December 16, 2012

When is a boat too tender?

MOST SAILBOATS lacking in initial (or form) stability lose their tenderness as they heel and become progressively (and reassuringly) stiff. But if a boat fails to start resisting heeling after she has reached an angle of 15 degrees or so, and continues simply to lie over and dip her gunwale in reasonably moderate winds, she’s probably too tender for safety. If she won’t stand up to all working canvas in 15 knots or so, don’t hesitate: seek professional help, preferably from a naval architect.

One of the paradoxes of boat design is that the boat that seems to be tender initially will likely be more seaworthy than the one that is stiff to start with. That’s because most capsizes are caused by wave action, not wind, and when a boat is upside down she will recover more quickly if she is reasonably narrow and has a deep, heavy, ballast keel. A wide shallow boat, while stiff to start with, tends to remain upside down a long time if she is capsized by a wave.

Liveaboard cruising boats often lose stability imperceptibly as their owners gather possessions over the years. This causes a boat’s ballast keel to become an ever-smaller percentage of displacement.

The situation can be improved substantially by lowering weights wherever possible. These include internal ballast, books, water and fuel tanks, batteries, outboard engines clamped to the aft rail, life rafts, dinghies, provisions, and anything higher than the boat’s center of buoyancy.

Keep your spars and rigging wire as light as possible. That goes for the sails and fittings, too. Hanked-on foresails improve stability more than roller-furling sails because they can be lowered in bad weather.

Incidentally, it pays to lessen weight aloft. Because of the effect of leverage, every ounce off the top of the mast is worth a pound added to the keel.

Today’s Thought
Remember that there is nothing stable in human affairs ...
— Isocrates, Ad Demonicum

Some of us believe a girl’s family tree doesn’t matter, as long as she has the right kind of limbs.

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

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