prop and rpm question

I have a buddy of mine and I told him I would ask here for some advice. He is running a 93 225 johnson mounted on a Armstrong bracket to a 25cc. Prop is a aluminum 15x17. He can only get about 25-30mph at 4800rpms. He said that some guy at the marine store took all the info I just gave you above and told him that was the right size prop for his boat. What do you guy think about this. Can that be done by the info can someone tell you more or less what prop to run. Thanks

Re: prop and rpm question

That's 30-40% slip...extremely bad.<br />I would expect that size of boat/engine combo to produce numbers in the 14-18% slip.<br />Two items we need to verify is what tach are you using, and how are you measuring speed????GPS???<br />We have one or the other (or both) off, and we need to make sure there isn't any growth on boat bottom.

Re: prop and rpm question

A 17 on a 225 should push a barge! Are you sure it's running on all 6???

Re: prop and rpm question

The tach is a teleflex and speed was measured on a gps garmin 76cs. We are going to start looking into the motor to see if it is running as best as it can. We are going to check comp. timming. etc. etc.

Re: prop and rpm question

What about my other question can someone tell you what size prop to run by the motor and boat size.

Re: prop and rpm question

AJC - they can only get you in the ball park unless they have experience with that exact boat with the exact setup, the exact same engine, and the same exact prop (including the manufacturer and style). Change any one factor and all bets are off.

Re: prop and rpm question

Teleflex....that may be an issue there...<br />Even if the tach is close we have something else wrong...we have a known GPS speed, so that more than likely points straight at the tach.<br />I would highly recommend an OMC/BRP tach only....we see alot of problems with teleflex, especially if they are more than a couple of years old. You may want to ask your shop if they can verify the tach against their shop tach if they have one...and checking at Idle won't do it...it's got to be at WOT.<br /><br />"What about my other question can someone tell you what size prop to run by the motor and boat size."<br /><br />The only possible way to do this correctly is with an accurate tach...

Re: prop and rpm question

Thanks Guys. I tell him about the tach issue and take it from there. I myself will also look into my tach since it is a teleflex. Is the OMC/BRP tach the only one you recommend.