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× Technical Discussions

Props

  • David
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3 years 10 months ago - 3 years 10 months ago #4377 by David
Props was created by David
So I don't think I really get this whole prop thing, 3 blade, 4 blade, pitch, diameter :-? My boat has twin 5.7 OMC's and I generally get up on plane at about 3000 RPM running between 21 and 29 MPH depending if I am going up river or down river. So what will I get with 4 blade props over my current 3 blade props. Is there a real difference in your opinion between aluminum, composite and stainless.

David
1990 Sunray Infinity
Twin 5.7 Cobra's
Last edit: 3 years 10 months ago by David.

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  • Roel Jansen
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3 years 10 months ago #4378 by Roel Jansen
Replied by Roel Jansen on topic Props
David,

That always gave fierce discussions in the past at BOC. Especially about the advantages of aluminium and stainless props.
Stainless is stronger than aluminium but can do harm to the drive if something is hit. That most of the times was the outcome! But also that you must have the right props with pitch and diameter for the boat. Advice given go to a prop shop.
I would say stay with the props that were originally on the drive(s) when the boat came from the dealer.

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  • kjs
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3 years 10 months ago - 3 years 10 months ago #4379 by kjs
Replied by kjs on topic Props
David, the most important thing is that the prop doesn't over-load the engine. In short you have to reach the recommended WOT RPM's for the engine.
The rest has more to do with blade cross section (or profile) and blade shape. A thicker blade if designed right can actually have less resistance than a thin blade (same thing as aircraft wings). It also has a lot to do with maximum rotational speed of the prop.
4-blade props are mainly better in smaller crafts as they usually provide a bit more stern lift which gets the boat faster on plane and you can stay on plane at slower speeds. Due to the lower pitch they also get the engine faster into the max torque point.
As you have trim tabs and a heavier boat most likely you won't see a difference with a 4-blade prop but you would lose top speed for sure as you have to go lower pitch with the 4-blade. Your acceleration may be a tad better with it.
If you don't have a problem with what you have I would stick with it........
Last edit: 3 years 10 months ago by kjs.

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  • David
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3 years 10 months ago #4380 by David
Replied by David on topic Props
Great advice guys. The props I have are Piranah composite props and the seem to work fine I was just wondering if I would gain anything by switching. As for the stainless steel its funny or not so funny that you mention damage to the drive when you hit something Roel. I was speaking to my brother-inlaw last night and he was telling me he hit some rocks on the weekend bending one of his stainless prop. Rather than run back on one engine he continued on at 3000 RPM until the engine with the damaged prop over heated. I suggested that when he hit the rock that he damaged the drive to the extent that the impeller was not working causing the over heating. He didn't think so :blink: What kind of damage can be done to the drive when you bend the stainless blades 2-3 inches :X

David
1990 Sunray Infinity
Twin 5.7 Cobra's

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  • kjs
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3 years 10 months ago #4381 by kjs
Replied by kjs on topic Props
Well, you can bend the prop shaft or even worse crack the lower unit housing. You can do that with all types of props but with an SS the chances are a bit higher.

Some weeks ago we did some intense prop testing and the Piranha HydroBytes in 4-blade did perform extremely well (we didn't have the standard model). This despite all the claims that composite props are poor performers.
However, I wouldn't want to generalize that. The tests were done with a 3l and 4.3l engine on 17.5/18.5ft boats only. Another interesting outcome was that the prop material deemed to have less impact on performance than the blade shape and profile. To me the interesting thing was that one of my props, a Vortech 3-blade, performed as well as many of the 4-blades but again, that was with the hull of my boat (1750). On the newer 175 the 4-blades were better.
My conclusion is that you have to try several props to find the right one for your boat and application.

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  • k-townguy
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3 years 10 months ago #4382 by k-townguy
Replied by k-townguy on topic Props
Dave, 4 blade props are generally for ski boats, wake boats, etc for a faster hole shot but you will not get the top end that you have now. As mentioned you must be able to reach WOT with your normal load, ie 2 ppl. If you are over, to low of pitch, if under too much. I think the rule for four blade is you need to drop 2deg of pitch. And as for stainless, yes there is an advantage, but as others have said, hit a rock and you may as well replace the drives. Ive had 2 ppl with stainless props hit shoals and is shreds the gear sets.

1985 Bayliner 2150 Ciera Sunbridge
VP AQ 125A/270


"FOUR PLAY'N"
Why is there never enough time to do it right the first time, but always enough to do it right the second time.

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