too large prop cause damage in Z drive?

[Ostsideend]

Hi Guys,

does a too large Prop pitch causes damages in bearings and/or gear wheels in a i.e. Alpha one Gen2 drive unit by overload (torque) or is it a bullshxx story???

Best regards
Oliver

 

[CR CRUISER]

Re: too large prop cause damage in Z drive?

Yes, it is very hard on gears and bearings as well as pistons are prone to cracking. It's the same as driving a stick-shift in high gear at slow speeds.

 

[Robbabob]

Re: too large prop cause damage in Z drive?

Hi Guys,

does a too large Prop pitch causes damages in bearings and/or gear wheels in a i.e. Alpha one Gen2 drive unit by overload (torque) or is it a bullshxx story???

Best regards
Oliver

Too small will cause higher RPM, and too large will cause the engine to bog down... you don't want either case to take place. Go small for faster hole-shot for wake board and skiing, but I cannot think of a case when you'd want to go too large but I don't know of how it might actually hurt the gearing (more of engine problems).

my $.02, experts will chime in before too long

 

[Bondo]

Re: too large prop cause damage in Z drive?

does a too large Prop pitch causes damages in bearings and/or gear wheels in a i.e. Alpha one Gen2 drive unit by overload (torque) or is it a bullshxx story???

Ayuh,... The drive will run abit Hotter when overloaded like that,...
You're more apt to burn Holes in the pistons from Detonation though.... Much more apt...

 

[steelespike]

Re: too large prop cause damage in Z drive?

Damage from an oversized prop would depend on the extent of the missmatch.
A 100 hp with a 1" oversize would likely perforn poorly but it would be a long time before you could measure the difference in wear.
An 18hp 1" oversize would be in more trouble.

 

[elkhunter338]

Re: too large prop cause damage in Z drive?

This is a ? I asked awhile ago and did not get any real world examples of where too big of prop actually resulted in engine failure.
Like someone else mentioned you want to make sure your engine is not spark knocking to burn holes in the pistons. I think a way over pitched prop will give poor enough performance that you will switch to one closer to the recommended range.
I found alittle higher pitch prop gave me better fuel economy, but was slower coming up on a plane. Running at the bottom of the recommended range gave better fuel economy than running at the upper end of the engine rpm range.
One think I thought of is a truck pulling a hill will use the highest gear possible to keep his ground speed up. Typically this is at max torque or slightly above.

 

[45Auto]

Re: too large prop cause damage in Z drive?

The reason your boat gets better fuel economy propped to turn at the bottom of it?s recommended RPM range is the same reason your car gets better fuel economy in overdrive.

Short and sweet: A given speed requires a given amount of power, which requires a given amount of fuel and air. A bigger prop (or taller gear ratio in your car) requires the engine to turn at a lower RPM to deliver the same speed. This lower RPM requires a bigger throttle opening to deliver the same amount of fuel and air through the engine, thus reducing pumping losses and increasing the engine?s efficiency.

Making the same power at lower RPM also increases cylinder pressures and temperatures. As Bondo pointed out, if you go too far then you destroy pistons and burn valves. This is why you don?t lug your car through the 15 MPH school zones in top gear or prop your boat with a prop that?s too big.

As a side note, one of the main reasons a diesel engine is more efficient than a gas engine is because the diesel does NOT have a throttle valve. It is running "WOT" (Wide Open Throttle) all the time and therefore has no pumping losses due to throttle flow restrictions. Power delivery is controlled solely by how much fuel is injected into the cylinder.

 

[QC]

Re: too large prop cause damage in Z drive?

Hi Guys,

does a too large Prop pitch causes damages in bearings and/or gear wheels in a i.e. Alpha one Gen2 drive unit by overload (torque) or is it a bullshxx story???

Best regards
Oliver

Back to the OPs question . . . I agree with steelespike that this depends on degree. What engine, gear ratio and prop dimensions are we talking about?

 

[Ostsideend]

Re: too large prop cause damage in Z drive?

The question was in general.

Let's say an chevy 5,7 carbed SB drives an Alpha one Gen 2 standard 1:61 or close to it wich is fine with 16 pitch will be operated with a 21 pitch for a longer period of time.

The boat will plane, but at WOT all the time.
Does this will overstress the A1 drive?

 

[Robbabob]

Re: too large prop cause damage in Z drive?

That sounds like a very large difference. Also sounds like you aren't inquiring at all about how the engine will react; only if the outdrive will be ruined.

Less problem with the outdrive as what you'll do to your engine and fuel economy.... over time.

 

[45Auto]

Re: too large prop cause damage in Z drive?

The boat will plane, but at WOT all the time.
Does this will overstress the A1 drive?

Won't bother the drive, but if you're in your recommended RPM operating range with the 16" pitch, then the 21" pitch will probably help you become very familiar with engine rebuilding techniques before very long ....