21p aluminum to 19p SS?

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21p aluminum to 19p SS?

Postby bunker108 » Sat Sep 29, 2012 5:46 pm

I have a stock Volvo 21p al on my 188 w/4.3gl. I can spin it to just shy of 5k wot at over 4000' elevation. But when loaded heavy camping at lake Powell it took a long time to plane...once on plane I could still spin 4600 witch is at the top of the operating range.

I picked up a used SS 19x14.5. I am hoping this will be a good compromise to plane quicker while not giving up too much on the top end. If I can save up some pennies I think the best prop would be a SS 4 blade 19p. I might pick up a cheap aluminum 4 blade to test this out my theory.

I will post the results when I can water test the 19p SS.

What do you all think about 4 blades?
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Postby AndrewT » Sat Sep 29, 2012 6:57 pm

With a 19" 4 blade prop , you will get better acceleration and the boat will stay on the plane at a lower speed, good for wakeboarding or pulling tubes.
There will be be some top speed loss may be about 4-5 MPH, nothing to worry about, overall it will be a lot better to drive especially for water sports.
I will also be easier for other drivers(the wife :) ) to maintain a constant speed (reduces the risk of divorce :D )
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Postby HOT PURSUIT » Tue Oct 30, 2012 11:41 am

Rule of thumb is 200 Rs per pitch change and 200 for 4th blade. Appears you will increase RPMs by 400 going to the 19P but 200 for the 4th blade. That means adding 200 Rs to what you currently have so be CAREFUL! Sounds like you will over rev the 4.3. Let us know.

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Postby Kimmel » Tue Oct 30, 2012 12:50 pm

HP: what do you mean by over rev the 4.3? I'm relatively new to boating. Our 187 came with alum. 23p. How does this apply to our situation. Thanks.
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Postby AndrewT » Tue Oct 30, 2012 4:11 pm

I think that the operating range of a 4.3 is between 4400 to
4600 rpm, at wide open throttle the revs shoul fall in that
rev range , if it overrevs it's no good to the motor if it's to slow
the motor lugs and does not perform in a optimal way.
You can adjust max rev by going up , down pitch size
on the prop like HP said in above post
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Postby HOT PURSUIT » Tue Oct 30, 2012 11:14 pm

Hey Kimmel! I had a 4.3L in previous boat but don't recall what the wide open throttle (WOT) revs were. Like Andrew said, the book will tell you what the maximum RPMs should be. Most newer engines have rev limiters which will not allow the engine to over rev.

A 23P on a 4.3L sounds like you are over propped Sir. What RPMs are you turning at WOT?

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Postby Kimmel » Fri Nov 02, 2012 2:30 pm

The boat is winterized....I'll have to look into it next spring.
We don't run wide open very often and in the 60 hours we've put on the boat since purchace, I've never felt like we were overdoing anything. The guy at the prop shop said he thought a 21 would be better, but the boat came with a 23 and a 23 spare.
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Postby bunker108 » Fri Nov 02, 2012 9:45 pm

I'm not really sure what the limiting factor is with a 4.3GL and SX drive is. A 4.3 will spin 5000k without risk of damage although the torque does drop off. The Volvo SX drive can spin much higher and handle much more power than he 4.3 is capable of.

I can spin my 21 aluminum to a bit over 5K with a light load. With a heavy load of camping gear, coolers and four on board at Lake Powell the 21p took a long time to get on plane but once on step I was easily reaching my rated WOT. I want to be able to plane quicker without giving up TOO much up top.

A 20p stainless might be best or a 19p stainless four blade.

I think my 19p SS will be under propped but I should be able to plane effortlessly with a full boat. I suppose I will just need to experiment.

Keep in mind that Powell is over 4K feet in elevation.
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Postby HOT PURSUIT » Sat Nov 03, 2012 8:08 am

I had a 4.3L carbbed, 190 horse, in an 18' bowrider and ran a 21P Merc Revolution 4, 4 blade SS prop and performed AWESOME. FYI


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Postby bunker108 » Sat Nov 03, 2012 10:29 pm

Hey HP what elevation were you running the 4 blade 21p at?
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Postby HOT PURSUIT » Sun Nov 04, 2012 12:29 am

1256 feet :lol:

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