Mast Bend

[Bill]

The following link is to a discussion of mast bend on small boats (Thistle). Principles do apply . . .:)

http://www.northsailsod.com/class/thistle/thistle_mastbend.html

[ebb]

'Butt tuning, diamonds, shims and overbend wrinkles'...what are regattas coming to? I'm still at the stage where sails are likened to airplane wings and the fabric is more like sheet metal than bed sheet.

How much bend do we want to introduced into an A/C mast? Are we really looking for those UGLY 'wrinkles'? It looks more like the halyard isn't up all the way or the cunningham is loose! I thought SMOOTH and SHAPE was the most efficient...?

[CapnK]

Bill - Thanks, good link for thought-food! One of the pics in particular, looking up the sail from below boom level, made it easy for me to visualize the effects described of mast bend on sail shape as the wind varied. Makes sense. Interesting how they tune the rigs like that, it is similar to some effects that you can get with varying tensions on the sail of a windsurfer - inducing twist or flatness into the sail depending on differing wind speeds and/or conditions/intentions. Sail setup on a windsurfer that you intend to slalom race is far different from what you would use with the same rig going out into surf, for example...

But Ebb! No, I don't think that for our boats you should try to go out and try to crank a nice curve into the mast - and certainly not to the degree you see on the boats in that story! :)

Those boats have relatively bendy, whippy spars, intentionally tuneable/bendable rigs, and rather larger sailplan ratios. Our Ariels have spars and standing rigging originally spec'ed for a vessel with 2/3 again the displacement - you could think of our rig as the proverbial rock... :D

I've read that for an average mainsail, turbulence around the spar destroys effective lift from developing along the forward 15% of the sail, the leading edge. It's probably arguable that our relatively larger spars have an even more deleterious effect - maybe 20%. This area would 'grow' in a gust, extending farther aft along the sail chord when the windspeed picked up. Add to that a slight veer in wind direction which is common in gusts, and suddenly, if your sail was trimmed only for the 'normal' wind with no anticipation for any change, when a gust hits your sail isn't trimmed properly at all.

It seems to me this is what the technique in the linked article is working towards dealing with - having a sail shape which can power up to meet the changes in a gust, de-powering back to a 'most efficient' even if not overall-perfect shape afterwards. You'll note that the author makes a point that the wrinkles should extend no farther aft than midway along the boom; the wrinkles down there are both farther out from the spar than the wrinkles above, and being lower, have less effect than if they were up the mast. They may not reflect the best sail shape for the entire sail, but having them there means that the parts of the sail above them *are* trimmed to make the most of the available average wind as well as the gusts.

Don't know if this'll make sense to anyone else, but it does to me. :D

Thanks again, Bill - I'm sure I'll remember this in the spring once I get the mast back up and start sailing against whatever other boats I can find out on the Bay. :)

[commanderpete]

Good sail trim site, with fancy diagrams


http://www.northsails.dk/UK/fast/default.asp