Does Length Matter?

Seahorse
November 2015

Much has changed in raceboat design with the advent of better materials and new stability solutions. In fact many of the old rules now no longer apply - advocates Juan Kouyoumdjian.

Within the context of racing sailboats, length has always been treated as a major performance parameter. Therefore, most racing rules either limited length or dealt with it in one way or another. It has also always brought some magnificence with it that was not necessarily only related to pure speed. But there is now a case for a rebirth of the Maxis at the top of our sport, something that is taking place not only for monohulls but also multihulls with the Ultime Class. Beyond this magnif­icence, the Maxi 'raison d'etre' is that it represents the environment within which the fastest monohull can be conceived. The intention of the following words is to try to explain why this is the case.

In the case of the largest racing mono­hulls, they are today gathered around the International Maxi Class [IMA] which, based on tradition, stipulates a few rules of engagement. Among them, length is restricted to between 24m and 30.5m, which is the reason why most modern maxis are at the long end of this range.

Indeed, one could be forgiven for believ­ing that length is still everything in terms of performance, but this is not the case. The appropriate relationship between length, weight, power and the capacity to use it properly is what makes the differ­ence. No element on its own can output high performance without balancing with the others. This is precisely why a smaller [shorter] sailboat such as a dinghy can outperform a much bigger (longer) one.

The single most influential parameter in this equation for sailboats this size is draft because it impacts weight and power; and it is obvious today that if we are talking about high performance at this size of boat, we can only consider a canting-keel set-up.

The relationship with RM is logical and directly scalable with the loads that a sail­boat sees, therefore the 'price' you pay for RM under this standard is close to what it 'should' be when compared to the struc­tures one could do freely if no build standard was applicable.

But this is not the case with length - in fact, I believe that GL today places too big an emphasis on this once dominant factor (as do popular rating rules). So, for example, if you compare two boats with exactly the same sailplan and RM, but one boat longer than the other, GL stipulates that this extra length requires a heavier structure - and the longer design would only overcome this weight disadvantage in upwind [and therefore slow] conditions.

This means that if RM is somehow restricted, either by hardware load limits or draft, then length should be an outcome of this limit rather than the other way around. This was the case for our recent maxi, Rambler88, when length was an output from an imposed draft. If, however, one starts the conceptual procedure the other way around, starting from maxi­mum length, then RM becomes an output based on what is required to compensate for the GL-imposed weight for that length.

In my humble opinion, but based on very extensive calculations, if you start from a maximum length of 30.5m as per today's IMA limit, the resulting draft to yield an ideal combination of weight and power for this length is about 7.2m, prefer­ably using a hybrid keel fin made of steel sideplates and carbon, plus fore and aft fairings, with shims to allow fine-tuning of draft depending on final keel and boat weights. But a carbon keel this size is only an ongoing 'case study', since GL does not entertain such a keel construction today -although it certainly will in the future.

With pure performance in mind, one could think of doing a sailboat even longer than the IMA limit of 30.5m. But for this solution to work, draft has to increase as well and here is where one finds the limit! Keels have to withstand several load cases imposed by GL (as well as real-life sailing conditions). Primarily one transverse case: bending of the fin due to heel/cant plus a further longitudinal case due to grounding. Both will dictate a natural limit as to how much draft one can achieve efficiently without weight increases defeating the original purpose.

But it is this second case (grounding) that is the most powerful influence. If, for example, one imagines a 130ft sailboat, she should have a draft of about 8.5m (assuming a canting keel) to be within acceptable ratios of weight, length and power. But if you analyse such a keel under grounding conditions, the keel - and keel structure - have to be so big and heavy that it is impossible to achieve those ratios in the first place, hence an intrinsic physical limit of length and draft.

This absolute physical limit, if analysed extensively, would come up not far from the IMA's current 30.5m length limit and 7.2m draft limit. The original reasoning or intention of these parameters might have not been this one, but the IMA's outcome is almost spot on! And this is the underlying concept of our latest Maxi design K99 ... right at this limit. (Note that for multihulls this reasoning does not apply because they don't rely on draft for RM). Therefore, I would propose that anybody building a monohull longer than around 100ft is no longer looking for pure performance. If you are, then build a multihull instead.

Beyond this general conceptual principle lies another level of refinement that con­sists of a good combination of cant angle, bulb weight and ballast weight. These are directly driven by the ISO stability require­ments, consisting primarily of a minimum angle of vanishing stability [AVS] - which represents the limit beyond which the centre of gravity moves outside a point at which the boat will naturally right itself. On top of this, there is a 'recovery from knockdown' calculation [FKR] that assesses a sailboat's natural attitude and response following a knockdown event.


In simple terms, a sailboat loses its capacity to right itself when the CG moves beyond its canoe body 'buoyancy' plane at large angles of heel. ISO requires this large heel angle limit to be 100° (as can also be seen on our graph) and therefore a sailboat needs to be able to right itself in sailing conditions up to that angle, beyond which (say 101 °) it can keep going and capsize.

Canting the keel or carrying lateral water ballast shifts the CG of a sailboat sideways to windward, giving obvious advantages at low angles of heel (the angles at which sailboats sail normally), but as such it also reduces the angle at which the boat is capable of righting itself. Curve 2 on the graph shows the effect of adding water ballast on top of the keel cant, increasing RM at lower sailing heel angles but reducing it at the higher angles where the ISO A VS limit is tested.

So for a given draft and hull form there will always be an optimum combination of cant angle, bulb weight and water ballast weight and location. This optimum is also impacted by the sailing conditions for which one wants to optimise the boat, using the nine basic conditions generated by a standard 3'''3 matrix [TWS: light, medium, heavy against TWA: upwind, reaching, downwind].

At the lighter end of our target condi­tions, irrespective of TWA, one would tend towards a lighter bulb and smaller canting angle but with larger ballast tanks, since water ballast can be emptied as required to reduce displacement. Mean­while, keel cant is used only relatively mildly, particularly downwind.

For upwind heavy-air conditions our sailing speeds will generally be under 15kt so the extra leverage of water ballast has a substantially smaller impact than in faster, planing conditions - plus at low heel angles every bit of righting moment helps. In practice, for reaching conditions, other than in the light, this trade-off between water ballast and keel cant is more delicate and is also heavily affected by sea state -which is another matter altogether.

When one does bring sea state into the equation, and without getting into too much complex detail, two main cases should be considered depending upon the direction of travel of the waves: opposite to the direction of sailing, hence increasing the speed at which waves are 'encountered', and along the direction of sailing decreasing such speed or even, as for the Maxis, turning it negative when boat speed exceeds wave speed.

In both these conditions a Maxi will sail more slowly than in flat water. Upwind, the sea state will dictate how fast you can sail, simply because things will reach a point where if you let the boat sail any faster, the slamming becomes so violent as to cause breakages that are not justified by a small gain in average speed. Nor is the structure of our optimal boat sufficiently robust as to be able to withstand the vio­lence of sailing too fast in a difficult sea­way; and to make it robust enough you'd be adding so much extra structural weight that the boat would not sail much faster anyway. In my experience, for typical wave conditions, the boat speed reduction required can be close to 10 per cent, ie sail­ing 10 per cent more slowly than in flat water with everything else being equal.

Downwind, the solution changes com­pletely between sailing more slowly than the waves in marginal surfing conditions and planing faster than the waves. The ideal shape of the hull as well as the longitudinal position of the CG changes dramatically between these two cases and therefore needs to be accounted for within the range of conditions for which the boat is optimised. What would make you faster in marginal surfing conditions would slow you down when outpacing the waves.

This is a very tricky design challenge to resolve when trying to achieve an 'all­round' boat. Going purely for records, hence above 25kt boat speed, makes the design task substantially easier ...

This is where the appendages play a big role since they can be used to lift the boat up, particularly the bow. Since the keel is already immersed, it makes sense to use it as much as is efficiently possible and load it upwards for this purpose, but this does come with some loss of righting moment.

This lifting effect, reducing a boat's 'virtual' displacement, is achieved by tilt­ing the cant axis [front up] of the fin so that the keel is consequently loaded upwards when canted. The amount of this tilt also depends on the structure of the keel, since one can easily overload an inclined canting keel with too much tilt and/or cant. In practice I believe that tilt should be kept fairly modest and, instead of using larger tilt angles, it is better to incorporate an adjustable trim tab on the fin to adjust this lift effect as a function of speed and particularly sea state.

Meanwhile, higher up the hull, a side or lateral foil can help with the boat's vertical lift without losing that much righting moment. But one needs to be careful with the weight of all these features as well as the speed at which they start working. It is difficult to give them any significant impact on performance below around 16kt of boat speed - and more usually it is closer to 20kt before they really kick in. So one needs to make sure that downspeed losses are not significant when these appendages are not being used but their weight is still being carried.

Finally, the rig should of course be considered carefully since these types of sailboats rarely sail with apparent angles wider than 70°. Apparent wind speeds will also vary a lot and sailing close-hauled we will often see double the wind speed. Therefore, the drag of the entire rig as well as the capacity to carry the appropriate sails correctly have a significant and increasing performance impact.

The weight and height of the rig's CG is also very important, more so in this type of monohull than in any other since a lower CG for the rig will have a significant impact upon the boat's design weight, righting moment and ISO stability calculation.

The combination of a large chord, aero­dynamically efficient wing section that only requires two shrouds per side, fixed to deck-spreaders, offers the best current solution. The improvement in terms of mast weight is not big when this configura­tion is compared with a 'classic' rig (with the same headstay tensions) but the lower CG may be significant. At the same time, our deck-spreaders allow for the headsails to be trimmed much further outboard, making for a nice all-round package.