Power Calculations

[McB]

How difficult is it to calculate how a engine change would effect a design? An example. What would the launch Whio, which has I believe a 50hp engine, do if it was built with a 15hp engine? Or converting to electric. Not that design but any. I am interested in rough estimates not exact calculations. Is that possible? How?

[L.W. Baxter]

It's not terribly hard to get a good estimate of boat performance with a particular plant and propeller if you have reliable measurements of the boat in question.

I would recommend Dave Gerr's The Nature of Boats or Sorensen's Guide to Powerboats for overviews on how such things work.

[john welsford]

It's not terribly hard to get a good estimate of boat performance with a particular plant and propeller if you have reliable measurements of the boat in question.

I would recommend Dave Gerr's The Nature of Boats or Sorensen's Guide to Powerboats for overviews on how such things work.

There are also several graphs in Jay Benfords "Small Ships" which are useful for displacement hulls.

John Welsford

[mcdenny]

Here's the formula in Excelese:

=10.665/(POWER(B4/B12,0.33))*SQRT(B3)*1.15

B4= weight in pounds
B12= Engine HP
B3= waterline length in feet
The result is speed in MPH, leave off the 1.15 on the end to get knots.

This works well at displacement speeds.

Wouldn't a Whio with a 15 hp high thrust 4 stroke OB hidden in a well be sweet? 9 kts WOT; 7 kts at 50% power; 6 kts (sailboat speed) at 25%. Quiet, fuel efficient, even cheap. A solar panel on the roof for extra house electricity?

Hmmm, maybe I'll take another look at the Whio website.