Is the Math Right?

[Traceyh]

I have contemplated changing props because mine is not in to good of shape.
I currently have a 14'x13P aluminum prop.
I am thinking of going to a stainless prop for better performance?

Heres my math.
A 14"x13P prop theoretically displaces approximately 2001 cubic inches of water per revolution.
A 13"x17P prop theoretically displaces approximately 2256 cubic inches of water per revolution.
I am currently achieving 5300 RPM at WOT on my 1988 Mariner 90HP 3 cylinder.
I normally run about 3800-4000 RPM cruise, and I am NOT looking for more top end performance, I am looking to achieve better midrange performance.
The math I used is this... prop radius squared, times pi, times pitch. That is the volume of the cylinder it would make. I did not subtract hub displacement because it should be about equal from prop to prop, so would not make a difference in the outcome.

 

[QC]

Re: Is the Math Right?

Very nice exercise, but probably over thought. What you need to concentrate on is pitch and RPM. You should probably ignore diameter although I can see by your calcs that you will want to understand why . . .

Define midrange performance. What do you want? No matter what you do, you need to be within the WOT RPM recommendations with any prop.

The calcs you need to concentrate on you'll find here: prop calculator

 

[glengold]

Re: Is the Math Right?

I have contemplated changing props because mine is not in to good of shape.
I currently have a 14'x13P aluminum prop.
I am thinking of going to a stainless prop for better performance?

Heres my math.
A 14"x13P prop theoretically displaces approximately 2001 cubic inches of water per revolution.
A 13"x17P prop theoretically displaces approximately 2256 cubic inches of water per revolution.
I am currently achieving 5300 RPM at WOT on my 1988 Mariner 90HP 3 cylinder.
I normally run about 3800-4000 RPM cruise, and I am NOT looking for more top end performance, I am looking to achieve better midrange performance.
The math I used is this... prop radius squared, times pi, times pitch. That is the volume of the cylinder it would make. I did not subtract hub displacement because it should be about equal from prop to prop, so would not make a difference in the outcome.

So what is the question? The boat manufacture should have a recommended Motor with WOT and prop specs. If your prop is all messed up you may not be getting max WOT for that particular prop.

When going to stainless there are several things to consider:

Stainless is heavier and harder, meaning probably less top end and more likely to cause shaft damage if you strike something.

Stainless doesn't flex like aluminum so low end should be better, but since its a heavier prop, it may be a wash.

Stainless is 3 times more expensive.

I've been told that diameter doesn't make too much a difference if its less than an inch. Basically you don't want a prop larger than the manufactures recommendations.

If I were you I'd get your prop remanufactured, and see what WOT is. Aluminum props can be remanufactured for cheap, check out Admiral props in Tampa Fl.

If you are still a ways below WOT, get a lower pitched prop. I'm guessing when that 13P prop is reman, you'll be turning more RPM's and may be above WOT range. This would be a good watersports prop or heavy load prop. You could then get a 15P prop for better top end and light loads.

Bottom line, there is no single prop good for everything. I stay away from stainless because it's expensive to buy, fix, and riskier for the LU.

BTW, I have a 13P and 15P for my 55hp. I used the 13 P mostly because I like the hole shot.

 

[Traceyh]

Re: Is the Math Right?

Very nice exercise, but probably over thought. What you need to concentrate on is pitch and RPM. You should probably ignore diameter although I can see by your calcs that you will want to understand why . . .

Define midrange performance. What do you want? No matter what you do, you need to be within the WOT RPM recommendations with any prop.

The calcs you need to concentrate on you'll find here: prop calculator

Wow, my bad, I defined everything except the boat. I have a 1988 Lowe windcruiser 24 pontoon boat. I mainly cruise along about 3800-4000 RPM.
The current prop gives me about 5300 RPM, so I within engine WOT design.
I have tried contacting Lowe Boats, and no reply from them so I am at my own recourse here. I have another similar thread in the Pontoon forum.
It seems that a 13" prop is easier to find than a 14", that is why the question.
It seems that a 13x17 is really close to a 14x13, but I dont know where to find a comparison chart that crosses props in that manner, or any manner for that matter. I have read some that say on a pontoon you should run a 4 blade, and I cant find that in a 14x13 prop, but can find that in 13x17.
Than again, I have read some that say a 3 blade stainless is better than a 4 blade aluminum. I simply dont know.

 

[glengold]

Re: Is the Math Right?

I know that pontoons have their own style of prop. Basically larger blades for better cruising and take off. At least the yamis have this, I dunno about merc.

Anyways, I used the online prop finder at www.dansdiscoutprops.com and put your boat with a 6 person load and 20 gallons of gas (don't know if it really matters) and they recommend a 13.75 by 15 pitch for watersports and 13.25 by 17 pitch for speed.

Either way your WOT range is 5000-5500 so you want to be there. I think the fact that your prop is all bugged up could be affecting performance. If you like to load the boat up, I'd get the 13P you currently have rebuilt by admiral (cheap) and see what WOT is. If you are turning over 5500 with the "good as new" 13P. Invest in a 15P for light loads.

 

[steelespike]

Re: Is the Math Right?

Disregard diameter except in the extreme ie if you have a 13" diameter now and a similar prop is 14" diameter it will likely lower rpm when compared to a 13" diameter in the same pitch.The weight of a ss prop doesn't enter in the equation except when shifting at excessive idle speed.
If your present rpm is 5300 with a 13" pitch if you go to a 17 it will lower your rpm
"about" 600 to to 800 rpm."About" 200 rpm per inch.If you lower your rpm you will be lugging the motor.Not just at wot but all through your rpm range.It will cause premature repair problems. And the 17 is comepletely wrong for a pontoon.
It is generally considered best to run near the max rated rpm especially with todays doctored fuels.Your 90 will use about 9 gph (gallons per hour)at wot
whether you are pushing your pontoon or a light planing boat.The idea is to get the most speed for your rpm.
A 4 blade prop should push more efficiently with less slip and ventilation.
possibly an 11".When considering a prop for your pontoon a larger diameter
could be considered.