May 15, 2012

Beware the overhang

LONG OVERHANGS look very elegant on a sailboat. Many racing designs used them at one time or another, boats such as the Thirty Square Meter class, which is limited almost entirely by sail area. Designers searching for the fastest hull for given sail area came up with slim, narrow boats with minimum underwater surface, and long, long, overhangs to increase speed when the boat heeled. But these were never ocean racing boats.

Long, low overhangs are dangerous in open water. They pound badly in a head sea and are easily boarded by large breaking waves. It can be very treacherous to work on a long narrow foredeck awash in heavy weather.

In large quartering seas, such a boat is almost uncontrollable. Each passing swell lifts the long aft overhang and tries to screw the boat around to the broadside-on position. The lever-arm provided by the long overhang means that it takes comparatively little power to spin the boat around. If that happens at speed, the boat will broach and probably capsize, with a good chance of losing the mast. For that reason, most seaboats have only a moderate bow overhang and very little stern overhang.

Thirty Square Meters and others of that type are wonderful performers in calm water but don’t be tempted to take them too far out to sea.         

Today's Thought
Danger comes the sooner when despised.
— Publilius Syrus, Sententiae

"How's that book on anti-gravity?"
"It's great. I can hardly put it down."

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


KevinH said...

The 30 square is of course an extreme of design. For pounding to weather, modern canoe hulls with fin keels are a close contender. A traditional lead mine type cruising hull will have a forefoot that will part the water and provide bouyancy up front that a canoe hull does not, but results in the pounding to weather that gives the crew practice in retrieving masts from the water. I'm not convinced that stern shape has too much relevance. The stern will lift to a following sea by virtue of the quarter's bouyancy long before the presence of a flat/sharp/wide/narrow/canoe/pinched transome makes any difference. said...

This is a timely post in our search for that ever elusive boat that is the perfect compromise of sailing fun, sea worthiness, and accommodation, not necessarily in that order. With myriad of choices and variables to consider when looking for a voyaging boat, it's almost a relief to have something like long overhangs to rule out, especially when they make such pretty boats that it's easy to get sucked into loving them! We enjoy your books very much. We have found them to be delightfully entertaining as well as usefully informative. Thank you for writing.

BillH said...

You already know this, but "For your life, build no fantail overhang on a craft going offshore."

J. Slocum, in the appendix to "Sailing Alone"