same displacement.
The point of interest is that good light air performance is potentially achievable without automatically resorting to a rounded and pinched hull shape. This – incidentally – describes quite accurately the hull of my 1968 Dufour Arpège, which had remarkable light wind ability.
General Performance Aspects
When I started designing the Nordkyn sloop, I didn’t constrain the length of the vessel. Here is why.
The inputs into the problem were:
- I had a very good idea of the payload I need to carry for the type of cruising I am interested in from the voyage of the Yarra. The essential I need was already there. This amounts up to about 1500kg, plus 500kg as a temporary excess allowance.
- I knew what performance class of boat I wanted. Waterline length versus displacement and sail area versus displacement are the parameters that broadly set the behaviour of a design in terms of sailing performance. The former can be seen as a measure of speed potential and the latter is simply a power-to-weight ratio. Assigning targets for these parameters was a simple step and a lot of statistical data is available to help understand where this positions the design in relative terms.
The problem was then a matter of matching up the boat size so lightship displacement + payload versus waterline length and sail area all worked out at acceptable values. Since the payload weight was fixed, making the boat longer automatically reduced its relative contribution and pushed the performance metrics upwards. I wrote earlier that I couldn’t make Nordkyn smaller than 13 metres in spite of some efforts in this direction, and this is why.
In simple terms, I didn’t want a small, heavily loaded boat and so design constraints were added to control this aspect.
Hull Resistance Considerations
When a displacement hull travels at speed through the water, a high pressure zone is created at the bow, and another one around the stern, as illustrated by the bow and stern waves. In-between, a trough forms in the water, denoting lower pressure. Pressure at the bow creates a resistance to overcome and pressure at the stern can help with pushing the boat if the hull shape is conducive to it. Ideally, we would like them to cancel out completely, but this is never quite achievable.
The goal is minimising pressure build-up at the bow and losing as little of it as possible at the stern. The overall mechanism governs the wave-making, or residuary, resistance of the hull
Hull resistance normally becomes very significant once speed is high enough to create the wave pattern described above, once the Froude number climbs around 0.45. The longer the hull, the faster it needs to travel to reach this value, favouring waterline length. Waterline length also opposes pitching of the hull, something highly desirable. A steep stem enters the water and starts lifting the hull earlier than a raked one and a sharp entry with good freeboard leads to a lower bow wave than a blunt one with a raked stem.
In order to benefit from the pressure build-up at the stern, we need a hull shape that can be lifted by the stern wave, rather than leaving it behind: we want to see some area in the waterplane aft. This absolutely rules out pinched or narrow stern shapes. Another critical goal is preventing the flow from separating completely from the hull around the stern for the same reason; this happens readily when the buttocks lines feature excessive curvature or become too steep.
Those are excellent reasons to reduce hull overhangs and straighten the stem. Overhang should be understood in the sense of a part of the hull length that does not normally become immersed under way. We see many yachts with a “maximum waterline”, in other words with the bottom of the transom immersed and no stern “overhang”. Short of being extremely fast all the time, this is first and foremost a good way of achieving maximum wetted surface and turbulence issues past the stern.
Hull resistance increases dramatically when pressure on the hull rises at the bow while being “lost” around the stern. Those are the drivers for adopting a fine entry forward and a straight, relatively flat bottom aft. A hull around which most of the pressure shifts forward above a certain speed becomes very difficult to steer or keep on track.
Course Stability
Course stability is related to keel length in an extremely undesirable way. Keel length is detrimental to manoeuvrability and poor manoeuvrability usually improves course stability
A long keel is the worst way of achieving course stability, because it does nothing to address the root cause of instability in the first place: large variations in yaw moments.
As a yacht heels, the sail forces acting up in the rig fall to leeward, above the water, while the hydrodynamic resistance remains with the immersed volume of the hull. This creates a moment that causes the boat to try rounding up. Good transverse stability is a prime contributor to minimising yaw moments; a yacht that is not prone to heeling far and rolling tracks much better. This favours flatter hulls that stiffen up more quickly.
In the case of a relatively flat and beamy hull, hull hydrodynamic resistance also tends to shift to leeward with heel and the turning moment can be further minimised. This also speaks for a flatter hull with some beam against a narrow or rounded one, provided its resistance does not increase significantly with heel – which is achievable, but requires careful attention.
Course stability has absolutely nothing to do with hull lines being somewhat symmetrical forward and aft, or any other concept of “balanced hull lines”. In most instances, those hulls develop severe flow instability issues at speed in the stern region because pressure recovery behind the midship trough can’t occur properly.
Low hull resistance and preventing the hydrodynamic pressure from shifting forward are essential to achieving good tracking, otherwise yaw moments develop between the resistance force forward and the driving force behind it. Furthermore, it is an unstable situation: a yaw angle results in a turning moment that tends to further increase yaw.
While rig forces fall to leeward with heel, the drag from the hull appendages swings out to windward and adds to the turning moment. High keel or rudder drag also contribute to course stability issues. In fact, keel drag can be a major cause of tracking problems downwind as it also leads to a bow-down moment and build-up of pressure forward.
Efficient appendages are important for good course stability. Good separation distance between keel and rudder also makes a far more positive and powerful contribution to tracking than any long keel, because we are dealing with two efficient foils opposing course changes.
If good course stability is achieved through an efficient fin keel and spade rudder arrangement, then manoeuvrability literally comes as a free extra due to the natural pivot point created around the keel foil.
Keel and Ballast Considerations
Nordkyn uses a hollow foil and bulb arrangement for the keel and a high-aspect spade rudder. The keel is by far the single heaviest component on such a monohull. Carrying weight is always detrimental to performance, there is no such thing as good weight on a sailing yacht. For a given hull shape, hull resistance is directly proportional to displacement.
The concept of ballast ratio makes very little sense, it is the vertical centre of gravity achieved that matters
Ballast is needed to provide stability when the heel angle becomes noticeable in a hull such as this one, and for safety as it has a great deal of impact on the angle of vanishing stability. Stability at low angles originates from the hull shape mostly. The most effective way of reducing ballast requirements is carrying it as low as possible, hence the bulb keel design. A bulbed keel of some sort is a primary enabler for designing light, stable and efficient yachts.
Draft is the single most important parameter for upwind performance. However, with a bulbed keel in particular, root stresses become unreasonable at some point and the intended sailing programme of the boat needs to be linked back to structural considerations. Another aspect that prompted controlling draft is dynamic: when a yacht punches close-hauled into a heavy breaking crest in a huge sea and gets buried in, it also gets pushed laterally quite violently and heels over. The deeper the keel, the further it is likely to heel while getting shunted sideways.
It became a matter of designing it “deep enough”, rather than as deep as possible. Nordkyn draws 2.35m and I can live with this quite easily. There were very few places where it would have been a restriction during the voyage of the Yarra.
Deep bulb keels contribute to increasing pitching inertia – or longitudinal gyradius – just like weight in the hull ends. While this is not ideal, hulls with a good waterplane area (i.e. flatter hulls typically), a long waterline and clear separation between centre of flotation and centre of buoyancy typically offer sufficient pitch damping to mitigate it and reap the benefits of the low centre of gravity and/or lighter displacement. Some narrow yachts with fine ends retrofitted with deep bulb keels developed such a tendency for hobby-horsing close-hauled as a consequence that the change had to be reverted immediately; they just wouldn’t sail.
Nordkyn’s hull strongly opposes pitching because of its stern shape, making very suitable for this type of keel.
Rudder
Nordkyn uses a single spade rudder. It is a concession made to resilience to damage, because twin rudders would be more efficient. A single spade rudder is effectively protected by the largest and strongest skeg a yacht can have: the deeper ballast keel in front of it. Rudder skegs are detrimental to flow, rudder balance and steering performance. In addition to this, they tend to come with much smaller rudder stocks and the strength argument is not always that clear-cut.
Here, the rudder blade was balanced to allow using a tiller for steering, even when the foil loads up.
Should Nordkyn’s massive rudder stock ever get bent and jam the rudder blade, provisions were made to allow dropping it down a short distance from within the boat to restore hull clearance until repairs can be carried out.
Twin rudder options were not progressed due to their very real exposure to damage:
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Any junk afloat at the sea surface potentially comes straight into the line of fire to hit the windward rudder. No matter how the rudder is engineered, hitting a log or growler with it at speed on a beam reach would result in very serious damage and complications.
- Another classic case of windward rudder damage is by wave impact in breaking seas. While somewhat easier to mitigate through construction, when bad luck gets involved, the force behind the blow in areas where seas are very long and very fast can be astounding.
Rig
One fantastic consequence of designing a low resistance yacht is that the driving force required remains modest. This means that the sail area doesn’t need to be as large as what is found on a heavy boat of the same length.
In the case of Nordkyn, I took advantage of it immediately and almost eliminated the overlap between genoa and mainsail. Now that the headsail didn’t need to get past the lateral rigging, it became possible to push the chainplates outwards and lengthen the spreaders while narrowing the headsail sheeting angle to its optimum value, so benefits everywhere.
A wider staying base for the rig greatly reduces vertical loads when the boat heels and a non-overlapping headsail is smaller, more efficient and more manageable.
A masthead rig was retained because it is stronger and simpler, it helped achieving a better balance between the areas of the main and headsail and is more flexible as more drive can be obtained from a headsail alone at times. This is not to say that there is anything wrong with a fractional rig; such an option could be developed very easily with other benefits.
Under no circumstances was I going to accept runners or an inner forestay. Inner forestays can be very handy, but simplicity in manoeuvring and the elimination of the risks associated with runners was more important.
As long as the foot of the forestay remains in place and the aft end of the boom doesn’t move, one can generally shift the mast and trade sail area between the mainsail and foretriangle without great consequences on balance, especially with this type of sailplan. I wanted headsails that could be replaced as wind strength increases and reefing a mainsail is never too onerous in terms of effort, which favoured a large mainsail.
Allowing the mast to move forward of the keel also meant that on a run, with the boom fully paid out, all the drive originates forward of the appendages and this also contributes to improve tracking.
The remaining task was achieving fore-and-aft rig stability so the mast would not pump upwind. This was resolved by using a longitudinally stiff section, slightly raked back and pre-cambered. A mast that gets into a reverse bend situation is at risk of collapsing. The double swept-back spreaders design originated from there as it creates the stability needed.
Another benefit of this type of rig is that capshroud tension on the weather side tends to naturally pull the masthead back and tighten the forestay. The magnitude of the contribution is higher than one would think.
Strength, simplicity and resilience aspects also led to using a fixed backstay. This determined the amount of roach that could be designed into the mainsail.
Lastly, the mast was keel-stepped. This arrangement is much stronger and does wonders for rig stability, as no pivot point at deck level is created. It also enables using a lighter mast section in most cases.
Finally
After one of the last hulls of the last series had remained unchallenged for several months, a 1:10 scaled fibreglass model was built in three days over New Year and then towed to validate wave patterns at different speeds and heel angles.
The outcome is a design that is not extreme in any way. There were no benefits to be found in adopting somewhat radical hull shapes, well on the contrary; instead the Nordkyn hull ended up as a very subtle balance of a large number of variables that delivered remarkable seakeeping characteristics and sailing ability.
Building the boat was always the intent. For every day in the construction of the scaled hull model, it took about six months – part-time and single-handed – in the workshop to achieve the same. A yacht being more than just a hull, the project took a little longer again after that.
Hi Eric,
You say……” it is the appraisal of those qualitative aspects and the fundamental understanding of what matters, how and why based on years at sea that was going to differentiate the project.
At times, this involved taking some design risks and departing from common trends.”
What were the risks?
Regards – Peter
Peter,
When you depart from what is already out there, the risk you are taking is getting a boat that doesn’t perform to expectations, or exhibit surprising, undesirable characteristics. Innovation is not a decision to be taken lightly, due to the potential cost of a mistake.
This explains why, in the field of yacht design, we see very, very little real innovation. There are lots of names, but most merely make their own version of what already exists. Then the whole discussion revolves around looks and features: the actual sea-going aspect of the boat is quickly swept away. This is not interesting.
When everything goes well, the styling and features don’t actually detract from seakeeping. Unfortunately, most of the time, they do, considerably.
Once you start making true design decisions and deviate from the norm, you can bet that you are going to get feedback about your performance as a designer: sea trials will ensure that. It also means that the outcome of the project can suddenly look less certain, and even more so when you consider that most designers are not hydrodynamicists and can’t predict the flow around a hull shape or its performance from physical principles, by a very long way.
Yacht design is first and foremost a game of copycats: basically the same, just looking different. In the cruising arena, the competition is on “features”. In the racing world, much more effort goes towards low weight, construction and material aspects than shape. It is easier and safer. Many radical “go-fast” shape options proved to be nothing less than disasters: they were found to have drawbacks. Often, a better understanding of hydrodynamics and the physics of sailing could have spared the experiment altogether too. In the early days, I spent a lot of time looking at “failed” racing yachts on the hardstand: there was so much to learn at zero cost!
The sloop Nordkyn is quite different in many ways than common modern yachts and it was even much more distinctive when created back in 2001-2003. I wanted some specific characteristics that were problematic to find in an existing boat and I had spent a lot of time thinking about those things at sea in all weather conditions.
I developed my own performance prediction software while designing it, because I needed this to explore the interactions between design parameters. It is too complex to guess them. I also created a large number of hulls over almost 3 years and assessed each one of them at various angles of heel before finally settling on one. It was about balance, course stability and performance both up and downwind. After retaining a design, I took the time to build a model of the hull and test it. The launch Wild South was also model-tested: at first, making a light displacement, unballasted and yet fully self-righting motor yacht looked like a risky bet in terms of motion and comfort at sea.
In comparison, the design of most yacht hulls is something that gets done and dusted in a few days at the most, and often in less than a day with 3D CAD and hydrostatics instantaneously available. If you are experienced at drawing hulls, you can come up with something comparable to what is out there in a matter of hours while also hitting your displacement target and all other hydrostatic constraints.
A great deal of “design” time then gets spent fluffing around the look of the boat, the features, the interior, the “systems” and other gadgets to be offered. This is not naval architecture, it is just a styling and marketing exercise to sell the boats. It does nothing to make good sea boats.
If instead you want your hull to achieve specific properties at different angles of heel, keep other aspects constant, ensure there is no coupling between heel and trim and also meet conditions dictated by hydrodynamics or design choices, then hull design can turn into a fairly challenging and non-trivial exercise.
Some offices do carry out design research – like Bray in Canada – they think about the physics and make their own decisions, but those are few and even more so when it comes to cruising boats. In fact, the more “cruising” boats are, the worse they tend to get.
What people don’t realise is that sailing a tub day in and day out is unrewarding hard work at best, and dangerous at worse; no amount of features and gadgets can offset that either. Cruising on board a brilliant sailing machine instead is a lot of fun, it gets you there in less time, with a fraction of the effort and at a lower cost.
You can always dial a high-performance boat down if you feel like it, but there is nothing you can do to improve a tub. The reality is also that, most of the time, conditions are decent and there is nothing to dial down.
I mentioned Bray Yacht Design because I fully share their views about ballasting: departing from “accepted” ballast ratios was one of the many things I did with the sloop Nordkyn and it didn’t prevent it from achieving a high AVS and plenty of power.
Best regards,
Eric
Hi Eric,
How much of the development time was spent evaluating and testing keel bulb options? Nordkyn’s reminds me very much of the few published photos of post-12metre AC boats. And did you have to spend much time choosing a foil section?
Thanks – Peter
Peter,
Keel bulbs: a bit. There are design trade-offs with bulbs. Long and skinny gives more wetted surface and less pressure resistance up to a point, but the advantage is more prominent at high speed. Designing for high-speed has been the downfall of many racing yachts, because it easily makes them poor in all the more common conditions where high speeds can’t be obtained: not enough wind etc.
Flattening the bulb allows lowering the centre of gravity a little further for the same draft, at the expense of a slight increase in wetted surface.
Bulbs should also have their maximum girth as far back as practical. Maintaining a positive pressure gradient for as long as possible helps with delaying boundary layer transition (in other words, more of the bulb keeps operating in the laminar flow region) and it lowers the drag.
Without the option of using ballast material heavier than lead, I needed a volume of around 220L in it. I considered various options and I ended up modifying a NACA 66-series profile.
Today I would run some CFD on it, but that wasn’t an option for me back around 2004 when the keel was designed.
I spent a fair amount of time working on the foil. The racing crowd wants keels as thin as possible: the drag reduction is ridiculously small; the strength reduction, on the other hand, is huge. Narrow keels also stall very easily. A thicker foil doesn’t hurt the design much, is much stronger structurally and can keep operating well in conditions much less favourable.
I found that I usually outpoint other boats and I don’t seem to suffer from a speed penalty. Last year, between Christmas and New Year, I was sailing upwind in light weather towards Cape Colville on my way to White Island when one of those racing crews with Kevlar sails decided to take me on. I watched them battle for an hour, tuning and sitting on the weather rail. I was having a cup of tea and the Windpilot was steering. When they were pointing as high as I did, they were falling further behind and when they matched my speed, they were falling to leeward. I was steadily pulling further away effortlessly. They couldn’t do anything about it. In the end, they threw a tack and disappeared towards Great Barrier.
Foil sections must be chosen for the anticipated leeway angle, which itself depends a lot on the foil area and there are quite a lot of things to think about. My VPP code was extremely valuable to assess the various trade-offs as I had access to all the data from the keel model.
Assuming that the boat will always be going fast is just wishful thinking and a mistake again in most cases. Unless you design a machine with enormous sail area/displacement and sail area/wetted surface ratios, it doesn’t really happen and the water is not always flat. When you are ocean cruising in difficult conditions, a bad keel can stall/fail to produce lift when you need it the most and put you on the shore. It is silly to try and cut it right at the limit.
The keel design must however be slippery and able to handle a lot of speed without any issues, otherwise it can pitch the bow down with disastrous results on the run.
Like with hull shape, the keel design is one of those areas where you can make big decisions and take risks one way or another and you are going to get feedback about what you have done once on the water.
Regards,
Eric
Hello,again.
The more I read of your design development the more questions come to mind.
For the rig did you ever or do you consider that a Carbon Fibre Mast and and Boom would have advantage over the Aluminium one that you have.
I do appreciate that finance might, in the end have been a persuading influence upon your decision. If you had had the finance available would you have fitted one?
I see today that there is some movement towards standing stainless steel standing rigging being replaced by Man made Fibre,supposedly to reduce weight too.
Hello James,
Of course, carbon spars would be very attractive for this boat, but keep in mind that the Selden rig – excluding standing and running rigging – represented over 20% of the total material cost already. Going to carbon would have doubled the price of the spars at the time. From a different angle, carbon has its long term issues as it can galvanically corrode almost anything else including stainless steel, but I don’t know enough about what happens in practice to comment further.
Cost in general wasn’t really an issue when I built the boat, I built for material costs and no labour and I frequently just went for the best (and not the most!), but within reasons. Carbon spars would have been beyond justifiable, but with the alloy mast I also knew I was going to get an extremely reliable and long lasting solution. I still tend to think today that an alloy mast will outlast a carbon one, rightly or wrongly. Even if I had been able to get carbon at no extra cost, I would have studied the matter carefully before going for it, and the outcome is not certain.
When you face the coils of wire and all the fittings in a crate, the weight of stainless steel rigging is amazing. My alloy mast section itself weighs 160kg and from memory the standing rigging competes with this. Stainless steel still has the advantage of being extremely resilient to abrasion and it doesn’t care about UVs. The lateral rigging on modern superyachts is usually continuous carbon fibre and they peel strands off at the each one of the spreaders. These things are being done, but I don’t think I would be too happy with a very thin little bundle of fibres leaving the deck to hold the mast on a 43′ ocean cruising yacht in the high latitudes and remote places. Even stainless rigging is hard to repair in many parts of the world unless you happen to have everything you need on board.
If the context/intended use for the boat was different, then of course all these considerations would come on the table. If you were not going to carry a couple of tonnes of gear like I do, then you could also carry a little more ballast in the bulb, reach even deeper with the keel and fit an even larger rig on the boat. It is impressive as it is already and it could go beyond that. A few months ago, I clocked 23 knots SOW with long periods over 20 knots while sailing up Cook Strait in a gale. You stand in the cockpit watching the spray blasting past the hull side like a giant fire hose wide open as the apparent wind gets amazingly low for the sea state and you tell yourself that you don’t really need to be doing that. It is all fine and under control, but you can’t help but think that your margin for error could be quite narrow. I used to feel the same around 14-18 knots and now I don’t pay much attention, so maybe it is a matter of getting used to it, but there has to be a limit somewhere.
Best regards,
Eric
23 knts is quite terrifying / exhilarating as the best I have achieved over any length of time was 12 knts down the Portuguese coast in a gale.
Whilst I had a very reliable and powerful autopilot I hand steered, one wonders at your speeds whether you trust to a windvane or prefer to “keep your life in your hands”?
Krs
James
I think the answer depends on the boat. If it tracks well at any speed like Nordkyn, there isn’t much of an issue. If you get pushed around and need to anticipate the yawing motion caused by the waves, then only a person at the helm can do that (but there can be ways of improving/fixing this with the sail plan to get it to self-steer).
In this case, the vane was holding it fine and I was inside most of the time. There wasn’t much for me to do outside and hanging around outside in heavy weather is always more risky than being down below. I never stop the boat and never stay outside for no good reason in bad weather. Later, I arrived in a narrower spot between the coast and some islands and rocks (the Brothers) and I took over steering because I needed to be almost dead downwind for a while and quite precise with my heading.
Coming from a late-1960s 30′ sloop with a top speed of 6.8 knots (the maximum I ever saw was 11 knots coming down a big swell sailing towards Staten Island), averaging 10 knots almost effortlessly and climbing up to 18 knots was a huge shift in the beginning. There is a lot less apparent wind on the run, but you can’t slow down or stop, or worse try to turn around. That morning in Cook Strait, I had the whole place to myself. The average wind speed was around 45 knots according to the weather people, but you wouldn’t have thought so on board. It was almost pleasant with 15-20 knots knocked off by boat speed. When I had gone from one to two reefs in the main earlier, it slowed the boat down momentarily with a large increase in wind speed and I was really pleased to accelerate again as I was winching the reefing line in. It is a different way to sail.
Kind regards,
Eric
Where is Nordkyn now, Eric? Have you done any major passages? Also, how is your experimentation with ultrasonic antifouling going?
Hello Dave,
Nordkyn is currently anchored in Paterson Inlet at Stewart Island as I type this and the NW is picking up outside. Work/projects keep coming in the way of long ocean passages, but I have clocked the 1000NM NZ North to South and back a number of times in recent years. I no longer bother stopping much along the way, I have seen the coastline. I just sail with whatever I get some 40-50NM out where it is nice and quiet.
No further changes to the ultrasonic system in nearly a year now. Down here the sun is 10 degrees lower than in the Hauraki area and it is not good for my horizontally-mounted solar panels, so lately I took the unprecedented step of turning it off at times. The idea is having more energy to feed into the computer of course. The water is clean and cold and the antifouling is only 8 months old, but I do not like the result at all! I had a look at the rudder blade today and powered it up again right away. I think I was starting to take the result for granted, but it is a necessity for me now. I might try tweaking the low-power mode so it pauses longer between bursts and see what happens. That would be a lot better than nothing. Last time I increased both the power density and the maximum frequency and it was much better, but I can only conclude about the combined effect, so lowering the power density only would be a worthwhile experiment.
Kind regards,
Eric
Interesting, thanks for responding. What are your plans long-term for Nordkyn?
A long voyage back to Alaska and beyond has been on the cards for quite a while.
It is also looking increasingly likely that a slightly bigger version of the same will get built before too long. This would be remarkable, because the new build market has been extremely thin for ocean going yachts since 2008 and this is more a boat for discerning sailors.
That’s great to hear. Where would it be built? I assume it’s not for you and you’re happy with Nordkyn as is?
I certainly wouldn’t bother designing a boat if it wasn’t going to be built! It is only being discussed at the moment, somewhere in Europe. As far as I am concerned, I am perfectly happy with what I have got and this can be my last boat. In terms of volume and length, Nordkyn is a fairly big boat for a single-hander or a couple and it would easily carry a small family. It only gets too small if you want to carry more weight than what was intended.
Eric:
I have, with great interest, read most of your writings on this site.
It is very generous of you to avail your audience of your narratives and conclusions. Your plain statements are refreshing.
I am not an experienced sailor, but have always loved sailing, and am grateful of your comments.
I am curious: was your prior vessel, the Yarra, a Dufour Arpege? Your opinion of the Arpege?
It appears you consider it’s seakeeping traits better than many other boats of similar size?
If you were unable to build or have built the boat you wanted, are there any production boats the size of the Yarra, in addition to her, for your purposes, for which you would settle?
Many thanks,
Mike Warbington
Hello Mike,
The Yarra was a Dufour Arpege indeed. It was an excellent sea boat (I wouldn’t be here if it hadn’t been) and it was particularly fast in light winds. It wasn’t perfect in the sense that the hull hit a speed limit around 6.8 knots on the run and it gave the vane a fair bit of work steering downwind in a seaway, but it never did anything stupid like burying the bow. It never broached too badly either, but I was careful in this direction. In bad weather, at least 50% of the result – good or bad – comes from the decisions made by the skipper.
It was quite strong structurally, but not entirely bulletproof, and it held together with the help of some specific strengthening around the attachment of the bulkheads.
Your second question is a little bit harder to answer because I don’t look at production boats these days, but I did inspect a Dufour 325GL closely for someone not too long ago and I must say that I was favourably impressed by its construction. It is a more modern design and I think it should deliver better overall performance than the Arpege with improved downwind characteristics.
Another very seaworthy older boat with sailing characteristics similar to the Arpege is the Miura, very common in South Africa and it is quite modestly priced now. It is about all I can say I think.
Kind regards,
Eric
Hello Eric,
Thank you for sharing all of this most valuable information; I have read everything on this site (as well as a lot of your comments in forums) in regards to hull design and how different forces come into play in heavy weather, and it has truly taught me a lot. It has challenged some of the popular (mis)conceptions out there as well as reinforcing some of my own feelings on certain ideas. I have long been doing hardstand tours and, since happening upon this site, have been able to garner a lot more from this practice. The same can be said for observing wave patterns made by boats as they enter or leave the marina.
The point of all this is trying to educate myself as much as I can before making my own yacht purchase. I am bound by a fairly low budget (~$60k — alas, no Nordkyn for me just yet), and have some fairly specific criteria to consider in the design department. If you have the time, would you share your opinions/estimations/reservations about a few (locally) well-known boats? I would find it most valuable — primarily for my ultimate cause, but also to apply some of this theory to hulls that I already know (which may be interesting to others as well).
– Farr 1020, 11.6/38, 1104 (IOR distortions) and 1220
– Beale 33 and 11.2
– Van de Stadt 37 (hull too deep behind keel then rising too steeply aft?)
– Lotus 9.2 (rudder), 950 and 10.6 (something iffy about the stern?)
– Young 88, 11
Some of these are slightly out of my price range, but I’d still appreciate any of your thoughts on any of these boats (or any others) if you have the time.
Kind regards
Daniel
Hello Daniel,
Thank you for your comment. There is lot to be learned indeed from visiting hardstands and talking to the sailors, especially when there is quite a modern, competitive fleet around. I couldn’t comment on designs without having a close look at the hull shapes and it is not automatically easy to find suitable views for each of them. I don’t have the time at the moment to try and get such data either and it is not where my interest lies anyway.
Old IOR boats are usually best left alone because of their poor downwind characteristics, and even more so for the later ones with distorted hulls. Many had discontinuities faired out afterwards because of the course stability and steering issues they caused. The only good thing about IOR is that it was such a fiasco that it is unlikely to ever be repeated again.
The Farr 38 is a boat I have seen up close, old design, but quite a nice hull. The deadrise in it has no advantages and leads to somewhat slack bilges and less stability (and power) than it would otherwise have. Many of them were built in South Africa in fibreglass and they were of excellent construction.
The rudder arrangement of the Lotus 9.2 is atrocious and a few years ago I actually assisted with the design of a new one for a boat based in Nelson. We eliminated the skeg completely and reduced the lateral area, using a transom-hung blade, and it was a new boat after that: speed and manoeuvrability. The rig is also too small, but it is a different issue. It is a hull with good handling characteristics.
A Beale 40 took me on a couple of years ago just as I sailed out of Islington Bay at Rangitoto. It had a clean hull and a crew of three, dead keen. Stiff southwest, 20-25 knots. I was only going a few miles upwind (Orakei) and hadn’t bothered with a headsail, so just hauling upwind steadily and tacking under full main at 6 knots or so. They tried everything: pointing higher, lower, trimming one way or another. No answer, I just pulled away steadily at every tack while standing in the companionway with a cup of tea. No idea what is wrong with it, but it was a ridiculous show and I don’t think I would even bother looking at the smaller ones.
The Youngs could be more interesting than all the others.
Kind regards,
Eric
Hello Eric,
Thank you for your reply. Apologies: I did mean, but failed to say, only if you already had opinions of them.
I hadn’t actually seen a Farr 38 out of the water before, but after reading your comment I did find some pictures online and I can certainly see what you mean about the deadrise angle and consequent slack-ish bilges. I have only one picture of a 1020 out of the water but it looks like it may be similar — I will investigate further.
Currently on Trademe is a Lotus 9.2 that has been extended ~400mm and now sports a Wright 10 rig; a very interesting prospect, though I can’t quite make out from the pictures whether the rudder has been modified or not. Good to know that that particular issue can be successfully remedied though. The hull is over 40 years old now, which isn’t ideal, but is priced accordingly I suppose.
The Beales are interesting because they look like they should sail well, but yours is the second account I have heard detailing one performing in an underwhelming fashion: the other was from a guy I got talking to who had built one — a 31, I believe — as a racer but couldn’t get it to plane in conditions where it would have been otherwise expected. One thing I have noticed on the hard is that they often *appear* to to be deepest (slightly) just aft of the keel; though far from uncommon in other designs, I understand that this is less than ideal. I’m really not qualified to speculate, but it does intrigue me as they are a nice-looking hull otherwise.
The Youngs generally perform very well from what I gather, but I have no first-hand experience with them. Certainly for their day they are competitive, though I get that this does not automatically translate to good heavy weather behaviour.
Something I have been pondering for a while is how, in design, you achieve a well-balanced helm. Or, maybe more to the point, how you would achieve an imbalanced helm. I’ve noticed that some boats need a lot of windward helm when sailing upwind in a breeze, which obviously generates drag, but I cannot quite comprehend the cause. Initially I thought it might have a little to do with rig balance and the placement of the mast, but that can’t be the entire solution. Is it simply the arrangement of the underwater appendages in relation to each other and the WLL, such as the keel being too far forward?
Thanks again for sharing your perspective.
Kind regards
Daniel
Daniel,
The Farr 1020 doesn’t have a discontinuity on the centreline astern. Forward of the keel, I don’t know.
Extending a Lotus 9.2 is a good idea as it would improve the flow around the stern at speed. Having the transom on the waterline or even slightly immersed is hardly ever a good idea, it increases wetted surface and most yachts are just not fast enough to benefit from it. Here, it would also allow transforming the transom-hung rudder into a spade underneath the hull. Having an immersed surface above the rudder and a very small gap increases its apparent aspect ratio and effectiveness compared to a surface-piercing rudder.
When I was watching the Beale 40, the exact same thought crossed my mind: it looked like a boat that should sail acceptably, but it was a tub. Copying the looks is not enough.
The curvature of the lines in the hull, the buttocks aft in particular, and its position are very important. Every time the flow gets deflected, it produces a high pressure zone on the convex side and a low pressure zone on the concave side of the streamlines. This explains the bow wave and curving the run of the lines aft causes a low pressure in this region to appear and it depresses the free-surface. The stern gets sucked into a hole as a result when the speed increases. One particularly awful example of this is the Hallberg Rassy 44 (https://sailboatdata.com/sailboat/hallberg-rassy-44) where they clearly wanted to lower the engine to gain interior volume.
Balance is something discussed above already and it is a lot more complex. Hull shape, the position of the rig and its height, the appendages and stability all have a role to play. Weather helm arises from having to produce a force-moment with the rudder that cancels the force-moment produced between the resistance of the hull and the driving force from the sails to keep the boat on course. A balanced design is a design where the driving force tends to stay quite closely above the overall resistance force.
Kind regards,
Eric
Hi Eric,
I’m a sailor with vague ambitions to build my own boat one day; I’ve found your page a mine of information and I very much agree with your design philosophy. Thanks for your willingness to share your expertise with the rest of us. Just wondering if I could ask you a few questions:
You often talk about sail area and how it is designed around parameters like displacement and waterline length. Had you considered adding a bowsprit to increase the fore triangle area and what were you reasons for not (structural, balance issues, aspect ratio, etc)?
You say that your usual expedition payload is a ton and a half or so. What does this look like in practice? Is it mainly provisions and hardware for the boat or is a large proportion made up of things like shore going kit, dive gear, etc?
How would you change the design of Nordkyn if you wanted to carry more or less payload?
What are your thoughts on 1970s era cruiser racers such as Contessas and S&S design (the less IOR oriented ones) in terms of hull form, stability and seaworthiness?
Cheers,
Greg
Greg,
Adding a bowsprit would unbalance the rig unless the boom was also lengthened and it is very long already. The total sail area would also become unreasonable. When I first went out under sail with Nordkyn, I had a near-empty boat and it was quite flighty even with the deep bulb keel. This was expected, because I designed the boat in ocean cruising trim. A bowsprit would be useful for flying an asymmetric, but it is not a necessity and features that rarely used are often best left out. It would need to be retractable in order not to become a problem and it complicates the situation around the bow and with the anchor locker.
In terms of payload, a year’s worth of provision for one or even two people is not as impressive as one would expect. The weight is made of cruising gear (dinghy etc), tools (very important on long voyages in remote places), various materials, books, charts and other documents on top of the provisions and water.
If you start carrying scuba-diving gear, compressor and the like, you can quickly blow your weight budget, but there again, it is a matter of priorities. Someone who sails to dive could decide to do it, but carrying it just to have it makes no sense to me. I do have a self-built hookah-type arrangement however, it is important for cleaning the hull.
The payload to be carried is what led to the hull length of Nordkyn. I would have been happy with a smaller boat. If it was going to carry hardly anything, it could be as small as 38′ maybe and a bigger payload would lead to a longer boat. As a boat gets longer, its length/beam ratio increases naturally, because stability increases more rapidly. I wouldn’t expect much else.
S&S designed quite a few of the Swans in that era and they made very decent sea boats. I owned and sailed a 1968 Dufour Arpège for many years and it was an excellent (but not perfect) sea boat overall with its fin and bulb keel. Its very high sail area/wetted surface ratio made it outstanding in light weather. The Contessa 32 is a dangerous boat in a fast following sea and I don’t think you could even pay me to go to sea in one of them. Too much wetted surface and sluggish when it would be the most enjoyable to sail.
Kind regards,
Eric
Hi Eric, I have studied, read and reread all of your articles. I have learned so much. It’s almost a higher education on sailing and hull dynamics. Hope you’re doing well and enjoying the adventures sailing. Question: do you know the design of the Salona 41? Although in GRP (solid under the waterline with vinylester resins, steel frame) it has – in my thinking – more or less the same hull shape (also 2,25m lead bulb keel option). I really like the design, build-quality and such. Could you express your opinion on this vessel? Would it be seaworthy enough? I have a bit of doubt about the weight though (7,4t). https://sailboatdata.com/sailboat/salona-41
Hans,
I didn’t know this design, but I found a profile drawing. You are right about similarities with the Nordkyn 13, they are around the same area in the design space. I don’t see an issue with the 7400kg displacement advertised, but this is an empty boat and they really mean empty. It is not a realistic sailing condition. The hull shapes are quite different however. The Salona is deepest under the engine/companionway as many cruising boats are. It creates more headroom and volume further aft, but it also results in a strong curvature upwards towards the transom after that. At high speeds, it can cause flow separation and the hull resistance to shift forward. Because the longitudinal volume distribution in the hull must be preserved, it also prompts for a narrower waterline aft to compensate. This makes it easier to obtain a hull that won’t trim bow-down when heeling, but it is not as powerful and it prevents lifting some of the hull out of the water upwind. The upright wetted surface will be slightly less, but it won’t reduce much when heeled.
The Nordkyn 13 is deepest just aft of the mast and it has a lot longer and much flatter run into the stern. All the ballast is also concentrated into the keel bulb and it makes a more powerful boat that can reach very high speeds while maintaining perfect control at the helm. I designed it this way to optimise the handling in long fast following seas and gain more power and upwind performance, but it was very difficult to achieve. It took 3 years of sketches and analysis to gradually get there.
Kind regards,
Eric