Cure for porpoising flats boat?

Waterrazor

Seaman Apprentice
Joined
Apr 5, 2002
Messages
49
I have a 95' Key West Stealth with a 115 Yamaha 2-Stroke. At WOT (5200-5400 rpm), I start to trim up the motor to get the MPH up (38-41 mph), but then the boat starts to porpoise. I compensate with my trim tabs but then the MPH start to drop and I am back were I started (35-38 mph). Does anyone else have this problem? Any ideas on a cure?
 

SpinnerBait_Nut

Honorary Moderator Emeritus
Joined
Aug 25, 2002
Messages
17,651
Re: Cure for porpoising flats boat?

Waterrazor, I had the same problem on my 16' boat and all I did was just trim it up some more and it just got up on top and started a walking then.<br />Got a 115 Mariner and at 4800 it is running 47mph and that's not a WOT but is good and smooth in the ride department though.
 

magster65

Commander
Joined
Sep 1, 2002
Messages
2,573
Re: Cure for porpoising flats boat?

Try placing some weight in the bow, like your fuel tank, it helps.
 

Waterrazor

Seaman Apprentice
Joined
Apr 5, 2002
Messages
49
Re: Cure for porpoising flats boat?

This boat has a 40 Gal. fuel tank in the bow already!
 

Gadgetman

Cadet
Joined
Feb 5, 2003
Messages
17
Re: Cure for porpoising flats boat?

Waterrazor<br /><br />I don't think there is a sure fire way to fix that problem.I screwed around with a 22' flats boat for 2 or 3 years with the same trouble.<br />What is the length and HP rating on your boat?<br />Mine was rated for 200,but I started with a 175 which porpused just like yours.At first I thought my prop was losing bite letting her fall off.Went with a deep cup prop,and it got worse,telling me she didn't have enough power to hold the bow up.<br />With the 175 the combination what worked best was a set of shims to reduce the angle of trim,and raising the motor up on a jackplate.Ran smooth and gained 4 mph.<br />Later on I put a 200 on her with a deep cupped prop,and took the shims back out.She then had the power to hold her up.Ran smooth and gained 15 mph.<br /><br />Hope this helps
 

boatingfool

Chief Petty Officer
Joined
Nov 30, 2002
Messages
610
Re: Cure for porpoising flats boat?

Spinner_bait Nut ,<br /><br />Love the signature ;) .
 

Trent

Captain
Joined
Nov 17, 2001
Messages
3,333
Re: Cure for porpoising flats boat?

A Sting Ray fin helps with this problem.
 

Waterrazor

Seaman Apprentice
Joined
Apr 5, 2002
Messages
49
Re: Cure for porpoising flats boat?

Trent, once I'm on plane the Sting Ray fin will be out of the water!
 

skydad

Cadet
Joined
Sep 27, 2002
Messages
24
Re: Cure for porpoising flats boat?

A Doel- type fin cured the porpoising problem I had with my boat.
 
Top