Figure 1: Lift as function of angle of incidence
Figure 2: Drag versus angle of incidence
Lift
Figure 3: Incidence = f(t)
boat is essentially in quasi-Archimedean mode (more or less displacement sailing) its speed is basically limited by the hull, plan- ing up to some 16-18kt; meanwhile, the pitching and rolling in a seaway is limited to a few degrees on a modern Imoca hull with a relatively fat nose. Hence the varia- tion of flow angle of incidence is small. Above 20kt speed a current Imoca foiler
will generate sufficient lift to start climbing up on its foil. However, as the Imoca rule in force for the next Vendée Globe in 2024 does not allow foils on the rudders the transom will remain stubbornly in the water (Figures 8 and 9). The net effect is an escalating nose-up
attitude with the angle of incidence of the flow over the foil increasing, which results in a further increase of the lift generated which in turn increases the angle of inci- dence… until the immersed foil area is suffi- ciently reduced to just maintain the neces- sary balance between foil-lifting force and gravity acting vertically downwards on the mass of the boat. Meanwhile, the induced drag has also
increased, requiring the boat to bear away to generate more thrust from the sail. How- ever, that situation is unstable in a seaway, as the boat entering a wave will see a sudden increase of the immersed foil area which will generate an additional lift force, pitching up the boat which will then find itself essentially airborne after passing the crest of the wave. But gravity is still acting with the foil
airborne, so thanks to Newton the vertical descent velocity of the boat will easily reach 7 metres/second at the level of the foil. For a boat travelling at 30kt (15m/s)
when the foil re-enters the water the angle of incidence will be of the order of 25° meaning the flow will be totally detached (stall) and the resulting drag enormous. The boat will inevitably slow down, the flow will finally reattach itself on the foil which by then will be fully immersed, thereby creating an excessive lift which will launch the boat upward again… And so on. In the absence of a rudder elevator (which will not be introduced at the
58 SEAHORSE
earliest until after the next Vendée Globe in 2024), to improve the situation we need to find a solution to limit the positive feed- back of the boat lifting on its foil, causing that further detrimental increase of angle of incidence of the foil flow and resulting in excessive lift and drag. In terms of engineering the best solution
would be an active autopilot control of either the foil shape (changing its camber, for example, with a flap) or rotating the foil to adjust its angle of incidence. However, the Imoca rule restricts the use
of autopilots to acting on the rudder only. Nevertheless, the rule still allows the foil to be free to rotate around the transverse axis as a means to allow the skipper to change the effect of the foil. The other degree of freedom available is the amount of foil sur- face extending from the boat, but adjusting that is a serious task. If we want to reduce the coupling
between a boat lifting up on its foil and the resulting increase of incidence of the flow on the foil we need to think out of the box (and the rule!). The essence of any solution to improve the behaviour of the current foilers in a seaway (an Imoca or a yacht with DSS-like foils) is to find a way to keep the foil-lifting force as constant as possible. To achieve that without an active
system one solution is to design the foil structure such that when the lift-load on it increases the foil will twist and reduce the angle of incidence of the flow (similar to the flexi front wings on a modern F1 car), hence reduce the lift; this solution has the disadvantage that the response or deflec- tion of the foil will be ‘set in stone’ and very challenging to adjust again after man- ufacture. A set of Imoca foils already costs upwards of 500,000 euros, so building multiple foil sets each with different deflec- tions will be a very expensive exercise. My preferred alternative solution is to
use the forces at the inboard end of the foil to induce pitching of the foil with respect to the hull in order to reduce the angle of inci- dence as the lift and drag forces increase. This can be achieved by introducing a
flexible element combined with a carefully positioned pivot at the attachment of the inboard end of the foil (think of the reverse arrangement to a gybing centreboard on a dinghy). Figure 10 demonstrates how the flexible attachment at the inboard end of the foil can be arranged. The inner end of the foil is rigidly fixed
in a head-block pivoting off its rear edge and restrained by springs at the front edge; the pivot and springs are secured in a box rigidly connected to the hull’s internal
Figure 4: Lift dynamic range
Lift coefficient
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