Originally Posted by scoobert
so using your formula, and i doubt it is linear, if the boat i am looking at does 20KTS on 550HP 10KTS should take 70HP, and 5kts should take 9hp?
i dont see how that would be the case.
Our 34' has 25hp. We only need about 1/4 to 1/3 throttle to do 5kts. That's likely somewhere around 6-8hp (not sure exact hp to throttle settings). So, yes, 9hp is probably correct at least in terms of magnitude of HP.
Ideally, you don't want to go massively oversized on your engines. A pair of 400hp diesels that never take the boat above 5kts, will cost a lot to buy, a lot to maintain, they won't be as efficent as an appropriately sized engine
and they won't like idling long term.
On the other hand, a 9hp engine is probably too small. While it will likely maintain 5kts in calm conditions, with a head wind
or punching thru waves, it will likely have trouble maintaining 5kts. Also in docking
situations, it may not give you enough of a burst of power
to make some manuvers. While I don't like the option, if you will use the engine to power
a big alternator
, that can eat up a few HP also.
Looking at displacement sailboats and trawlers in the 30-40' range, you typically see HP in the 25-150hp range with assumed cruising speeds of 6-9kts depending on power and hull shape.