Piranha Composite Propellers

[SeaSpray]

Dan,

Those are very interesting numbers. With my Johnson 90 and stock prop I ran a little faster before adding the permatrim. A couple mph on the top end does not matter that much to me so no problem.

I have a compro prop that fits but like you said earlier it is too small. I picked it up cheap to have as a spare.

Thanks for doing the initial trial. Now, I think I would like to get one. Where did you get yours?

Also I am wondering if the 18 blades are going to be too much?

Steve

[dogon dory]

Sorry, Folks - Post Deleted By Author

[dogon dory]

Sorry, Folks - Post Deleted By Author

[SeaSpray]

Okay guys, I spent the money and got the Pirahna prop with 2 sets of blades. I only had a short time to try them out Sunday but the first numbers look good. My boat is fairly light right now, only about 25g gas, no water, no food, no cloths, no toys. Just pots, pans, and 2 people.

My stock prop is a 3x14x19. I bought the 4x14x18 and 4x14.5x16 Pirahna.

RPM S19 P18 P16 (speed in knots)
2000 6 6.2 5.9
3000 8.6 9.6 8.8
3500 12 14.2 12.7
4000 17.9 17.9 16
4200 18.7 20.5 NA

WOT:
Stock 5200/25.6
P18 5100/24.5
P16 5400/24.1


These numbers are for a single run with each prop with unknown current. I am sure they would change for different conditions and loads. It does seem that the Pirhana 18 provides more speed for the same RPM/fuel than the stock prop. I am thinking that the P16 will be a good prop for Lake Powell.

How do you calculate the prop slip? Any comments on these numbers?

Steve

[Sea Wolf]

Steve-

Here's how to figure prop slip:

1. Take the engine RPM and divide it by the gear ratio to get the RPM of the prop:

5000 RPM divided by 2:1 = 2500 prop RPM

2. Multiply the prop RPM times the pitch to get the forward travel w/o slip:

2500 RPM x 16 inches = 40,000 inches per minute.

3. Change inches to feet by dividing by 12:

40000 divided by 12 = 3333 ft per minute.

4. Knowing that 60 mph = one mile per minute, and one mile = 5280 ft,

divide your ft per minute by 5280 to get your fractioal speed of 60 mph:

3333 divided by 5290 = 0.6313 of 60 mph, so

5. Multiply 60 mph by the ratio (decimal) above to get the speed in MPH w/o slip:

60 MPH x 0.6313 = 37.8 MPH (theoretical speed, w/o slip)

6. Find the loss due to prop slip in MPH by subtracting the real speed from the theoretical speed:

Your observed speed = 30 MPH, so

37.8 MPH minus 30 MPH = 7.8 MPH loss due to slip

7. Find out what fraction (decimal) is lost by dividing the theoretical speed by the loss due to slip:

7.8 MPH divided by 37.8 = 0.206 fraction/decimal lost to slip.

8. Multiply this fraction/decimal by 100 to get percent:

0.206 x 100 = 20.6% slip

***When the propellers are pretty well matched to the engine and the load, the slip will be about 10-15% in planing hulls at speed. The amount of slip at displacement speeds will usually be much greater.

****************************************************************************

Hope this helps! (Was lots of fun!)

Joe.

[Papillon]

Joe here is a link to a prop calculator: Try your siffering out on it and let me know how it works out.

http://continuouswave.com/cgi-bin/propcalc.pl

[Sea Wolf]

MIKE-

YUP, IT WORKS FINE!

JUST PLUG IN ALL BUT ONE NUMBER (IN THIS CASE SLIP %), AND THE CALCULATOR FIGURES THE REST!

EASY!!! (Almost TOO easy!)

JOE.

[thataway]

You would definately want the 16" or maybe even a 14" prop to get WOT at Powell. I don't know what your engine's WOT should be, but many modern outboards are in the 5500 to 6000 RPM range.

[SeaSpray]

Thanks Joe for the run through the numbers. Makes sense once it is explained.

Bob, The Johnson has a WOT range of 4500 to 5500. Don't know why such a large range but that is what the book says.

Steve

[dogon dory]

Sorry, Folks - Post Deleted By Author

[mikeporterinmd]