Twilight sailing and positive stability

[khiemtran]

Thursday night, and it's twilight sailing again on Botany Bay...

The winds were quite strong for most of the day. This is what I saw when I went out for a walk at lunchtime (albeit in a different city: this is the Blue Mile in Wollongong).



But by the time I left work, the wind had begun to drop. By the time I made it to San Souci, this is what the bay looked like.



Where would you rather be on a Thursday night? Stuck in traffic or out in your own little boat?



You might also have noticed the waves in the last picture. The tide was going out and the waves were rolling in the other other direction. This was another new experience for me: getting to surf the waves in towards the bridge. I didn't really get a picture that really captured the waves properly. It was quite different from swimming or body surfing, because the waves weren't breaking, just rolling on. With the wind behind me, the boat seemed to be moving effortlessly, surging ahead each time we caught a wave and then halting when we fell off the back. Hmm. I notice I just used "we" there.



Here's a view over the stern as the sun sets. The wind is actually blowing roughly in the direction of the sun, which means we're on a "close reach" - we're heading into the wind, although not as tightly as we could, which would be "close hauled". If we were close hauled, I'd probably be out on the rail watching for sudden wind changes, not taking photos out the back.



As I mentioned earlier, the first big thing I had to grasp when learning to sail was angle-of-attack and how the sails would tell you how to set them. The next big thing was all about the boat's inherent stability. It's easy to imagine as you start out that your job is to keep the boat stable; but it's actually a bit more subtle than that. Your job is to help the boat keep itself stable.

Everything in sailing is to do with the balance of opposing forces. At its deepest, most elemental level, it's all about playing off the two domains of air and water to enable you to do what you couldn't with one alone. And since the forces of the air will only ever work on one side of the boat; and the forces of the water on the other (all being well), there are always going to be uneven forces that the boat will have to deal with. The good news is that after centuries of design evolution most of these forces will cancel each other out.

For example, if a gust of wind hits the sails, there will be a heeling force which tries to make the boat roll over (since the sails are connected to the boat by the mast, which acts like a giant lever). But, as boat starts to heel, the volume of the hull under water begins to change. More of the hull on the low side goes underwater, which means the centre of buoyancy moves over to that side and creates a force trying to right the boat. The more the boat tilts, the greater the force becomes. (There is a point-of-no-return, however, when the boat has tipped so far that the force begins to reduce again as water starts flowing into the hull.) To help the boat, you move your weight out as far as possible. On the one hand, you're obviously pushing down on the high-side, but on the other you're also shifting the boat's centre of gravity away from its newfound extra buoyancy on the low side, giving it an extra long lever to pull on.

At the same time, if you have set the sails carefully, the boat will have "weather helm". That is, without input from the rudder, it will naturally turn towards the wind. You can do this because the force from the sails isn't exactly through the centre (in this case, the "centre of pressure") of the boat, so there is always some turning motion induced by each sail. If you have two sails and one (the jib) pushes forward of the centre of pressure and the other (the main) pushes aft of the centre of pressure, they will balance each other out. All you have to do it make sure the force from the aft sail is slightly greater and that sudden gust will make the boat "round up" and point into the wind, which is exactly what you want.

Meanwhile, as the shape of the boat under the water changes, it makes the boat want to turn away from the direction it's tilting. Left alone, this will point the boat into the wind (again, exactly what you want if you're heeling too much). The only catch is if you try to fight it with the rudder and pull the tiller towards you (which would turn the boat away from the wind). Since the rudder acts below the boat's centre of gravity, it would then add to the heeling force that the boat was doing its best to counter for you. (The simple rule is "Tiller towards you, boom goes towards the water; tiller away from you, boom goes away from the water", which assumes that you're sitting on the opposite side of the boat from the boom.)

And, finally, you yourself are also part of this wonderful balance. When you're hanging out over the rail for example, and the wind suddenly dies, what do you think would happen? My first guess was that I'd plunge into the water, but it also turns out that you pull like mad at the two things in your hands - the mainsheet and the tiller extension - which just happen to be the two correct inputs you need to stop the roll to windward. Just as long as you weren't mucking about with a camera at the time...

Anyway, once again, I escaped without capsizing and had a very pleasant evening. As the sun set, the last of the wind seemed to die and the waves vanished, leaving the water almost glassy. A perfect end to the evening.

Comments

[mnfaure.livejournal.com]

I noticed the "we" too and wondered who you were sailing with. :P

[khiemtran.livejournal.com]

It turns out sailing is always a team effort - even when it's just you and the boat!

[asakiyume.livejournal.com]

it's all about playing off the two domains of air and water to enable you to do what you couldn't with one alone

This is so cool--you're a magician, playing with air and water. And I loved that you said we too--I was thinking, you and the boat, but it's not just you and the boat, it's you and the boat… and the wind and the water.

[khiemtran.livejournal.com]

It is magic. It still seems astonishing that I can potter back and forth across the bay without spending any effort (apart from concentration).