This is a really old post, after making this post I took the suggestions of the guys and made a totally new thread called The ultimate Tandem Island (hydrofoils, spinnaker,jib,etc) viewtopic.php?f=69&t=33720
There I pretty much described all my escapades, trials and tribulations.
I'm not recommending to re-enforce the mast anymore, instead I just added a rear stay line, that helps the sail from putting too much stress on the mast holder.
The hydrofoils do work, but later on I added a bow sprit, that angled the front sails at a more acute angle which creates lift to the bow, thus eliminating the need for the hydrofoils. With the angled fore sails, the bow no longer dives, actually in strong winds the bow sprit and big sails actually lift the bow out of the water about 6 inches or so.
Here is a pic of my first mirage drive foils:
Later on I made some more out of that white 1/4" thick kitchen cutting board material you can get at Walmart (for about $4 bucks), I use that stuff for everything. Making those foils is kind of a touchy feely thing, using a hack saw, file, and a dremel tool. Basically you start with a rectangle then drill and dremel the shape of the mirage fin out of the plastic, then just cut the top off at an angle to get the desired angle of attack (about 8 degrees) by having the edge of the cutting board up against the bottom of the hull with the mirage drive locked in the forward position.
Here is a pic of the foils themselves with a scale to get an idea of the sizes needed for a TI.
Once everything is sculpted and fit, I just gobbed clear silicone caulking over all the joints to hold everything in place. Of course the dimensions would be slightly different on an AI. When pedaling or with the fins pointed down, they do nothing, they only engage when you lock the pedal to the forward extreme position with your foot. The way I did it was just sail like normal, if I felt the bow diving, I would just put my foot on the pedal to raise the bow back out of the water. Worked like a champ.
Of course there is a little drag from the foils, but at least on my boat with 260 sq ft of sail, without the foils the boat would dive and stay completely underwater (what I called nautilus mode), until I either backed off on the sails (not about to do that), or I changed direction. Having the bow completely submerged didn't seem to slow the boat down a lot, but you sure got wet in the cockpit (like a waterfall), with the foils just a tap on the mirage drive with my foot raised the bow back out of the water.
Like I said earlier, once I angled the fore sails, I no longer needed these foils, they have been just sitting in my drawer for 3 1/2 years now unused.
Hope this helps