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

[rasel02232129]

Dear All,

Can you please help me regarding the Maxsurf Hull Speed, what is the value of Overall Efficiency should I consider? My boat data particulars are: L= 4.8 M, B=1.45m, D= .45m, d=0.125m. Pls see the attached photos.

Reards,
Rasel

 

[Mr Efficiency]

Your boat has very little freeboard, you need much more to have much use outside of a calm backwater.

 

[rasel02232129]

Dear Mr Efficiency,

Well noted your comments with thanks! Can you please help me with my question?

Regards,
Rasel

 

[TANSL]

For an outboard motor I would use a 70% efficiency, being optimistic.

 

[rasel02232129]

Dear Mr. TANSL

Thank you very much for your valuable reply regarding 70% efficiency. As per the attached, Can you please reply regarding the method to be computed for this type of boat?

Regards,
Rasel

 

[TANSL]

I'd use Savitsky planing and Lahtiharju, choosing an average value between both results. But keep in mind that these are mere estimates. In practice the boat will never sail under the conditions assumed in these tests.

 

[Ad Hoc]

... your valuable reply regarding 70% efficiency....

70% is unrealistic and almost impossible to get .....50% is a far more sensible value.

 

[TANSL]

50% not for outboards. Being cautious I would use 63-65% and depending on whether it is a propeller or a waterjet. A current, modern engine, not the ones from 1988.

 

[DCockey]

70% is unrealistic and almost impossible to get .....50% is a far more sensible value.

50% not for outboards. Being cautious I would use 63-65% and depending on whether it is a propeller or a waterjet. A current, modern engine, not the ones from 1988.

I've heard 50% recommended as a generic overall efficiency for an outboard, with overall efficiency defined as thrust x vessel speed / shaft power. The 50% overall efficency assumes a properly match propeller and is at the "design" condition, not slow speeds, etc. I would not be surprised if propeller efficency in isolation was over 60% but that is without accounting for the drag of the outboard lower unit.

Outboard power is rated at the propeller shaft, not at the engine crankshaft so engine installation losses and gearing, etc losses are already accounted for.

 

[TANSL]

The overall efficency, on a boat with a traditional shaftline and fixed pitch propeller, with appendages, etc., can be around 50%, or even a little less. (You can check with Holtrop-Mennen, for example)
For an outboard planing hull, like the one shown by the OP, the propulsive efficiency (EHP / BHP), which is what to apply in Maxsurf, can be 65% or more. Imho.

 

[Mr Efficiency]

Different outboard motors with the same advertised HP vary widely, I think they operate under a code that allows plus or minus 10%. Anecdotally, it has probably been more, in practice, there have been very similar engines in a wide band of power rating, that were not much different in performance in the range of normal use, but the higher HP end of the offering would wind out to higher RPM, due to different carbs, porting, tuned exhausts etc, but the cubic capacity the same. An outboard is not like a car engine, where the ability to rev higher gets a performance car through the gears more quickly, in normal sane use, you never rev an outboard to the limit.

 

[gonzo]

"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%." ICOMA
That is no more than about +/- 0.6HP difference.
Classic Whaler: Boston Whaler: Reference: ICOMIA Engine Horsepower Standards http://continuouswave.com/whaler/reference/ICOMIA28-83.html

 

[Mr Efficiency]

Hmmm....funny we don't see 17 hp or 39 hp motors, if they have to comply with that.

 

[gonzo]

We see 9.9 and 48 HP motors to comply with maximum power or taxation laws.

 

[Mr Efficiency]

That's a different matter, my point is if there was a regulatory change that mandated such a slender margin of error, every existing model in production would have needed altering, was it only for new designs, not existing ones ?