All that seems to be discussed by windfoilers is how they attach at the top and bottom.... and maybe how long they are. For the America's Cup they didn't want to strain the brains of the competing developers so they made them an over-engineered, off- the-shelf one-design component.

So the competing design parameters for windfoilers would be

.reliability
.attachability
.minimum drag
minimum weight
.cost
.provide necessary lateral lift for operating conditions.
any more?

Carbon foil arms are tapered. Is that a cantilever engineering consideration or for better hydrodynamics as they get higher?
Are extruded aluminium arms noticeably compromised?

Do you get higher top speeds by riding high?

If you're curious about any aspects of T foils the moth sailors are the folks to go to.

This is a good read.

hq=Full+Scale+Measurements+on+a+Hydrofoil+International+Moth&rlz=1C5CHFA_enAU873AU873&oq=Full+Scale+Measurements+on+a+Hydrofoil+International+Moth&aqs=chrome..69i57.8640j0j4&sourceid=chrome&ie=UTF-8

In moth terminology a "Foil arm" is known as a "strut", a "dagger board" or a "rudder".

On struts they say
"The strut analysis brings home several points:
.......
.? Thinner is better. Certainly, in straight ahead tests thinner foils performed better, the limit being structural. It is unknowen if this trend would hold if yaw were considered. With the section and wave and spray drag coefficients, the foil drag for the 18 inch strut immersion could be calculated. If this is then subtracted from the T-foil drag, the result is the total lifting foil drag, including junction drag, section drag, induced drag, and wave drag. "

Moths seem to use struts with thickness to chord ratios between 12 and 14%
The NP pinkie is 11%.
I could find the dimensions of Fanatic and Slingshot aluminium extrusions on the web 15.5 % and 13.4 %

The tank testing produced some interesting drag results. Riding high produces less strut drag but the lifting foil produces more wave drag as it gets closer to the surface. They also like banking the craft to windward having the lifting foil do a lot of the lateral work.

Can't find anything on the ideal taper of the strut on a T foil though?

It is an interesting question, the windward heel/strut(mast) immersion ratio is definitely a big area for tuning. There are certain times when you can get the system into the 'zone', the right amount of windward heel and the right foiling height and the thing will just hammer upwind. That said it's a difficult thing to quantify and really sometimes what 'feels fast' isn't particularly fast VMG wise; I'm talking windfoil racing here.

Moths are definitely right down there in terms of chord length, the thicknes to chord ratio might be similar to the race foils. Where the strut attaches to the foil is a much smaller chord than a windfoil, I reckon it's around 70-80mm where a windfoil would be 100mm+ I think (i'll measure mine when I get a chance). There foils are generally much higher aspect ratio although having the flap on the rear main foil may give them this luxury, having such high aspect foils for us may not work as well.

Moths are right up there in terms of development, many smart minds have spent a lot of time in CFD etc optimising their foil designs, I don't think the windfoil world at this stage has anywhere near the technical development. I reckon the big game changer would be an adjustable on the fly rear stabiliser (not active just passive), would increase speeds around the course a fair whack in my opinion.

Ian K
"Riding high produces less strut drag but the lifting foil produces more wave drag as it gets closer to the surface."

Thanks for that piece of information, I was told the higher the foil is the faster you go, but I had many time the distinct feeling I was going slower at max mast height on my AFS W95 foil (i.e.,close to breeching), know I know why!

Select to expand quote

Sandman1221 said..
Ian K
"Riding high produces less strut drag but the lifting foil produces more wave drag as it gets closer to the surface."

Thanks for that piece of information, I was told the higher the foil is the faster you go, but I had many time the distinct feeling I was going slower at max mast height on my AFS W95 foil (i.e.,close to breeching), know I know why!

There could be an additional reason riding too high is slower. With less strut in the water the required lateral force has to be generated with a pretty short section of strut. A small foil running at too high an angle of attack puts it at the wrong side of the drag bucket.

Although you can relieve the strut of doing too much lateral lifting by canting to windward if you look at the moments at the fin box around the lengthwise axis you can see the strut always has to do its share.