Help selecting prop

[blacksvt]

Hi

I have a Maxum 18' 3.0 Mercruiser. I had a 19 pitch 3 blade and I was looking for something to give me more power "out of the hole". The PO gave a me prop that he said was a power prop....seems it was a 21 pitch 3 blade.

WOT on the 19 gave me about 4500RPM at around 40mph

the 21 is about 4000-4200 RPM at 3/4 Throttle at 32MPH. I say 3/4 throttle because I think if I go WOT I will over rev the motor. Im not sure what the max rpm is but I never like to take it over 4200rpm.

The 21 gives me a much better start for wakeboarding, however I dont want to risk someone driving me over doing the engine. Props are also aluminum and when I find the correct one I am getting stainless. Looking for someone to guide me in the right direction - should I consider a 4 blade?

Nathan

 

[Tail_Gunner]

Re: Help selecting prop

You need to run @ wot to determine the overall pitch needed to run your boat @ its peak performance....wether that's just off plane...mid throttle or a full on run..

Yes a Solas brand 4 blade would a very good prop or the huslter 3 blades that are now clearanced priced.

 

[hwsiii]

Re: Help selecting prop

Blacksvt, if you would tell us the year and horsepower of your boat we can be more helpful. Also the manufacturers and models of the props you are currently using, as well as the number of people, gas and extra weight you have in the boat. You seem to have overrevved the motor with the 19" prop to 4,500, but on the 21" prop you said you were at 3/4 throtlle and turning about 4,100 RPM. That doesn't seem to be very feasible, unless there is a huge difference in blade geometry and/or diameter, and usually with aluminum props there should not be that much difference between the two props. If you will give us the information requested we can be of more assistance.

H

 

[Tail_Gunner]

Re: Help selecting prop

Some food for thought when you prop a boat your finalizing the final overall all gearing ratio. Running a engine to the top of the rpm band as in this case 4800 is not so you can hot rod the motor quite the opposite. It's done so your overall gearing presents a lesser load on you engine at all operating speeds

In effect propping a eninge to 4800 will make for lighter loads on a engine @ lower planing speeds...Here's some numbers that might pertain to you boat..19p @4400 or 17p @4800...Using the 17 will be eaiser on your engine @ all speeds..there is less of a load.

Fast FactsPropshaft HP 135-hpConfiguration Inline-4Aspiration NaturalDisplacement 3.0 L / 181 CIDBore & Stroke 4.00Fuel System 2V Carb with Turn Key Start & Mechanical Fuel PumpBase Engine Weight 635 lbs.Length/Width/Height 29 x 26 x 21Max RPM@WOT 4400-4800....Warranty 3-year

As to the prop itself and 3 blade made easy is a speed prop...it is a simple as having less blades in the water and blades create drag. 4 blades have more blade area so they will create more thrust at lower speeds elevating a boat to plane eailer..That is a general rule

Some things about SS while more expensive they offer more features...To make it simple as in this case a 16 pitch 3 blade ss should give a better hole hole shot (less pitch) and yet give you the same if not higher top end speed...As to top end speed SS does not deform like alum will under high stress and more complex design can be built in giving increased effecincy.

Remeber a prop only determines the final overall gearing but operates in a constantly changing enviroment nothing is linear becasuse of that.