sailing - page 5

2.5 Stability

A gust sweeps in and the boat heels sharply. The new sailor's eyes go wide and his knuckles whiten as he clabbers for a handhold, any handhold, in the face of imminent capsize. In choosing to take up sailing, he didn't bargain on capsizing. But the boat doesn't capsize. The gust passes, and the boat comes back up to an even keel. So why didn't it capsize? It felt like it was going to, lying on its side like that, the mast maybe 30 degrees shy of vertical. To answer that good question, we need to look at the unseen parts of the boat below the aterline. But before we do that, let's examine why sailboats heel in the first place. After all, motorboats don't heel.

The wind, even when it's relatively light, exerts real force on the sails as they translate wind force into motive power. You will feel that force spread throughout the boat like a bolt of electricity. Touch a jib sheet under load, and it feels more like an iron bar than a rope. It's really quite impressive, the pwer contained in those pieces of fabric we call sails. The trouble is, the laws of physics dictate that most of the wind's force-when the wind blows from over the front of the boat-wants to push the boat over on its side. Only a relativey small portion of the wind's force devotes itself to moving the boat forward. In the language of boat design, a degree of force is referred to as "moment", and so here we are talking about heeling moment. So what compensates for the inevitable heeling moment? The answer is that part of the boat you can't see because it's underwater. It's called the keel. 

If it's useful, think about sailboat stability in terms of a seesaw. Put a heavy kid on one end, a light kid on the other-the seesaw will be out of balance; it will capsize to the heavy kid's side. But put two kids of even weight on the seesaw, and you have balance. The seesaw is stable. In boats, the force (weight) of the wind in the sails naturally causes the boat to heel, but it won't, under reasonable conditions, capsize, because this heavy chunk of lead, the keel, mounted on the bottom of the boat counterbalances the heeling moment.  

There are some helpful hints for the helmsman in determining the wind speed and direction and  therefore to be able to predict the heel.

In case of sailing upwind in high winds, you can de-power the mainsail by easing the main sheet, traveller or reefing. 

  • The wind speed indicator, anemometer, provides constant information that may be evaluated as either apparent or true wind speed at the push of a button. This instrument is very accurate and should be mostly trusted.

  • Less technical signs of wind direction and velocity include; flags, telltales, your hair, and birds. Birds, when given the opportunity, generally sit with their beak into the wind.

  • Ripples on the water.

  • If you put your nose directly into the wind, you will hear the wind equally in both ears. 

There are many ways to judge how the wind is hitting your vessel, and these tend to become intuitive with experience - you simply sense the wind. This is when you truly have the "feeling" of sailing.