Above: Swiss lake racing rules dictate a maximum air draft of 21m which is far from modest on a 35ft boat but nevertheless every effort is still made to cram in as much area as possible. After early rig troubles with the previous non-foiling D35 cats the designers went a little more conservative with the TF35, particularly as there are high demands in terms of rig load to ensure this big gennaker can be flown efficiently upwind in light air – the same reason that the chainplates ended up set further inboard than would be typical. Above right: the 26ft test mule and (right)… cut-away mainsail for human traffic, single wishbone boom and massive gennaker foot
with an expansion pack easily added for longer courses like the Bol d’Or. We have now entered an era where we
can substantially change the behaviour and performance of your boat with a mere software update/download in less than 20 minutes. Much like your phone. There are endless opportunities here. From changing the take-off strategy, pitch behaviour in a straight line and in the corners. And the use (or not) of rudder differential. Once we had settled on a basic configu-
ration we set out to verify the concept on a smaller scale to reduce the risks. We had soon identified a 26ft daysailer with roughly the right proportions. We then built a set of foils for it and sailed it on the lake throughout late summer and autumn 2018. The boat behaved largely as expected,
but being more manageable and relatively less powerful than the ‘real thing’ it gave us a much more friendly platform with which to experiment and later fine-tune the sizing and geometry for the final boat. The devel- opment of the flight-control system in par- ticular benefited greatly from this exercise. The control forces and actuator speeds also got a reality check, as full-on vehicle- dynamics modelling was not on the cards.
Performance The extra bits dragging in the water for the foiling configuration have obvious nega- tive performance consequences when sail- ing in Archimedean mode. So that is a problem that is difficult to overcome. If you are 5% slower in Archimedean mode,
54 SEAHORSE
but 30% faster in foiling mode, will you win the Bol d’Or in varying conditions? A few measures were taken to hedge
against the drifting scenario. The hull-centre beam is slightly less than on the D35, which will allow earlier hull flying. This will ini- tially result in a lower righting moment, but this is not an issue once foiling because the foils extend outwards significantly beyond the hull beam (conversely, our chainplates were located on the inner edge of the hulls, allowing the very big gennaker to be sheeted much tighter and making it effective for use sailing upwind). Lastly, if the forecast guarantees a
drifter, then a more (or less) conventional C-board is available for a long course such as the Bol d’Or. It must also be pointed out that, due to these boats’ light-air home venue, the TF35 carries much more sail area than a typical foiler. For this reason the top-end speed will not be nearly as high as other foilers, such as the AC boats, AC50s etc. As wind increases above our target range of use the aero drag of this excess sail area very quickly becomes a liability. For the same reason (aero drag), pulling off foiling gybes and tacks will be more difficult on the TF35. It will be inter- esting to see if the sailors develop better sailing techniques to overcome this.
Safety With the drastic boatspeed increases of the last 10 years or so, safety aspects cannot be dealt with in a ‘business as usual’ manner. Basic physics will tell you your body needs
to absorb 25 times more energy when hitting something at 30kt than at 6kt. The primary items to consider are:
falling off and getting hit by a foil; losing grip, and flying into something on the boat; falling off the front is a particularly bad scenario in any foiling boat, so we extended the forward net as far forward as practical. Also, the racks are wider than the foil and elevator foils, in part to min- imise the chance of falling in the water forward of the foil. Impact clothing and helmets are a must on any fast boat like this. When sailing at these high speeds oper-
ating from a cockpit like the AC75/50/72 is of course preferred. Unfortunately, this boat is small enough that this was not easy to accomplish, so we ended up without cockpits, putting more emphasis on the impact clothing and helmets. Finally an inflatable airbag is installed at the mast head to help prevent full-turtle capsizes.
The foils There are lots of challenges in designing foils for boats like this. Not least of which is designing them so they can be built for a reasonable price… The T-joint is a detail where a lot of
effort and money can be spent. We opted for a relatively simple metal bulb into which the strut and foil are inserted. The horizontal foil is solid, but the vertical foil has multiple shear webs. The control system actuation of the foils
is done with flaps rather than by moving the entire foil. This will keep the control
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