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Old 03-03-2005, 11:34 PM
joebobb joebobb is offline
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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
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Old 03-04-2005, 06:40 AM
FAST FRED FAST FRED is offline
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Location: Conn in summers , Ortona FL in winter , with big dock & room for O'nite stop .
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
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Old 03-04-2005, 05:51 PM
PowerTech PowerTech is offline
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Yanmar 1GM 10 or a 2GM 20

Last edited by carlwun : 03-04-2005 at 06:28 PM. Reason: ad some thing
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Old 03-04-2005, 06:21 PM
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gonzo gonzo is offline
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Semi-displacement hulls are not economic at displacement speeds. For economy you should find the design's target speed.
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Old 03-05-2005, 04:30 PM
CDBarry CDBarry is offline
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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.
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