Don't know if this works with outboards(it should) but i know it works well with inboards. My boat comes out at 81% efficiency.
Just note that this is a method for checking the efficiency of the hull and prop package and really has nothing to do with efficiency of the motor and gear. But it all ties together. An educated guess would be that motor efficiency x gear efficiency x hull efficiency = a somewhat overall estimate of boat efficiency.
Here is the formula for figuring out your boats efficiency:
RPMs divided by Reduction Gear Ratio X=s Pitch
X=s 60 (minutes) divided by 12 (inches per foot) divided by
5280 (feet in a mile) ==s MPH Divided by 1.15 =s Knots
Example my 31 JC 370 HP Yanmar@ 3300 RPMs 2:0 gear with a 23 X 21 propeller a WSOTY speed of 22.5 knots.
3300 (RPMs) divided by 2 = 1650 X's 21 (pitch) = 34650.00 X's 60 = 2079000.00 divided by 12 ("s per foot) = 173250.00 divided by 5280 (feet in a mile) = 32.81 (miles per hour) divided by 1.15 =28.53 knots @ 100 % efficiency
22.5 knots (Boats actual speed divided by 28.53 knots ='s .79% efficiency
Therefore the only things you need to get your boats efficiency is the follow:
speed at wot @ RPMs
pitch of your propeller