January 15, 2012

Think inverted

IT'S THE OPINION of a British expert that cruising yachts should be designed for minimum stability when they're upside down. I mentioned this in my book The Seaworthy Offshore Sailboat, and I believe it's as valid now as it was when I wrote the book.
John Lacey, former honorary naval architect of the Royal Naval Sailing Association, and a member of the Royal Corps of Naval Constructors, said in the fall 1982 issue of the RNSA Journal that until the disaster of the 1979 Fastnet Race, few people had explored the stability characteristics of yachts sailing on coastal waters, beyond a 90-degree knockdown.

But on the night of August 13, 1979, that complacency changed. Sixty-three yachts each experienced at least one knockdown substantially farther than 90 degrees. Many did not right themselves quickly, and remained upside down for considerable periods.

Lacy said the influence of the then-popular International Offshore Rule (IOR) for racing yachts had radically changed the shape of yacht hulls by greatly increasing the proportion of beam to length.

"Increase of beam gives great sail-carrying power without additional ballast," he pointed out. "It also provides the benefit of greatly increased accommodation in a given length."

But the shape of such a hull also makes it very stable when inverted.  In other words, if it is turned upside down by a wave, it tends to stay upside down. To bring the boat upright again would require about half the energy needed to capsize the boat in the first place.

"Since the initial capsize may have been caused by a once-in-a-lifetime freak wave, one could be waiting a long time for a wave big enough to overcome this inverted stability."

By way of contrast, Lacey calculated that a narrower cruising hull with a lower center of gravity, such as a Nicholson 32, would require only one-tenth of the capsize energy to recover from a 180-degree capsize.

Beamy, shallow-bodied boats, he said, "may increase the size of the wave needed to initiate capsize, but in the end the sea will still win if the wave is awkward enough. It therefore seems in my opinion that we should tackle the problem from the other end, and design yachts for minimum stability when upside down."

So, to recap, here are the main requisites for fast recovery from a 180-degree capsize in a monohull:

Ø Moderate to narrow beam

Ø A low center of gravity  (mainly from a deep, heavy, ballast keel)

Ø A moderately high cabin top with reasonably wide side decks, and

Ø Watertight hatches, ports, ventilators, and cockpit lockers so little or no water gains ingress while the boat is upside down.

Finally, I always advise anyone contemplating taking a boat into blue water to "think inverted."  Try to imagine all the chaos that can happen when a boat is forcibly dumped upside down. And take steps to prevent that chaos.

Today's Thought
If the danger seems slight, then truly it is not slight.
— Francis Bacon, De Augmentis Scientarium: Principiis Obstare

"Why did that sailor buy drinks for all those girls?"
"He likes to have a port in every sweetheart."

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)


andre said...

Small Boat to Freedom by John Vigor(2nd Edition,2011)
Got and read this book over the New Year. Enthralling.Could not put it down.
Should be compulsory reading for everyone contemplating a multi week deep water voyage,sailing the Caribbean, as well as armchair "deepwater sailors"/dreamers(like me?) The description of the passage between Durban and Cape Town is particularly interesting, informative and could be very useful to sailors .
The section on the Caribbean and the vagaries of Cruise Ships (I hesitate to call these floating hotels ,Liners) would help those sailing there (especially relevant with the news of the Shipwreck on the Italian isle this last week end)
Should be recommended reading for Naval and Coastguard personell

Anonymous said...

So you are basically against the whole idea of multihulls?

John Vigor said...

Dear Anonymous:

No, I am not basically against the whole idea of multihulls.

John V.