About 15 years back or so i bought a cheap 12ft Duranautic at a yard sale. I thought it would make a good small pond boat. For $100, for a boat that looked like it had never seen the light of day since it was new I couldn't go wrong.
I had a 6hp Evinrude, about a 1974 model, the boat was also that same year but rated at 10hp max.
With the 6ho, it topped out about 11 mph and unless I put a hundred or so pounds up front it rode bow high and did nothing but plow water.
I hung a 9.8hp Mercury on it a new in the box 1973 motor I bought at an estate sale. With the 2 blade prop is was slow to get moving, I switched to a three blade prop off a 7.5hp Merc and that did a lot better it took off quickly but would not get on plane and topped out at 12 mph on the GPS. (With the 2 blade prop it would wash out under the boat and the hull would sink below the surrounding water as it started to move but never get going with any speed, it just pushed the bow up and the transom down even when trimmed all the way in. The three blade stopped this but the lower pitch meant is moved less water. With the three blade prop it was able to reach full RPM though.
I picked up a 1985 15hp Johnson for cheap and gave that a try, that was better by a long shot, it got me up to 21 mph but it still struggled to get on plane unless I put weight up front. A tiller extension helped but only a little.
I added a hydrofoil, (Dolphin brand with a Kmart tag on it) and it would pop right up on plane in a matter of maybe two boat lengths and stay there but with the foil my top speed was only 17 mph. I also had to add a tab to correct for some pull to one side. The kit came with two little stabilizer tabs that screwed to the bottom of the plate for just that purpose.
I didn't I moved that motor to a 14ft hull the following year but got a 9.9hp Evinrude for the 12ft boat. With the same hydrofoil it did nearly the same out of the hole as the 15hp, maybe even a bit better, but I lost 3 mph in speed topping out at only 15mph. Which was fine for that boat. I still needed to add ballast weight because at rest the motor weight and my 350lbs made the boat sit dangerously low in the stern. A long throttle extension would have helped but I still had to sit in the back to start the motor and with only an inch of freeboard it was an accident waiting to happen.
I was never a big fan of those hydrofoils because of some handling issues I saw with one on a 16ft runabout a buddy had. The boat had a fairly flat bottom with little for a keel and almost non existent chines. With a 50hp on that boat and a Quicksilver brand hydrofoil, it the boat would spin out on turns if you weren't careful and that hydrofoil would dig in as the boat came to a stop jolting everyone out of the boat. Without it, the boat wouldn't get on plane with more than two people in it, with it that boat would plane with four adults and two kids or pull a skier with two onboard.
I've had four or five of those hydrofoils over the years, ranging from one that was no more than a stainless steel plate cut into a delta shape with a sight down turn on each side, to one that had 9 ribs on each side called an "Excalibur II" and several other renditions of the plastic bladed design.
The flatter style foils lose the least amount of speed but don't have as much lift at low speed with a smaller motor. They still work but don't work as well at lower take off speeds. I tried one on a 6hp and all it did was hinder the boat still not fully getting on plane.
The Dolphin style version is shaped more like an airplane wing and I think it provides more lift with less speed.
I do recommend making real sure the motor is firmly attached when using a hydrofoil though. They push hard in the upward direction and tend to really put a lot of force on the motor clamps. I found myself having to really torque down on the clamp screws over and over. I eventually put a serrated plate under the motor clamps on the inside to they didn't eat up the wood transom panel. On my current boat, I put two bolts through both for security and to prevent it from coming loose.
I run the 15hp on my 14ft Seafarer 14L now with the Dolphin foil, the motor is not bolted but I replaced the motor clamp screws with stainless steel aftermarket versions with hex heads so I can crank down on them with a wrench, which I carry in the boat. (Using SS screws in place of the original aluminum ones requires liberal amounts of anti seize compound so they don't fuse to the threads due to being dissimilar metals.
The 1999 14ft boat does nearly the same speed as the 12ft boat, likely because it drafts less water with the same weight. The 12ft Duranautic sat low despite being nearly completely flat on the bottom, it handled stern weight poorly. The Starcraft SF14L has more of V further back but the added width and hard chines on both sides make it much more stable in the water. Its also rated for 35hp on the CG plate and 1140 lbs where as the 12ft is only rated at 560 lbs total weight and 10hp. The 20" increase in length makes a huge difference due to an added 15" of beam width.
