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Hydrofoils

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DrCoxie

Contributing Member
So I got a 15 hp evinrude and it has problems getting on plane (really barely wants to) with only one person in it, with some weight at the front it's no problem it planes out fine. So i added some hydrofoils and again with two people in it, one in the back. The issues come when I plane with one person in the boat, it does it more easily now and the speed drastically increased, it's proper on plane and going like nuts but it's gotten crazy unstable, after a few seconds and with increasing speed it feels like the boat just wants to dig into one side, like the foils suddenly create drag on one side.
I'm wondering if that sounds like a set up issue? Like is the engine trimmed wrong? is the engine too high? or are hydrofoils just bad? or the boat?
The trim im using is 3 holes up so the engine is about as horizontal as i can get it and the engine is sitting slightly above the bottom of the hull.

I've been thinking aboout grinding the transom down some to get the engine lower in the water but im worried it'll make the problem worse... anyone got any experience or ideas to share?
 
What boat ??-----What pitch is the prop on the motor now.----Are you sure the motor is running on both cylinders ?
 
racerone said:
What boat ??-----What pitch is the prop on the motor now.----Are you sure the motor is running on both cylinders ?
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its a 1982 evinrude 15 hp with the standard aluminium prop it came with and the boat is a 4.5 m ärnvik from like 1970-1980, really heavy thing tbh hence the need for some help getting it planed.
The eingine is running fine lol
 
I assume that you have attached the hydrofoil to the anti-ventilation plate (aka anti-cavitation plate.) If so, on that boat the engine should be raised or lowered so that the hydrofoil is even with the lowest point of the transom.

Having said that, you are likely very under-powered in the first place. Looks the the maximum rated HP is 50. So, based on that alone, you're underpowered.
 
chawk_man said:
I assume that you have attached the hydrofoil to the anti-ventilation plate (aka anti-cavitation plate.) If so, on that boat the engine should be raised or lowered so that the hydrofoil is even with the lowest point of the transom.

Having said that, you are likely very under-powered in the first place. Looks the the maximum rated HP is 50. So, based on that alone, you're underpowered.
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Sorry thats not what it was at all, I dont have the exact model and name but its similar to a uttern 455, so a fiberglas boat. It's probably true that a 20-25 hp would work much better with the boat but when it does get up on step it actually goes really well, or with two people in it, i'm very happy with the speed in those situations its just the instability with the foils that isthe problem atm
 
I should add that when the foils are on with two people in the boat its suddenly very stable again, I was thinking that's why the engine might need to come down a bit, that make any sense?
 
As I said previously, the engine should be raised or lowered so that the hydrofoil is even with the lowest point of the transom. That's where you will get optimum performance and optimum stability.

Once that is accomplished, and the instability persists, then you need to consider permanently redistributing the weight in the boat - more weight up front - batteries, cooler, seats, gear.
 
chawk_man said:
As I said previously, the engine should be raised or lowered so that the hydrofoil is even with the lowest point of the transom. That's where you will get optimum performance and optimum stability.

Once that is accomplished, and the instability persists, then you need to consider permanently redistributing the weight in the boat - more weight up front - batteries, cooler, seats, gear.
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Ok thanks, one more question, if the foils bend upward above the cavitation plate, would you suggest lowering it so far that the tips of the hydrofoil are at that lowest transom point?
 
Hmmm. I've not seen a hydrofoil for a boat where the tips are not on plane with the main body, so I have experience with that. But I would suggest that the main part of the foil, where it attaches to the anti-turbulence plate, be even with the lowest part of the transom.
 
chawk_man said:
Hmmm. I've not seen a hydrofoil for a boat where the tips are not on plane with the main body, so I have experience with that. But I would suggest that the main part of the foil, where it attaches to the anti-turbulence plate, be even with the lowest part of the transom.
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Yeah all the ones ive seen are level, ill try lowering the engine and if that doesnt work i might just buy a better hydrofoil without some fancy gullwing design. Thanks for the advice!
 
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