Change the Prop or Not?

Sevcrist

Seaman Apprentice
Joined
Apr 16, 2005
Messages
30
I have a new Polarkraft aluminum modified V fishing boat and a Mercury 40hp four stroke EFI engine. This setup weighs about 1100 pounds at normal load.<br /><br />I currently have an aluminum 10-1/8x15 prop and the motor is rated at 5500 to 6000 WOT.<br /><br />I can get the motor to go to 5700 if I trim it out quite a bit but the boat porpoises alot. Trimmed in more the boat rides great but I only get 5300 rpm's out of it.<br /><br />Should I go down a pitch or two so as to increase the rpm's or leave this setup alone?
 

05GlastronSX

Chief Petty Officer
Joined
May 29, 2005
Messages
437
Re: Change the Prop or Not?

well you are not in the recommended RPM range for the engine so i would say to change the prop to a smaller pitch to fix that. you might stay the same or actually lose a few mph, but you will not be lugging your engine.try a 13P prop and see how that works out. it should raise the rpms by roughly 400, so then you will be up to 5600-5700 rpms without trim and then you will also no longer be porpoising.
 

umblecumbuz

Lieutenant Junior Grade
Joined
Sep 25, 2004
Messages
1,062
Re: Change the Prop or Not?

Hang on there fella's,<br /><br />Before playing 'propscotch', check your motor height and weight distribution on the boat.<br /><br />Where you put the weight in the boat makes a big difference to trim, and to porpoising, and this has an effect on WOT. Move for'ard what you can.<br /><br />The motor height makes a difference to WOT also. Check your AV plate and see where it sits in relation to your keel. Try and get it to sit level or a little above the keel by raising the motor a notch or two if necessary. <br /><br />Do these things first, then test and see what results you get. Watch for proper water intake if you raise the motor, and trim in on turns if you start getting prop blowout.<br /><br />The time to play with props is after you've optimised your setup, not before.
 

pwiseman

Petty Officer 1st Class
Joined
May 4, 2005
Messages
207
Re: Change the Prop or Not?

you might try moving some weight to the bow and see if this helps with the porposing. Smaller boats with 4 strokes are often stern heavy by 50-80 lbs.<br /><br />If your boat falls of plane real fast when you slow down, this is a good indicator that you have some weight distribution issues.
 
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