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MAST BEND - HOW MUCH IS ENOUGH?
By Greg Fisher

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It seems confusing we find our mainsails perform best when set up with a hint of wrinkles... But there are a couple important reasons that these speed/overbend wrinkles need to be apparent in our mainsails. These wrinkles serve as a great guide to ensure our mast is bending properly and enough. If the main is too full it will never allow the boat to "shift" into the next higher pointing and faster gear. Without enough bend in light winds the main would be too full and stall. In heavy winds the boat will simply be overpowered. It is one of those situations where we need to do whatever is necessary shim-wise to help encourage the mast to bend. In some cases different masts need more shims and maybe looser diamonds than others to achieve the same bend and wrinkles.

I think looking at the initial prebend can we a nice way to compare rig settings as a starting point (1/2" to 3/4" seems to be close) but the acid test is the actual appearance of overbend wrinkles-diagonal from the mast and ending halfway back on the boom. They should just be appearing in light winds and quite evident in bigger breeze, but always in the same position.

There are some Thistlers who change their shims as the conditions change. On 3839 we've not found that necessary or desirable. However, hypothetically, if we found we weren't getting enough wrinkle in lighter winds we'd shim more and conversely in breeze if we were developing to long of overbend wrinkles, all the way back to the clew, we'd remove a shim from behind and maybe even stick a shim under the front to restrict the bend. Again, shim to whatever it takes to maintain the "cool wrinkle"guide. On our boat we've had the same 1/8" shim under the back of the mast from MWE to the Nationals and in all conditions and crew weights for that matter. For a sort of a technical explanation, I think that the slight wrinklediagonally in the main tends to help "trip" open the lower leech the breezier it gets. This loaded area from the mast to the middle of the boom helps allow the aft sections of the main to taper off easier and depower the main more...in light winds less tense of a wrinkle will help maintain a rounder back end of the sail which will help maintain power and pointing. It is pretty nice when the mast and sail work together like this as it almost makes the sail "self-adjusting".

Too much bend as demonstrated by the wrinkles running farther aft on the boom over flatten the main and rob the boat of valuable power, too little and the boat never will "wind up" and hit optimum speed and height it's capable of. In regards to the "bumps" caused by the wrinkles, I believe there is a boundary layer in the main and as long as the main shape is fair, these slight wrinkles ( even though they may be a good inch or more deep at times) are not deep enough to disturb this boundary layer and ruin the efficiency of the sail. And as we know, the boat just plain sails faster and higher when the wrinkles are there and doing their job! I am sure this may create more questions...and I will do my best to answer the best I can! Check the images below!

Bryce sailing upwind Many have seen this shot of Byrce Dryden before but he exhibits the "ideal" overbend wrinkles. The light is such that they really stand out - but the length and position are perfect which ensures his butt and diamonds have to be pretty darn close!

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