How fast will it go?

Building a pontoon boat. 18 foot long, the pontoons are (2) 18 foot by 15inches wide by 22inches tall. the pontoons are flat on the bottom and have a 16 degree pitch on the front end. I am putting a 65 hp engine with a direct drive shaft that is attached by a universal joint off the transmission. the shaft angle in the water is 7 degrees. the boat weights 1,200 lbs fully loaded. The pontoons will sit 6 inches submerged in the water at stationary. I will run the engine 1x1 ratio at 2500rpm, (this can be changed). What prop would you recommend for that rpm and stats for a 3/4 inch shaft and what speed do you think you could run at? thanks
Mark

 

Since max RPM is fairly slow, Prop will need to be 13 1/2 to 14 dia. with a pitch of 21 or 22, 3 Blade, min D.A.R. of 55% providing 11% slip.
Max speed should be 45 mph.
Prop size and predicted speed is based on max weight of 1200 lb with Driver and fuel etc.

 

http://www.go-fast.com/boat_speed_predictions.htm

http://www.glen-l.com/how-fast-chart.html

 

Thank you Jango,

What if we increase the rpm to 3000- to 3500. Can we put a smaller prop on her and maybe a 2 bladed and get somewhere around 20 to 25 mph? Is so what would you recommend?

 

Yes, As rpm increases, prop dia can decrease. Normally, props are sized for max efficiency. Is 65 hp max rated? at what rpm?
If you want to size for 20 - 25 mph, will need to know HP and rpm at that speed which can be estimated resonably close if max hp and rpm are known.

 

We can change the cam to slow the rpm down at still keep the hp at 65. I was under the impression that most props run at 2000 to 2500 rpm. Maybe I am wrong? We want to 20 mph with the smallest prop that we can turn. I was thinking of a 10inch or 11inch prop. What do you think?

Mark

 

If you're saying 65HP @ 2500 rpm? That's what I used to determine prop size of 13 1/2 x 22. Why would you want to limit max.rpm,s to 2500?
Allowing the engine to run up to 4500 rpm would calculate to a 10 x 12 Prop with the same 45 mph max. Higher max rated rpm would further reduce prop size - again, what is the rated rpm?
Slowing the engine down by decreasing the throttle to 2500 rpm (10 x 12 prop) would provide approx 24 mph with slip at abt 16% and utilizing approx 25HP.

 

Jango,

We can run the engine at 3500 rpm and we will use a 12 inch 10 1/2 pitch prop. So what will that come out with.

Thanks
Mark

 

With a 12x10 1/2 3 bladed prop @ 3500 rpm, speed should be 30 - 31 mph

If you're looking for small dia prop, @ 1200lb max, you don't need to go to 12" dia x 10 1/2. The 10 dia x 12 p. previously recommended will provide approx 34 mph at 3500 rpm and 24 mph at 2500 rpm.

 

J,

Thank you for your help! I appreciate your expertice. We will use this prop in mud and weeds. What would you recommend as a prop manufacture for something with that kind of use. I am thinking of a 3/4 inch or 7/8 inch shaft for the prop.

Mark

 

Unless your shaft is turning 4500 rpm+, stainless steel is not really necessary and is very pricey - 500 to $1000 or more. Alum. is not readily available for inboards, so I would use a std. Brass or Bronze propeller. I don't think you'll find a 3/4 shaft prop which I wouldn't recomend anyway. Stay with a 7/8 or 1" shaft. Any manf. should be OK . Search the web, look on Ebay.

 

NOW EXPLAIN IT TO ME SLOWLY..

If you put a 65 hp outboard on this thing with its 2 :1 gearbox it will drive a 13/14 inch X 15 pitch and your speed will be 1mph x 15 x 3000 = 45 mph .Now no way will a 65 hp running at only 2500 rpm and with direct drive turn a 22 pitch prop more like a 10 pitch so you only get 10x2500= 25 mph.
AND I don understand changing the prop to slow down ..whats the throttle for ???

 

I agree, but the original assumption was 65HP AT 2500 RPM. hence the 13 x 22 prop. A highly unlikely situation unless the engine produced more like 150+ max.hp.

 

Ok, Just to make it clear. We use the boat specs at the start of this thread. Now, as for the engine. It is a vw engine and I was told that we could grind the cam and slow the rpm down to 2500 rpm, and still produce 65 hp. My bad, I thought that most props ran at 2500 rpm. That is why I wanted to slow the rpm down to max hp. Now forget that. Let us say that the vw engine will run all day at 3500- 4000 rpm. Back to the first question of the prop turning 3500-4000 rpm, what would be the best prop to run and what speed do you think we could get out of her? Please use the boat specs at the start of the thread. I would like to have a 12 inch prop and a 10 1/2 pitch?

thanks
Mark

 

The 65 HP VW probably developes max HP between 3600 - 4400 rpm.

The ideal Prop for a max of 4000 rpm would be a 10"dia x 13 1/2 pitch, 3 blade which will provide 45 MPH with 11% slip.

At the same 4000 rpm,a 12" Dia x 10 1/2 pitch prop will get you to 36 mph

Of course this is all based on a max weight of 1200# as previously stated