Saturday, January 23, 2010

How much Horse Power do you need in a sailboat?

In the case of a 32ft WLL, 26,000 lbs. sailboat the short answer is 36HP for a nice cruise on Biscayne Bay and 100HP when trying to claw off a lee shore at 3am after you have screwed up your navigation.

Good old Nigel Calder has an excellent table on page 226 of his Marine Diesel Engines (3rd ed). He uses the example of a 32ft WLL, 26,000lb auxiliary sailboat, which is close enough to a fully loaded Whitby 42 to make no difference. Please refer to Calders book for details, but the basic figures are;

Hull speed = 7.58 Kts.
Shaft HP to reach hull speed allowing for 40% prop efficiency = 36.25HP.
Add 33% for power train loss = this gives a BHP of 48HP.

This ties in with all other tables I have seen. Obviously add a few HP for alternators and other bolt on's.

1 comment:

rj said...

Scotty! I need more power! lol.

I hear so many people say "You can never have too much power". But I think you really "can" have too much. We repowered our heavy displacement 28 footer with a 3-cyl Yanmar 3GM (replaced an old 2-cyl Volvo MD-7A). But we had to pitch the max-prop back twice and the boat is still a bit overpowered. I kinda wish we went with the 2GM, but the 3GM cost only $800 more so we thought it was a no-brainer.

The trick these days is finding a naturally aspirated engine in the target HP range. It is hard to avoid turbos and intercoolers these days.