Short spreaders

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Static, no wind Almost all the IOM's I've seen have spreaders whose length is around 66% of beam, like the boat on the left side of the diagrams.  The boat on the right of the diagrams shows a rather different idea, spreaders whose length is around 45% of beam.  The diagram exaggerates this for illustrative effect, as they say.

The conventional spreaders do an excellent job of triangulating the mast and stiffening it up, so when the boat is heeled the mast stays reasonably upright.  Conventionally, the spreaders are wider than 50% of beam, so that they make a larger angle between shroud and mast at the hounds.  This larger angle allows the shrouds to run with somewhat less tension than would otherwise be needed to keep the mast upright, and is important for full-size yachts and dinghies, where people's safety is an issue.

The "short" spreader idea has little going for it when the boat is sailing in light airs, but the picture changes when the wind gets up, as illustrated in the diagram on the right.

With conventional spreaders, as the boat heels and the wind fills the main, the weather shroud tightens up and the leeward shroud slackens.  The tighter weather shroud pushes the spreader, and hence the mast, to leeward, which is what the mainsail wants to do anyway.  The end result is that, as the old salts will tell us, "the slot closes", and we might say that the angle of attack of the main increases and its twist decreases.  None of these things is what we really want when the wind gets up.

The wind is up... With the "short" spreader, we have the opposite effect when the wind gets up.  The weather shroud tightens up, but instead of pushing the mast to leeward, it stabilises the mast or even pulls it to weather slightly, depending upon spreader length chosen.  In any case, the slot opens, the centre of the mainsail reduces its angle of attack to the wind and it twists off.  In theory, this is all good news, the right response to a rising wind or a gust.

I'm trying this theory at the moment, and so is Trevor Bamforth.  His experience, he tells me, is very promising.  I saw his "Stealth" sailing at Poole in top-of-B-rig conditions, and he won most races by half a leg of the course against very respectable opposition.  My experience in top-of-A-rig conditions is also promising.  There are a couple of things to watch.  The most important is that the mast is no longer so well stabilised against a twisting force, and a really strong gust could pop a pre-bent mast into a reverse bend.  Ouch!  It might be worth fitting a longer than normal backstay crane to help out here, and then positively fixing the crane to the head fitting with a screw.  Oh, and keep an eye on the IOM Class Rules when you fit the spreader.  It may well need a different method of attachment to the mast from the conventional, and we don't want anything that won't pass measurement.

Gary Cameron has made these suggestions in a post on the WindPower discussion forum on or around 6/7 Oct 2001:

I think it is worth thinking about the rigs and what happens in the gusts as our group has found that more improvements have been made in boat speed via rig development than hull development. Jib pivot position as well as forestay tension (provided via the backstay and mast) have a large effect on when the jib leach opens under wind pressure alone (eg takes over from the jib leach line, topper, whatever you want to call it). If this opens too early you do get weather helm. But you do want it to open.  As the wind builds you need it to open in concert with the main leach opening. If you look at an IOM rig there are two types I normally see. One is the style you see on many UK boats with the shrouds attaching to the mast at the same height as the forestay, long spreaders, a lot of forward mast prebend and then a lot of backstay tension. The GBR rigs frequently have the jib pivot a long way aft on the jib boom. This style of rig is quite "bound up", as the forestay tension in combination with the aft position of the jib pivot minimises the ability of the headsail to twist open in the gusts. The long spreaders with high shroud attachment limit the middle of the mast moving to windward, preventing the middle of the main leach opening. The top portion of the mast does move to leeward in the larger gusts helping to de-power these rigs. The other rig is the type you see on a TS2 or one of Trevor Bamforth's or Geoff Smale's boats. These rigs have the shrouds attaching to the mast well below the forestay attachment point, they use shorter spreaders with less forward mast prebend, and usually have the jib pivot forward of the UK rigs position. Craig Smith, Geoff, Trevor and myself all use this style of rig to encourage the mast to move to windward in the middle (only slightly) and to get the tip to lay off to leeward. The trick is to get this happening at the same time the headsail leach is starting to open. Too much mast bend too early means a loss of power and height and maybe even some lee helm creeping in. Too little and you will get weather helm. Get it right and you go fast with a boat that is easy to sail. If you have a TS2, follow the rigging guide. If you don't, the principles still apply, just persist and ask for help from other good sailors using that type of rig. Mind you, Craig and I are still learning about the rig even after 8 years of using it. Geoff and Trevor are both good to talk to if you get the chance.

To go with the idea of short spreaders, I've come up with some crude adjustable spreaders to allow the "V" angle to be changed relatively easily.

2005-12-18


2011 Lester Gilbert