Aug 222013
 

This article deals with some of the physics involved when a yacht is running before the sea, especially in heavy weather when waves are long and their front faces become steep.

Yachts running in heavy weather and following seas face challenges that are very dependent on the characteristics of the design; the ability of the crew to fully control the situation can be much reduced and this is why understanding the dynamics of the situation becomes quite important in order to anticipate properly.

Driving Forces in Following Seas

One immediate point of distinction when running before the sea with any small to mid-size craft is the acceleration usually imparted by approaching waves. Not all boats accelerate the same, but a constant is that – provided the wave face is steep enough – all boats will try staying ahead of the crest. Some do so elegantly, and others very poorly.

When a wave approaches a vessel from the stern, it is not so much that it pushes the hull forward – it doesn’t, but instead it simply creates a slope and the boat is carried down that slope by its own weight, like a cart rolling down a hill.

Driving component of vessel weight in following seas

The red force is the driving component of the weight of the vessel. The steeper the wave, the greater it gets.

This contribution is surprisingly large when compared to sail driving forces. We can compute the approximate sail force from a jib of 14m² downwind in a 50-knot apparent wind as follow:

Wind speed v = 50 knots = 25.7 metres/second Air density ρ = 1.29 kg/m³ Sail area A = 14 m² Approximate sail drag coefficient CD = 0.66

Driving force = 0.5 x CD x A x ρ x v² = 0.5 x 0.66 x 14 x 1.29 x 25.7² = 3936 Newtons

To get a feel for this, 3936 N is the force required to support a mass of 3936 / 9.81 = 401kg. In simple terms, the jib is pulling the equivalent of about 400kg (or 880lbs). The windage of the hull, mast and shrouds further adds to this.

Now, if the wave slope is a mere 3° only and the displacement of the yacht is 10 tonnes, the component of the weight that is aligned with the inclined sea surface – and the speed of the boat – is 10000 x 9.81 x sin 3º = 5134N, equivalent to 523kg or 1155lbs!

Downhill in following seas, gravity very quickly exceeds sail forces as the wave starts lifting the boat

If the wave face is very steep, ultimately, a large fraction of the weight of vessel can get converted into driving force; this is why even heavy hulls with unlikely shapes can be seen getting on the plane in front of steep waves in heavy seas.

For small angles, this drive originating from the weight of the craft is essentially proportional to the slope of the wave: for a 6° down angle, the force would nearly double. If the boat doesn’t accelerate readily, the wave keeps catching up with the hull, lifting the stern further and the driving component of the weight keeps increasing, trying to accelerate the boat anyway.

The crew’s ability to control driving forces is essentially limited to sail forces and – unfortunately – those may at times represent a small fraction only of the total propulsive force incurred by the craft. Hence the well-known risk of “losing control” while running in heavy weather. The answer, however, is not automatically looking for ways to “apply the brakes”.

The Effect of Hull Resistance

Understanding the concept of hull resistance in the context of running downwind is crucial: ultimately, hull resistance is what determines how far the driving force is allowed to rise.

As the total driving force grows larger with the slope of the wave getting steeper, boat speed increases markedly, and so does hull resistance. Once hull resistance matches the driving force, an equilibrium is reached and speed stabilises.

Forces on yacht running before the sea

Once hull resistance (purple) equals the sum of the driving forces (red), speed stabilises.

If hull resistance increases quickly enough in relation with speed, the driving force may not be sufficient to keep the boat in front of the wave crest; this happens all the time to all yachts in light and moderate conditions. As long as the face of the wave is not too steep and the crest is not breaking, the wave can pass underneath the boat without issues. However, if the wave rears up, curls and breaks, the near-vertical front face and crest area are dangerous places where to be.

In the case of a breaking sea with a steep front face, the more easily the boat accelerates, the further away it remains from the turbulent water in the crest. A high-resistance hull requires a higher driving force and will ride higher up on the wave, down a steeper slope, often surging forward only as the crest arrives behind. This is a characteristics that is normally easily detectable long before sea conditions become dangerous.

Yachts in following seas

Hull resistance is the most important factor in determining whether a yacht will keep ahead of a dangerous crest or not.

Different hulls present different resistance characteristics as speed increases. The key determining factor in terms of how resistance increases with speed is hull shape. Displacement in itself adds to resistance, but it also contributes to driving forces downhill.

Hydrodynamic resistance of hulls depending on shape factors

Easily-driven hulls with a suitable shape see their resistance increase more gradually and flatten out as they reach planing speeds. A high-resistance hull may or may not accelerate; in all cases, the forces involved are much more considerable.

Deep hulls with slack bilges and narrow sterns usually are the worst when it comes to building up resistance due to poor stern hydrodynamics and their inability to lift and accelerate regardless of the driving force; those essentially follow the solid orange curve above and will not accelerate, no matter what. Most small yacht heavy hulls still feature reasonably firm bilges for stability reasons and their bottom surfaces are able to provide some dynamic lift above a certain speed. A lot of cruising yachts fall into that category. At some point along the orange curve, they lift and accelerate quite suddenly, following the dashed line.

Modern hulls with flatter stern sections and medium displacement progressively move towards the purple curve. Those are easier to drive at all speeds and don’t face such a marked transition between displacement speeds and a semi-planing regime. Provided other conditions such as course stability are also met, those are by far the safest and most predictable for running in following seas. It doesn’t take a light racer to run at speed in front of a steep sea, mainly suitable stern shapes and a moderate length/displacement ratio.

Many cruising boats have been designed under the assumption that speed was unimportant, or even that the boat was not and should not travel fast in the case of some well-known double-ender designs. This was overlooking the simple fact that the crew has little or no control over driving forces in a heavy following sea, and by extension little or no control over the speed the hull may actually achieve.

While a yacht doesn’t need to be generally fast to remain safe, it does need to be able to accelerate and reach high speeds at times without becoming dangerous

Typical Accident Scenarios in Following Seas

There is no good or safe way of getting caught into a breaking crest; there is too much energy being expended and too much uncertainty about what the vessel might do.

Some designs fail to accelerate as seas get steeper and do get caught in the crests; at this point they experience various difficulties such as maintaining steerage while surrounded by confused white water, or shipping seas over the stern. Another hint is hull trim in relation with the sea surface: is the hull riding bow up or burying the stem when accelerating? Those are very powerful leading indicators of what is likely to take place in bigger, steeper seas. As always, careful observation and understanding of the dynamics of the boat in the sea are everything and so-called “knowledge” is hardly relevant.

Pitchpole

A pitchpole happens when a boat rolls over longitudinally, end-over-end. It is the most publicised, feared and also least common cause of accident at sea.

The most dangerous situation develops in heavy following seas, when the front face of the waves can become overwhelmingly high in relation with the length of the vessel. In this instance, a boat that hasn’t managed to accelerate and outrun the crest can pitch down by the bow severely, and even engage the foredeck in the sea.

I once met the ex-skipper of a Contessa 32, who had left South Africa bound for Australia via a moderately southern route across the Indian Ocean. Along the way, a severe westerly blow developed, accompanied by heavy following seas. She described to me the boat planing in front of the waves out of control with the bow and part of the foredeck under green water. Shortly after, it pitchpoled and was left dismasted and seriously damaged structurally. Reaching Western Australia under jury rig took several weeks. Over a year was spent on the hardstand repairing the damage incurred.

A very interesting and detailed account of accident in following seas was provided by Miles Smeeton following his attempted passage from Australia to England in 1956 in the classic “Once is Enough“. His heavy 46′ ketch rode up the near-vertical face of a huge following sea in the South Pacific and pitchpoled. It couldn’t outrun it and couldn’t ride it either. On a subsequent attempt, following ill-advice, he stopped and hove-to in heavy seas and was readily rolled and significantly damaged again in conditions that were far less extraordinary than the first time.

In spite of the examples above, yachts that have actually been pitchpoled are very rare exceptions. Most yachts don’t track straight long enough to get into this situation, or they engage the forebody into the sea, yaw and then partly fall to one side before getting rolled and capsized more conventionally.

Broach

Broaching takes place when a vessel running before the sea no longer tracks straight and comes beam-on into the sea. This is the most common incident while running before steep seas.

As developed earlier, as the wave approaches and the slope increases, a fraction of the weight of the vessel adds to the driving force from the rig and windage, causing the boat to accelerates. If everything goes well, the hull rides bow up and keeps up with the crest until the wave has finished breaking.

Key forces on a yacht running down a wave

Hull resistance (purple) is equal to the sum of the driving forces (red). If the boat travels at the same speed as the wave, it will run until the wave slope reduces; otherwise the crest will keep catching up.

When hull shape is inadequate or when a bow-down attitude develops as speed increases, hull resistance can shift forward drastically while the driving component of the weight is still acting through the centre of gravity, further aft.

Key forces on a yacht running down a wave as resistance increases

This boat failed to accelerate enough and is now running down a steeper slope. The driving component of the weight acting through the centre of gravity is dominant; resistance has shifted forward due to a bow-down attitude or the characteristics of the hull shape itself.

If the boat yaws for any reason, the component of the weight parallel to the surface is no longer aligned with the direction of travel and contributes to amplifying the rotation.

Key forces initiating a broach in following seas

Any small tendency to yaw is immediately amplified by an unstable force moment between the weight of the boat still acting downhill and the hull resistance that opposes forward speed. If the rudder is unable to overcome the yaw moment, the boat will come beam-on to the wave crest.

An inadequate hull shape is one where hydrodynamic pressure moves towards the forebody as speed increases. This compromises course stability and the key risk is a broach that brings the vessel beam-on into the crest. At this point, the boat is no longer running before the sea and this case is discussed in another article dealing with yachts caught in beam seas. The most common outcome following a high-speed broach in a following sea is getting dumped into the trough from the crest, or rolled, or both.

Unlike often suggested, in a following sea, it is not the oncoming waves pushing on the stern of the yacht that cause course instability. Most yachts that are unstable downwind start yawing long before the crest gets anywhere near the stern. It is the displacement of the hydrodynamic resistance forward that leads to an unstable equilibrium of the forces acting on the vessel.

Course stability in following seas and hull resistance are also the object of a related and more detailed discussion in the design of the Nordkyn hull, where superb seakeeping properties were sought not only downhill, but at all wind angles.

Practical Implications

Designs featuring high-resistance hulls generally become dangerous in following seas at some point. They either end up tangled in the breaking crests or accelerate at steep downward angles on the faces of the waves, at the risk of engaging the bow into the sea. Conversely, experience shows that vessels able to accelerate and keep up with problematic seas don’t ship water over the stern and simply stay ahead of the waves for as long as they break.

With considerable drive originating from the weight of the boat, sail choices made by the crew in following seas primarily impact the minimum speed the vessel keeps on the back of the waves and in the troughs. Steep seas still drive the hull close to the speed they travel at and it is not unusual to see even rather sedate cruising designs surging to 20-25 knots while carrying little or no sail at all in a stiff blow in the high latitudes. As long as they ride bow up, track and remain steerable, all is as good as it gets.

This is not always the case however. Most of the problems experienced running in following seas originate from hull shape, with a special mention for the geometry of the stern sections, sometimes compounded by poor rudder design. This means such issues are not readily overcome by choices the crew can make, other than staying within the capability limits of the design. Attempting to run before heavy seas in a vessel that buries its bow or yaws uncontrollably is a near-guaranteed accident before long.

My 9-metre sloop Yarra was once hauling upwind in a huge sea under #2 jib alone in the middle of the North Atlantic. This sail configuration left the helm largely neutral. Half-way into the night, one of the steering vane control lines attached to the tiller somehow parted, the helm moved around and by the time I had realised something was happening and jumped outside, the boat was way off course.

Now this is where things became interesting. I found the sloop broad-reaching at high speed down huge seas and the small jib sheeted in hard forward was now filling from the leech, sometimes taking aback momentarily, and essentially steering the boat in spite of the loose tiller. I couldn’t really improve on that so, working as fast as I could, I mended the steering line in the dark. During all that time, it never once showed even a hint of a tendency for broaching; the sail forward wouldn’t let it. I returned the boat on its upwind course, but also made a mental note of this interesting discovery. Subsequently, I used this concept at the odd occasion by sheeting a storm jib from both sides on the centreline of the boat, using it for steerage rather than propulsion straight downwind. It provided a huge improvement in course stability and safety in following seas, but only worked in very strong winds. Provided the sail was sheeted in very flat, the inevitable and repeated gybing wasn’t much of an event.

Accidents in following seas happen very quickly and are often violent as boats tend to be thrown forward from the crests. Inexperienced skippers are often fooled by the relative peace and comfort that comes with running before the sea and don’t read the warning signs, or won’t act on them, sometimes by lack of an alternative strategy.

One alternative strategy is turning around and punching into it under sail; some designs that don’t handle at all in following seas may simply have no other safe option; designs that won’t run and won’t sail upwind either can end up out of options altogether and shouldn’t be there in the first place.

Towing Drogues

Drogues are anything attached to the stern that will cause increased drag at speed. It can range from a long warp, some warp and chain or more sophisticated and purpose-designed devices like a Jordan Series Drogue, a collection of little parachutes attached to a long warp.

Attempting to tow drogues in heavy weather is a double-edged sword:

  • By increasing drag and overall resistance, a drogue pulls the yacht further up the slope of the wave as it approaches, into a steeper gradient and closer to the broken water of the crest.
  • By introducing an additional resistance force located as far aft as practical, the drogue can mitigate or cancel the shift in hydrodynamic resistance towards the forebody of some hulls. This can greatly help with preventing excessive course instability from developing.
Using drogues in following seas

Towing a drogue shifts the average position of the resistance forces closer to the centre of gravity and helps cancelling unstable yaw moments, but at the cost of increasing total resistance and pulling the vessel up the wave slope.

I personally don’t favour the use of drogues in general. I wouldn’t think of deploying anything alike unless there was a clear-cut and observable reason for doing it. I will rather stabilise tracking using aerodynamic means as described further up than slow the hull down in the sea. Different boats exhibiting different properties in following seas may be forced to opt differently.

Towing a suitable drogue can provide considerable benefits for boats that lack the course stability required to run in following seas. How much and how long for is a matter that will still depends on the hull design; drogue or not, the boat will still accelerate in front of a wave steep enough and may still bury the bow.

Once deployed, most drogues aren’t practical to retrieve before conditions subside due to the enormous forces involved. If the strategy doesn’t work out or becomes unsuitable, there are few options but cutting them away.

Historical Considerations

Historically, a variety of designs demonstrated sufficient capability for avoiding damage by outrunning heavily breaking seas. One of the first adepts of such a tactic was the Argentinian Vito Dumas [1] in the early 1940s on board the 9-metre Legh II who seemed to have acted from instinct and observation, followed by Bernard Moitessier some 20 years later on board his 12-metre ketch Joshua as he got on the verge of an accident towing warps and trying to slow his boat down [2]. Both vessels were relatively flat-bottomed double-enders with a very firm turn of the bilge, which essentially enabled them to plane ahead of exceedingly steep waves, and neither were overloaded, an aspect Moitessier was adamant about as he already had to contend with a heavy steel hull.

A body of rhetorical justifications was developed long ago about how a hull should not accelerate and split the breaking crests by the stern. While these theories succeeded in relatively gentle and well-formed seas with modest crests, they never managed to cater for a steep wall of water chasing a yacht at high speed and – unsurprisingly – the designs they produced also failed to cope spectacularly in such situations. The concept was flawed and the argument is senseless today. Knowledge, materials and construction methods all readily allow producing strong moderate displacement hulls with shapes that are far superior for accelerating and keeping ahead of waves faces that are simply too steep to be successfully negotiated in any other ways.

Since the early 1990s, the short-handed offshore ocean racing circuit has probably put more small boats in huge following seas than ever before by venturing eastwards deep into the Southern Ocean for weeks at a time, but the strategy that was first explicitly documented by Bernard Moitessier in the late 1960s using an unlikely design still stands unchallenged. Today’s boats are better at implementing it.

Running before the sea in heavy weather does remains the ultimate option for a vessel once sea state rules out all other strategies, so if the accent is placed on survivability and sailing in latitudes where truly extreme conditions are encountered from time to time, selecting a design that does handle following seas in a sound and predictable way is essential.

Read additional articles about seamanship and heavy weather dynamics.

References

[1] “Alone Through The Roaring Forties”, Vito Dumas
[2] “Cape Horn: The Logical Route”, Bernard Moitessier

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