Why leave a dagger board partly up?

I'm new to sailing catamarans so I've been watching a lot of videos on line. I've noticed a lot of boats sailing with their center boards half way up. A part from a lack of water depth to handle the extra draft why would one leave the dagger board partly up?

I don't sail a boat with dagger (or center) boards but the purpose is to reduce drag when downwind sailing.

Happy Sailing,

David

It not only reduces drag, but also reduces the tendency of the boat to "trip" over its boards when flying the spinnaker.

In heavy air, if the boards are left all the way down, they produce too much lift, making the boat hard to handle.

I understand and agree with all the answers, but this last one about lift I don't understand. Can someone please enlighten me on that. What is the "lift" you are talking about?

Lift in this instance is not vertical, but (nearly) horizontal.

Any foil moving though a fluid with an angle of attack > 0 produces lift. The amount of lift is proportional to the area of the foil. By definition, the direction of the lift vector is perpendicular to the span of the foil.

For catamarans such as the Tiger, the foil span is the long dimension of the daggerboard.



In the image above the "Induced angle of incidence" is the angle of attack. For catamarans, this is less than 5 degrees.

I see whatyou are saying. Though in this case you have a fully symetric foil so the only lift is as a result of lateral (in this case, or falling in the case of an aircraft). If that is the case then you will reach an equilibrium where the lift equals the force (the force that is pushing the foil lateraly inducing the lift). The part I don't get is how that makes too much lift and how that makes it hard to handle.

I thought the boards on the cat were along the CL axis. If that is the case you are saying the side slip induces a 5 degree (more or less) incidence angle, do I read that right?

Nice diagram and explanation BTW.

If the boards were not aligned fore-aft, they would cause too much drag (Nacra's F18 boards twist in their case to add more angle of attack, somehow, I don't get it). Downwind, the boards are "lifting" in the wrong direction, as well.

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Yes - although I would venture to say that 5 deg is the maximum.

To explain the "hard to handle" aspect:

The amount of lifting force from the daggerboard is proportional to the incidence angle and the area of the foil (at small incidence angles).

For a fixed lifting force, you can either have a large area/small angle or a smaller area / larger angle.

When headed downwind, the incidence angle is small to begin with, since not a whole lot of lifting force is required to keep the boat from side-slipping.

With the board fully down (large area), very small changes in the incidence angle produce large changes in lifting force. These large dynamic swings are what makes the boat hard to control, because you are trying to balance the rig forces to the hull/foil forces by steering and/or sheeting (essentially changing the incidence angle of the sails).

With the board partway down, changes in the incidence angle do not produce large dynamic swings in the lifting force, therefore, it's easier to balance the rig forces without having hair-trigger reaction times.

Jibing boards are a whole other topic. Suffice it to say that they are designed to "twist" in their trunks to produce more lift at a lower incidence angle. You could accompish the same thing with an asymetrical daggerboard.

Here is my attempt. Lift is created by the lateral motion of the foils in the water. The later motion is caused by the sails trying to push the boat sideways. It does not matter if the foils are symetrical on both sides becasue the leading edge is not the front of the foil, but instead is off to one side slightly do to the lateral movement of the boat in the water. This is the same principle of a keel on a keel boat. So how does this all relate to raising the board when off the wind? Lets put it in an example. The boat is sailing downwind on the port gybe. This is when the wind is coming from the left side of the boat. The wind is pushing the boat to the right, the centerboard is generating lift and trying to push the boat to the left. When you reach the velocities that Tigers and other spin boats go down wind you end up with a lot of pressure. The sail is creating lift to the right causing the mast to want to go to the right while the daggers are creating lift to the left, causeing the bottom of the boat to go left, net result it wants to flip. In heavy air this can create a wild ride that can be felt in the helm. We some times call it tripping as in "the boat feels like it is tripping on something", that something is the dagger, so you raise it to reduce that tripping.

Another reason to raise the boards down wind is to promote slipping. When going downwind the leward slip is only bringing you closer to your mark, so in this case it is a good thing. Upwind side slip is taking you away from your mark, so then it is bad. I have seen some I20 sailors raise the board part way when going up wind in very heavy conditions, I suspect to help keep the boat stable.

I don't have a niffty diagram, so now you understand it, or I have confused you even more. Hope it was the later.

Thanks to both of you for the clarification. I knew there was more to this, and the verbal explanation of the other vectors involved was just waht I needed.

Now all I need is for the water around here to get a little softer (then the solid form it has now).