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post #1 of 5 Old 05-16-2009 Thread Starter
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High aspect?

What does the term "high aspect" mean when referring to keels and masts? I think I know but I can't find an actual definition.

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post #2 of 5 Old 05-16-2009
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The term is actually "high aspect ratio", and is used with aircraft as well. A high aspect ratio sail is one that is high and with a shorter boom; with aircraft a high aspect ratio wing is that of a glider, low aspect ratio is that of a crop duster (the former is highly efficient but very critical, the latter is not as efficient but great at low speeds).

A high aspect ratio keel will be deep and skinny, low aspect ratio keels are the full-keel kind. The wetted surface of a high aspect ratio keel will tend to be lower, as would that of a sail.

Sails are measured with IJPE numbers, so a high aspect ratio sail would have the ratio of P:E being as high as possible.


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post #3 of 5 Old 05-16-2009
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High aspect refers to the shape of a foil as it cuts through its fluid. A deep keel with a short chord where it attaches to the boat, and a tall mainsail with a short boom would be high aspects. Gliders have long, narrow wings: high aspect. A shallower keel with a long keel/hull joint, a mainsail on a short mast with a long boom would be low aspect. Supersonic fighter planes have short, wide wings: low aspect. A solent jib is considered high aspect. A genoa would be low aspect. The term is relative. It comes from "aspect ratio", which compares the leading edge length of the foil to its chord length. CCA rigs were designed typically with low aspect mainsails. IOR boats with their short booms generally have high aspect mains. High aspect foils develop more lift for their size than low aspect foils. This is one of the reasons deep keel boats do better to windward than full keel boats. There are points of diminishing returns however. If the keel's too deep, you start to hit things with it. If the mast's too tall, you heel too much, and the weight you have to carry to compensate for it slows the boat down. It's all a balancing act.
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post #4 of 5 Old 05-16-2009
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High aspect wing: that on a unpowered glider
Low aspect wing: delta wing on a supersonic jet fighter

High aspect sail: Volvo Ocean Race boat
Low aspect sail: Sunfish.

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post #5 of 5 Old 05-16-2009
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Quote:
Originally Posted by paulk View Post
High aspect foils develop more lift for their size than low aspect foils.
I'm going to suggest modifying this statement a little. It's not that high aspect ratio foils produce more lift (for a given amount of area), they create less induced drag. Here's a bit from North Sails:

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Another factor that must be considered is induced drag. This is the drag that a wing generates when it creates lift. Over most of a wing, the low pressure above the wing is kept isolated from the higher pressure underneath by the physical presence of the wing. At the tip of a wing, where the wing ends, there is nothing preventing air from flowing around the wing tip from the high pressure beneath to the lower pressure above. This results in the standard tip vortex that is often seen spinning off the tips of airplane wings and flaps. When the flow takes this alternate path around the tip instead of over the airfoil surface, energy is expended that does not develop lift, but does cause drag. This is called induced drag and it increases exponentially with lift, so a wing, such as a sail, that is producing substantial lift, experiences much more induced drag than a wing that is producing a lesser amount of lift.

The most effective way to minimize induced drag is to increase span, as induced drag is inversely proportional to the span squared. Highly efficient airplanes like gliders have very high span for the amount of lift they are producing. Winglets are a way to create the effect of higher span without actually increasing the physical span. They are useful when there is an artificial constraint on wingspan (like a draft limitation on a keel).
This article was using an airplane wing as an example, so for a given wing area, increasing the span increases the aspect ratio.

Often boats are offered with a regular rig, or a high aspect ratio (aka tall) rig. The high aspect ratio rig will point better, since sailing to weather is where the maximum lift is being generated by the sails and lowering the induced drag lets the boat go faster.

On the other hand, for downwind sailing you want the sails to generate lots of drag, because now the drag is powering the boat. So the high aspect ratio rig with its lower induced drag is at a disadvantage here.

A higher aspect ratio rig also puts the center of effort higher in the air so will tend to heel the boat more, or require more ballast to compensate. The taller rig, on the other hand, can catch higher speed wind further up.

Like everything else, selecting rig aspect ratio is a compromise.

Tim
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