Prop selection

[Bratcher4079]

Hello all, I need help with prop selection. The boat is a 2005 rinker captiva, 5.0 MPI motor, with a alpha 1 gen 2 out drive, 1.62 gear. Current prop is a 14.25x21 pitch. Current wide open throttle is 45 mph at 4500 rpm on a smooth lake. My issue is that we want to use for wake boarding and the take off acceleration is poor. I would like some prop suggestions that would increase acceleration while maintaining the correct rpm range. I'm not worried about top speed. Any advice would be great. Also, the existing prop was used at 50' above sea level and the boat is now being used at 3500' elevation.

 

[SkiDad]

Re: Prop selection

if it were me I would be looking at an 18 pitch 4 blade.

 

[Bratcher4079]

Re: Prop selection

if it were me I would be looking at an 18 pitch 4 blade.

How did you come up with that prop? Will it keep my rpm's in correct range?

 

[Bratcher4079]

Re: Prop selection

How did you come up with that prop? Will it keep my rpm's in correct range?

Mercury marines prop selector recommends a Alpha 4, 14.5x18 four blade prop too.

One last question is were planning on going to lake at 7500' elevation. Would this prop be adequate for a week of tubing and wakeboarding? More then likely this will be the only time I go to this lake.

 

[SkiDad]

Re: Prop selection

i just did my educated guess of 200 per pitch - so I figured going from 21 to 19 would raise you to 4900 and then to 18 4 blade would raise you another 100 - i think you max out at 5000. Just watch your rpm if you go fast - if it over-revs just back off a hair. As for elevation i don't know anything about that, sorry. If your not comfortable with an 18 then just go with a 19

 

[Bratcher4079]

Re: Prop selection

Thanks for the input. Anyone one else have ideas on how to compensate for another 4000' in elevation?

 

[jestor68]

Re: Prop selection

The correct prop pitch at "normal" elevation would be 19; based on your current performance with the 21.

To compensate for the high elevation, you will need to drop 2 more inches of pitch for that location.

It's best to stick with the same brand/material when changing props(for more consistent results).

A 4 blade prop will usually run about 100 rpm SLOWER than the same pitch 3 blade. That's why it's recommended to reduce i inch of pitch when switching from 3 to 4 blades in order to MAINTAIN rpm. If you can afford to lose 100 rpm, use the same pitch.

Your rpm range should be 4600-5000 rpm.

 

[Bratcher4079]

Re: Prop selection

The correct prop pitch at "normal" elevation would be 19; based on your current performance with the 21.

To compensate for the high elevation, you will need to drop 2 more inches of pitch for that location.

It's best to stick with the same brand/material when changing props(for more consistent results).

A 4 blade prop will usually run about 100 rpm SLOWER than the same pitch 3 blade. That's why it's recommended to reduce i inch of pitch when switching from 3 to 4 blades in order to MAINTAIN rpm. If you can afford to lose 100 rpm, use the same pitch.

Your rpm range should be 4600-5000 rpm.

How do you come up with the correct prop pitch? Is there a website for it or is it a math calculation?

 

[jestor68]

Re: Prop selection

The 19 pitch is from Mercury's calculator based on current performance(21P, 4500 rpm, 45 mph). It then says to use a 19 pitch and says 4800 rpm @ 49 mph.

OK; here's the dope on altitude: HP loss = elevation in feet / 1000 X 0.03 X HP at sea level.

It boils down to about a 3 % loss per thousand feet of altitude.

Your prop slip % with the 21 figured out to about 18%; a little high due to being over propped.

The 19 at 4800-4900 rpm should yield a better slip factor(10-11%) and a higher top speed(48-49 mph).

 

[fishrdan]

Re: Prop selection

How do you come up with the correct prop pitch? Is there a website for it or is it a math calculation?

Mercury's Prop Selector is pretty good. I was thinking a 18P 4 blade too, from your numbers, altitude and wanting better holeshot.

Going to 7500' will require a different prop. Test out what ever prop you're going to get for 3500', get good numbers, then pop them into the prop selector to see what it spits out for 7500', I'm guessing a 15 or 16 pitch.

 

[jestor68]

Re: Prop selection

Do not depend on the Mercury prop selector to make altitude adjustments with accuracy. Use the formula( it does work out to 3% per thousand feet).

In your case, that means a 58.5 HP loss at 7500 feet compared to sea level. At 3500 feet, it'd be 10.5%(27 HP). So you're going to lose 31.5 HP more when you go to 7500 ft. That won't require more than a 2 inch pitch reduction.

How much pitch to adjust is pretty much a judgement call.