Whyfor : Forward central daggerboards ?

Boo2

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Question as title : was idly browsing the web and came across an IMOCA 60 that had one of these and wondered what it is for ? Obviously (?) an FCD has the effect of moving the centre of lateral resistance of the hull forward and so will increase weather helm, but can't the designer just achieve the same effect by moving the keel forward ? Or is there another reason behind them ?

Boo2
 

Motor_Sailor

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All keels are a significant proportion of the all up weight of the boat, more so with race boats. So the ballast element has to be placed close to the centre of buoyancy for it to trim correctly fore and aft. As a result, you don't have that much scope for moving the keel fore and aft.

However the centre of effort of the sail plan in various configurations can vary enormously. The dagger boards help balance this and provide lateral resistance when the keel is fully canted.
 

flaming

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Question as title : was idly browsing the web and came across an IMOCA 60 that had one of these and wondered what it is for ? Obviously (?) an FCD has the effect of moving the centre of lateral resistance of the hull forward and so will increase weather helm, but can't the designer just achieve the same effect by moving the keel forward ? Or is there another reason behind them ?

Boo2

IMOCAs have canting keels. So you do not rely on the keel for any lateral resistance, the daggerboard (or more usually daggerboards) do that job. So the daggerboard is the centre of lateral resistance that the designer will be working to.
 

Dino

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On a canting keel boat it is known as a Canard. The main reason is to offset the increased leeway. The Cookson 50 was originally designed with a trim tab on the back of the keel to reduce leeway but it wasn’t very effective. The successful Cookson 50’s all had canards.
 

markhomer

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why dont they stick end plates on the canting keels , they could even articulate them and produce down force , think they are missing a trick here .
 

flaming

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why dont they stick end plates on the canting keels , they could even articulate them and produce down force , think they are missing a trick here .
They don't want downforce, they want lift from the keel fin.
It seems counter intuitive, but in the latest IMOCAs the keel pin is inclined slightly so the front is slightly higher than the back, so you get a positive angle of attack once it is canted. So the fin acts as another lifting foil to windward to balance out the foil that's to leeward.
IMOCAs rules also state that any foil can only have one axis of movement, so a keel with a flap on would not be allowed to cant. By inclining the keel pin you can get the same effect with only one axis of movement.
 

Foolish Muse

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When I wrote my singlehanded tips book, I interviewed the designer of an IMOCA 60 and we discussed this in detail. The answer is that the dagger board actually twists the sailing direction of the boat into the wind.
As you know, most sailboats crab to leeward slightly. This is how the keel provides lift to the boat. Adding the dagger board twists the boat so that is no longer crabbing to leeward, but is shooting straight through the water "like a dart". Obviously makes the boat point much higher.
 
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