dagger-board sweep?

[daryl_wilkinson]

I have been thinking about dagger-board positions / sweep lately. And I looking to sweep my elliptical tipped board by around 3˚. This is for a few reasons. To help position the board in the right place without moving the board excessively backward. With the idea that better balance between foil and rig can be achieved across the wind range. To increase the relative leading edge length to enhance lift. To encourage downward directional flow over the board to minimise induced drag and minimal end losses. And to try and increase the cavitation inception speed. As well as practical considerations like weed shedding.

My thinking is based on some sources ( small yacht and craft design - Gordon Trower / and web sources: http://speedsailingdesign.blogspot.com/2007/01/tutorial-article-on-keel-design-methods.html) as well as anecdotal evidence ( looking at fast windsurfing fins etc )

I just wondered what peoples take on this was....

[ross_burkin]

Will has his board raked back as far as the slot would allow (I know he wanted more) due to the fact that the boat will be planing a lot of the time, bow up. I think thats the reason.

[daryl_wilkinson]

yes... that is one of the reasons I was thinking about also.

[Will_Lee]

We thought about 'variable geometry' sweep like many centreboard classes do but abandoned it because of the extra complexity.

Remembering my Marchaj, doesn't more rake make the board seems to the water to be like a  more tapered board stuck out straight down? Small adjustments in rake therefore change a number of properties quite apart from the balance issue and the slight change in section.

Hmmmm. Suddenly sounds like a really good idea!   

[daryl_wilkinson]

There's a bit about that trickery here: http://www.sailinganarchy.com/YD/2005/dealonkeels.htm

"The sweep angle is the amount of fore/aft rake in the keel, and the taper ratio is the ratio of the root chord to the tip chord. The two are altered together in order to maintain “elliptical loading”, an aerodynamic term referring to the ideal, highly efficient distribution of lift on a true elliptical foil. So what the hell does that mean? Well, an elliptically shaped keel may not be practical for several reasons, so the shape may need to be distorted to correctly balance and trim the boat. Fortunately, the foil can be “tricked” into thinking it is elliptical with the right combination of sweep angle and taper ratio. For instance, if the keel is swept aft 20 degrees, then it should have a 20% taper ratio (tip is 20% as long as the root) in order to maintain elliptical loading. Unfortunately, a 20% taper ratio is not ideal for stability reasons, as it raises the vertical center of gravity of the keel, so a compromise must be made to increase the taper ratio and thus reduce the efficiency."

Obviously this is with relation to keel boats, but if you are not using an elliptical foil for other reasons you could sweep it to maintain elliptical loading.

There's more on it here: http://www.tspeer.com/Planforms/Planar.htm

I must admit some of this starts to get beyond me - having spent my further education dealing with ART and not Physics but I feel that sweep angle to a point is has a lot of positives. Not least the likely-hood of increasing lift without the need to increase righting moment.

[Phil Alderson]

Some random thoughts on sweeping foils

As we are unrestricted about where on the centreline to put the centreboard, there is no need to sweep the board to balance the rig as long as it is put in the right place.

If the board is swept aft as you pull it up in strong winds the clr will move forwards giving a smaller decrease in weather helm than a vertical foil(assuming the rudder is sharing the side load)

Our foils work in a very different flow regime to yacht foils, which are tilted 10-30 degrees away from the vertical so there is a flow down the foil from root to tip. Ours are vertical and much higher aspect ratio so the only downward flow experienced is due to higher pressure on one side than the other, this then leaks around the tip. This is tiny compared to yacht keels which is good as a downward flow increases the tip vortex and so the drag..

We make much less leeway thanks to much higher aspect ratio foils.

We are a long way away from cavitation speeds, we get ventilation but not cavitation.

By sweeping the foil you get an effective increase in chord length and decrease in thickness and also a slight reduction in aspect ratio, but you could just make your foil in the shape you want as there are lower structural and no ballast issues to worry about.

[daryl_wilkinson]

This is quite an interesting bit of information http://www.culnane.net/dc/sailing/moth/rake/index.html which suggests that 3˚ of rake is an optimum, certainly for, as then low rider Moths sailing with a 5˙ angle of mast rake. Obviously raking the foils to high values isn't a good idea but I'm interested in a small value, as from sources I have found seem to suggest the lowest levels of drag at this amount of sweep. What complicates this ( and is the reason for my thinking ) is that my boat is a little different to all other Cherubs so far in that it will be sailed in different rig set ups and has two mast positions. I am thinking that a compromise of 3˙ makes sense across a range of wind conditions, as a variable foil system ( as Will already suggested ) would be quite complicated ( I was originally thinking of different angle boards ( straight foil and raked foil ) in a raked slot ). Given that you can use the water line to aid lift in the light by keeping the stem in and you tend to sail with a little heal also, the marginal lack of lift from a 3˙ raked foil I guess will be compensated for, with the reduced drag being beneficial. And as it is better to start to increase mast rake as the wind gets stronger the rig will be better balanced as the wind increases. And as I have found sources that suggest a slight rake is optimum I started to see how I could configure the sets ups without increased complication and cost, and seemingly without performance loss.  And then there is the introduction of the rudder 'T' foils with variable rake. You can see why I asked the question!