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.
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 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.
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:
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.
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.
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.