HullSpeed Overall Efficiency and Method for a Boat with one 25HP Outobard.

I think you are forgetting the marketing department bit.

If Suzu wants to compete with Merc's 50 horse; they don't want the customer comparing the Suzu 49 hp with the Merc 50 because the Suzu loses. I don't want to tell someone I bought a 49 horse.

So they want the engines to get to the nearest 10..25..50..100, or in the case of the 9.9; I have never understood..

 

The story I read a long time ago was that some ponds and lakes prohibited engines of 10HP or larger. So the manufacturers offered 9.9HP engines.

 

Indeed... and is generally a good start whether I/B or O/B engine.

Indeed.. open water prop efficiencies can be as high as 70%..but this has no hull, no skeg, bracket etc etc in front of it..and all the associated issues that brings, once it is in front.
Hence 50% is about "right"..

 

As we all know, or should know, there are various "efficiencies" to take into account when passing the total towing resistance to engine power and I think that each one is talking about the one that seems best to him, being their values totally different and independent. Therefore, I do not doubt that you are right but your answer is not responding to what the OP requests.
I insist on saying that using the Savitsky method for a boat in planing conditions (without keels, bow bulb, consoles and other strange artifacts), the efficiency requested by Maxsurf should be between 65 and 70% (pessimistic value / optimistic).

End of my intervention in this "deep" and "passionate" discussion.

Edited to correct or complete some points

 

It seems this discussion started without a clear definition as to what is the meaning and/or purpose of the input request of the program. Now, if the program uses some algorithm for resistance prediction, a probable use for an input value for "overall efficiency" will be to make an estimate of available net thrust with a given engine power.

In this perspective, the "gross efficiency" of the propeller in open, undisturbed water is not relevant, only the resulting net thrust after the deduction of drive leg drag, hull wake influence and thrust loss due to drive leg wake and cavitation/ventilation is of interest. In this case, a standard propeller would be something like 10.4" dia x 13" pitch for a speed around 22-24 knots. Gross thrust comes at roughly 69% efficiency, but there will be a drag penalty at 22 kn of about 140 N (calculated from a down scaling of test data for standard i/o drives with 16" props), and a thrust loss of about 5-7% of gross thrust due to the leg wake.

Edit: I would add that the nominal thrust loss due to "leg wake" is depending on degree of cavitation/ventilation; at high cavitation coefficients (=small or no cav.), the loss is slightly lower than the values cited, and vice versa.

 

There are plenty of things that can change the equation, e.g. stainless steel propellers, which also allow a little more latitude in raising the motor height for less drag, if the engine can be trimmed underway, that too can alter top speed.

 

Classic Whaler: Boston Whaler: Reference: ICOMIA Engine Horsepower Standards http://continuouswave.com/whaler/reference/ICOMIA28-83.html

 

It is a bit confusing, perhaps a lawyer getting paid for it, might read and understand it clearly. This says 10% as I read it.
7 Manufacturing tolerance
The corrected power at rated speed of any individual marine propulsion engine or propulsion system must not deviate more than ±10% or 0.45kW, whichever is greater, from its declared power, except that for governed engines or systems of more than 100kW [134-HP] the tolerance shall be ±5%.