QUESTION: I have noticed a few small trimarans with foils in each ama. What are the pros and cons of this arrangement compared to one in the center hull?
ANSWER: It really depends on the type of trimaran we're considering. As I write this, I can only think of three cases for which a foil in each ama should be considered. The main one is when one is sailing a VERY light trimaran (probably of all carbon fiber) will high buoyancy amas, that will frequently be pushed to sail on one ama. Then one foil per ama will be needed. The second reason, would be when one is considering asymmetrical foils (or even hydrofoils) for the reason of developing extra lift from the leeward ama. In such a case, the foil would be almost flat on the outboard side, with a cambered surface on the inboard face to provided lift to windward from the leeward foil. Such boards might even be toed-in 'very slightly' to add to the lift but from my own experiments, there is a real risk that overall resistance will negate any added lift.
The foil section will be a compromise shape as the design speed could vary considerably and therefore require some experimentation for best results—but if I were asked 'where might I start?' over a beer at the local yacht club bar, I'd probably suggest an arc of a circle with a radius about 2.5 to 3.0 times the chord of the foil. The third possible reason is when one is sailing a very long race where redundancy of foils could be a useful 'get-there' factor should one become damaged.
But even then, it might be noted that while early ocean racers did use such foils, later highly successful trimarans such as Sodeb'O and IDEC decided not to; perhaps because of the stress added to the amas—a source of past damage.
Generally, there are more positive reasons to having one central foil in the main hull. For starters, one foil and one box are lighter and require less work to construct. A foil under the deeper hull is also less likely to take air and cavitate. Further, a single foil in the center hull is more accessible for adjustment and can also be left in the same position for each tack—compared to two foils that require constant attention if one is to not accept more wetted surface than necessary.
Cost and added complexity is also a factor against having two foils and generally, an ama is more vulnerable to structural damage than the main hull, should a foil be impacted at speed. Further, getting an ama foil caught in an anchor line or lobster pot, could cause a sudden veering, a gybe and a capsize. And getting out to retract a heavily loaded foil on the ama in bad weather is no fun and definitely carries additional risk and even the akas (cross beam and/or waterstay system) will be loaded higher. Some may argue that the foil box gets in the way in the main hull but if it's worked in with the accommodation arrangement or placed off center or under the floor (in the case of a centerboard), then that disadvantage can be offset. Typically, a dagger board base is also used to provide mast support and add extra rigidity between the cabin top and the keel.
Anyway, as I don't promise to have covered all the possible arguments here, please feel invited to submit your own pros and cons that as justified, I'd be glad to add to these thoughts.
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