Can the Wrong Prop cause drastic RPM loss

[ryanicus]

I have a brand new bay boat and am dying to get it running. It came with 11 5/8 15 pitch prop and it wont get more than 1500 rpm. If you just use the throttle to rev the engine it will go up to 4000 and seemingly more but this was my first use.

When you try to go forward it acts like there is a piano tied to the bottom and we are dragging it.

15ft 10 inch 975 pounds 3 people and one dog. Full of gas with a brand new yamaha 50 hp. Please tell me I have a prop problem and not a horsepower issue for this size boat.

 

[ivar]

Re: Can the Wrong Prop cause drastic RPM loss

I'd suggest a 12-13" pitch for that size boat, but 2" more should not cost you more than 5-600rpms. Seems to me like your motor not making the HP it is supposed to.

Brand new, I'd call the dealer and let them take care of the problem, they should also know what prop to use and what kind of speed vs rpms to expect.

 

[WillyBWright]

Re: Can the Wrong Prop cause drastic RPM loss

Those generally take 11 to 13 props for planing boats. If the boat is rated for around 50Hp, the 13. If it'll hold a much larger motor, 11. 15 would be for a very light boat and load like a jon boat with a hunter and a dog.

You want the motor to turn 6000 with a light to average load. If it's not getting on plane, it'll lug and not wind-out.

You did unload it from the trailer and not just unhitch the whole thing from the car, right? (old joke)

 

[WillyBWright]

Re: Can the Wrong Prop cause drastic RPM loss

Come to think of it, if that's the High Thrust model, 15 would be about right. But those are made for pontoons and not planing boats. Did the dealer sell you one of those? They should know better! They use a larger prop than the standard model. There are cases where the High Thrust would be advantagous on planing boats like heavily loaded workboats and for waterskiing.

 

[ivar]

Re: Can the Wrong Prop cause drastic RPM loss

If it was the Hi Thrust model , it would use a larger diameter prop, like a 13.5"x15"