Dagger Boards --- I don't quite get it

[David M]

Originally Posted by Limpet

Forgot I started this thread long ago.

Aren't most dagger board shapes symmetrical...in which case there is no lift because the water flowing over the surfaces on both sides is moving at the same speed.
An airplane wing provides lift because the air travels faster over the longer top surface thus creating a low pressure on top surface of the wing relative to the pressure below the wing.

This makes for the most efficient shape but it is the angle of attack that determines the amount of lift. You could take a sheet of plywood, give it an angle of attack and it would produce lift, no flat and rounded side necessary.

A rounded backside helps to preserve the laminar flow, without it there is a greater inclination for there to be a vortice in the same area if it were a flat backside. A laminar flow creates the greatest pressure differential, for the amount of drag, between the low pressure and high pressure sides of the foil.

Originally Posted by ElGatoGordo

Not sure what you're asking. I haven't heard of asymmetrical boards. It would only generate lift on one tack...sounds like a pain to me.

An asymetrical board with the high pressure side being on the rounded side would still generate lift, otherwise an airplane would never be able to fly inverted. Doing it this way is just less efficient with there being a lot more drag.

[ElGatoGordo]

Originally Posted by David M

This makes for the most efficient shape but it is the angle of attack that determines the amount of lift. You could take a sheet of plywood, give it an angle of attack and it would produce lift, no flat or rounded side necessary.

Hence:

[dan360]

It's worth noting, that depending on who you ask you'll get different recommendations on how to use your boards.

Windward board down when sailing upwind for safety first, OR
Leeward board down when sailing upwind for performance first.
Some of both boards down when sailing downwind for stability.

I have a table from VPLP with recommendations at each angle/windspeed combination for what do do with each board (full down, full up or partial), but Outremer's recommendations are in some cases opposite with a preference for safety.

[Steve_C]

Chris White Atlantic 57.

[dan360]

On that Chris White picture - which hull is that? There's an explicit requirement with that asymmetry which tack to use which board on. Looking at the picture it is Port? (as the lifting lines presumably lead to the cockpit).

[Steve_C]

Yep, sorry I meant to state that. port side with the bow to the left of my feet. very clearly these boards are meant to be used one at a time.

[Tar34]

Rethink the obvious....You want lift? You got lift. Less drag? That too! No it's not Broken

[ElGatoGordo]

There you go. I'm way to lazy for that, but out would clearly be a race/performance advantage

[44'cruisingcat]

Originally Posted by Limpet

I can certainly understand that the Dagger Board approach yields greater speeds sailing down wind when the boards are up, but I don't quite understand the claim that the boards allow closer pointing to the wind.

With the Main sheeted all the way in, as you put the bow closer and closer into the wind, there is a smaller and smaller component of force pushing the Cat forward. The Main does not know if the Hull has a Keel Or Dagger boards and the forward forces are the same.

So how is it dagger boards can yield closer pointing? Is it simply the reduced resistance thus the component of forward force is more effective? If this is the case, then I don't see how Monohulls can point closer, as they have a heavy displacement will lots of resistance to motion.

Getting back to the original question, dagger boards won't neccessarily let you POINT higher. What they do is let you SAIL higher by reducing leeway much more effectively than mini keels.

[cal40john]

Originally Posted by 44'cruisingcat

Getting back to the original question, dagger boards won't neccessarily let you POINT higher. What they do is let you SAIL higher by reducing leeway much more effectively than mini keels.

And high lift to drag devices (daggerboards) allow you to have much less surface area, smaller area board is as effective or more effective than much bigger surface area keel, which means less drag, higher speed and so better progress to weather overall.

[Tar34]

Originally Posted by cal40john

And high lift to drag devices (daggerboards) allow you to have much less surface area, smaller area board is as effective or more effective than much bigger surface area keel, which means less drag, higher speed and so better progress to weather overall.

Sparkman Stevens applied the high aspect foil in it's Tartan 34C design