changing hp

[kmac]

I am wanting to know how much mph increase I would see if I changed from a 50hp to a 60hp or to a 70 hp on a 16' Bayliner. Any opinions?

 

[KCLOST]

Re: changing hp

5 mph maybe...

 

[JB]

Re: changing hp

Using the formula:<br /><br />Speed=(square root of [HP/weight])X 200<br /><br />If your BL weighs 2000lb WOT with the 50 would be about 31.6mph.<br /><br />Replace the 50 with a 70 and you come up with 37.4mph.<br /><br />If your boat actually weighs less the speeds would both be higher, of course, but these calculations verify that KCLOST's guess is very close.<br /><br />The formula assumes that you are ideally propped and rigged with both engines, and that you have a clean planing hull with very little dead rise.<br /><br />Good luck.

 

[kmac]

Re: changing hp

help <br /><br />square root of 50 is (7.071/2000)=.0035355<br /><br />.0035355x200=.7071

 

[JB]

Re: changing hp

Take the square root of (50/2000), not the 50.<br /><br />50hp/2000lb = .025.<br /><br />Square root of .025 = .158113883<br /><br />.158113883 X 200 = 31.6227766mph<br /><br />I forgot to bracket the 50/2000

 

[kmac]

Re: changing hp

Thanks, that works much better. I should have known better

 

[kmac]

Re: changing hp

By the way, where did you come up with this equation?

 

[JB]

Re: changing hp

This equation was developed by another engineer who was a frequent contributor to another site I used to frequent. As far as I know it was never published except on that site and here.<br /><br />He worked it out by measuring results on many boats and working it backwards to determine the mathmatical relationships and find the range of constants that fit hull types.<br /><br />The "200" is a constant factor for low deadrise planing hulls. For deep "V" hulls use 175, for flat bottoms like a jon, use 225. It does not work at all for displacement hulls.<br /><br />If you use the right constant and accurate weight and have the boat ideally propped and rigged it is amazingly accurate.