Need help with porpoising

[Flex]

Hey guys, need some help. My boat is a 17' mod v aluminum. I'm running a 70 Evinrude with a 6" jack plate no tilt/trim.

I have been through multiple props and here's my issue. If i trim it down it doesn't porpoise at all, but i lose speed. If i trim up i gain speed but porpoise, and it gets worse as i accelerate. If i trim motor down one notch i lose speed and bow is too low but no bounce.

I've tried about 4 different props, just had a racing prop guy add some cup, adjust rake and shave tips on my 13 3/8 17 pitch stainless.

I swear it gave me even more stern lift!

I do have gas tank up front under decks but even when I'm only on a few gallons it handles the same.

I came across some old posts about adding hook in the rear. Does this sound like it may help?

I run 40 at about 57-5800rpms porpoising. I think I'll lose water pressure if i crank jack plate any more.

I can attach pics if anybody needs to see what I'm working with.

Sorry for the essay guys. Beating my head against the wall two years on this thing!

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[Broke Down]

Is the back of the bottom of the boat dented up at all? Closer to the transom?

Have you tried adding some welded tabs, something like a fixed trim plate? I used to weld them on boats all the time with issues like yours. We would take them out an keep bending the tab until it felt right.

Also, did it do that before the jack plate?

[Flex]

Originally Posted by Broke Down

Is the back of the bottom of the boat dented up at all? Closer to the transom?

Have you tried adding some welded tabs, something like a fixed trim plate? I used to weld them on boats all the time with issues like yours. We would take them out an keep bending the tab until it felt right.

Also, did it do that before the jack plate?

No dents in the hull at all.

It didn't do it before adding the jack plate, but it also only had a 35hp on it so didnt run much faster than planing speed.

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[Feesherman]

Originally Posted by Flex

Hey guys, need some help. My boat is a 17' mod v aluminum. I'm running a 70 Evinrude with a 6" jack plate no tilt/trim.

I have been through multiple props and here's my issue. If i trim it down it doesn't porpoise at all, but i lose speed. If i trim up i gain speed but porpoise, and it gets worse as i accelerate. If i trim motor down one notch i lose speed and bow is too low but no bounce.

I've tried about 4 different props, just had a racing prop guy add some cup, adjust rake and shave tips on my 13 3/8 17 pitch stainless.

I swear it gave me even more stern lift!

I do have gas tank up front under decks but even when I'm only on a few gallons it handles the same.

I came across some old posts about adding hook in the rear. Does this sound like it may help?

I run 40 at about 57-5800rpms porpoising. I think I'll lose water pressure if i crank jack plate any more.

I can attach pics if anybody needs to see what I'm working with.

Sorry for the essay guys. Beating my head against the wall two years on this thing!

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Yes, a hook would help

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[jl8200]

I used to have a boat similar to this. I was a 17ft Alumaweld with a 115 Yamaha on it. I bought it with a wale tail/hydrofoil thing on the motor and it made a huge difference in porpoising. How do i know this? Well the first thing i did when i got the boat was take it off and it ran like ****. I put it back on right away and never took it off again. The boat just ran better with it on. Maybe that might help.

[Flex]

Originally Posted by jl8200

I used to have a boat similar to this. I was a 17ft Alumaweld with a 115 Yamaha on it. I bought it with a wale tail/hydrofoil thing on the motor and it made a huge difference in porpoising. How do i know this? Well the first thing i did when i got the boat was take it off and it ran like ****. I put it back on right away and never took it off again. The boat just ran better with it on. Maybe that might help.

Cav plate is already drilled from previous owner. May be worth a shot.

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[redaddiction]

Doesn?t that mean your motor is too high?

[Flex]

Originally Posted by redaddiction

Doesn?t that mean your motor is too high?

On plane the cav plate is 1/4" -1/2" out of the water.

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[Dogface]

Weld some tabs on the back and adjust as needed. We do this all the time.

[wishin i was fishin]

I had a 1648 Alweld with a 75, added a whale tail and it helped alot.


Start with a whale tail before welding or hooking your boat.
power trim would make a world of difference.

[MattMigues]

try a prop with less rake

[B Shark]

Whale tail

[Cappy]

i am no expert as I understand porpoising its because the motor is too angled up. on my triton hull the motor has to be completely all the way down I mean straight down to prevent this. trim it up a lil the boat goes to up and downing. When all the way down it dont. is it possible your mods changed the flow dynamics requiring the motor to be lowe?? Thats what my boat needed and solved the problem.

[jkcckc2002]

Buy a whale tail. I had a 40hp on my boat ans was doing the same thing. Whale tail helped. Get a small one I dont think the bigger ones would do any better performance but may add more drag than needed.

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[Flex]

Never updated. So i ended up raising the jack plate another inch and it solved my issue, also gained a little top end speed.

I guess the motor was just set too low as the new prop is designed to run at the surface and give bow lift.

I do think I'll have him chop the tips a bit next time i bring it in. With the added cup im still not quite at the WOT rpm i want. But its very close.

The prop is a 17pitch stainless sst. On the block it measures more like an 18 pitch with a lot of rake. The prop guy added a good bit of cup and did something to the rake i think. Boat ran 35 with me(260), brother(260) and dad (170) plus 12 gallons of gas and two ice chests last weekend. Also got to run it skinny a few times.

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[Broke Down]

Good deal! That was a lot easier than welding some tabs...lol

[Flex]

Originally Posted by Broke Down

Good deal! That was a lot easier than welding some tabs...lol

Yeah it was. But I'll probably continue to tweak. I can't leave stuff alone!

I really want a bigger rig. But i love mine, its been paid for since 1998 and it fits in the garage.

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[Broke Down]

Nothing wrong with that, no note and still put fish in the boat is a good thing. I have a little Alumacraft like yours, it is a great boat and handles stumps better than my Sea Fox.

[Greentrout1959]

Put a Whale tail on it.... I think your problem will go away... You'll get a plain much easier too....

[wishin i was fishin]

nice to hear you got it figured out.

i know what you mean you cant leave it alone! get every ounce of performance you can!