Relationship between keel shape and rig balance question.

I am thinking of making a new daggerboard for my boat for a variety of reasons and I want to know how the foil shape I choose will affect the boat's rig balance as in weather helm or lee helm. Currently I am using a NACA 0013 shape and after reading about NACA five digit 6***** series, it sounds like the point of minimum pressure can be moved depending on the foil shape. If I were to choose a foil where the minimum pressure is further aft than the currrent foil would it tend to correct some weatherhelm I have? Am I completely off base here or is this correct thinking? Thanks Todd

 

Your thinking is correct but changing keel section will have a minor consequence, like a few percent of chord length, on the line of lift. There are likely to be easier and more effective means of reducing weather helm.

There is some discussion here that can provide ideas:
http://www.boatdesign.net/forums/showthread.php?t=18926

If you are thinking about appendage changes then a smaller dagger and bigger rudder, if permitted, will bring the centre of lateral resistance further aft. Note though that a stern hung rudder is not quite as efficient as a daggerboard beneath the hull.

Rick W.

 

Thanks for the info. I should clarify my thoughts. I am thinking of replacing the daggerboard primarily make it lighter. It is a steel fabrication filled with lead and I am looking at a bulb on the end of the fin that should give the same righting moment with less overall weight. Also being steel even though it is epoxy coated the rust is problematic. The weather helm isn't severe so I was thinking that all else being equal I might choose a different foil shape to help correct it. It sounds like it can't hurt. Thanks Todd

 

How does the foil section improvement do more than increase lift? A small amount of weather helm is normal, but in any case, the position of the dagger is fixed and unless you either angle the case or shift the case aft, the weather helm won't be affected. Forward raking of the mast may be a far more effective means to reduce weather helm, and not much forward-tipping would be needed going by what you've said.

Alan

 

Alan
If you open the Applet on this site:
http://www.mh-aerotools.de/airfoils/javafoil.htm
Set the Options to suit water you can play around with different foil shapes.

You can look at the pressure distribution or go to the analaysis page and look at the moment. You will see that you can get a larger negative moment by moving the position of maximum chord backwards. This is indicating that the centre of pressure moves backwards.

The effect is quite small though. It is also likely the operating range of the foil will be reduced.

If the foil is being completely redesigned you could alter the shape so the leading edge is further aft. There should be no need to have it aligned with the frint of the supporting case.

Rick W.