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post #26 of Old 12-16-2010
Jeff_H
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While you are right that at some point heel angles need to be controlled with sail choice and trim, rail weight figures prominently in how much drive and how much drag the boat produces. While there is an optimum drive and optimum heel angle for any wind, how you attempt to achieve these ideals is a balancing act of sail trim and crew weight.

When you talk about the impact of rail meat, you really need to look at the whole equation and not just crew weight. Righting moment is the center of that weight times the righting arm. The righting arm is the dimension between the center of the weight of crew and the center of buoyancy. In most boats as a boat heels the center of buoyuancy moves to leeward.

So If we use the boat in your picture as an illustration, it would appear that the boat is heeled approximately 25 or so degrees (measured with a protactor rotated so that the measurement is adjusted for the focal point of the picture appearing to be off axis with the boat), (which, BTW 25 degrees is probably too high a heel angle for optimum speed. They seem to know that onboard since you can see that the mainsail trimmer has dropped the traveler and is vang-sheeting the boom to leeward in an attempt to flatten the boat and compensate for the weigh of two crewmen who are off the rail and up on the bow)

In the picture, the center of the crew weight is probably something like 5 feet off the center line of the boat. I'm seeing 9 guys on the rail and two on the bow. Assuming, all of the 11 guys are on the rail that is probably something approaching 2,500 lbs as they are dressed.

Again, eyeing the picture, the center of buoyancy at that heel angle has shifted approximately two feet to leeward. So that means a lever arm of roughly 7 feet, which when multiplied by the crew weight results in a righting moment that crudely would be somewhere around 17500 foot pounds (7 feet x 2500 lbs=17,500 foot-pounds).

To put this in perspective, lets assume that the boat in question has a 4500 lb bulb on its keel and that bulb is 8 feet below its center of buoyancy. At a 25 degree heel angle that bulb would be approximately 3.4 feet to windward of the centerline of the boat, which when you add the 2 feet that the center of buoyancy has shifted to leeward, results in a righting arm of 5'-4" and a righting moment around 24,000 foot-lbs.

In other words, the crew has achieved roughly 75% of the righting moment of the keel bulb with only 55% of the weight. That is pretty efficient, especially since the boat is not being penalized in their rating for the crew weight.

To touch on a couple of the fine points raised above. These days virtually all racing rules limit crew number and/or crew weight to prevent people from putting together a 'beef trust' and kicking butt in heavy going and from going too light in light air.

As has been pointed out, on crack race boats, this crew weight is used very strategically. In light air race-boats are often heeled to leeward and trimmed slightly bow down. This does a number of things. One of the big ones is that heeling a modern boat perhaps 10 degrees moves the wetted surface to the round of the chine and away from the flat of the bottom (a cylindar has less wetted surface per capacity, than an flattened oval) which reduces wetted surface pretty dramatically. Bow down also reduces wetted surface pretty dramatically by moving more of the wetted sruface to the cylindrial bow sections and away from the flatter stern sections.
Trimming to leeward and bow down trims some 'feel' into the helm and a bit of lift, so that the helmsperson can use a minimum amount of helm and yet hold a course.

Another advantage is that when a boat is heeled over gravity pulls the sails into a more proper shape so that any wind that encounters the sail is encountering a foil shaped object. Lastly, very often in light air the wind has a vertical component, falling slightly downward, and the heel angle exposes the sail more perpendicular to the wind.

Normally larger heel angles mean more leeway, but in light air, there rarely is enough side force that leeway is as much of a problem as simply maintaining forward motion and building apparent wind speed.


Jeff



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Last edited by Jeff_H; 12-16-2010 at 05:33 PM.
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