Thread: Effect of hydrofoil on speed

I can recall having the same issue with my 40 hp Nissan on a 15 ft Alumacraft wide bodied jon. I loved how the boat would jump up on plane... but I actually had to drop the motor two more holes to offset the hydrofoil's tendency to load up the back end of the boat. In other words the back end of the boat was plowing a bit causing a loss of top end speed. Hard to believe what difference such a small adjustment made.

16ft alumacraft 40 hp Yamaha. Put one on and gained hole shot, lift and about 3-4 mph. Can’t see how it would hurt top end unless trimmed down to far. If no trim and tilt you might try tilting up one notch and see how it performs. Sometimes you just have to play with motor a little when you make a change. Do you know for sure you lost speed?

Originally Posted by Cray

16ft alumacraft 40 hp Yamaha. Put one on and gained hole shot, lift and about 3-4 mph. Can’t see how it would hurt top end unless trimmed down to far. If no trim and tilt you might try tilting up one notch and see how it performs. Sometimes you just have to play with motor a little when you make a change. Do you know for sure you lost speed?

I actually meant to say that I also gained top end speed very similar to what cray describes above... a total improvement over the performance prior to installing hydrofoil.

I had one on my 25 horse 16’ stick boat and it was a dog without it. I’ve heard of the loss of top end with bigger motors but not with the smaller ones. The hole shot has always been such an advantage that I never gave the loss of speed a consideration.

I've had one on every boat I've ever owned but my current one (only because there isn't a need for it). I've personally never seen a drop in speed. As others have said, It was a huge holeshot/up on plane improvement and the main reason for the installation. It also eliminates porposing which robs you of top end speed. I've had it on a 25hp 2 stroke, a 4 cylinder I/O, and a 40hp 4 stroke. I've only ever utilized one brand (Doil Fin) so I cannot comment on other brands, their design, or their operation increases or shortfalls.

Originally Posted by SeaRay

I've been told by several sources that a hydrofoil can reduce your top end by 4-7 MPH.
I put one on my 20 HP Tohatsu to get on plane faster but I need the speed more than getting on plane.
Any of you guys know this to be true??
I'm thinking about taking it off to see, but was looking for other's experience.

Lots of variables but I doubt you will see 4-7 mph decrease on a 20hp. Assuming you don't have power trim, your motor could be tucked in too far. One of the main reasons for top end loss is mainly for rigs where the foil is under water when planed out which creates drag. I've had foils on 2 boats (both had power trim) and haven't seen any speed reduction. You could check your speed with and without to see.

Dropped a couple mph when installed on mine


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I pulled mine off. Will check it ans see if it changes.