Shrouds
από Geoff Moore
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Whenever I am doing well I am almost always asked how I have my shrouds set. Usually the question is asked in roughly an inverted proportion to the ease and sensibility of giving an answer. Like its blowing fire hydrants over on the street, the five minute gun just went off, my jib halyard cleat is slipping, and some guy I just met, sails by our transom and casually yells out "where are your shrouds". He is then immediately out of ear shot. I can see him smiling politely and I have a natural desire to give him a detailed description on all my little tuning tips, but I know in just a few minutes all hell will break loose and I will probably forget to look him up after the race. As new sailors evolve into better sailors there is a point where they start to learn about sail shape. From there it is only a small leap to learning about how mast tuning affects sail shape. Mast tuning can certainly affect balance (see my article on mainsails), but lets explore how tuning affects performance. It is ironic that much of the post race talk is about shroud tension. No sails are attached to them. So why all the fuss. The importance of shroud tension is related to how far back the spreaders and chainplates are swept. In boats with in-line spreaders shroud tuning is relatively simplistic. The fore and aft mast bend is usually controlled by running back stays, so all the shrouds have to do is keep the mast straight side to side and hold it that way. There is always the danger of bending the mast the wrong way so some boats are equipped with baby stays. It is interesting to point our that most masthead rigs are built this way. Since mast head boats usually carry a bigger genoa there is an incentive for the mast builder to make the spreaders short so the genoa can be sheeted closer to the mast. The shorter the spreaders the tighter the shrouds have to be in order to hold the mast straight. Hence the current use of hydraulic mast jacks that are capable of putting more tension on the cap shrouds than you could possibly get by twisting a huge turnbuckle. Fractionally rigged boats like the J/24 are very sensitive to shroud tension. This is partly because rigs built with swept back spreaders don't rely on running back stays to limit the amount of mast bend. Instead lower or intermediate shrouds restrict forward bend because they are attached to the deck slightly aft of the mast. The tighter the intermediates the straighter the mast. They also help to pull the headstay tighter by shifting the mast aft. Another interesting point to the new mast tuner is that we usually try to "straighten" the mast as the wind increases. Actually, we aren't sailing around with a straighter mast. In heavier air, backstay, mainsheet, and vang tension all work together to add mast bend. The problem then becomes too much mast bend so we have to take measures to restrict the bend. Without runners we have to "over tension" the lowers or intermediates. Twenty years of very competitive racing has left the J/24 class with a wealth of tuning information. Nowadays every J/24 carries a Tuning guide complete with absurdly accurate and complex shroud tensions for every wind strength and surface condition. And the J/24 has only two shrouds!! The trend in new grand prix one design race boats is multiple swept back spreaders with non overlapping jibs. This is true in both the Mumm 30, Corel 45 and many other new racers and cruisers alike. The appeal of these kinds of rigs is two fold. One the headsails are smaller, therefore, less expensive and they don't get beat up on the rigging. They can also support battens. The second appeal is they don't need running backstays, so they are thought to be simpler. It will be interesting to see how these classes evolve. If the J/24 proves to be any kind of model, I would expect to see a lot of complexity in tuning the shrouds. ******************************************** * Geoff Moore * * Shore Sails Ltd * * 7 Merton Road * * Newport, RI 02840 * * 401-849-7700 * * fax 401-849-7952 * * gmoore@shoresails.com * ******************************************** |