SAIL to POWER

[tschienque]

Well I'm getting to the age when I'm considering giving up SAIL and converting to POWER.

Now my boat just about does everything I want of her at present (she's a 40' Wharram catamaran) but will want to up my cruising speed from current 9.0-9.5 with 25hp outboard. I'd like to get about 13-15 knots cruising.

I bought Dave Gerrs Propellor handbook to see how much power I'd need to achieve that kind of cruising speed. Well I couldn't understand hide nor hair of it! (Sorry Dave if you're on here). Far too many variables!

Also it seemed not to address the sleeker LWL/Beam ratios of catamarans that make them easier to drive through the water.

SO - is there a formula that I can apply to determine power requirements for catamaran hulls?

Basic data 34' LWL, Beam@WL 3', Cruising displacement 6 tons

Help!

[FAST FRED]

Search the archives for Long Skinney boats , and you will find a link to a British computer that will work the entire drag ( therefore Power required) out for you.

FAST FRED

[Richard Petersen]

Is the beam really 3' ? I have never heard of L to W ratio of 11 to 1.

[tschienque]

ratios of 10+ : 1 are commonplace in catamarans & tris; that, along with no ballast to drag around everywhere are a couple of reasons for their ability to go beyond the 1.4 * √LWL speed restraint on displacement vessels

[Richard Petersen]

Now I don't feel bad about a 2 seat wood runabout 20' X 4' X 3' with a Cleaver bow. A 40's woody. Plans look great. Was not sure about L to W ratio being " normal".