Sail Plan Boom Angle

[kenwstr]

Hi,

I sail an MG14, Australian 14 foot fractional sloop development class dinghy.

My rig is old and my mylar main beginning to crack up. I am planning to modernise the rig with an over rotating wing mast and get new sails for next season. One of the questions is boom height, obviously the lower the better. I think I need 750 mm between the boom and floor at the skipper position so I can tack OK. I am mostly sailing single handed now and am interested in developing a viable single hand option. I can go lower at the tack and higher at the clue while maintaining the same boom height and heeling moment. This would give a lower span load and reduce induced drag due to the longer luff. I am told that where you have to have a gap in a lifting surface, such as the main foot, it is better to raise the clue to reduce the tip pressure leakage (induced drag again). You can see this in designs like the Musto Skiff. There seems little doubt that raising the clue above the tack is more efficient allowing area to be distributed more efficiently in the sail plan.

The question though is how much is optimal for performance?
Can anyone run aerodynamic modling on this or know where I can go with it?

I guess like anything optimised, there would be little variation near the optimum point so even a ball park rule of thumb answer would be better than I have now.


Regards,
Ken

[Doug Lord]

Here is a rough sketch of the solution I'm experimenting with on my new boat: approximately the forward third of the main will kiss the deck. To do that I'm using a modified wishbone boom that allows for mid-boom sheeting.
It would use dual vangs...

[PI Design]

Hi Ken,

Short answer - get the foot as low as possible and the main take off from the boom (roughly half way along the boom?) at about 750mm above the floor.
Some other things to consider:
- The more the boom slopes up, the less effective the vang is (unless you use a ram vang, difficult on a wing mast)
- On a restricted sail area class, like the MG, shoving area in a little triangle down low may not be the best use. Granted, the aero efficiency increases, but the wind speed is less down there and the short horizontal chord of the sail in that area will not be efficient.

[kenwstr]

OK, the restrictions are on area and height above keel or deck.
Any double layer of sail is counted twice for area so the 29er style solution proposed by Doug is not an option. The sail must be set between the mast and boom. Some crews may be agile enough to go quite a bit lower but in my 50s 750 mm is about as low as I think I can reliable go. However this is only necessary at about the mid foot position. I can go lower at the gooseneck which would extent the span and reduce span load. I could for example take the gooseneck down to the desk, then the clue would be quite high. I take the point about the vang but I can just put the vang attachment points further along the spars. It is a known aero theory that raising the clue above the tack is more efficient than a level foot. A level foot means the entire foot is subject to tip loss. A level foot also has no camber and the aft region is in turbulance so that is a lot of very inefficient ineffective area. Raising the clue gives camber to the entire area as the foot becomes a trailing edge rather than an uncambered chord.

The only unknown is where the optimum point is.

I am really after value for this optimum. What I really need is access to some aerodynamic modling program or someone who can run an analysys. Anyone know where I go for that? Once the optimum is known I can make decisions about practical compromises. Without numbers, no logically reasoned decision can be made.


Regards,
Ken

[Doug Lord]

By double -layer do you mean sock luff? There is no reason you couldn't use the wishbone/low main solution on a non- sock luff design-unless you mean that the foot must be in constant contact with the boom?

[tspeer]

Hi,
...
The question though is how much is optimal for performance?
Can anyone run aerodynamic modling on this or know where I can go with it?
...

http://www.tspeer.com/Planforms/Planar.htm
http://www.tspeer.com/DesignTools/vortex95.xls