Hi Folks!
I discovered this thread whilst googling on foil moths, and as others have said, this is a goldmine! I'd like to thank everyone who has posted so much information so freely.
I'd like to contribute initially by summarising what has already been said and offering some ideas based on that.
Consolidating what has been said here, the goal is for a sailing boat which lifts off onto foils in the lightest wind possible, maintains stable flight thereafter, and which can attain the highest possible top speed.
Other less-common goals include jumping and safe landing.
Summarising others again, liftoff occurs when the power produced by the sail(s) is sufficiently larger than the total drag that the boat achieves the speed needed to generate the required lift. It has also been pointed out that this is not a binary switch, but a continuum in which the foil(s) begin to lift, reducing some drag (eg of the hull), allowing further acceleration.
So liftoff is predicated on maximising lift whilst minimising drag, or put another way: maximising the lift/drag quotient.
I believe this explains some of the effects that have been observed, such as heeling the boat to windward helping liftoff. When the boat is heeled to windward, it reduces the pressure on the centreboard thereby reducing the drag caused by the centreboard. I don't know if the drag caused by the foil stays constant (the boat speed has) or whether the extra downward pressure on the foil has increased its drag, but it seems most likely that the overall drag has been reduced.
I also know from a limited amount of waterskiing that the best way to get liftoff with a single ski is to first bend the knees, then straighten them (ie push out and down on the ski), causing the skier to "pop" out of the water.
So heeling to windward is probably doing something similar as well, although I expect on a smaller relative scale.
So for liftoff, I think we should focus on a hull design with the following properties:
1. minimal drag when sailing in low wind strength (high-wind drag is almost irrelevant)
2. minimum increase in drag, or even a reduction, when heeled to windward
There are two others which *may* be also be useful:
3. minimum increase in drag, or even a reduction, when sailed bow-up
4. some lift when sailed bow-up - particularly when heeled to windward.
I can see good potential for some or all of these in both skiff and scow forms.
For stable flight, we want to regulate the lift as speed increases. I don't know enough about the various compromises in terms of drag between trim tabs, pivoting foils and surface-piercing foils, but I would like to look at piercing foils as they represent a very simple design (no moving parts).
tspeer said:
But class rules have all but precluded the use of surface piercing foils.
I don't actually agree with this. Moth class rules effectively only prohibit foils which are affixed to the wings, but surface-piercing foils attached to the hull near the sides of a scow would be legal, as would surface-piercing anhedral or dihedral foils attached to a centreboard on a skiff or scow.
Top-end speed gets back to reducing drag. Again, I don't know the relative compromise of a surface-piercing design, particularly compared to (say) a ladder design where top-speed is attained by lifting all but the bottom-most foils out of the water. However, it seems clear that top speed will be achieved through reducing wetted area.
On this point, if the boat is being sailed heeled to windward, then we can presumably reduce the area of the centreboard. According to what's been said here, it is best to maintain the span (length), and reduce the width (distance from LE to TE). Perhaps this could even be quite tapered, so there is relatively more area when in displacement mode, and relatively less when flying.
I'll post more concrete ideas separately.
Cheers!
Nik.
