OB Props - What Pitch do you Have?

[montenido]

Hi all,

I have a 10' Caribe RIB with a 1994 Johnson 2-stroke 15hp OB. The motor runs well, but has trouble getting my boat on plane. I added those Dole-Fin things with no real improvement. Looking at the prop I noticed that it is 9.5" x 12.5 pitch. That seems like way too much pitch, which might be causing my planing problems. I don't have a tach, but I would say that the motor is not running at its intended speed at WOT.

I am curious about the props being run by any of you on 2-stroke 15hp motors. Would you mind responding with your prop size and pitch?

Thanks and cheers, Bill

[deckofficer]

Bill,

Not much can be offered without knowing the lower unit's gear ratio. I believe that vintage 2 stroke for the 9.9 to 15 hp range had gears of 12/29 so the ratio is 2.42. Peak power for that engine is 5500 rpm, so 5500 / 2.42 = 2273 prop rpm X 12.5" (prop pitch) = 28409" / 12 = 2367' per minute (no slip) X 60 = 142045' / 5280 = 26.9 mph no slip or around 22~23 mph with slip.

Does your hand held GPS tell you your going that fast at WOT?

[Cheechako]

That engine should be planing your boat like a hot rod. I wonder if something else is wrong? is it possible the hub in the prop is slipping? just two people in the boat normally? Any unusual vibrations in the engine?(main bearings) Fuel filter clean (is there one of those little pourous hard filters in that engine also?)

[colemj]

That is too much pitch for sure. I'm guessing you want something in the 8.5-9.5" pitch range. Many engines use the same prop spline pattern and depth - can you borrow someone else's prop for a test spin? I've loaned ours out for that reason before.

Mark

[Cheechako]

A quick check on ebay for that engine revealed that the majority of props for sale were 9.25 to 9.5" diameter and 9-10 pitch. That might say something. There were few outside that zone one to 7" pitch and some higher. Does sound like you are overpropped. Not sure how much the normal rules of pitching apply with a light dingy and those engines though. My 15HP Yamaha 2 stroke spun the hub on the original propellor, I found a bargain Yamaha prop on ebay for $25 new but it was a couple inches higher pitch than my original. I tried it and could tell no difference from my original... but maybe that only applies when you have a lot of extra HP. My Caribe 10 with a 15 yamaha was a rocket ship.

[montenido]

Thanks for the replies folks.

I ended up ordering a 9.25" x 9.0 pitch prop. I also ordered a electronic laser tachometer so I can see what's what when I get back to the boat, which is in La Paz (I am in the states). Right now, my dinghy is far from a rocket ship, and planing takes a long time, with much movement of bodies. I need a spare prop anyway, so one of these will be the back up.

I will post results when I get down there again.

Cheers, Bill

[deckofficer]

Bill,

This reduction in pitch will give a better hole shot to get you up on plane.

A 9" pitch prop should get you 18~19 mph at an engine rpm of 5500. Your engine will tolerate 6000 rpm, so you could go 20 mph. With the tach you bought you won't be over revving your engine but should for some reason the tach doesn't work, I wouldn't go over 20 mph with the 9" pitch.

[montenido]

Hi folks, just got back from the boat last week. The new prop made a world of difference. Of course, I didn't hook up the tach (lazy). I would say that top speed has dropped a bit, but the trade off in acceleration from a stop is so worth it. The dinghy now planes quickly and effortlessly.

Thanks for all the input and ideas.

Cheers, Bill