August 26, 2010

The magical centerboard

I WAS SAILING a dinghy the other day and thinking what a clever invention the humble centerboard is. In some ways, it’s the equivalent of a wing on a plane, but on a boat it’s a mostly invisible part of the magic of sailing.

I say magic, because the centerboard, like a fin keel, stops a boat making leeway by making leeway. That’s right. If a centerboard didn’t make leeway of between 3 and 5 degrees, it couldn’t work. It wouldn’t provide the “lift” to stop a sailboat drifting off to leeward so fast on the beat that it would never be able to make way to windward. And it has to be moving forward through the water to provide lift, of course, otherwise it will be stalled and allow the boat to slide sideways.

According to The Oxford Companion to Ships and the Sea, the centerboard for small craft was invented in America in colonial times. “The need to be able to sail to windward close-hauled, with an entirely flat bottomed work boat arose from the great stretches of shallow waters found in the Chesapeake Bay and along the Atlantic seaboard from Long Island Sound to Florida, and so the centerboard was born.”

Although some famously successful ocean racing yachts have had centerboards, naval architect Ted Brewer says the pure centerboard cruiser has fallen out of favor now, although it still has much to offer the sailor.

If you’ve sailed with a pivoting centerboard, you’ll know how useful it is in balancing the helm by moving the center of lateral resistance forward and aft. If you raise the board partly to angle it aft, for instance, it greatly reduces the tendency of a sailboat to round up while on the dead run.

To take this a step further, some boats have two centerboards, one large one up forward, and another smaller one aft. The task of the forward board is to reduce leeway, while the aftermost board is raised or lowered to attain neutral helm. This is particularly handy in heavy weather, when the changes to sail balance caused by reefing can by compensated for by adjusting the boards.

Like a fin keel, the efficiency of a centerboard usually increases with its aspect ratio. The longer and thinner it is, the better it will perform, especially if it is given a streamlined shape that provides more lift for its area.

It seems so simple when you look at it. You simply stick this piece of board down into the water through a slot in the boat and it stops you going sideways. But if you care to think about it, there’s a lot of interesting science and hydrodynamics going on down there. Like many aspects of sailing, we don’t normally give it much thought. It just works when we want it to, and that’s that. But it’s magic all the same.

Today’s Thought
‘Tis frivolous to fix pedantically the date of particular inventions. They have all been invented over and over fifty times. Man is the arch machine, of which all these shifts drawn from himself are toy models.
— Emerson, Conduct of Life

Boaters’ Rules of Thumb, #88
Fuel consumption, diesel. An inboard four-stroke diesel engine consumes about 1 gallon per hour for every 18 horsepower used. In other words, a 36-hp diesel running at full throttle will use 36/18, or 2 gallons an hour. That same 36-hp diesel tootling along slowly and developing only 9 hp will use 9/18, or 1/2 gallon per hour.

“What happened to that guy who tried to cash your check?”
“They took him away in a strait jacket.”


Nikolay said...

Morning John,

I have 2 question for you.
1. I am considering building a small boat, i.e. the Marissol Skiff. I have the book with all the plans and details but there is one number that I am still looking for: Maximum Safe Load Capacity (LBS). I can calculate roughly the displacement volume of the hull and using 62lbs/cu.ft of freshwater, the absolute maximum weight it would take without taking water over the gunwales.
My question is whether there is a standard minimum freeboard that designers use for small boats to determine the maximum load carrying capacity.
2. The Marissol Skiff is a daggerboard design. I intend to launch and recover it from a beach. I know a centerboard will swing up if it hits something on the bottom. Is there a way of adding a similar feature to a daggerboard, or in fact, is it safe to modify a daggerboard design dinghy to a centerboard?

Thanks very much,

John Vigor said...

Nikolay, you pose an interesting question or two. Strangely enough, the U.S. Coast Guard Boating Education Branch ignores freeboard altogether when it considers the maximum safe carrying capacity for a dinghy.
Here's the way it determines the number of persons of average weight (160 pounds) a boat under 20feet long can safely carry in calm weather:
Number of people = overall length multiplied by beam (both in feet) divided by 15.
For example, a 10-foot dinghy has a beam of 4.2 feet.
10 x 4.2 = 42. Divide 42 by 15 and you get 2.8 people. Round up to 3 people.
Logically, and for very obvious reasons, freeboard should come into the equation somewhere, but I have never seen a formula for it. Common sense must rule, I guess.
As for replacing a daggerboard with a centerboard, there doesn't seem to be any reason why you shouldn't do that, provided the added length of the centerboard case doesn't interfere with anything in the cockpit. I once saw plans for a centerboard conversion of the popular Mirror Class 11-footer, which normally comes with a daggerboard.
You need a longer slot in the keel, of course, and you should probably brace the top of the casing against sideways movement, possibly with a thwart or other suitable construction.
Plan to have the center of lateral resistance of the centerboard in the same place, fore-and-aft, as the daggerboard, so the sail balance isn't affected. A deep narrow centerboard is more effective than a shallow, wide one. And finally, keep the area of the board roughly equivalent to the area of the old daggerboard.
Good luck,
John V.

Nikolay said...

Thanks John,
Unfortunately after I looked at the plans more carefully there are too many things in the way aft of the daggerboard trunk to easily extend it for a centerboard.

How about a question on mast construction? if i cannot get rough lumber long enough for the entire spar, is there an area of the mast where a spline/splice in the wood would be subjected to the least amount of load stresses?


John Vigor said...

Nikolay, box-beam wooden masts nearly always need to be scarfed, and exerience has shown that a good scarf will render the joined area stronger than the rest of the wood. That goes for solid wood masts, too, so it doesn't seem to matter where you place the scarf, though usually it's lower down on the mast where (if you're paranoid) you can keep a good eye on it while sailing.
A good reference is "Wooden Boat Renovation," by Jim Trefethen (International Marine). He shows and describes how to make all sorts of mast scarfs.

John V.