Sail power and suchlike...

On blaming the victims, Kunstler muses about a window yet to be broken, and for those thinking things can't get any worse, think again...

Way back when, Tom Colvin said that a good rule of thumb for an auxiliary engine in a sailboat was about one HP per ton... Over the years I've come across no compelling evidence that Mr Colvin was wrong, so for the moment let's just say that one HP per ton is a good starting point.

So here is a question you might want to ask yourself... What sort of HP does your sail area produce?

In our case, the CAL 34 with its Simplicity rig (basic working sail area of 592 sq feet) horsepower figures look something like this...

8.5 HP @ 10 knots of wind, 11.5 HP @ 14 knots, and 23.5 HP @ 20 knots

For what it's worth the rig as designed by Bill Lapworth (basic working sail area of 515 sq feet) works out like this...

7.5HP @ 10 knots of wind, 10 HP @14 knots, and 20.6 HP @ 20 knots

Right around 20 knots of wind is where "So It Goes" tells us she'd be happier with a reef so, for all practical purposes, our rig tops out at 23 HP.

For those keen on checking their rigs HP the formula is HP at 10 knots of wind = .015 X square feet (use .020 for 14 knots and .040 for 20 knots). Not perfect but it is accurate enough to tell you what you need to know.

The CAL34 having just a kiss less than five tons of displacement would, under Colvin's suggestion, need a five HP engine though our Electric Yacht motor delivers 8HP @48V and the Atomic 4 it replaced produced  about 15HP in normal use (30 HP @ 3000 RPM) and due to it's ill performing stock prop I doubt it actually ever delivered more than 10HP of thrust.

So, when all is said and done the actual HP for the standard Atomic 4 is not all that out of line with what Colvin suggested... In fact, we have powered "So It Goes" with a 5HP outboard on several occasions and found it worked just fine...

The conclusion that I draw here is that 1HP per ton works but there is nothing wrong with a "little" extra oomph but that exceeding the horse power of your sail rig does not make a heck of a lot of sense unless you really want a motor boat.

The French, who know a thing or two about sailboats, call motorsailers "50's" (seldom used as a positive) to designate that they share sail and power at a 50% ratio. These days, with cheap HP available, nearly all modern sailboats would qualify as motorsailers... Something to keep in mind.

By the way the books "The Propeller Handbook" and "The Nature of Boats" by Dave Gerr have all the needed info to help sort out your rig, engine/prop, and HP questions.

Listening to Spirit

So it goes...