| The physics of sailboats |
Accurately analyzing the behavior of a sailboat is exceedingly difficult.
Unlike an airplane or submarine, both of which operate in one fluid medium,
a sailboat has to contend with two fluids, both air and water.
Even worse, it has to deal with the interface between the two media,
i.e. the waves, which turn out to be an even bigger problem.
Since a sailboat is propelled by wind power alone, the forces acting on a boat going in any direction other than dead downwind are not always intuitively obvious. Looking down on the boat from above, the sail and the force of the wind acting on it, are both at an angle with respect to both the boat's hull and the wind, pretty much all the time. Since the wind is always pushing the boat at an angle, and since that force is applied to the sail high above the water's surface, the wind will essentially always be trying to tip, or "heel" the boat over toward the leeward the side. This is called a "rolling moment". If you're an engineer, be careful not to call it a "torque". If you do, you'll confuse all the old salts who have no idea what that is. Since we don't want the boat to tip over, there has to be something to counterbalance the heeling moment of the wind. A dinghy sailor does this by sitting on the upwind, or "windward" edge of the boat, effectively trying to heel it over the other way. A yacht, on the other hand, is too big for that method. A yacht typically has a heavy keel full of lead for that purpose. In order for the lead in the keel to work, the yacht has to heel quite a bit, usually about 30 degrees or so. This kind of yacht, or "keelboat", is sometimes called a "leadmine", "leadbelly", or "leaner". |
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| Multihulls |
Multihull sailboats use a different approach to counter the wind's rolling moment.
When a multihull heels over slightly, the downwind, or "leeward"1, hull sinks into
the water more, and the windward hull comes up out of the water.
This means that the center of flotation is now underneath that leeward hull.
Moving the center of flotation leeward has the same effect as moving the
weight of the crew or keel to windward:
It creates a "righting" moment (opposite to the wind's heeling moment)
that prevents the boat from capsizing.
A multihull has two big speed advantages over a keelboat of the same size. First, the dramatic shift in the center of bouyancy allows the multihull to sail much closer to upright. Trimarans heel about ten degrees or so, while catamarans normally heel less than five degrees. This increases the horizontal component of the wind's driving force, which is the part that generates propulsion, and decreases the downward component, which just slows the boat down by pushing it into the water. The second difference is simply not having all that lead weighing the boat down. A keelboat sails with the wind spilling over the top of the sail, pushing the boat down, with a hunk of lead pulling it down from below. Multihulls are dramatically faster than keelboats.
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| Terminology |
1
If you want to be accepted as a true sailor, you have to pronounce this word as "LOO-erd",
even though it's spelled like "LEE-ward".
In fact, correct pronunciation of any nautical term in the presence of sailors
immediately brands the speaker as a "landlubber", or simply "lubber".
Remember especially these important terms:
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