Am I over propped? What should be on this boat?

[XcellR8]

I just purchased a 1990 Thundercraft 15"6' open bow f/g boat with a 90 hp Merc (I belive it's a 91). It came with a 13 3/4 X 21 Trophy Plus 4 blade SS prop. The boat didn't come with a tach but the first time out it appeared that the motor was under revving (it seems to labour when travelling along at very slow speeds and doesn't really scream at WOT). <br /><br />I bought a Tach and took it out for another ride and found that it would only rev to about 4400 rpm under WOT. Being relatively new to the boating scene I need some advice. From the research I have done so far it sounds like this may be too much prop for this motor (I'm not sure how long the previous owner had this prop on or why he went with such a big prop but that's what came with it)? What should be on this boat/motor combo? Also what should the rpms be at if the motor is placed in gear with barely any throttle applied (the motor started to run rough when I slowed down to a crawl to dock the boat)?

 

[JB]

Re: Am I over propped? What should be on this boat?

There is good advice in the Engine FAQs, XcellR8.<br /><br />Good luck.

 

[b.gagnon]

Re: Am I over propped? What should be on this boat?

I would think you should running more like a 15-17p,<br />you need to get more than 1000 rpms back @wot

 

[XcellR8]

Re: Am I over propped? What should be on this boat?

Originally posted by JB:<br /> There is good advice in the Engine FAQs, XcellR8.<br /><br />Good luck.

Thanks I have been reading in there. I have also been asking some of the dealers in my area and have been getting different answers so I thought I would see what others on here had to say.

 

[XcellR8]

Re: Am I over propped? What should be on this boat?

Originally posted by b.gagnon:<br /> I would think you should running more like a 15-17p,<br />you need to get more than 1000 rpms back @wot

That's what it's starting to sound like. I should probably also mention that a lot of my boating will be slow cruises down a local creek which eventually opens up into a river (where you can then travel at whatever speed you like).<br /><br />One thing that puzzles me is why the previous owner had such a large prop on it and I also wonder how long they drove it like that. Isn't it extremely hard on it to be over-geared like that or only if it has seen lots of WOT(I didn't need a tach to tell me it wasn't pulling the rpms, I just needed one to confirm it)? The other strange thing was there was no trim plate installed. The dealer I bought it from put one on as he said I need it. I noticed that with it on there was only about 1/8" of clearance between it and the prop so I cut a notch out of it to allow for more clearance (something a couple of the dealers told me to do).