Getting your rig right

Andy Rice advises a Fireball sailor on the mysteries of prebend and mast rake

Fireball sailor Ralph Gillam wants to know: What is the difference between prebend and mast rake?

Prebend and mast rake are two very different things but also very inter-related factors when setting up your rig for optimum speed.

Prebend is the amount of bend in your mast after you have pulled on the rig tension. Mast rake is simply the term used to describe the angle that your mast sits fore and aft in the boat with the rig tension on.

Measuring prebend

To measure prebend, pull on the rig tension to the amount specified by your sailmaker or an acknowledged expert in your class. For the Fireball, 400lbs is a typical setting. The Loos Tension Gauge is considered the benchmark tool for this job, and is available from most chandlers. On a conventional dinghy like a Fireball, make sure there is no tension in the deck-level mast control, whether it be a mast strut or wooden chocks. The purpose of measuring prebend is to measure the amount of bend the spreaders are applying to the mast, without other factors coming into play.

Next, take the mainsail halyard and tie it off tightly to the gooseneck fitting on the aft face of the mast. The halyard needs to be taught for accurate measurement, but not so taught that it affects the bend of the mast.

Now roll the boat over on to its side so that you can measure the 'deflection' at spreader level. This means using a tape measure to record the distance between the back of the mast track at spreader level (bottom set of spreaders on a boat like a 14 or RS800) and the tensioned main halyard. For a Fireball dinghy, a typical amount of prebend at spreader level would be between 20-25mm. 2001 world champion Stevie Morrison uses 22mm. When the boat is on its side, get somebody to support the mast at hounds height, rather than letting the tip sit on the ground, as this could induce unwanted side bend in the mast and throw your measurements out.

Measuring rake

This time, shackle your tape measure onto the main halyard and hoist the halyard to the top of the mast. Check with your sailmaker's tuning guide what the reference point on the back of the boat is for measuring rake. On a Fireball or a 470, this is usually the top of the transom. On a modern-style dinghy with an open, self-draining cockpit, the base point can either be the centre of the transom bar or the lip of the transom itself, so check you are measuring to the right point on the boat.

Once you have ascertained where to pull your tape measure to, pull it taught - again without pulling so hard that you are inducing extra bend in the mast - and take the measurement. On a Fireball, a typical medium airs measurement is 22'5. Generally, a sailmaker's tuning guide will come with a range of rake settings - say 22'7 for light airs, 22'5 for medium, and 22'3 for strong winds. So the windier the weather, the more you angle your mast back in the boat. In general terms, having a more upright mast puts power in the rig, and raking it back helps depower the rig.

Prebend and Rake working together

You will find your prebend measurement changes for different rake settings. The more rake you apply, the more bend the spreaders will induce in the rig, which helps flatten off the sails - just what you want for strong winds. So you are achieving a win/win here, without needing to alter your spreader angle. For this reason, your spreader angle shouldn't need to change too much through the wind range, with the possible exception of really light winds.

Here, with the mast upright, your normal spreader angle will be holding the mast at its straightest, which puts lots of shape in your sails. In any breeze where you are beginning to come in off the side deck and easing mainsheet to promote airflow over the sails, you may also need to increase prebend by increasing your spreader angle by, say, an extra 1cm on a Fireball. This will help flatten your sails and promote airflow in the light winds.

Rig settings in the real world

Sailmaker's tuning guides are a great start in helping you get your boat properly set up, and unless you are confident in what you are doing, it is not wise to veer too far away from the accepted wisdom. If you are going slowly, it may not necessarily be the rig that is at fault.

But that said, it is worth noting that tuning guides are based on what settings have worked for the author. So check that you have the same crew weight and a similar boat and foils to them.

If you are heavier than the average crew size for your class, the chances are you can stay powered up on your medium air settings for longer than other crews. You may also want to hold the mast a little straighter with less prebend, as your extra crew weight on the trapeze may be inducing more bend when you are sailing along. So try angling your spreaders slightly further forwards.

On the other hand, if you are lighter than average and struggling to keep up in the stronger winds, getting on to your strong wind settings early will help you depower (ie carrying more mast rake and prebend).

Good sailing

The relationship between mast rake, prebend and rig tension is complex. But get your tuning guide, find a good average medium airs setting, and then start experimenting with one small change at a time. If you keep a log of your rig settings for every race and practice session you will soon build up a good picture of what works for you and your sailing style.