Auxiliary engine size

[bg9208]

Is there a rule of thumb for determining an optimum engine size auxiliary for a sailing cruiser, I'm thinking in terms of a, somewhere between 38 and 42ft boat, some on offer seem to have small engines around 20HP or less and some have up to 80HP. I appreciate that hull form and displacement will have an effect.

[sluissa]

It really depends on your use. If you are impatient and expect to motor any time you can't get full hull speed out of your sails, you'd probably want a bigger motor. If your motor is probably only going to be used for pulling in and out of the dock, then you don't need anything huge.

[never monday]

lots of variables here.
I start with dividing the LWL by 2. Then round up and a few other adders for HP.

[neelie]

ahem....Shouldn't that be LWL x 2?

[SkiprJohn]

Aloha Never,

I like your formula. My water line is 34 divided by 2 equals 17 plus 4 for an alternator and that equals 21 or there abouts. Economical and perfect.

bg9208 that is really good advice.

Kind Regards,
JohnL

[sluissa]

My LWL is about 20. divided by 2 is 10. However, I get along fine with a 6HP outboard. Gives me more than enough power. The boat is designed to handle something closer to a 3.5HP outboard. So... I guess my boat is weird, or that formula doesn't scale down well.

[dcstrng]

((((Displacement in lbs)/2240)x75)x(1.4x(((LWL)^0.5)))x0.003)/0.45 = hp

Or at least that's what I've had in my spreadsheet for some years... think I pirated it from Skenes or somewhere... takes into account dispacement, and hull speed as well (displacement hulls only)

[never monday]

Originally Posted by sluissa

My LWL is about 20. divided by 2 is 10. However, I get along fine with a 6HP outboard. Gives me more than enough power. The boat is designed to handle something closer to a 3.5HP outboard. So... I guess my boat is weird, or that formula doesn't scale down well.

It's not designed to be a rule. I use it for a starting point. That way I keep from putting an 80hp in a 38' IP. Like I said earlier. I also round up, add for alternators, refidge compressors, water maker pumps. Then add the human factor and normal cruising grounds.

[jjt]

at least 4 Hp each 1000 kilos for a pure sailboat (not a motorsailer)