Did you notice any change to top speed with hydrofoils?
I don't and here's why. First, many years ago I used to recommend the SE Sport 200, which is a full
'flat' hydrofoil wing design (meaning straight across from each other, or tips 180-degrees apart), although recognize that the top profile of the wing is shaped like an airplane wing, i.e., parabolic or curved shape on top, with a flat running bottom. That did co$t one 2-3+ MPH, depending mostly on the
OB mounting height, due to any inherent drag from the wing.
I now use and recommend that Attwood
'gull wing' foil design, less
maybe not on a really FAST or high powered rig (like Tall Tiller has, but that's his choice). In use, the gull wing model gives you the stern lift to keep the bow down, get on plane faster, and stay on place at a lower RPM/speed. From a safety standpoint, it prevents you from positioning the OB trim pin IN so much so as to cause 'bow steer', which is a dangerous, if not deadly condition. In a turn, the gull wing portion also contacts the water to help stabilize the boat and keep you securely in the turn ...
no squirrely boats here! Regular foils have all these benefits too, BTW.
But once on plane,
IF the OB is mounted at the correct height (I advise having the vent plate level to the bottom of the hull or higher), the raised 'gull wing' portion of this foil is OUT of the water - hence no drag. On my hull, if I peer over the stern, I can also see the flat portion riding right at the surface of the water exiting from the stern of the hull. That is where you want the ideal OB mounting height. I had to add a 2" board to the trim cap of my ProCraft to raise the OB and then had to bolt the engine to the transom, as the screw clamps were 1/2 on/off the existing transom (don't worry, I have a 3/4" piece secured to the transom that the clamps also bite into).
I run in saltwater and with current and wind, it is really difficult to get an accurate if not repeatable speed running one way or the other or when with the wing on or off, as conditions change with time. But with that said, I doubt I lose anymore than 1 MPH and if it is 2?
Well so be it, as for the benefits the gull wing gives me ... I'll gladly take it!
I personally
would NOT run a small tin skiff without a foil. I've owned, used and/or have rigged way too many Lund, ProCraft, Starcraft, Smoker Craft, Sea Nymph, Duranautic
(my favorite tin hull!) and other brand tin hulls, from 12 to 18', to believe any differently.
But as with life ... your mileage (and opinion) may vary.