Onderwerp: gunboat

Dit wordt bij onze zeilclub een nieuwe klasse... Pff..

Wat is er gebeurd??

One of the issues with foils in general is the raised CoG when flying, and with the J foils this is exacerbated by the offset of the lift centre inboard. Looking at the G4 going over, the point of vanishing stability for this design is relatively low when flying, only about 32 degrees, so they have very little margin.

When they approach this angle the boat falls off the foils and the stability limit correspondingly increases, but then the boat slows, causing the AWA to increase, highlighting the slow sail release.

As such, this type of system relies on the boat being kept relatively level within a narrow margin. Sailing it like a beach cat is not appropriate, since the crew weight can't be used to offset the CoG, thereby increasing the stability margin, in the same way. Looking at an A cat, for example, the point of vanishing stability when flying is nearer to 70 degrees, and so a different set of parameters need to be applied by the helm in judging the safe limits of the boat.

My calc is based on VCG being 4m above the foil, about at bridgedeck level, and the foil centre of lift being 1.5m inboard from the extreme beam. I've ignored TCG effects of crew. This gives vanishing stability at 32 degrees when on the foils, and 63 degrees when on the hull.

It's worth noting that the boat doesn't seem to be able to foil at this angle, as it comes off the foil at 22 degrees, before any sheets are released, so beyond 22 degrees the hull is in the water, and the stability correspondingly increases. However, the effect of dropping off the foil increases the heel angle from 22 degrees to 34 degrees in just 2 seconds (43 to 45 seconds in the video). The rotational inertia created by this rapid angular acceleration, combined with the main still drawing with increased AWA, is greater than the residual stability, even with the hull in the water. The gennaker was dumped at 45 seconds, so good response from the crew, but not enough residual stability so too late.

The helmsman has the steering over to turn downwind at this point, so the correct manoeuvre was attempted, but the boat did not respond to the helm, I suspect due to the starboard rudder (only one in the water) was in the hole created by the foil diving into the water. At 48 seconds the helm is over to turn upwind, I'm not sure if this is an attempt to re-engage the rudder, or if they are falling at this point. At 49 seconds the boat is near to its static stability limit, and there is no way back.

One interesting aspect is seeing the effect of the combination of foil geometry, wave geometry, and heel angle. We've seen these foils operate in relatively flat water (SF) with the AC72's, and as they rise due to speed they side slip and stabilise, typically heeling to windward as they slip, in a self-stabilising manner. However, in this case the boat rises on the foil, not due to speed, but because the local water level drops away due to the wave profile. The boat immediately drops, and hence rotates to leeward. As it rotates, the increased immersion of the foil is offset by the increasing angle of the lifting surface, and hence the foil is unable to recover lift, but even if it could, the angle is such that the lift component from the foil is too far inboard, so isn't helping.

Without wanting to be negative, I would suggest that the G4 is showing the limitations of the J foil arrangement when combined with a relatively narrow, high VCG vessel when flying, and waves with amplitude around half the beam. With so little residual stability they will need to sail as flat as possible in future, reducing sail when necessary to ensure they still have control as wave size increases. They can open the limits out a bit with automated foil control, but there are inherent limits to what can be achieved with the same basic geometry.

Alternatively they could just keep driving it like they stole it, and allowing us all to learn so much more. It is a brilliant boat, but it has confirmed what my calculations have been telling me for my boat, so I'll be using a different geometry, and processing the heave signal to include roll and pitch before feeding into the foil control.