October 13, 2011

Seaworthy keels

MOST modern sailboats are not designed to be the most seaworthy vessels on the open ocean. They're designed as coastal cruisers, whose characteristics make them lighter, faster, more closewinded and often roomier below.

The old rule of thumb is that a boat with a long, traditional type of keel is more resistant to capsize than is a hull with the coastal cruiser's deep, narrow fin keel. At least, that holds true when neither yacht is making way through the water.

A yacht at sea is a dynamic system that receives most of its overturning energy from waves. A traditional keel and hull shape are effective at dissipating this energy gradually, absorbing the blows of the seas more gently.

A fin-keel boat has less underwater area in which to pass the energy on into the sea, and is more vulnerable to capsize when lying still in the water. But a fin-keeler becomes more resistant to capsize if she is kept moving, and can thus dissipate the incoming wave energy into a greater area of water.

A traditional cruising boat, having a more effective roll-damping keel, can better look after herself when lying almost still in the water, or hove to.

Tony Marchaj, a small-boat sailor who is also an internationally renowned aerodynamic and research scientist, says the keel of a truly seaworthy boat should be designed primarily for the survival situation (that is, zero speed) which implies a traditional keel with large lateral area and depth of the hull underbody.
In his book, Seaworthiness: The Forgotten Factor, Marchaj quotes a former editor of Rudder magazine, T. F. Day, who crossed the Atlantic in 1911 in the 25-foot yawl Seabird:

"My long experience in small boats has taught me this: that if a boat is a good boat, when real trouble comes she is best left alone. She knows better what to do than you, and if you leave her alone she will do the right things, whereas nine times out of ten you will do the wrong thing."

Today's Thought
Extremely foolish advice is likely to be uttered by those who are looking at the laboring vessel from the land.
--Arthur Helps, Friends in Council

Middle age is when you find yourself doing one bend-over to pick up two things.


Hugo said...

Hello Mr Vigor! As per your book "Seaworthy offshore sailboat", boats with a combination of keel and centerboard may not be seaworthy. Since I am considering buying a Jeanneau Sun Rise 35 with a combined pivoting centerboard and stub keel, I am writing to you for advice. Would you consider this particular boat safe and seaworthy enough to cruise from Bermuda to Polynesia and sail around the islands, with side visits to New Zealand and Australia?. I shall very much appreciate your expert opinion. Best regards, Hugo; hfcarrer@gmail.com

longroute said...

Dear Mr Vigor,
this article of yours was just what the doctor ordered…

It’s since several years now that I’m trying to have a more precise, objective and conclusive view about the seaworthiness of the most recent development of racing and cruising yachts: a delta shape, a flat bottom hull, a super wide and open stern like that of ferry-boats. I’m about your age and sailing since my youth (in the ’60) with quite a different kind of boats (my first experience was with a 36 ft Swan S.&S.) and needless to say I looked with suspicion and bewilderment the evolution of modern designs. All the reference books I read supported my doubts about the seaworthiness of such a light displacement and super-fast boats, but in speaking with dock fellows ( I live in Italy) I found to be in a minority of one. All are conquered by these new designs and all swear about their seaworthiness and superiority towards the old ones. Having never sailed aboard of one of such vessels I have no arguments to oppose. There is a widespread attitude which takes for granted that everything which comes out of the experimentation in the races courses is “the bible” and if you are not “up-to-date” which means if you don’t believe to what the modern builders advertise, you are just an old fogey…

I’ve met a lot of people from all over the world which were circumnavigating the world with their boats and most of them had sturdy “classical” designed boats (I’m using “classical” here to define boats prior to 1990, which is in my opinion about the time when real flat bottomed hulls were introduced in cruising yachts). But recently I’m seeing more and more people who choose those overgrown dinghies for sailing off-shore. So a doubt crept into my mind, what if they were right? What if I’m really an old fogey? (:-) They say that when a boat is lighter she will go over the waves when close-hauling and so she will not be damaged by them and she will be as firm as a rock when broad-reaching and planing.

(One example here: http://www.walkabout.it/barca-vela/walkabout43/
And another even more interesting (force 11) here: https://www.youtube.com/watch?v=NIUuuy8BNdk&index=11&list=LLqaYtgQM8T70Nb8Q0LE_P8Q )

As you say:
“a fin-keeler becomes more resistant to capsize if she is kept moving, and can thus dissipate the incoming wave energy into a greater area of water.”

So, perhaps is it only a matter of switching to a new way of tackling rough weather and long distance cruising? Or instead it’s all a folly as the Economist wrote in the following article?

“They put to sea in ever faster, more extreme and more expensive yachts. […]
“This has, as a by-product, littered the sea with yachts whose designs are evolutionary blunders.”

A comment from you will be much appreciated.