Stainless vs. Aluminum- A few questions

[Bigprairie1]

I just acquired a stainless 19P. prop for my o/b motor and I was wondering if anyone had seen any advanced mechanical issues show up on the lower unit due to the excessive weight (3x) of the stainless?
I have to think that I'd be losing a few hp just by spinning that heavier mass around....but hey, first I'll ask a few people 'in the know'.
My motor is a 90hp (Force) in good condition and runs out right now over 5000rpm with a 17 pitch aluminum prop and currently is pushing my 17' around 40mph.
Another thing about this newly acquired stainless 19P prop is that it appears to be 'cupped' on the outer edge. What does this offer?
Now, I did get a great deal on this prop and I'm wondering if it is worth it to change it out or just keep it as a spare, particularly if it is going to compromise something without adding much value.
Any feedback on this would be great Iboaters!!
Many Thanks
BP

 

[Bifflefan]

Re: Stainless vs. Aluminum- A few questions

I just acquired a stainless 19P. prop for my o/b motor and I was wondering if anyone had seen any advanced mechanical issues show up on the lower unit due to the excessive weight (3x) of the stainless?
I have to think that I'd be losing a few hp just by spinning that heavier mass around....but hey, first I'll ask a few people 'in the know'.
My motor is a 90hp (Force) in good condition and runs out right now over 5000rpm with a 17 pitch aluminum prop and currently is pushing my 17' around 40mph.
Another thing about this newly acquired stainless 19P prop is that it appears to be 'cupped' on the outer edge. What does this offer?
Now, I did get a great deal on this prop and I'm wondering if it is worth it to change it out or just keep it as a spare, particularly if it is going to compromise something without adding much value.
Any feedback on this would be great Iboaters!!
Many Thanks
BP

The cupping helps it hold on to the water better, ie: less slip.
I dont think the weight is an issue.
In an ideal world you will either loose a couple hundred RPM at the same speed, or gain a couple MPH at the same RPM.
Hole shot may suffer a tad bit, or may not.

Really, you just need to put on and try it. Thats the best answer.

 

[QC]

Re: Stainless vs. Aluminum- A few questions

I dont think the weight is an issue.

The weight is a NON issue . . . hydrodynamic drag and less flex make stainless more efficient than their aluminum counterparts. Compared with moving weight, moving anything through the water is a much higher load. Compare a lightweight oar with a heavier one. The issue is still pulling it through the water, not the weight.

 

[Frank Acampora]

Re: Stainless vs. Aluminum- A few questions

Stainless runs about 200-400 RPM faster than aluminum in the same pitch.

Cupping drops RPM about 400

increasing pitch drops rpm by about 400.

So increasing the pitch to 19 from 17 and cupping the prop will drop RPM by 800 leaving you at 4200 RPM. Stainless will run 200 faster leaving you at 4400 RPM which is just below the rated range of 4500-5500. Stainless cupped can be run higher in trim and on the transom which will add another 200 or so RPM putting you at about 4700.

In short, using the stainless 19 pitch prop MAY drop speed a couple of MPH or it may equal current speed.

Stainless is much longer lived and looks much better. It does not affect longevity of the lower unit in any way. Choice is yours.

 

[Bigprairie1]

Re: Stainless vs. Aluminum- A few questions

Wow, good info....all around.
I'm going to give it a...whirl (pardon the pun), but not this season...having just started my winterizing. It will be well worth the try next spring.
Frank- as usual....unbelievable detailed feedback (wealth of knowledge dude)
QC- very interesting info regarding the weight of stainless vs. alum. when underload.
Many thanks.....ideally some feedback on this in the spring!!
Collin