Ideal Power for displacement operation

How could one determine the appropriate engine size for a small motorboat (25' LOA and about 3 - 3.5 tons displacement) that would be operated at displacement speeds (up to about 6.7kt)? The hull is a round-bilge, semi-displacement hull.

Obviously, the boat would be expected to easily cruise at this maximum displacement speed.

Kind regards,

JoeBobb

 

6.7 is a bit high for a boat with a short LWL , many cruise at 1.15 of the Sq rt of the LWL to save noise /fuel/.

And old rule of thumb is 3HP for every ton of displacement or 5 hp per ton if your from the USA and have boodles of cash to burn.

A brigs or ONAN would be a gool long term engine , if youre into gasolene.

FAST FRED

 

Yanmar 1GM 10 or a 2GM 20

 

Semi-displacement hulls are not economic at displacement speeds. For economy you should find the design's target speed.

 

Calculate the resistance using whatever standard method or model series that fits, select an optimum prop that fits on the shaft, see how much power you need to spin it.