In years past, I raced Snipes, Lightnings, other one-design boats. Whenever we had the wind aft, we raised the center/dagger board to decrease drag and increase performance. With the wind dead astern, we raced with no board down at all. So now I wonder about the performance of various keel boats that come in shoal and deep keel models. No one seems to disagree that the deeper keel boat will outperform the same boat with a shoal keel when the wind is forward of beam and the keel provides lift. But what happens when that same deep keel begins to add to drag when the wind moves aft? Is it possible for the shoal keel boat to outperform the deep keel boat in such situations since it's keel present significantly less drag going down wind?
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Significantly less?
That term significantly less is the key. On the H37C can the thirteen inch difference in keel depth be considered "significant"? As a percentage of total wetted surface I think not. And the keel is fatter and weighs 300 pounds more. Not sure how those factors figure in the equation.
That term significantly less is the key. On the H37C can the thirteen inch difference in keel depth be considered "significant"? As a percentage of total wetted surface I think not. And the keel is fatter and weighs 300 pounds more. Not sure how those factors figure in the equation.
Drag
I'm not a racer and I'm not sure I understand how a keel adds lift but I know a little about drag. There are 2 types. Pressure drag is a function of the surface area projected by the object perpendicular to the direction of travel. The more you slope a windshield the more you reduce a car's pressure drag. Friction drag which is far less significant is a function of the total surface area of the object in contact with the fluid through which the object is traveling. When the wind is behind, the keel produces only drag. In any other direction however the desire to reduce drag must be balanced with the need to transform the wind's energy into forward motion for the boat. The most efficient design for a racer would seem to be a deep slender keel with little projected surface area. The longer it is front to back the more friction drag it will produce but again this is less significant (although it becomes more significant in turns). The length needs to be proportional to the boat's length. A shoal keel makes shallower areas more accessible to cruisers like me.
I'm not a racer and I'm not sure I understand how a keel adds lift but I know a little about drag. There are 2 types. Pressure drag is a function of the surface area projected by the object perpendicular to the direction of travel. The more you slope a windshield the more you reduce a car's pressure drag. Friction drag which is far less significant is a function of the total surface area of the object in contact with the fluid through which the object is traveling. When the wind is behind, the keel produces only drag. In any other direction however the desire to reduce drag must be balanced with the need to transform the wind's energy into forward motion for the boat. The most efficient design for a racer would seem to be a deep slender keel with little projected surface area. The longer it is front to back the more friction drag it will produce but again this is less significant (although it becomes more significant in turns). The length needs to be proportional to the boat's length. A shoal keel makes shallower areas more accessible to cruisers like me.
Shoal keel will outperform better down wind....
unless the boat is in 'survival' mode conditions, then its (sometimes) easier to broach the shoal keel. With a shoal keel (downwind) less skin friction (less wetted surface), less projected area drag (difference in frontal area of the keel).
unless the boat is in 'survival' mode conditions, then its (sometimes) easier to broach the shoal keel. With a shoal keel (downwind) less skin friction (less wetted surface), less projected area drag (difference in frontal area of the keel).
Rich,
Your parameters would be valid for boats with identical keel forms except for depth. But in the real world that doesn't happen. A shoal keel is heavier to make up for the lost lever-arm moment. That means it takes up more space under water and invalidates most of your treaties. I think that in the real world the shorter keel looses on all points of sail.
Your parameters would be valid for boats with identical keel forms except for depth. But in the real world that doesn't happen. A shoal keel is heavier to make up for the lost lever-arm moment. That means it takes up more space under water and invalidates most of your treaties. I think that in the real world the shorter keel looses on all points of sail.
Numbers
My Ericson 23 has a shoal keel and a centerboard that has a decently shaped foil. With the board up, the boat draws 2'; board down, 5'. I have checked her speed off the wind and ddw and generally see that with the board up she gains 0.2 to 0.4 kts. This is not particularly significant for pleasure sailing, but makes a BIG difference when racing. There is 1300 lb of lead in the shoal keel, and the boat is still very stable with the board up.
My Ericson 23 has a shoal keel and a centerboard that has a decently shaped foil. With the board up, the boat draws 2'; board down, 5'. I have checked her speed off the wind and ddw and generally see that with the board up she gains 0.2 to 0.4 kts. This is not particularly significant for pleasure sailing, but makes a BIG difference when racing. There is 1300 lb of lead in the shoal keel, and the boat is still very stable with the board up.
retractables change the problem
As Bob indicates, having a fully or partially retractable board of some kind changes the dynamics. The centerboard and swing keel boats should have the ability to reduce the most friction, all other things being equal. A fixed shoal keel with no accessory board probably has wings or extra weight that hurts its performance downwind; if the shoal keel was designed to be used in conjunction with a board, those problems may be less (because the designer used less weight in the fixed part, for instance). The use of swing keels on traditional schooner hulls became a hallmark of American racing yachts in the late 19th century and is believed to be responsible for our America's Cup victories throughout that period. The turn of the new century saw the rise of narrow-beam sloops with fixed, weighted keels.
As Bob indicates, having a fully or partially retractable board of some kind changes the dynamics. The centerboard and swing keel boats should have the ability to reduce the most friction, all other things being equal. A fixed shoal keel with no accessory board probably has wings or extra weight that hurts its performance downwind; if the shoal keel was designed to be used in conjunction with a board, those problems may be less (because the designer used less weight in the fixed part, for instance). The use of swing keels on traditional schooner hulls became a hallmark of American racing yachts in the late 19th century and is believed to be responsible for our America's Cup victories throughout that period. The turn of the new century saw the rise of narrow-beam sloops with fixed, weighted keels.
Displacement Hull vs. Flat Bottom
I would think a sailboat designed to plane will do so quicker downwind with the board up. On displacement boats such as my C&C 33 MKII, Canadian Yachts Mag. evaluations reported better upwind performance with the fin, and no noticable improvement downwind with the board up on the CB model. Also, I've heard sailors may experience noticably more leeway with shoal draft keels.
I would think a sailboat designed to plane will do so quicker downwind with the board up. On displacement boats such as my C&C 33 MKII, Canadian Yachts Mag. evaluations reported better upwind performance with the fin, and no noticable improvement downwind with the board up on the CB model. Also, I've heard sailors may experience noticably more leeway with shoal draft keels.
Just one thing,
This is dated, but racing rules required all underwater stuff that moves, to be down and locked. But I don't know if that's changed.
This is dated, but racing rules required all underwater stuff that moves, to be down and locked. But I don't know if that's changed.
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