How Much Prebend?

Author: Tof Nicoll-Griffith. Reprinted from The Firezone, 1993
On my first Fireball, KC6884 there was an old golden Proctor D section, a stiff mast with little skinny spreaders. I had no idea what the spreaders did except that if you didn't have them it wasn't very good. I also recall looking up my mast track when the rig was loaded and seeing the mast invert (bend forward) but I really didn't know if it was a bad thing or what effect this had on my main. Despite my ignorance I managed to have some success in racing, but I soon reached a point when I was no longer rising in the fleet.

My real breakthrough in Fireball sailing was when I learnt what to look for when you're looking up the mast and then later, what to look for in the mainsail as you sailed. By adjusting my spreaders and strut I saw what changes took place in the depth and draft position of the main. Through racing and experimentation I found out what mainsail shpaes were best for various conditions. Knowing how to adjust your mast so it fits your main properly is imperitive to improving your performance on the racecourse.

To understand mast tuning one must know a little about how a sail is designed and what gives it its shape. Seam taper and luff curve are the two main components to sailshape and depth. Seam taper (also known as broadseaming) is the depth built into the sail. Seam taper has a percentage depth (percentage of chord length), a maximum draft position source of the shape you see in your main when you are sailing. When you put a positive luff curve on a straight mast the extra cloth (beyond the straight luff) is cloth added to the depth of the main. Most of added shape appears in the luff. How much luff curve is the question most often asked by sailmakers.

If little is there and the mast bends more than the luff curve, over-bend creases (wrinkles from the luff to clew below the spreaders to the gooseneck) appear because the cloth is being pulled out of the sail. As a result the sail becomes much too flat off the mast and insufficient power is generated for most conditions. If too much luff curve is designed into the sail then the maximum draft remains too far forward for most mast settings. When the luff curve is right hten it is up to you to set your mast up with the proper prebend to maximize performance. Base prebend is the aft curvature the mast has when the rig is under load. This prebend is caused solely by the length and angle of your spreaders. If you have a strut it should be neutral when setting up base prebend. For many current designs base prebend falls within 2.5 - 6 cm. Your sailmaker should be able to tell you where to start but from there you should fine tune your rig (prebend) though angle and the length of your spreaders and try different set-ups to see what works best.

Sailshape diagram

To find proper prebend settings, both the sailor and sailmaker face similar problems. Too much prebend (ie: over bend the mast) and you begin to make the mainsail entry too flat. Too little prebend and the maximum draft is too far forward and the main is very deep. For the Fireball we want to see our maximum draft near 45% aft. Remember that these prebend measurements are without strut influence. This, of course, is only the beginning of mast tuning because for every different kind of wind and wave conditions one can always make small adjustments to their spreaders to optimise performance. Once you've got the prebend for the conditions you are going to sail in you can start to use your strut when you're out on the water (because you can't reach your spreaders) to make adjustments as the conditions demand. Your strut does make some mast adjustments (especially down low) but does not give the same prebend distribution as the spreaders, however this is better than nothing.

I hope this will help you move closer to the front this summer.


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