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OMC Prop History: 387334 to 763588

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MonsterFarmer

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Hello everyone, new member here to annoy you. Maybe I just don't understand propellers (actually I most certainly don't understand propellers), but in what world does it make sense to replace an 8x7 (387334) with a 8-1/4x8 (763588)? According to everything I can find, that is what OMC did at some point, but does that make engineering sense?

So I'm working on my 3rd early 70's 6HP which I got with a broken plastic 2-blade propeller (385319, probably). My other running 6 has the aluminum version of that 2-blade (386955) and has worked well with it, but actually originally had a 3-blade aluminum (presumably 387334) which I ran into a rock years ago, but before that it also worked well. According to this chart in another forum, that 3-blade was possibly original and standard to all these motors, so I figure for this new project motor why not put a 3-blade since I already have another one with a 2-blade. The weird thing is that everywhere I look for a compatible 3-blade propeller, they want to sell me an 8-1/4x8 (763588) because that replaced the 387334 at some point. Is that somehow effectively the same as an 8x7? Does it even matter at this power level? I did see an old thread on here that made it sound like any of them would probably work fine, so I don't know. Should I just get the bigger prop? A Solas is only $60, not that big of a risk if it sucks I guess.
 
A 7" pitch is about the most you can put on a 6 hp, the 8" is for a 9.5 hp. I put a 8" on a 6hp in a pinch and it was way too much propeller.
 
Right, which seems to be the thinking of OMC back in the '70s, but they themselves replaced a 7" pitch with an 8" pitch. Every vendor selling props for this motor is also selling that 8" pitch 3-blade design. Perhaps it's a case where there's more to it than just these two dimensions?

Now to be honest my contact tach hasn't worked in a while so I don't really know how my motor (with the original 2-blade 8x7) handles its current duty on a loaded 12" V-hull. I know even less how it worked when it still had a 3-blade, presumably original 8x7. I'm leaning towards just ordering one for science and doing some tests of my own. It seems conventional wisdom suggests more propeller does better under heavier loads, and lately all this boat does is carry a small family and 3 days of camping supplies, so it definitely falls in that category.
 
Sure looks like it, at least for this year range which is, what, 66-76? I only see one choice for each gear in the crankcase, and the same part numbers appear in multiple years. So many propellers though. Just now noticing that my aluminum 2-blades (386955) are 8x7, but the plastic 2-blades (385319) are actually 8x7-1/4. If you get one today (763745), it's 8x7-1/4, but it's aluminum! Still not as crazy to me as adding another blade AND upping both dimensions, but I've gotta try it and see what the difference is.

Finally got to take a shakedown cruise with my main motor, a '73 Johnson. It ran great after the carb rebuild, much better low speed stability, but I could not get up on plane with ~425lbs of family. This was not only to test out the motor, but also the family, specifically the 2 small children. The baby didn't mind, but I couldn't get full cooperation from the toddler in balancing the boat, so the best I saw on GPS was 9-10mph wide open. This was with the 2-blade 8x7 that I've had forever. I've seen it plane the boat anywhere from maybe 14-17mph depending on load, but I'm wondering if a 3-blade would be better. Our camping trip coming up will add quite a bit of weight (mostly in the cooler :)), so I think the extra grip will be more helpful than top end speed. If I understand propellers at all, that is...

The backup motor ('73 Evinrude) does not seem to pump water at all so it will need some more work. Runs well though and is in great condition in other ways, so I think it's gonna be a great part of the team! Man I love these little things!
 
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