Prop Replacement - Excessive Bow Lift

deafwish · **Posted:** Sat Jan 12, 2013 7:30 pm

Hey all,

I replaced my lightly damaged OEM Volvo Penta prop (3850302) with a VP prop of the same spec (3817469), as the 3850302 is discontinued.
The original prop is a 14.25 X 21 and used to max out at 4500RPM.
The newer prop is a 14.3 X 21.
My Horizon RS now struggles to plane at lower speeds, needs more throttle control and revs to almost 4800RPM...
The newer prop looks to be more streamlined than the old but this seems crazy!

Is there really any difference between a 14.25" & a 14.3" diameter prop?
Thanks in advance...

Surface Interval · **Posted:** Sat Jan 12, 2013 8:39 pm

Does the new prop have cupped blades (curved lip on the trailing edge of each blade)? A prop that is cupped will rev 200-400 rpm less, and usually produce more thrust. How does the size of each blade compare? Props with smaller blades will usually produce less thrust and rev higher. How does the rake angle compare between the props? This is sometimes a little to measure and compare but props with a higher rake angle will usually produce more bow lift. I had a '95 190 horizon with a 5.0 EFI. The 14.25 X 21 Stiletto Stainless prop I bought gave me the exact same performance as the OEM Volvo/OMC cobra 14.25 X 21. Check your boat's and engine's specs to see what max rpm is. Generally, the engine should be within a couple hundred rpm of redline at trimmed out top speed. It sounds like you may need to keep looking for a prop that better suits your needs. Hope this helps.

deafwish · **Posted:** Sun Jan 13, 2013 12:35 am

Surface Interval wrote:

Does the new prop have cupped blades (curved lip on the trailing edge of each blade)? A prop that is cupped will rev 200-400 rpm less, and usually produce more thrust. How does the size of each blade compare? Props with smaller blades will usually produce less thrust and rev higher. How does the rake angle compare between the props? This is sometimes a little to measure and compare but props with a higher rake angle will usually produce more bow lift. I had a '95 190 horizon with a 5.0 EFI. The 14.25 X 21 Stiletto Stainless prop I bought gave me the exact same performance as the OEM Volvo/OMC cobra 14.25 X 21. Check your boat's and engine's specs to see what max rpm is. Generally, the engine should be within a couple hundred rpm of redline at trimmed out top speed. It sounds like you may need to keep looking for a prop that better suits your needs. Hope this helps.

Thanks for the technical response!

Both prop's have slightly cupped blades.
The blades on the newer prop (14.3 x 21) look to be a tiny, tiny bit larger - which contradict your suggestion!

There might be a tiny bit more rake on the newer prop, but it's very difficult to guage.
Volvo Penta state that the newer prop I'm using is a direct replacement for the older part number...

RPM range is between 4200 - 4600.

Surface Interval · **Posted:** Sun Jan 13, 2013 10:40 am

A quick check of the Four Winns website shows a 14.25 X 23 prop came on that boat/engine package. Was the 21p the original prop? I recall that my 5.0 EFI Ford/Volvo had a max rpm range of 4200-4600. Some of the other engine packages had a 4400-4800 rpm range. Since the early to mid '90s several changes occurred in the propulsion packages. Somewhere in 1994-1995 the industry changed from rating engines by horsepower at the crankshaft to horsepower at the prop. This caused HP ratings to be reduced by typically 25hp. In the late '90s General Motors came up with several improvements to their line of engines that netted significant hp rating increases. Many 1980s Chevy 5.7s (350 cu in) were rated at 260hp. The mid '90s version was down to about 225hp at the prop, while current models are in the 280 and 320hp range. My 2002 280hp has a redline of 5000rpm. If the outdrive has been changed to a different gear ratio that will affect prop selection also.

Walt · **Posted:** Sun Jan 13, 2013 11:51 am

deafwish wrote:

The original prop is a 14.25 X 21 and used to max out at 4500RPM.
The newer prop is a 14.3 X 21.
My Horizon RS now struggles to plane at lower speeds, needs more throttle control and revs to almost 4800RPM...
The newer prop looks to be more streamlined than the old but this seems crazy!

Is there really any difference between a 14.25" & a 14.3" diameter prop?
Thanks in advance...

Diameter wise shouldn't be causing issues. Look at the rake; that's the angle the blade has to the shaft centerline. As long as they are the same, then no issue there either. I have only run into a bow-rise issue when going to a 4 blade prop, but even my Hi5 doesn't have excessive bow-rise. How about blade material (SS vs Alum?) and blade thickness... the same I hope? A thinner blade will cause less drag, thus higher RPM. Another possibility is ventilation. Are there holes in the prop hub on the leading edge of the blade? Do they have caps in them? If so, try it with them removed. This will allow the prop to cavitate slightly, allowing higher RPM out of the hole, but not at speed.

From your results, it doesn't look like we are comparing apples to apples.

deafwish · **Posted:** Sun Jan 13, 2013 3:42 pm

Both prop's are run of the mill aluminium and neither has any hub vent holes.
I'll get a photo of both, this evening.
I wouldn't have thought that VP has made the newer prop with thinner blades.
Only a micrometer could tell the difference...
I'll see if anyone local will lend me their prop' for a few hours of water testing...
SX gear ratio is 1.66 - if this makes any difference?

<Edit> The images on the SX Drive are the old prop & the freshly painted one is the newer style. Note the streamlined hub on the newer prop.

ric · **Posted:** Mon Jan 14, 2013 8:31 am

Well that's your first problem: aluminum. I went from steel OEM to aluminum and was HIGHLY disappointed. Same thing happened that you described. Felt like I was driving on ice, zero traction.

If I'm not mistaken, I thought the OEM VP props were mostly steel. Not stainless, but some kind of pot metal. Both on my 2000 FW and my 2009 SR OEM VP props are both magnetic.

The Kid · **Posted:** Mon Jan 14, 2013 2:29 pm

ric wrote:

Well that's your first problem: aluminum. I went from steel OEM to aluminum and was HIGHLY disappointed. Same thing happened that you described. Felt like I was driving on ice, zero traction.

If I'm not mistaken, I thought the OEM VP props were mostly steel. Not stainless, but some kind of pot metal. Both on my 2000 FW and my 2009 SR OEM VP props are both magnetic.

umm no a OEM Volvo prop is aluminum... I have 4 of them on the shelf and 5 FW in stock and a magnetic doesn't stick to any of them

ric · **Posted:** Mon Jan 14, 2013 2:41 pm

The Kid wrote:

ric wrote:

Well that's your first problem: aluminum. I went from steel OEM to aluminum and was HIGHLY disappointed. Same thing happened that you described. Felt like I was driving on ice, zero traction.

If I'm not mistaken, I thought the OEM VP props were mostly steel. Not stainless, but some kind of pot metal. Both on my 2000 FW and my 2009 SR OEM VP props are both magnetic.

umm no a OEM Volvo prop is aluminum... I have 4 of them on the shelf and 5 FW in stock and a magnetic doesn't stick to any of them

Tell that to my prop! Not SS but very magnetic

deafwish · **Posted:** Mon Jan 14, 2013 3:15 pm

Both prop's are aluminium.
What OEM's supply a steel prop' for marine use?!

Ric - This subject has been answered in one of your own threads:
viewtopic.php?f=3&t=8691&p=85986&hilit=prop+is+magnetic#p85986

deafwish · **Posted:** Sat Feb 09, 2013 3:36 pm

I fitted a Solas Amita 4 (14x21p) and sea trialled it, yesterday.
For the $250 it cost me, this has been the best bang for buck upgrade I've done to any toy I've ever owned!

I've managed to reduce my planing RPM by 400 and increased my top speed by 5mph!
Via GPS, I managed 53 mph with 1/2 tank of fuel, two adults and fully trimmed (from 47mph).
I'm not far off having a 50kn bowrider! :lol:

www.iFourWinns.com

Prop Replacement - Excessive Bow Lift

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