When they were up they were up, and when they were down they were down… painfully so. These two shots of Charal two-handed training for the Transat Jacques Vabres illustrate the violent vertical velocities in play, particularly in the forward part of a current semi-foiling Imoca as raced in the last Vendée Globe. The constant slamming, and staccato loading and unloading of the foils, is a hideous structural case for the designers and engineers, while for skippers it is an energy-sapping experience. Rudder elevators are not allowed until after the next VG, at the earliest, so many are looking at alternative solutions for 2024. In the Fastnet Charlie Dalin on Apivia showed that steady flight is possible without an aft elevator, but only for the best boats and only in flat water. The ‘sprung’ solution discussed follows the principles of F1 wing technology – substituting mechanics for variably flexible composites
of incidence and Figure 7 showing the drag coefficient as a function of time. We note that the drag function over time is no longer a perfect sinusoidal function and it shows a serious peak at maximum inci- dence. We also note that the average drag over a cycle is 0.0153 ie 18% more than the 0.0129 drag at 4° – which is the aver- age angle of incidence over the period! Clearly over the period the additional
drag resulting from the time dependence of the angle of incidence of the foil is a penalty in a seaway compared to that of a boat without any foils at all. This has two consequences:
point inducing a high vertical velocity at the level of the foil located some 11m for- ward. That vertical velocity by itself amounts to a significant dynamic variation of the flow angle of incidence on the foil. To illustrate the case let’s look at the
response of a foil to a variable flow inci- dence angle. The figures on the following two pages are included as examples. Figures 1 and 2 show respectively the
lift and drag coefficient of a typical sym- metric profile as a function of the angle of incidence in the range -16° to +16°. Note that when the angle of incidence is above 12° the flow starts detaching itself from the profile and the lift ceases to increase before starting to drop above 13-14°. At 4° incidence, taken as a reference, the lift
coefficient is 0.433 and the drag coefficient is 0.0129. Figure 3 shows the variation of the
angle of incidence of the flow over the foil against time. The initial setting of the foil is 4° incidence, the amplitude of the varia- tion is +/-4° and the period of the sinu- soidal function is 4 seconds. Figure 4 shows the lift coefficient in the
above incidence range. The lift coefficient at 4° incidence is 0.433 and it will oscillate between 0 and 0.877. Figure 5 shows the lift function over time, noting the sinu- soidal incidence function. The average lift over the period is the same as for the foil working statically at 4° incidence. Now let’s look at Figure 6 showing the drag coefficient in the same range of angle
l The additional foil drag will require more thrust from the sails (hence the need to sail more distance). l The additional foil drag being applied several metres out from the side of the boat will induce a time-variable yaw moment which will need to be cyclically corrected by the autopilot, thereby inducing additional drag from the rudder. This is the main reason why foilers in a
‘mer agitée’ do not deliver the expected performances upwind and when sailing very deep downwind. They need to gener- ate more power from the sails and that means in general sailing a longer course.
Can we improve the situation? As described above, a big issue stems from the additional drag induced by a foil sub- jected to variable flow incidence. When the
SEAHORSE 57
BERNARD LE BARS
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