November 17, 2014

Full keel versus fin keel

IN THE EARLY STAGES of falling in love with sailing, the question most frequently asked is: “Should I buy a boat with a full keel or a fin keel?”
The answer is that it depends on whether you want to do serious deep-sea sailing, out of sight of land and well away from sheltered waters. That’s when you need a full-keeled boat. If you’re doing coastal sailing or round-the-buoys racing, a fin-keeled boat will do you fine, and it will probably be faster, nimbler, and more weatherly.

So why buy a full-keel boat at all, especially since there aren’t many of them being built nowadays? Well, in short, they’re better at handling storms at sea if you’re short-handed.

The eminent research scientist Tony Marchaj, a champion racing sailor, tells us that a boat at sea is part of a dynamic system. The large surface area and shape of a traditional, long-keeled underwater hull can damp rolling better than the small surface area of a fin keel. This difference becomes marked when the boat is stopped in the water.

When the boat is stationary, after a few rolls the water in which the keel is swinging back and forth becomes filled with random eddies and swirls that offer less resistance to the keel. But if a boat is moving forward, the rolling energy (that is, overturning energy)  can be dissipated more efficiently into a much greater area of less confused water.

That’s why it’s usually necessary to keep a fin-keeled yacht running in heavy weather, whereas a boat with a full-length keel can lie hove-to, or ahull, and still dissipate the wave energy that is trying to roll her over through the greater area and superior damping qualities of her underwater shape.

Racing boats with fin keels usually carry crews large and skilled enough to man the helm at all times in heavy weather, and they can therefore benefit from staying on the move. But mom-and-pop boats must often stop while their crews cook, navigate, or get some rest. A full keel will then be more of a safeguard against getting rolled over than will a fin keel.

In other words, a traditional long keel will look after you when the boat is dead in the water; but a fin keel needs to be kept moving. Nevertheless, Marchaj points out that even a full keel will have more damping action if it can be kept moving. “In a survival situation, active rather than passive tactics are usually successful,” he says. “Those who are able to maintain some speed and directional control fare better.”

Ø If you’re interested, there’s much more from Marchaj, and a primer on how to handle storm conditions in a full-keeler, in my book, The Seaworthy Offshore Sailboat (International Marine).

Today’s Thought
When clouds appear, wise men put on their cloaks;
When great leaves fall, the winter is at hand;
When the sun sets, who doth not look for night?
Untimely storms make men expect a dearth.
— Shakespeare, Richard III

“They tell me O’Riordan stayed up all night to see where the sun went.”
“Oh did he now? And what happened?”
“It finally dawned on him.”
(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)


SailFarLiveFree said...

The old keel debate is sure to bring some lively discussion. Don't forget, full keels also have some advantages when near shore too: 1) Shallow draft for anchoring/exploring 2) Protection for the prop/rudder 3) Ability to withstand a grounding (sometimes).

I think Bob Perry had some technical gripes with full keels: "Go ahead and love your crab crusher full keel boat but don’t try to justify the design on technical terms."

57 degrees North said...

I have to say I'm a staunch disciple of full keel boats, though I freely admit it has less to do with technicalities, than a deep rooted sense of self-preservation...

Lee said...

It's an old article, but if you're still checking comments here's my 2 cents. After reading/watching/following many sailboat blue water passages, and particularly the Golden Globe 2018 revival race, it seems that full keeled boats are knocked down or rolled at a much higher rate than fin keels. The lower righting moment, higher center of gravity, and the huge surface area that seems to catch undercurrents and spin the boat on it's axis are all factors that contribute to the same result. Not a fan.