HP vs speed vs weight

[Bill kubiak]

I have a 25 ft sailboat that weights 2600 pounds<br /><br />I have a troublesome 7.5 sailmaster that will give me 6.5 mph gps speed at wot<br /><br />I borrowed a friends 9.9 johnson and it pushed me at 8.0 mph gps speed at WOT<br /><br />I am considering a new Johnsom 15HP because it weights the same as a new 9.9 and I have been told this extra HP will push the sailboat past it's hull speed to maybe 10 or 12 MPh<br /><br />any comments, true or false or just wishful thinking

 

[Luna Sea]

Re: HP vs speed vs weight

Get the 15. It will go faster than the 9.9, how much you'll find out soon soon enough! Go for the 15! And the new Bombardier Johnsons are supposed to be nice. I just bought the 8 myself. Good luck and post back with the gps speed........

 

[Paul Moir]

Re: HP vs speed vs weight

At 8 mph you are at a speed/length ratio of 1.4, which is at the corner where resistance increases exponentially for a displacement hull. I doubt you'd be able to reach 12mph with any sized engine because your speed/length ratio would have to be 2.1 for that, ie your boat would have to plane off. <br /><br />I'd be surpised to see you gain 1mph with a 15hp outboard. Try before you buy!

 

[jim dozier]

Re: HP vs speed vs weight

Paul is right. That's why they call it the "hull speed" on a displacement hull. You will have to get it on a plane to go much faster, and to do that you're going to need ~100 hp or more depending on your planing area and hull design. However, what the 15 hp will do is push your boat fully loaded at its hull speed against a 30 knot wind which the 9.9 might not. If you go with the 15hp don't expect to go much if any faster. Make sure that whatever you end up with has a prop that will let the engine turn at its rated rpm, many sailors are overpropped and turn their outboards at too low an rpm.

 

[Bill kubiak]

Re: HP vs speed vs weight

Johnson offers a sailboat fat power prop for an even swap of the one that comes on the engine. They have good response in reverse when you are headed toward a concrete dock and want to stop right now