Ideal Power for displacement operation

[joebobb]

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

[FAST FRED]

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

[PowerTech]

Yanmar 1GM 10 or a 2GM 20

[gonzo]

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

[CDBarry]

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.