WOT vs Pitch vs Hull vs Blades?

[WhalerSailor]

Cheers all,
I have a friend who has just retro fitted his 13' MFG runabout with a 1962 Merc 500. This is one clean Merc 500. Great compression! Looks and runs like new. We fitted it to the boat and took it out last week with the old two blade prop which came on it. The engine ran great all day but would only reach a top speed of 23 MPH and the motor was sounding like it was revved too high. As of now we haven't installed a tach (coming soon). Yesterday he put a three blade Michigan wheel prop on it and could not achieve WOT (nowhere near it) and a top speed of 12 MPH. Both props are 13 pitch. Is there such a thing as setting the dwell or point setting to match a prop (ie. two vs three blade)? I used to have an old VW with no power - cleaned and set the points and she took off! I know these props were sold as standard props for as little as 35 HP and his boat is small and light. Any insight would help immensely. Thanks, Dave

 

[Chris1956]

Re: WOT vs Pitch vs Hull vs Blades?

Dave, There is no changes to the motor based upon a different prop. The motor should be set up with the standard dwell, timing and plugs. 23 MPH is a bit low for that setup. I would think it should go at least 30 MPH. if you create a post asking about the differences in the 2 blade and 3 blade props, in the Prop section of iBoats, you will get an intelligent answer.

 

[Silvertip]

Re: WOT vs Pitch vs Hull vs Blades?

People have a tendency to want to blame a prop for a lack of performance when that's the last thing that should be looked at. First, make sure the engine is in a proper state of tune. That means new plugs, compression check, nice fat spark at each cylinder, fresh fuel, clean carbs, lionk&sync peformed, idle and WOT timing properly set and the engine height set so the AV plate is flush to an inch above the bottom of the hull. Make sure the hull is clean and that you are not dragging around a boat load of water soaked foam. Next install a tach as without it you have no idea what the engine is doing and what effect various props have. Having what you consider acceptable speed with the engine running below its recommended WOT RPM is a good way to destroy a perfectly good motor because it would be considered under propped and therefore "lugging". When all of that is done, then start experimenting with props.

 

[WhalerSailor]

Re: WOT vs Pitch vs Hull vs Blades?

Thanks for the response guys,
All that has been done except for installing a tach. This guy is a very meticulous person as you can tell by the pics.

http://members.cavtel.net/zxsstudios/RobsMFG/RobsMfg.html

Everything has been gone through short of splitting the powerhead. Carbs, Fuel pump, compression (123, 125, 123, 123). The boat has new stringers, transom and floor not to mention refurbishing everything from the cleats to the logo. Just can't figure out why the motor won't get into the power range with a three blade prop rated at the same pitch as the two blade.

 

[Rudy Brown]

Re: WOT vs Pitch vs Hull vs Blades?

I cant help you on the problem with your prop, but I can say WOW, thats one NICE boat.

 

[WhalerSailor]

Re: WOT vs Pitch vs Hull vs Blades?

Thanks Rudy,

Yeah, this is the guy that got me started doing my whaler. After finding that my old 20' Seaswirl cabin cruiser was - although comfortable - too much to tow around, I went to a very small 15' ArrowGlass bass boat. It was a LOT of fun but only on smooth water. After a grand search for the perfect light weight small craft that would go anywhere I came up with the Boston Whaler Montauk. That was two years ago and I'm about ready for the water - only now I realize that my choice should have been the BW Outrage. Maybe there's still one more boat on the horizon.