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Lift
I just got back and I am reading the posts after I left. I would like to add some comments. When I mentioned that lift did shorten the waterline length, it was because anytime the boat is being heeled because of forces on he sail, the keel and rudder are generating vertical lift. The tangent of the boat heel is equal to the ratio of the vertical lift to the side forces. For example with a 15 degree heel the vertical lift is just over 1/4 of the side forces. At a 30 degree heel, the vertical lift is over 1/2 half of the side forces and at 45 degrees they are equal. Since a boat in the 20 to 25 foot range can get a side force in the hundreds of pounds range, the lifting force can be significant. At low heels, the geometery does not cause waterline length increases. At higher heels, the lifting tends to offset it. In other words, you won't get a significant much in water line length due to heeling in most of the boats in use today.
I just got back and I am reading the posts after I left. I would like to add some comments. When I mentioned that lift did shorten the waterline length, it was because anytime the boat is being heeled because of forces on he sail, the keel and rudder are generating vertical lift. The tangent of the boat heel is equal to the ratio of the vertical lift to the side forces. For example with a 15 degree heel the vertical lift is just over 1/4 of the side forces. At a 30 degree heel, the vertical lift is over 1/2 half of the side forces and at 45 degrees they are equal. Since a boat in the 20 to 25 foot range can get a side force in the hundreds of pounds range, the lifting force can be significant. At low heels, the geometery does not cause waterline length increases. At higher heels, the lifting tends to offset it. In other words, you won't get a significant much in water line length due to heeling in most of the boats in use today.
Speed is fun, Oh Ya!
Franklin, Most modern hull configurations actually slow down with excessive heeling, but it sure is fun ain't it! The only time a lot of heel is good is when you need to go where the boat doesn't want to go. Jon McClain
Franklin, Most modern hull configurations actually slow down with excessive heeling, but it sure is fun ain't it! The only time a lot of heel is good is when you need to go where the boat doesn't want to go. Jon McClain
Shift incenter of bouyancy
Without doing the math I'd venture that heeling by shifting the center of bouyancy would be more important than any lift generated by the keel or rudder. The original question was a beam reach and speed. In my macgregor I would go faster on a beam reach with the daggerboard in the up position!!! The reduction in drag I suppose. With my Laser and buccaneer pretty much the same effect but sometimes I'd leave the boards about half down. Of course the laser and buccaneer (chyrsler) would plane easily. Tom
Without doing the math I'd venture that heeling by shifting the center of bouyancy would be more important than any lift generated by the keel or rudder. The original question was a beam reach and speed. In my macgregor I would go faster on a beam reach with the daggerboard in the up position!!! The reduction in drag I suppose. With my Laser and buccaneer pretty much the same effect but sometimes I'd leave the boards about half down. Of course the laser and buccaneer (chyrsler) would plane easily. Tom
The problem with heeling a modern wide beam hull
is that the foil on a wide beam boat is a thick foil compared to traditional narrower beam boats. Thicker foils generate more lift. The foil described is not the keel or rudder foil, but the foil made by the hull chines which become asymmetrical with heeling. When flat, that thick foil of modern cruisers of course is symmetrical and offers no lift, but when heeled it becomes asymmetrical and starts generating lots of lift and unfortunately its to leeward. This is true because the differential of flow is greater for a thicker foil and generates a greater pressure differential. And, the greater the heeling, the greater the asymmetry and the more leeward lift. Compounding the problem is usually the wide beam has a flatter bottom and less rocker so that the asymmetry of heeling is even more pronounced. The big curved chine of the leeward hull compared to the flattish low rocker bottom means lots of asymmetry. And, if the center of this leeward lift of the asymmetrical heeled hull doesn't correspond with the CLR, then it also fouls up boat balance and induces helm, usually weather, sometimes enough to round some designs up when they heel too far. While there are new designs that actually produce windward lift with heeling (less hull curvature and more rocker, made possible by deep bulb keels, our cruisers ain't them and we are better off with reasonably small amounts of heeling to avoid the leeward lift. Leeway forces of the sailplan are bad enough but when the hull starts lifting to leeward things get outa hand, then leeway becomes great enough to have the keel and rudder tripping up the boat which really adds heeling and things have gone very sour...
is that the foil on a wide beam boat is a thick foil compared to traditional narrower beam boats. Thicker foils generate more lift. The foil described is not the keel or rudder foil, but the foil made by the hull chines which become asymmetrical with heeling. When flat, that thick foil of modern cruisers of course is symmetrical and offers no lift, but when heeled it becomes asymmetrical and starts generating lots of lift and unfortunately its to leeward. This is true because the differential of flow is greater for a thicker foil and generates a greater pressure differential. And, the greater the heeling, the greater the asymmetry and the more leeward lift. Compounding the problem is usually the wide beam has a flatter bottom and less rocker so that the asymmetry of heeling is even more pronounced. The big curved chine of the leeward hull compared to the flattish low rocker bottom means lots of asymmetry. And, if the center of this leeward lift of the asymmetrical heeled hull doesn't correspond with the CLR, then it also fouls up boat balance and induces helm, usually weather, sometimes enough to round some designs up when they heel too far. While there are new designs that actually produce windward lift with heeling (less hull curvature and more rocker, made possible by deep bulb keels, our cruisers ain't them and we are better off with reasonably small amounts of heeling to avoid the leeward lift. Leeway forces of the sailplan are bad enough but when the hull starts lifting to leeward things get outa hand, then leeway becomes great enough to have the keel and rudder tripping up the boat which really adds heeling and things have gone very sour...
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