Damaged Prop, GPH, Prop Performance Question

[Cameron351]

Hello, I have a 56' pontoon house boat with a 130hp 4cyl with a SX Cobra drive. At 2400 RPM she cruises at around 6 mph with a 15.5x13 aluminum prop. I have logged 122 hrs and I average 2.9 GPH. This last trip out I hit a piece of wood and damaged a couple blades on my prop (and that sucks but, it's my fault and that sucks more). Anyway, I am looking at a new stainless prop and I wanted some opinions on the the GPH I am getting. Does this seam reasonable? This 4cyl is a 1997 with a 2bbl holley. Any thoughts comments or suggestions would be greatly appreciated.

Thank you,
Cameron

 

[jestor68]

Re: Damaged Prop, GPH, Prop Performance Question

I am really curious to know what rpm the little motor would turn if you put the throttle wide open. If it does not achieve between 4400-4800 rpm, you require a prop with less pitch when you go to purchase a new one.

 

[steelespike]

Re: Damaged Prop, GPH, Prop Performance Question

I can only speak in generalities.
A 130 hp motor will use 10% of its rated hp in fuel at wot. thats 13 gallons per hour.
2.9 seems really reasonable to me and calculates to a little over 2 miles per gallon.
Your throttle setting will determine your gallons per hour. Your boat speed will determine mpg.
a ss prop might reduce slip a bit.
If you boat where a hard strikes are common a ss prop might not be a good idea.
its possible that a slight reduction in throttle might noticeably improve your gph. without greatly reducing your mph.
Your present slip is 59% probably reasonable for your setup.
A 4 blade prop intended for a very heavy application would help.
Also a slight reduction in pitch might increase rpm at the same or less throttle setting.
The higher rpm could reduce slip. A 13" pitch seems high for a 56 ft pontoon.
Perhaps you can find a dealer that will let you try some props with the understanding your buying.

 

[Cameron351]

Re: Damaged Prop, GPH, Prop Performance Question

Good input. My WOT is only a little over 3000 maybe 3200-3400 at the most, I don't really test that on this boat. I was also thinking about a four blade as I think it would help slip at these lower RPM's. I will have to see if I can find someone to work with me on prop selection; If not I will go with something smaller for sure in the four blade configuration. Is it better to drop diameter or blade angle for this low speed application?

Thank you guys for the input,
Cameron

 

[steelespike]

Re: Damaged Prop, GPH, Prop Performance Question

If your motor is struggling at wot it is struggling all through the rpm band. You need as much diameter as you can find and I think an 11" pitch. A 4 blade will also help reduce slip.Though it will be high on a 56 footer.
A pontoon is ideal for displacement speeds about 6 mph.