This brings up a sour point with my wife, she is a little mad at me right now. As everyone knows I love to just tinker with my TI, when I read that post about someone on an AI getting 30 knots on their foils I went out to the garage and broke out the old foils that I built for my TI 3 1/2 yrs ago and put them on this TI (it's not hard they just clip on).
Here is a pic
The front foils are the main foils, and typically all that I put on, the rear ones were added for only when I have a passenger in the back and are seldom used. The best speed I ever got from the foils on my TI before retiring them 3 1/2 yrs ago was around 20 mph on a strong downwind (20mph plus winds) with my huge spinnaker out and all 265 sq ft of sail out (no magic feat though, as the boat would easily do that without the foils anyway). Actually that's when I realized that spinnakers are not the way to go because you can never go faster than the wind with them, basically that's what got me started on learning everything I could about wing sails.
Actually when I retired the foils I figured out I could get the same lifting of the bow as what the hydrofoils achieved by adding a bowsprit and tilting the forward sails. The lift created by the angled sails lifts the bow out of the water and allows the boat to plane. Basically doing the same exact thing the foils were doing but without any drag of any kind. That's what I've been using for over three years now. Since adding the bowsprit I have never had a problem with the bow diving. Actually when pushing the boat really hard like in a strong 20mph plus downwind the hull planes quite nicely and the tip of the bow is around 6 inches out of the water, and only the rear tips of the AMA's are touching the water. The downside to going these speeds is the rudder system, if your flying along at 20 mph and crank on the rudder to turn hard, the rudder just snaps off (breaking the rudder pin) and flies up into the air.
I had my TI out again with the foils the next day after reading this post, after a frustrating hour of trying to get up on the foils I gave up, keep in mind I retired my spinnaker about two months ago (probably because it has a big old rip in it and I just don't feel like fixing it again), plus the winds were only 5-6 mph that day anyway. So I was unable to get up on the foils and remembered clearly why I retired them in the first place. For the next week I made some mods to the foils and added ailerons and invented a new automatic self leveling system (the reason my wife is mad is because I spent way too much time on it). The pic above is the old foils before making any mods.
I used to design/build/race 3 point hydroplanes as a hobby so my theory on the foils was to design them like a 3 point hydroplane, basically the front 2/3's of the boat rises out of the water, the back 1/3 of the boat is now at an angle so it can now plane like a regular speed boat or 3 point hydroplane would. Since the TI has a displacement hull the max speed is around 10 mph, and because it can't plane the horsepower required to get any faster goes up exponentially. With the foils and the ability to plane there should be no speed limit to the hull now beyond the cavitation point on my foils (around 40 mph currently), my old 3 point hydroplanes would do well over 60 mph. With this design I can still use the same standard rudder system and the boat is not as weight sensitive, and I can still use the pedal drives.
The foils are adjustable to any angle of attack desired however at anything over 8 degrees angle of attack (AOA) the drag from the foils is greater than normal hull drag would be so it's hardly worth using them. At 2 degrees AOA drag is minimal (less than 1 mph drag from the foils in the water) however lift off speed is close to 10 mph (at 8 deg AOA lift off is 4-5 mph).
Today I tried them out again with the new Ailerons and automatic levelers, in 7 mph winds I got up on the foils and foiled for a while at around 8 mph ( with a little assistance from my motor and pedaling like crazy). Afterwards I took the foils off and went out again without them and the TI performed about the same with just the wing jib and me pedaling and the motor running at 1/4 throttle, so bottom line in 7 mph winds the foils didn't buy me anything at all (except they look cool).
My plan is to retire the foils again until I get my main wing sail completed (probably next year sometime), I'll then get them out again and try to get to 30-35 mph in 15-20 mph winds. This kind of stuff is fun for me.
In the mean time I just plan to use and enjoy my TI as is for a while with the wing jib, it is faster than heck and works well as is in pretty much all conditions even the very low typical winds we have around here all summer. In my opinion currently it's the perfect boat for me, even if I never make the wing main sail, or put the foils back on.