I won’t pretend to say that I am an expert on this by any means, but I can offer a simple explanation of the basics. At first, I too was mystified about the strange performance aspects of a DN bendy mast. Here is my understanding and experiences:
1) When the mast bends out to leeward, the center of effort (CE) in the sail moves out horizontally from the center line of the boat and down vertically, as the mast compresses. This positions the CE to a better advantage to the offset sailors weight and hull. It’s like the sailor hiking out to windward by equal amounts, and these masts bend out 2 ft. plus!
2) The bendy mast takes the irregular gusts with this sideways bending so the boat has difficulty hiking up on 2 runners. This means more control and speed as a gust hits and smooth, safer sailing to beat! By comparison, a stiff aluminum mast will have you in a hike, then a slam down and possible skid before you know it. And besides, you let the sheet out to get back down too.
3) The masts do not control sail head twist that much. Twist is not as important on an iceboat compared to other sailing crafts, mainly because of lower wind gradient differences from the ice surface to the top of mast compared to soft-water-sailing, choppy conditions. (Many other factors here too about foil shapes, but I am keeping this real simple)Some mast designs do flex more at the top than others to create some twist in the sail, but the primary bending control is aimed towards the lower portions of the sail.
4) Most other aspects of the sail, shape across the chord, ect. are normal like any other sail. Chord shape mostly depends on mast/sail compatibility and actual loft cuts, fuller or flatter.A really bendy mast, with “not so matched, lesser luff curve” will result in a sail getting real flat at full sailing tensions. The mast sucks out the shape. Some sailors have sails cut for this purpose as a high end range cut. Most sails do get flatter under tension no matter what kind of mast you have.
The DN mast has gone through some interesting changes and probably will continue with refinement, but the basic “bendy to leeward” is a groundbreaking achievement and a full departure from any other sailing craft. My hat goes off to the developers of this concept.Maybe this post will bring out the real “techies” for a full blown explanation.
Jeff Brown – Sailmaker, JB Designs US 5232