Rigging - Struts and Chocks

Struts and chocks are intended to stop a variety of forces from messing up the natural mast bend. Natural mast bend is generated by the spreaders, and should amount to roughly one inch (backwards). This is measured by applying normal rig tension with a mast rake of 22'8", and pulling the main halyard taught to the mast at the gooseneck (pictured right). At the spreader height, you should be able to measure roughly one inch between the halyard and the mast. This is one inch of pre-bend, and indicates the amount of bend that your rig is inclined to achieve before anything comes along to mess it up. (If you don't get something between 0.5 and 2.0 inches, you need to alter your spreader angles or lengths)

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.

Now in fact, you don't need a strut to achieve this effect. In the old days, everyone used chocks, which are basically just bits of thick ply or plastic chopping board from Tesco, cut to a shape similar to this. You would stick these down in between the front of your mast and the front of the mast gate to fill the available space, and they would stop the mast going forward. Nice and simple, and increasingly popular these days.


	<< Front of boat

There are two snags with these. You generally have several of these of differing thicknesses, and can't always get the right permutations to fill the available space. Plus they don't stop the mast from moving backwards. The mast does this when you let the kicker off and put the kite up on a windy day, as the sails push the top of the mast forwards and therefore tend to make the bottom of the mast want to move backwards, and the spinnaker pole pushes backwards like mad on a close reach. Now this is a good thing in moderation, as we want a mast that is as straight as possible and as far forwards at the top as possible when we are going downwind, so some movement is a good thing.

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 fit a 'puller'. This is a bit of string that follows the line where the strut would be if you'd got one, pics to come later.

Struts come in two different flavours. These are:


	Front of boat >>


	Deck mounted traveller



		And Mast mounted traveller.


	Front of boat >>

Two things shall ye know about struts;

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.
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 is kinda medium, you may want to straighten the mast to counter the extra kicker that you will employ (kicker tends to bend the mast more).
If it gets really windy and you're overpowered, let the strut go forward an inch or two and pull on more kicker to take up the slack. This flattens the sail and allows the top to 'blade off', and makes going upwind a lot easier.