An interesting posting appearing in the "general Anarchy" forum "Sailing Anarchy"
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The repeated comment that is TOTALLY off base is that pre-bend has anything at all to do with how much the unsupported top of a rig falls off to leeward in gusts. The two concepts could not possibly be more orthagonal. If you want to argue that point, you just don't have a good grasp of why tubes bend, and how much they bend. The amount they bend to the side is totally independent of how much they are bent fore-and-aft. Totally!
Bethwaite has it right, in that a total system of mast+spreaders+boom+shrouds+sail works together in a coordinated manner. He points out that when a gust comes along, the increase in load on the leech of the main will cause the mast to bend more, and when you've got swept spreaders, then the distance from the chainplates to the hounds decreases, the rig falls to leeward slightly. As the hounds are only part way up, then a smooth curve develops to the masthead.
However, there ain't nothing there about needing pre-bend to make that happen effectively.
There are two reasons for pre-bend:
1) Pre-bend stabilizes the rig (this was mentioned above). The full and correct answer is:
When the full set of 'wires' (forestay, inner forestay, backstay, runners, jumpers, shrouds, etc.) and the full set of sail forces (luff tension, luff curve, batten pressures, outhaul, downhaul, cunningham, etc.) and the stiffness of the mast tube and spreader mountings all together balance forces such the mast has a consistent curve, then obviously you've got a bunch of forces working in many different planes, and the sum of the forces against the stiffness of the rig defines one curve. Very completely, in 3 dimensions.
If there is no (or little) pre-bend, then the mast will noodle around somewhat until it gets forces building up from some rigging or another against the stiffness of the tube. By bending first, you are already at that stressed state. You have essentially removed the slop from the complete mast system. The slop in the system consumes energy, instead of having that energy go into forward motion. So you go slower.
2) Main sail flattening (way off base in the answers above -- the unsupported mast tip falling off the leeward is totally independent of prebend). There are two correct answers here:
2a) On a rotating rig, the prebend causes the main to be flatter when the mast is rotated less, and you rotate less upwind and as the wind increases. The tip falls to leeward better when the mast is rotated very little too, causing the main to depower at the top. Also, the lift-drag characteristics of a rotating mast mainsail system is more tolerant of under rotation than over rotation, so you are adding some width to your groove, so you can better sail in seas and gusts and the larger boat speed changes you experience on a multihull in heavy air. You are erring to the side of low drag, over high lift. The right thing in a breeze.
2b) A pre-bend rig should always have somewhat more curvature towards the bottom of the rig. This means that when you reef, you are moving the main down and flattening the main due to the increase in mast bend. Not that anyone reefs anymore.
2c) On a non-rotating rig, the prebend is an automatic draft control mechanism for your mainsail: As you sheet out, as on a reach or downwind, the main gets fuller. Sheet in, and it gets flatter.
Like most things with boats, the value of pre-bend is not absolute, or even all that good. Pre-bend works within certain constraints. Outside of those rules, different approaches are far better.
For example: if you can have lots of smart crew, and there are no rules about the amount of adjustability of rake, then you really, really do NOT want pre-bend on a monohull. You are far, far, FAR faster with running backs with checkstays and lots of headstay length adjustment -- as in many feet! And no swept spreaders! Like an AC boat.
Or if you really want speed, go with a full wing mast.
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