Time for a new prop. What would be good?

[Rowroy]

So it's time for a new prop and I need some guidance as I've never selected a prop before.

Here are the vital stats:

Boat: 18' Ebbtide ski boat (1075 lbs dry / ~1475 lbs fully loaded)
Motor: 150 hp Mariner Mag III
Current prop: p/n 48-78122-21 (I think this is 21 pitch / 16? rake / 13.75" dia) aluminum

Currently, the motor will turn ~6300-6400 RPM @ full throttle which yields ~ 55-57 mph.

If I keep the motor around 5800 RPM, then it'll run about 50 mph.

The boat has an awesome hole-shot and planes very quickly, but I know I can get more top end. I would also like to stick with aluminum due to cost.

Based on these numbers, what do you think I should go with?

 

[steelespike]

Re: Time for a new prop. What would be good?

I'm no Mariner expert but I think you may be pressing things a little at 6,400.
Check the vertical height,the antivent plate just above the prop even with the bottom is considered a starting point.Once you have the height check the Turning Point Rascal prop.Once you have the hub a further prop change is fairly reasonable.

 

[Rowroy]

Re: Time for a new prop. What would be good?

Yeah, the max RPM for this motor is only about 5600-5700 RPM. This is one of the reasons that I'm considering going with more pitch.

 

[steelespike]

Re: Time for a new prop. What would be good?

Maybe you should verify that the tach is accurate.You would have to go with a 25" pitch to get to 5,600 Is your Ebbtide considered a fast design?
It sure does scoot; speeds above 50 with a relatively basic setup is pretty good imo.Is the speed by GPS?A speedo could be off by 10% either way.
Your weight figures seem a little light.The Mariner is over 400 lbs.that doesn't leave anything for gas,batteries,anchor,you etc.??
Just fishing around to see where we are at.

 

[Rowroy]

Re: Time for a new prop. What would be good?

The speed is by GPS. Good point on the weight . . . I didn't even consider the weight of the motor.

As for a "fast design", I'm not sure. The place from which I bought it said it should run about 65 MPH with the right prop. I'm not sure if this is true or just part of the sales pitch.

 

[hwsiii]

Re: Time for a new prop. What would be good?

Steele is right Row if you use the same model prop in a 25" pitch it will be doing 56 MPH at about 5,600 RPMs at 10% slip.


H

 

[steelespike]

Re: Time for a new prop. What would be good?

Steele is right Row if you use the same model prop in a 25" pitch it will be doing 56 MPH at about 5,600 RPMs at 10% slip.


H

It just seems to me that is a lot of pitch.I would check the tach then decide on a prop if necessary.

 

[Rowroy]

Re: Time for a new prop. What would be good?

I'm fairly certain that the tach is accurate. However, I it never hurts to double check things like this - which I will try to do this week.

Let me ask you this: How will increasing the pitch affect hole shot and top speed?

I would think that hole shot would suffer and top speed would be slightly higher (all things being equal, of course). Is this correct?

 

[bowman316]

Re: Time for a new prop. What would be good?

Yea, slightly less hole shot, more fuel efficent and better top end.
i would reccomend a 25 or even 27. Are you doing water skiing?
That is the only thing you really need hole shot for.

 

[Dhadley]

Re: Time for a new prop. What would be good?

13 3/4" props are usually 4 blade props. Is yours 3 or 4? There are some pretty good deals on ss props out there right now if you search a bit. That will drop some r's too.

The Ebbtide is a pretty good design. Should be a ton of fun......

 

[Rowroy]

Re: Time for a new prop. What would be good?

I don't do any skiing, but I must say that I really love the "feel" of the hole shot with the current prop. It will really put you back in your seat!

I'm just trying to drop the motor RPMs and maybe get a little more top end at the same time.

13 3/4" props are usually 4 blade props. Is yours 3 or 4? There are some pretty good deals on ss props out there right now if you search a bit. That will drop some r's too.

The Ebbtide is a pretty good design. Should be a ton of fun......

Yes, the prop is a 3 blade. I'm actually surprised at the handling characteristics of the Ebbtide. It really handles better than I expected at higher speeds and cornering is excellent.