gofaster said..
Fantastic picture!
I have been experimenting successfully with flexitip masts to relieve pressure during strong gusts but maybe I need to revisit the real bendy mast...
I wish the photo were mine! I don't even know who took it; it's been in the DN class archives for many years...
The DN rig is unique (to my knowledge) in how it uses mast-bend.
I just wanted to show one (extreme) example to get people thinking about bendy parts.
The Finn rig is probably a better analog to the typical unstayed landsailer rig.
Another place to look for inspiration about chassis flexibility is the go-kart racing community. In many cases they use frame twist/flex like a suspension.
Most sailboats and landsailers allow the tip to bend to leeward which increases twist in the sail and decreases the angle-of-attack at the top of the sail. This reduces the lift at the top of the rig (i.e. moves the center of effort lower). There are even some rigs that allow the top of the sail to generate "negative" lift (i.e. pushing the mast to windward) to move the CE even lower.
It's the opposite effect in the DN, the mast-tip is actually "hooked" to weather which keeps the top of sail in-line with the mast (i.e. reduces/eliminates twist). Usually when you look at a DN directly from the front or back along the boom the sail "disappears" because there is no twist and you are looking directly along the sail from the leach to mast (the boats in the photo look like they have much more twist than normal these days).
The bend in the mast and plank both contribute to effectively lengthening the side-stay and angling the mast to leeward. The curve of the mast/sail reduces the amount of spanwise flow (i.e. "spilling air") so the forward drive of the sail does not reduce much. However, the side-force vector now points down reducing the tendency to hike and putting more force downward on the leeward runner to keep it from sliding sideways.
gofaster said..
Question - in this yacht design are you allowed any mast? ie is this mast used because its found to be the best or is it because you have to?
The mast bend is not controlled by the class rules, only the length, weight, cross-sectional dimensions, and material are proscribed in the rules.
Super-bendy composite masts are the result of observations that wood and aluminum masts were "fastest just before it broke".
There was a period in time before the rules were changed to allow composite masts when breaking a mast during a race was a commonplace occurrence as people tried to balance the speed potential of bend against the strength limitations of wood and aluminum.
The rig is rreally the result of the antiquated sail dimensions. There is way too much sail-area for the size of the boat and no effective way to reduce it within the rules, so so sail and rig-development over the past 30 years or so has centered on controlling the power from the sail and reducing the drag.
Cheers,
Geoff S.
