Are plastic props junk?

[guy48065]

I imagine the answer has a lot to do with the hp of the motor.
I have a Force 5hp (1990 Merc) that I like for it's light weight. I've only run it in a barrel so I don't know how it performs on the lake. It has the original plastic prop. Do these flatten out under load?

 

[jimmbo]

For a 5 hp, a Plastic Prop is fine. Even a Plastic Prop does not Flex very much if at all. Even if it did, the flex would likely be in the range of 1/50", which would increase the engine speed by a couple of rpm

Added bonuses
Lightweight
Inexpensive
Non Corrosive

 

[KJM]

I recently saw an add where a guy was selling a composite prop with interchangable blades. Would that be plastic, or something stronger? Would the blades be different pitch's? Sorry, not trying to pirate the OP's thread, this is somewhat related!

 

[briangcc]

I ran a Comprop composite prop for a bit on my Bayliner as I was boating in rock filled areas. I didn't notice any performance hits but will admit I ran it for roughly 1 season - the one season I had a Force outboard on my boat which the following season I repowered with the Honda.

I still have the prop in pristine condition. They were rated up to a certain horsepower before they'd self grenade. 5HP isn't anywhere near that limit.

 

[QBhoy]

Be fine. But, given it’s likely components there…I’d absolutely take it off and have a check of the shear pin, it’s most likely to have behind it. If the engine is unknown to you, well worth a check and having a spare. Wouldn’t believe how often I’ve found shear pins only just holding on…or worse still…that have been replaced by a cut to fit nail or screw !! Resulting in defeating the whole purpose. Some folk !!

 

[jimmbo]

Edit: FUBARed. so Content deleted

 

[QBhoy]

In most OMC Outboards, it wasn't a Shear Pin, it was a Hard to Shear Stainless Steel Drive Pin. On the Plastic Props, I don't know if OMC varied from that.

Good to know. Think this is partly based on a merc though, perhaps. Any of the smaller merc engines I’ve had or had in bits, have a wee shear pin that goes through the shaft and sits home and mated to the back of the prop. This could be different of course. Never had a force before. Tried to avoid them, if I’m being absolutely transparent, haha.

 

[QBhoy]

In most OMC Outboards, it wasn't a Shear Pin, it was a Hard to Shear Stainless Steel Drive Pin. On the Plastic Props, I don't know if OMC varied from that.

Ah. You’ve maybe got a point there jimmbo. We’re some of these smaller force not actually a collaboration with omc perhaps. Just came to my head, they might be right enough. Just looking at the pic of it there. Top end of it doesn’t look too merc like. Some of the shaft components might be merc looking perhaps. The lower unit a little different for sure. Merc 5hp engines of that era had through prop exhausts for sure.

 

[jimmbo]

Oops I messed up, for some reason I was thinking OMC, not Force
I will delete my words

 

[QBhoy]

Oops I messed up, for some reason I was thinking OMC, not Force
I will delete my words

No. I think you may be not far off to be fair. Might be wrong…but for some reason it’s in my head that some of the small force shared certain parts with omc perhaps. Not sure why i think that at all. And could be way off. Only ever seen one of these in real life. Almost sure there is a pin in the front or back of the prop. Maybe behind the nose cone even. Not sure if it’s designed to shear or designed to sacrifice the prop side though. Worth a look anyway

 

[guy48065]

Well I'm glad to hear plastic is fine for small outboards. Thank you!

A related question:
IF I'm interested in peak performance and maybe want to try a different prop--what do you all use to measure rpm? I can't recall ever seeing a recommendation on what to use on a little motor. Is one of those little battery-powered LCD jobs where you wrap a wire around the spark plug lead the hot ticket?

 

[jimmbo]

Are there any Optional Props offered for that motor?

 

[QBhoy]

Well I'm glad to hear plastic is fine for small outboards. Thank you!

A related question:
IF I'm interested in peak performance and maybe want to try a different prop--what do you all use to measure rpm? I can't recall ever seeing a recommendation on what to use on a little motor. Is one of those little battery-powered LCD jobs where you wrap a wire around the spark plug lead the hot ticket?

Those wee tachometer are pretty good…as long as you can make sense of the settings menu…to dial in the correct setting for the engine you have…in terms of how many cylinders she has and also the ignition cycle too. But to be honest…probably not something that should be bothered with too much…especially given the absolute obscurity of such an engine, in terms of available parts for it, perhaps. Maybe there are lots of these things where you are..but certainly over the pond here. I’ve only ever seen one of them in my 44 years being around boats end engines most days

 

[guy48065]

The question of what to use to monitor rpm isn't motor-specific and can be applied to any portable OB or boat that doesn't have a tach.

 

[QBhoy]

The question of what to use to monitor rpm isn't motor-specific and can be applied to any portable OB or boat that doesn't have a tach.

Yeah. No doubt. But I was specifically referring to the idea of messing with the prop on this particular engine. That’s the reason he wanted to measure the rpm, I assumed. All the best

 

[nola mike]

The question of what to use to monitor rpm isn't motor-specific and can be applied to any portable OB or boat that doesn't have a tach.

I've had mixed luck with those. better bet is timing light or multimeter or the optical tachs.

 

[guy48065]

I've had mixed luck with those. better bet is timing light or multimeter or the optical tachs.

I don't have enough hands for meters & timing lights when operating a tiller-steered outboard. If the Tiny Tach isn't reliable maybe there's something like it that is?

 

[flyingscott]

Chances of finding a prop for that motor is going to be some slim. Treat that one like gold replacements will be hard to find.

 

[guy48065]

Is this "9-spline, pin drive" prop really that much of a bastard hub?

 

[flyingscott]

I would start looking for a spare.