Help.

[al-B-kerkey]

I just recently bought a used boat. 15ft Sprint 255 severly underpowered with a 40hp Yamaha. At present boat will not plane. Had motor checked out compression etc... with motor is fine. Boat has been sitting awhile and with a buddy in front we finally got it to plane but were only running 22-23mph and turning about 3100rpm. Both guages nay be off since it has been sitting. It has a 10 1/4 X 14 prop now. Mechanic said I needed to go down and the moe I read the more confused I get. Can someone recommend the best prop for max performance? Thanks

 

[Silvertip]

Re: Help.

Before you start buying props, you need to realize that to get the engine into the 5000 - 5500 rpm range you'd have to drop prop pitch by at least 8 or 9 inches which would leave you with about a 5P prop. Since that is not a likely route to take you need to check some things. Do the throttle plates in the carbs open fully horizontal? If not, you have a link & sync, cable, or control issue. The tach does not generally go bad just because its been sitting. But check the back of it to make sure the calibration dial is set correctly as the tach may just be reading wrong. Check the position of the AV plate in relation to the keel. It should sit even with or an inch or two above the keel for best performance. If it's not, raise the engine. Lastly, no prop can make a speed boat out of a severely underpowered boat. Having said that, a 40 HP engine on a 15 ft boat is not underpowered unless the boat is full of water soaked foam, the bilge is full of water, or the boat is simply a heavy weight.

 

[al-B-kerkey]

Re: Help.

Mechanic said motor was running and functioning like it should. Boat had been sitting out so foam may be holding water? I currently am keeping it in the garage. As far as I know the tach is correct but it could be wrong. I have gone down 2 pitch sizes and with some fresh gas and a little run time< hope that it will get in the reccommended WOT rpm of 45-5500. Any other suggestions? Thanks

 

[al-B-kerkey]

Re: Help.

OK Frustration has set in. Went down to a 12"pitch and boat ran about 37-3800RPM and about 27mph according to tach and speedometer. Took about 5 boats lengths to get on plane. So I figured if I went down 2 more pitches to a 10 then I would be closer to the recommended RPM range and pick up a cpl MPH. I called the boat dealer and the service dept. recommended an 11" 4 blade prop to help get out of the hole better. Just tried it out and I ran 3700 RPM and 23MPH and took longer to get on plane. Can someone PLEASE help me out here. Frustration has really set in I know I'm not going to break speed records and I also know that boat is way underpowered(because all boat dealer people/mechanics have told me so) but I do want to get the best I can out of it. Thanks and sorry for the rant.

 

[Silvertip]

Re: Help.

I still contend you either have an engine issue or a tach issue. You should be able to tell by ear -- whether or not the engine is running at 3000 rpm or 5000 rpm but the best check would be to verify that the tach is correct. 40 HP on a 15 foot boat is not underpowered -- unless as I said, its full of water soaked foam. With a 12P prop that engine should have no trouble reaching 5000 RPM.

 

[MikDee]

Re: Help.

I still contend you either have an engine issue or a tach issue. You should be able to tell by ear -- whether or not the engine is running at 3000 rpm or 5000 rpm but the best check would be to verify that the tach is correct. 40 HP on a 15 foot boat is not underpowered -- unless as I said, its full of water soaked foam. With a 12P prop that engine should have no trouble reaching 5000 RPM.

I'm in total agreement on all points!
By the way, that original 14" pitch prop sounds about right for that setup, if the motor was working properly. Is the bottom of the hull fairly clean? and is the cavitation plate on the motor, fairly even with the bottom of the boat?

 

[al-B-kerkey]

Re: Help.

Honestly it sounded alot better and performed better with the 12. I didnt gain any rpm and it performed worse with the 11" 4 blade. If all else fails then I will go back to the 12. What do you guys think of an 10 or 11 pitch 3 blade? I guess I need to buy a new Tach and speedo.