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How to Tension and Tune your Standing Rigging

A year ago we replaced all our standing rigging with Dynex Dux. Often one sees recommendations to tension the rigging to 20% of its breaking strain. With a breaking strain for Dynex that is 3x that of steel, those types of calculations don’t mean a lot.

So I went back to basics – and drew from my dinghy racing rig tuning experiences:

What is it that the rig is supposed to do? There are two main functions:

1) stop the mast falling overboard
2) create the right shape in the sails for the conditions in which one is sailing

Along with these are a few ancillary points:

1) mast should be upright – ie centered laterally
2) in column, ie not bent from one side to the other
3) probably have some rake, ie leaning back a bit
4) probably have some pre-bend ie fore and aft bend
5) be tight enough so that when under strain in some waves, the mast is not ‘pumping’, as this puts a lot of strain on it
6) especially in a cat, not be so tight that the boat folds in half

The mast rake affects your handling – rake further back and you will get more weather helm and probably better upwind performance. More upright and you get less weather helm. In practice, however, your rake is probably already determined by your forestay length.

The mast pre-bend is determined by your sail cut. Ask your sailmaker for the optimal amount of bend they recommend. They should have records on how much luff curve they cut into your mainsail.

More mast bend will flatten the sail, reducing power in a blow. Less mast bend will create a fuller sail, with more power. This works in tandem with luff tension – more tension brings the flow forward, making for better handling in a blow (the wind blows the flow aft), and opens the leach which helps to de-power the sail in a blow.

OK, so now think about what the various bits of wire do:

1) the cap shrouds hold the mast upright. Their main function is to stop it falling overboard. They also pull the top of the mast aft and down. That increases mast bend by putting the mast under compression load. The fractional forestay also contributes a bit to that mast bend. However, not a great amount. The cap shrouds also provide the forestay tension. Tight forestays are Good – they help you to sail up wind, as sag in your jib luff is Bad.

2) The spreaders, whether on your main shrouds, or on diamonds, normally induce mast bend if they are swept back. In rare cases the spreaders are not swept back, in which case their only purpose is to keep the mast straight, ie in column. That is also an important function of swept back spreaders.

4) The lower shrouds prevent excessive mast bend, and also help keep the mast in column.

5) If you have a boom vang, that pulls the boom forward and induces mast bend.

6) The mainsheet, when close hauled, tightens the leech, which pulls down the mast head and induces mast bend, thus flattening the sail. That is why you pull it tight in a blow – but ease out the traveller to spill wind – and tighten the luff (cunningham) to open the leach. It also tightens the forestay for better upwind performance.

Now put it all together:

1) Set up the cap shrouds so the mast is vertical. Take the spinnaker halyard from one side to the other to compare lengths to make sure you are upright. Tighten them to get the pre-bend recommended by the sailmaker. You can set up a topping lift on the boom and tighten the mainsheet to help you achieve that. To measure the pre-bend, take the main halyard to the gooseneck and tighten it. then measure horizontally from the mast track to the halyard at the maximum point – probably about where the spreaders are (yes, you have to go up the mast to do that).

2) Now set up the diamonds (if you have them). Make them tight. Sight up the mast and make sure it is in column. If you have two sets of diamonds, as we have, you may get funny s-shaped wiggles. If so you will have to adjust the two sets until you get it all straight.

3) Now tighten the lower shrouds to control the maximum amount of mast bend.

OK, that is the static tuning. Now you have to go sailing.

Go out in a nice fresh breeze. 20kts close hauled apparent is nice. Crank in the mainsheet and get some power on.

You will find that the leeward shrouds are loose. You don’t want that as in waves you will get some serious pumping. So tighten up the leeward ones. Count how many turns you put on the turnbuckle.

Then tack and put the same number of turns on the other side. If you can’t do it, then tack again, and loosen the other side the same amount. Repeat a few times.

Now sight up the mast again, and make sure you haven’t got any wiggles. If you have, you will need to think through what is too tight and too loose, and make adjustments.

If you have a big crease coming down the mainsail from the middle of the mast to the clew, then you have way too much mast bend. Tighten up the lower shrouds to contain that.

Once you have done all that, you should be nicely tuned.

The one wrinkle in all the above is the strength of your boat. On a cat the shrouds are wide apart. With the mast pushing down in the middle, and the shrouds pulling up at the sides, your boat will want to fold in half. If there are any issues with its strength then that can, indeed, happen. The signs of that are progressive loosening of the shrouds over time. Consult with the boatbuilder if you have any concerns about that.

Of course, your rig may have less stays that you can adjust. Or you may have in-mast furling, or a rotating mast, both of which will limit mast bend. You can only work with what you have and, in the end, we are cruising not racing, so don’t sweat it. But if you can, it is nice to set it all up as optimally as you can. And when you order new sails, make sure the sailmaker knows the rig you have, so he can accommodate its limitations and cut the sails to match.

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