Need Help !! What is the better prop for my boat?

DownToFish1

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Mar 2, 2014
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Hello, I have a 1984 24 ft rampage express with a 5.7 300 hp I /O alpha 1 .. Currently I am running a 14 1/2 x 17 3 blade . I can only get the boat rpm to 4100 @ 28.5knots. It will cruise at 19 knots @ 3400 rpm burning approx 1 gal per mile. Which seems very bad for economy. I also have a hard time maintaining cruising speed in seas. Boat will drop down to 15.5-16 knots. This prop was on my boat when I purchased from previous owner.

I found the original paper work specs on this boat. The original prop the came on the boat new was a 3 blade 15 1/4 x 15. Said the boat goes 37.5 mph @4650 rpm. I am looking for advice on the best prop. Should I try the original prop ? I am also considering a 4 blade I like the benefits of the extra blade in the water

Any advice is much appreciated thank you !
 

Maclin

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May 27, 2007
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Re: Need Help !! What is the better prop for my boat?

This question would best be addressed in the Prop forum, perhaps a Moderator will move it for you.

Your engine needs to be able to hit top RPM at light load for best performance and economy and engine life. Based on your numbers and the description of the stock setup you found it sounds like you could be over-propped. Going to a 15 would help but you may need to drop even more.

When using a prop calcualtor, and I had to assume a 1.5 drive ratio, I am getting 25% slip, that is high. But using that same slip number it shows that a 15 pitch would get you to 4600 rpm at about the same speed.

If I were to guess, not knowing much about the boat, going to a 4 blade 14 pitch would be a good thing to try.

You do need to verify that your engine is healthy and in top tune as if it is out of tune that will also keep it from reaching top rpm.
 
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jim909

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Mar 17, 2012
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Re: Need Help !! What is the better prop for my boat?

I would research what is best for your boat. I had to do the same thing when I purchased my boat. I found out that the original owner had an outboard prop on an inboard motor. To determine the ideal prop, I researched many manufacturers. However, Quicksilver, was best in regard to determining the spec's that I needed. I was also told that using a 4 blade will create more top end speed, and the 3 blade is better for hole shot performance. Hope this helps.
 

Maclin

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Re: Need Help !! What is the better prop for my boat?

WOW, the Mods moved it while I was posting, good job guys :thumb:
 

SkiDad

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Jul 18, 2010
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Re: Need Help !! What is the better prop for my boat?

sounds like your engine is not running well - i would think you could turn a 17 pitch no problem. But if engine is OK, i would definitely drop to 15 pitch if it were me.
 

jestor68

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Jun 12, 2012
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Re: Need Help !! What is the better prop for my boat?

A Solas aluminum 15 X 15 might do the trick.

Bear in mind that your test results sheet was dealing with a new fresh motor and was showing a 15% slip then, which is probably usual for that type hull.

Is your motor "fresh"?
 
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Bondo

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Apr 17, 2002
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Re: Need Help !! What is the better prop for my boat?

Hello, I have a 1984 24 ft rampage express with a 5.7 300 hp I /O alpha 1 .. Currently I am running a 14 1/2 x 17 3 blade . I can only get the boat rpm to 4100 @ 28.5knots. It will cruise at 19 knots @ 3400 rpm burning approx 1 gal per mile. Which seems very bad for economy. I also have a hard time maintaining cruising speed in seas. Boat will drop down to 15.5-16 knots. This prop was on my boat when I purchased from previous owner.

I found the original paper work specs on this boat. The original prop the came on the boat new was a 3 blade 15 1/4 x 15. Said the boat goes 37.5 mph @4650 rpm. I am looking for advice on the best prop. Should I try the original prop ? I am also considering a 4 blade I like the benefits of the extra blade in the water

Any advice is much appreciated thank you !

Ayuh,.... Welcome Aboard,.... You're Over-propped, or underpowered, or need a lower gear ratio,....

Ya gotta get the motor's rpms up to 'bout 4600/ 4800 rpms at Wot,...
 

DownToFish1

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Mar 2, 2014
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Re: Need Help !! What is the better prop for my boat?

Thanks for the help . The engine and out drive are 2 years old. approx 150 hrs on them. I do have a lot of slippage when running in some seas.
 

DownToFish1

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Mar 2, 2014
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Re: Need Help !! What is the better prop for my boat?

I am ok with the current cruise speed and top end speed, more would be a plus. But my first priority is to maintain my cruise speed in seas. Also my second priority is economy its horrible, compared to similar boats and set ups. That's why I am leaning towards a four blade. The merc prop selector suggested a alpha4blade 15x16.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Need Help !! What is the better prop for my boat?

Sometimes when the pitch is a little too high the boat may struggle to maintain cruise speed evenly.
You need to go with a prop like that in your paper work. 15 1/4 X 15 seems to work it should maintain cruise better.
You also may want to trim down for the best cruising speed manors.Slip at 37.5 is about 14% respectable for a big heavy boat.
 

Maclin

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May 27, 2007
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Re: Need Help !! What is the better prop for my boat?

DowntoFish, conventional wisdom when going from a 3 blade to 4 blade is to drop one inch of pitch. This is mainly due to the extra load the engine will see due to less slip. The drop in pitch will keep the engine top RPM's at about the same number as a 3 blade of the one inch higher pitch. Top speed may go down a little, but the engine can keep the boat on plane easier and dig in sooner from a dead stop if needed to get out of harm's way.

So if a 15p is correct for your boat with a 3 blade then a 14p would be a good place to start in a 4 blade.
 
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