Porpoising? What causes and how to fix

[JiMindy]

Put new props on the boat, same size, just not bent or chipped, took her out yesterday and now she porpoises like a wild horse. Tried trimming up and down with minimal help. WOT kept her flat but it was too rough to go that fast and if took her to half plane, no problem there either. Would trim tabs help? Running twin 90 Mercs on a 21 ft center console, ran light with 3/4 tank of fuel, just wife and myself. Got into flat water and was just fine for a mile or so, then the chop picked up and the wild ride started again. Only happened between 3500 and 4500 RPM.

 

[Bob_VT]

Re: Porpoising? What causes and how to fix

Same size props but are they the same design? ie cupped or not cupped were the old props worked on at anytime to add or remove cupping.

That does sound strange if you have not changed anything else.

I will move this to the prop section.

 

[emoney]

Re: Porpoising? What causes and how to fix

You don't mention what make/model of boat, but trim tabs are designed just for that. I've not met too many folks that weren't happy with that decision.

 

[1ArmBandit]

Re: Porpoising? What causes and how to fix

are the new props exact replacements? pitch etc. ?

 

[1ArmBandit]

Re: Porpoising? What causes and how to fix

You don't mention what make/model of boat, but trim tabs are designed just for that. I've not met too many folks that weren't happy with that decision.

i was thinking the same thing, but if there was no porpoising prior to prop change....sounds like something else maybe?

 

[UncleWillie]

Re: Porpoising? What causes and how to fix

Too much weight in the rear.
Trim DOWN, start with All the way down and trim up slowly until it mis-behaves.
If Full Down trim is not enough, Tabs will help. They are essentially Uber Down Trim devices.
Have the admiral sit up front. If that cures it. Move some equipment (Batteries?) forward.

Also, some sea conditions just require you to slow down!

 

[JiMindy]

Re: Porpoising? What causes and how to fix

Identical props, admiral hates the bow, Actually have tabs but need to replace actuator cylinder on starboard side since it leaks like a sieve. Even on flat water she will be fine at first but then start bouncing. Trim motors allll the wayyy down, no improvement. 21 ft Sea Fox CC, 85 Gal tank, batteries (2) are aft, Twin 90 Mercs. Maybe I just need to put on my big boy pants and fix those tabs....

 

[Tabman]

Re: Porpoising? What causes and how to fix

Jimindy,

Do you know what brand of Trim Tabs you have? If they are Bennetts we might be able to fix your leak at no charge with new O-rings.

Tom
Bennett Marine

 

[UncleWillie]

Re: Porpoising? What causes and how to fix

Identical props, admiral hates the bow, Actually have tabs but need to replace actuator cylinder on starboard side since it leaks like a sieve. Even on flat water she will be fine at first but then start bouncing. Trim motors all the way down, no improvement. 21 ft Sea Fox CC, 85 Gal tank, batteries (2) are aft, Twin 90 Mercs. Maybe I just need to put on my big boy pants and fix those tabs....

Are the Tabs and motor trim actually going down?
It could be you are pushing the switch but they are still up, Go and actually look!
If the tabs are actually up it could be worse than no tabs at all.

I can not see how the boat will porpoise trimmed down and tabbed down.
Then again, in some sea conditions you just HAVE to slow down.
Has this problem persisted over a few outings or just the first time after the prop change?

The admiral does not have to go and live in the bow.
I am sure if you ask nicely she would be willing to sit there for a minute or two just as an experiment.