Severe Porpoising Need help fast

[Budget33]

Hi All,

I have a 1989 Maxum 1700XR with a 1986 120hp Johnson. This boat is porpoising very badly starting at 25mph, with the engine trimmed all the way down. The engine is also mounted as low as it can. I have put up to 500lbs in the bow to compensate for the extra weight of the motor but it still porpoises at the same point.

Not sure what to do, I understand I am over powered as I believe these boats were only rated for 90hp but there must be some way to fix it.

Any help is greatly appreciated, this is only my second boat. I cant be the only one who craved a little more power


Budget

 

[Another retired guy]

Re: Severe Porpoising Need help fast

Hi. You say the engine is mounted as low as it can be. Check the position of the anti cavitation plate. With the engine lowered and the plate level, it should be in line with the keel. Can be 1/2" or 3/4 higher.

 

[dingbat]

Re: Severe Porpoising Need help fast

Trimming all the way down isn't helping. The prop shaft should be pretty much parallel to the surface of the water.

 

[Bondo]

Re: Severe Porpoising Need help fast

Ayuh,... Welcome Aboard,... Check the bottom of the hull for Straightness, 'bout the last 6' of the keel line,...

 

[Budget33]

Re: Severe Porpoising Need help fast

Thanks Retired Guy, I will measure and post back

DingBat, I tried trimming the motor up but the boat just held the bow way out of the water and was very hard to plane

Bondo-o, I will check and post back

Thanks All, what about trim tabs or a different prop??

 

[crabby captain john]

Re: Severe Porpoising Need help fast

Both trim tabs and a different prop could help cover the problem. Best to find the cause 1st.

 

[Budget33]

Re: Severe Porpoising Need help fast

Retired Guy, The cavitation pate is the adjustable plate above the prop correct? the fin on the lower unit that plate is mounted to is 1" below the bottom of the boat.

Captain John, I agree to finding the cause. Could the motor be too big? ( is there such a thing? haha) its rated for 90hp and i think the weight difference is about 80lbs at most

 

[QC]

Re: Severe Porpoising Need help fast

Usually weight, but yours is so severe that I would be getting a straight edge and referring to Bondo's post. Anything else in the stern? The little thingee just above the prop is steering trim. Forget that for now. The thing it is mounted to is the Anti-Ventilation plate. The plate should be even with the bottom. You said you moved it down, and looks like you did I don't think that's your problem. The issue with oversized motor is that it is weight that is all of the way outside of the boat, so a little means a lot.

 

[JimS123]

Re: Severe Porpoising Need help fast

Classic case of a motor that is too big for the boat coupled with a light hull that can't take the weight. You're not 5HP overpowered, you're 30% over!

How long has the motor been on the transom? It could also be that the extra weight coupled with a poor setup on the trailer has caused a hook on the bottom of the hull. The longer she stays that way the worse it'll get.

Been there, done that....LOL. In my case the motor was brand new so it was time for a bigger boat! The new owner put a 40 on the old hooked hull and it ran fine, albeit it wouldn't crack that 25 mph barrier.

 

[Budget33]

Re: Severe Porpoising Need help fast

Hi Jim, Thanks for the bad news, haha. This motor has only been on this boat a very short time and only used i the water 3 times. The boat originally had a 70hp but was blown up ( that is how i bought it ) i guess i should be on the hunt for a 90

 

[QC]

Re: Severe Porpoising Need help fast

Hook should help porpoise

 

[jestor68]

Re: Severe Porpoising Need help fast

Evinrude E tech 90=320; 115=375; 130=390. IMO, the weight of the motor is not likely the cause of the problem.

 

[Frank Acampora]

Re: Severe Porpoising Need help fast



It is not a weight issue. The engine is lifting the bow until there is not enough lift and thrust at the stern to keep it up, and the bow drops. Then it starts to lift again. This is repeated ad-infinitum--porpoising. Go over to the prop forum and see if they can recommend a stern lifting prop. It may help. Trim tabs may help too.

As far as overpowering and weight: This little 14 foot hull is powered with a 125 and she does not bounce. yet, with an older 85 that weighed less she did porpoise.

The last photo shows a tunnel hull that will porpoise until she passes a critical speed of about 30. If you have the guts and if the bouncing is not too bad, try to push through and see if the hull stabilizes.

 

[Fed]

Re: Severe Porpoising Need help fast

I agree a V4 90 will be almost exactly the same as a V4 120.
(I think the etec 90 is a 3 cyl)

The higher the motor is mounted the less likely it is to porpoise but if you have an external keel then you may be forced to keep it a bit lower.

Hook = Concave
Rocker = Convex

Check your hull for a rocker.

 

[crabby captain john]

Re: Severe Porpoising Need help fast

30% overpowered,, just a tad heavy.... sounds like a beginning ..... boats are DESIGNED by experts for certain things... Want to follow me out 65 miles to the Gulf Stream in your boat?

 

[Fed]

Re: Severe Porpoising Need help fast

It's porpoising at 25 MPH and at that speed it's probably only a 50 HP motor.
Weight should be the ~same as a V4 90.

 

[Budget33]

Re: Severe Porpoising Need help fast

Thanks All,

Someone mentioned trying to drive through the bounce, I tried that, only got worse till the boat was almost completely out of the water. Also the porpoising at 25mph isn't that much past being on plane but if i make a hard turn i don't bounce.

Fed, my thoughts as well, that a v4 90 and my v4 120(looper) are very close in weight. my engine is a 1986. I def haven't been able to use all 120hp

 

[Auger01]

Re: Severe Porpoising Need help fast

Sounds like there is some "rocker" in the hull. It is the opposite of hook. Like others have said, you can check with a straight edge on the hull.

 

[Pony]

Re: Severe Porpoising Need help fast

I tried trimming the motor up but the boat just held the bow way out of the water and was very hard to plane

Are you trying to get on plane with it trimmed up?? (not going to happen) The idea would be to be trimmed down when you are accelerating to plane to keep the bow down, and THEN start trimming up once you are on plane.

 

[NYBo]

Re: Severe Porpoising Need help fast

It is not a weight issue. The engine is lifting the bow until there is not enough lift and thrust at the stern to keep it up, and the bow drops. Then it starts to lift again. This is repeated ad-infinitum--porpoising. Go over to the prop forum and see if they can recommend a stern lifting prop. It may help. Trim tabs may help too.

^This.

Where are you boating and how rough was the water?