Dhadley: Prop Question

[Mercboy]

Dhadley- hey there. Mercboy AKA Busterb here. Our T1 is down at work- so I'm logging in at home under my old password! Question for you... My brother in law has this old 73' model 15 foot fiberglass trihull clunker called a cougar made by arrowglass. He has a 76'model 75 hp evinrude on it with a 15' pitch aluminum prop. The motor runs great but will only turn about 4000 rpm wide open. What pitch aluminum prop would you recommend for this old rig? What is the ideal rpm range for this motor? Thanks! (Btw, the boat gets up on a plane very quickly.)

 

[Dhadley]

Re: Dhadley: Prop Question

Good tip about the boat getting on plane quickly. That tends to make us curious about the tach. If the motor only turns 4000, the holeshot should be horrible. To answer your question about prop size - if you are turning a 15" 4000 RPM now and since you need to turn 6000, you would have to reduce pitch by about 10". I've never seen a 5" pitch prop for a 70. All kidding aside, the 15" should be pretty close. Let's verify the tach and go from there. If it's an OMC tach look at the back and see what it's set on. <br /><br />Good luck!

 

[Mercboy]

Re: Dhadley: Prop Question

****... now I'm embarrassed Dhadley! Uhh.. I was using my seat of the pants tach- my ears and the speed of the boat. I know, I know, I need a tach. But I do know how a 3 cylinder OMC sounds turning 5500 to 6 grand- heard plenty of others just like it and it seems this motor is only getting 3/4 throttle. Boat will only run about 25- know that for a fact. Used a GPS. We aren't missing a cylinder, powerhead was rebuilt a couple of years ago and only has about 5 hours total on it- awesome compression. Are carb adjustments the culprit here? I fiddled with the linkages running wide open down the lake and the throttle plates are wide open. I didn't see much in the way of carb adjustment screws on these carbs! ****- I thought it was just too low a pitch prop. As always, thanks for your help!