installed a different motor, now we have a porpising issue

I took off a 85 hp force and put a 90hp yamaha, now we have a porpising issue at 3/4 throttle,
the only way I can tame it is by running the power trim all the way down , but we also slow down then.
I have a 1990 20' blue fin. the old force motor would push it at 45mph,
the yamaha cant go but 34mph. any help?

 

is everything the same??,, same prop, same pitch, is prop same distance below,aft, as old 1,,,,could be almost any of them,,,,,,P.S. ,,im not a motor or prop guy what-so-ever.
and welcome to the forum

 

THis site will help you analyse the situation. If you click on the green "figure" text you can see what the parameters are. Beta is the deadrise.
http://illustrations.marin.ntnu.no/hydrodynamics/resistance/planing/index.html

I did a data test to show how it displays porpoising - see attached.

The things that may have changed. The new motor it heavier so this has shifted the CoG aft. The motor is set flatter or even prop out so this is lifting the bow. Motor is set deeper. The value of a and f in Savitky are therefore higher.

Anything you do to drop the bow will help control the problem. Moving weight forward will help.

Rick W.

 

fer a dumb@ss i was at least in the same ballpark,,,,,,,well,, more like same state,,hehe

 

I had this issue a couple of times when changing engine brands in the old days so..
more rake on the prop could have caused this...
or the gearbox is deeper in the water...
or the distance the gearbox is from the back of the boat due to the pivot pin being in a different location relative to the outside of the transom hence when trimmed level the prop is closer or further away,
or .....different gear ratio causing a thrust issuse at mid range but I doubt that

and of course a different weight engine....
If you where doing 80mph I might consider the shape of the gearbox but you ant there...yet
PS how are you measuring 3/4 throttle..by rpm?

 

Rick,
That is a very interesting site which can be very useful in dealing with hydro-dynamic issues. We use it often to develop the confidence to rear load our RIBs with big outboards - see www.explorermarine.co.uk
Regards
HM

 

The weight of the motor will be lighter , the leg is it the same length ?
Porpoising is usually from a curve in the keel / bottom somewhere possibly near the back .
The underside of the plate on the outboard needs to be about 5mm below the level of the hull as a start point , it can be adjusted a good 5 mm above.
I have a simular problem so i adjust the power trim at a lower speed and then open it up once its trimmed and the porpoising never happens again

 

After the basic checks on keel hook, longitudonal Centre of Gravity and prop lift, I would be looking to check the prop centrline is the same distance below the boat as it was before - or reduce it if you can. We build fast RIBs - see www.explorermarine.co.uk - and some models we build with a 535 mm or 21 inch transom height to fit a 20" = 508 mm leg engine.
This reduces porpoising, reduces fuel used and increases top speed.

After launch we sometimes lift the engine another hole = 19mm or 3/4 inch - to improve things further.
The only downside on our boats of having a very high engine, is that cavitation can occur when running at very high speed at full lock - but who ever does that.
Note that these comments may only apply to specific hullforms, and inparticular to hullforms with our distinctive transoms.
Good luck
HM

 

Can you please tell the exact model name of the old and the new outboard?
Does the new motor have the same shaft length?

 

Hmmmm... an old 2 stroke, fairly light engine to a newer 90 horse 4 stroke, much heavier and probably significantly more horsepower.