For several months now I have looked at many many different boats for sale on the net. And there seems to be a very large range of engine sizes for a given displacement of boat. And to me some of them don't make any sense...
For example a Hans Christian 38 of 12.5 tons has a 45hp engine where as a Hallberg Rassy 38 of 9 tons has a 62hp. And these are original equipment. So in this case the HC has a much smaller engine and yet is much heavier than the HR? And I understand that it's not all about the Hp and that power output can vary but still...
Obviously the designers did their sums and decided these were the correct Hp for the boat but how did they calculate this?.
And there are many others among the production boats as well where the Hp seems to vary a lot to a similar displacement and length.
I am also interested in this in the event of having to replace an older engine with a new one, which I assume with the advances over the last 30 or so years would be lighter and have more power for a given Hp?
have more power for a given Hp?
Hp is Hp. Might have a different torque/speed characteristic, but a Hp rating is what is. However, a new modern motor is likely to be lighter for a variety of reasons.
Some people just love to have big motors. It is in the psychology rather than engineering. You often see 14ft power boats with massive outboards on the back, developing obscene power.
Im sure there is some rule of thumb for sizing engines but after that it would be personal preference for how big you want to go. I'm sure the original boat designer had an ideal weight in mind so I don't think it pays to stray far from that.
A rule of thumb is 4bhp per tonne displacement. However Mb has a 20bhp engine, weighs 6.5 tonnes or so but still achieves hull speed with a clean hull and prop.