The idea is that, having achieved your inch or so of pre-bend, you use the strut to lock the mast in place. You need to do this because your sails are cut to go on a mast with 1" pre-bend, and anything which changes the bend will degrade the performance of the sails. In particular, you want to stop the kicker from bending the mast. You must pull on a fair amount of kicker to support the leech of the sail, particularly when sailing to windward. The kicker pulls the boom down, which tightens the trailing edge (leech) of the sail. But the leech of the sail pulls against the top of the mast, causing it to bend more. We expect this - you can't have any tension in the leech unless the mast is bent beyond its natural position - it is the mast trying to get straighter which provides the tension. However, the kicker also pulls the boom forwards. Given the chance, the boom pushes the mast forwards low down, which generates loads of unnecesary mast bend and wrecks the leech tension as it does so. Net result is a bent mast, a low boom end and still no tension on the leech worth mentioning.

So after you apply rig tension, and before you apply kicker (usually before you go afloat in fact), you lock the mast in place by cleating off the control lines to the strut. Now the kicker tries to push the mast forwards and the strut stops it. All the kicker's effort goes into lowering the boom and tensioning the leech, and we are happy bunnies again, 'cos that's what we wanted.

But if there is nothing stopping the base of the mast from moving backwards, it can overdo it and the whole thing can end up bending forwards. This is known as inverting and is not a feature of a race winning rig. An inverted mast is frankly not a big deal unless you are racing in pretty exalted company, but it does slow you down a bit, so we don't really approve of it. Bung a few chocks in behind the mast if it bothers you, or use a strut.

Struts come in two different flavours. These are:

Two things shall ye know about struts;

1. The ideal strut meets the mast at the same height as the gooseneck. This means that it directly counters the force of the boom without compromising the mast pre-bend at all.
2. The higher up the mast you put the thing, the more it interferes with the jib, so nobody much actually puts it as gooseneck height.

The shallow angle of the Mast mounted strut shown above provides some mechanical advantage. A relatively large movement of the traveller resulting in a lesser movement of the mast, which is why it is set so low down on the mast at a shallow angle to the deck. The Deck mounted strut shown is at about 45 degrees to the deck, as a shallow deck angle here actually works against you. On the face of it, the mast mounted article is mechanically superior, but if your boat is very light and has correctors fitted, it is better to have a deck mounted strut / traveller. Basically, anything screwed to the hulls adds to the hull weight, allowing you to ditch some of those correctors. Anything screwed to the mast never gets weighed at all, so you don't get to lose correctors to compensate, and your boat is a bit heavier as a result. For the 2Kg or so that this gains you, this is not a big deal frankly.

All struts need to be bolted to both mast and deck, and they need about 8:1 purchase on the control lines. Don't forget to uncleat the strut controls when you release the rig tension, or your strut will spend all week under tension which is bad news all round.

An advantage of struts over chocks is that you can adjust them more easily whilst on the water to alter your mast bend.

• If the wind drops right off, you will want more mast bend to stop the leech of the sail from hooking to windward.
• If the wind picks up, you may want to straighten the mast to counter the extra kicker that you will employ (kicker tends to bend the mast more).