Prop questions...please help ! Im lost

mtntrogger

Seaman Apprentice
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Apr 29, 2013
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Im sure this is simple question but I dont really understand too much about props.
So I have a 2001 Four Winns Horizon 180 bowrider. Its got a Volvo Penta 5.7 with Sx outdrive. It originally came with the 5.0, but the PO had the 5.7 put in in 2016.
It currently has a 4 blade 14.75 x 15 prop that is pretty dinged up. Im wanting to replace it. She came with 2 other 3 blades which I havent tried out yet. I believe one is a 19 pitch and the other is a 21, both are also 14.5" not 14.75 ( I think).
I opererate the boat at varied altitudes between 7,000 and 10,000ft above sea level, with the annual trip to lake powell at 3,700 ft.
I mostly use the boat for wake boarding, but occasionally just cruise around , most often at low speed. I like how fast the 4 blade gets up to plane, but dont really have a frame of reference, since I haven't tried the 3 bladed props yet.
My question is basically, what is the difference between these props performance and fuel economy wise ? I realize the higher pitch props should have a higher top speed, but is there a better size or pitch for my current application ? Which ones are best for fuel economy ? Could I go bigger (15") ? What advantages would there be ?
Thanks again for anyone who takes the time to rely to this, I appreciate all opinions
 

Maclin

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May 27, 2007
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Need to know some actual numbers like RPM and MPH at WOT. What is the highest RPM your engine can reach at the lakes you boat at the most? What is the MPH at that RPM? There is a recommended top RPM range that we are looking to keep the engine in at WOT. Need to know if the engine is a GL or GXI, or whatever model number (carbed v fuel injected). It would also help to know the drive gear ratio, the 5.0 may have had a different one than what Volvo would put behind a 5.7, makes a difference.

Did the previous owner boat at those elevations also?

A 4blade provides more bite at takeoff and planning speeds over a 3blade.

A higher pitch number moves the boat farther for each revolution, and will have a higher speed than a lower pitch at the same RPM. The higher pitch will load the engine more. A lower pitch will take off better at the expense of top speed, also too low can allow the engine to over-rev.

Just thinking out loud, the 19 and 21 were probably the previous enigne's props at lower altitudes.

At sea level a 5.7 in that boat may need a 23pitch.
 

Maclin

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I should have led with this... Read the "Read This First!" topic at the top of the Props thread in the Stickys. It is a guideline of what all information is needed to get good advice on props.
 

Sea Rider

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What's mentioned in the prop stickys will only be good for sea level boating, when boating at higher elevations say 5 to 10K KFt it's a different story as engine will run less powered due to working with much less oxygen compared to the full amount found at sea level.

Need to know min-max wot rpm range factory stated. OTOH, have not stated any wot rpm numbers achieved with current dinged prop, will need a tach in excellent accurate working order to play with props and their pitches, much needed when dealing with prop maximization at high altitude levels.

If can't possible go for a re jet, play with + time advanceº + a less pitch prop your last chance of performance improvement is playing alone with less pitch sizes, but boating at several altitudes with same prop will be an issue. For that will need to constantly swap current owned +,. prop pitches as combo increases/decreases mentioned altitudes to work OK, but not top though.

Much better would be boating lightly to medium loaded, too loaded along elevation issues will kill wot revs real fast. Test current 19 pitch at full throttle at each 7 & 10 KFt of elevation and check if engine achieved at least safe middle wot rpm range. Report your findings...

Happy Boating
 

mtntrogger

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Apr 29, 2013
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Thanks fellas,I appreciate the replys. The engine is carbed, im not sure what model that would make it, perhaps GL ? No idea what the drive gear ratio is, is there a easy way to identify that ? Since Ive only pushed it to WOT a few times and the speedo isnt working Im not sure I have an accurate assessment. BUT , The tach redlines at 6k...Ive seen it get very close to that at the lake at 10,000 ft,I had around 5 on board using the dinged up 15 pitch. The previous owner operated at slightly lower elevations, but likely not enough to make a big difference. Id say he was using it mostly at around 5,000 ft or lower. He lived near Denver so I presume thats mostly where he was running it. After reading your responses it seems like replacing the prop with the same size and pitch would be my best option for my application (mostly lower speed) especially since I can likely achieve redline or beyond with the current prop. It looks like Solas sells one for around a hundred bucks, plus an additional 30 for the hub kit.
 

WesNewell

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Jan 3, 2018
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Try the 19p prop. Should bring your rpm down ~800 rpm. See where that puts you in relationship to max rated rpm range. With a normal load of ?? people, you'd want the rpm' to be maybe 200 rpm below max. and you'd probably want the 21p when you go to Mead, and probably a 23P at sea level.
 

jimmbo

Supreme Mariner
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May 24, 2004
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The gear ratio is in the side of the outdrive. Being it was originally a 5 liter it is probably a 1.6:1. Is the 5.7 an actual Volvo Penta, or some car/truck Crate Engine 350?At sea level that engine-gear ratio would need a about a 23. At 10,000 ft... even a 19 will be pretty big. But give it a try. WOT throttle for the carbed 350 was 4400 - 4800

I run the same engine on an 18 1/2 footer, but at 2400 ft. My ratio is 1.5:1 and the stock 21 in AL prop gave me 4800. I rehubbed a 20 Raker I had and it gave me 4900. Adding a 4 bbl Holley spread bore carb gave another 500 rpm. A higher pitched raker brought me back to 4800. Small primaries on the 4bbl increased low end torque, which helped the hole shot with the taller prop
 
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Sea Rider

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Having the tach a 6 K red line rpm limit is no indication that engine can rev that high. For that need to know what's the min-max wot rpm range factory stated for that engine ? Other issue to have in consideration is : will you be constantly or sporadically be boating with 5 souls on board at both mentioned altitudes ?

Happy Boating
 

mtntrogger

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Apr 29, 2013
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Thank for all of your reply's. Lots of good information from you fellas. Interesting what pitch sizes you all mention. If 19 is big at 10k, then my 15 is way too big ? It works excellent for wake boarding, pulls people of all sizes up instantly. I will try the 3 blade 19 next time out just to observe the difference. Thanks jimmbo for posting the stated rpm range at wot for the engine. I will watch the tach much more closely next time out at wot. I can say that normal wake board speed is with the tach at 3k.This is about 18 mph
The 5.7 is a Volvo Penta. Its stamped on the manifold, and all off the components that I can find have Penta part numbers. The carb is a Holley 2bbl. I would say as far as load is concerned 5 souls is pretty average for me. 8 or 9 is as many as I ever have, but can roll as light as 2 or 3.
 

Maclin

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Not sure what Big means in your context. The 15p is doing what you want right now, although we did not get the WOT RPM's or MPH, just going by your observations. By your observations, a 19p at that altitude would probably result in a disappointing performance. Holeshot not as good, not able to maintain planing at lower speeds, etc.
 

mtntrogger

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Apr 29, 2013
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exactly ! I havent tried the others yet and please correct me if im wrong, but to me low speed planing on a boat that is mostly run at low speed would equal better gas economy, correct ?
 

Maclin

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Most economical speed is headway/displacement while nearly idling! With recreational runabout hulls the next best is planing at 3500 rpm generally. Lookup some boat reviews, the best gallons per hour vary a lot, but usually happens when well up on plane and trimmed out. Full throttle is worst, but not by as much as you may think.

Powering an accelerating then cruising boat is like a reverse tractor pull.
 

Sea Rider

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For better fuel economy engine needs to operate well trimmed in the middle to max wot rpm range as usually loaded at any given altitude if that's possible. Middle range will gain some extra MPH while losing top hole shot, full wot will gain a faster hole shot with slight top end decrease. Eeach boater has its own preferences. Once there can throttle at the speed you like most while combo doesn't go off planre and engine revs smoothly at whichever rpm that happens.

If will constantly be boating with 5 up, test your different props at the highest altitude and check which one achieves middle to max wot rpm range and stay with it. If plan using same prop at the least altitude with 5 up check achieved wot revs, should see an rpm increase, more with 3 up. How much, impossible to determine, a wot test will tell..

Happy Boating
 

mtntrogger

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Apr 29, 2013
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Thanks again gents. Would it also be safe to assume that with the boat fully loaded with 8 to 9 souls the 15 p 4 blade would be more appropriate than the 3 blade 19 or 21 ? or is that not correct...
 
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