Spreaders

An article from AES.

The number of spreaders on a rig is normally something that can NOT be adjusted and most people don't really know why they have a particular number of spreaders and/or why the boat next door has more or less. Certainly, there is confusion over just how big the weight savings are if you got yourself some more... There are several overlapping issues. Generally speaking it is lighter to have more spreaders. Take a rig with one spreader, if you add a second spreader the length of mast between supports is reduced substantially, this has results in a drop in the required mast inertia but only transversely, this means that a lighter mast can be used. Fore/aft there was no benefit. As a result while a lot of single spreader rigs are based on round tube as you move towards 4 spreader rigs the mast shape becomes increasingly elliptical. What this means is that because the mast inertia are lower transversely a narrower section can be used, but it still needs the same fore/aft inertia. The weight saved from the lighter section more than cancels the weight of the extra spreader and the rigging wire weight hardly changed at all. Once you get to three spreaders, the weight increment saved with more spreaders becomes much less and by the time you get to five spreaders you may well be going backwards but it depends on the case and the mast sections available. The IMS rule has hung (clung) on to the low rigging penalty (LRP) which means that the rigging and/or first spreader can not be below 25% of I. This effectively blocks rigs with checkstays from the gooseneck (Tim Stearn) or 5 and 6 srpeader rigs where you would naturally put the first spreader quite low down. So, despite the fact that it makes little sense there is also a rule block in play. As if that was not enough there is a 2:1 aspect ratio limit under IMS; well it is not a hard limit like the LRP, but it effectively makes it pointless to create a mast with a lot spreaders because it's not worthwhile from a windage point of view. Moving aside from the rules, there is a basic conflitct between complexity and weight saving. In this case a lighter mast should actually be cheaper, but the extra spreaders create extra labour hours so, for most yachts the optimum is around 3 spreaders, 2 for a small yacht and 4 for a very large one. There are all kinds of proviso's and exceptions that go with that so don't take this too far. The interesting bit is that transverse inertia for a typical sloop are based on panel length squared, compression etc etc. So, if you have no spreaders and you decide to put in one it will halve the panel length, this quarters you mast inertia. So, in theory if the spreader is properly designed and stayed, required mast inertia drops by 75%. This is pretty substantial I would say. Then if you put in another spreader (two) the improvement 66%. Three spreaders 44%, Four spreaders 36%. Of course the mast inertia fore/aft didn't really change so you didn't save any weight there. If you took that one spreader 75% case, if you remove 75% of the side wall then it is probably much too thin. So you go to a narrower mast with a thicker wall, this mast is of course heavier than the big one with the really thin wall. So, after going around in circles you wind up with modest overall weight savings despite increasing levels of complexity in design and construction. You didn't find that in the least bit interesting(?), well you're probably right, but its my job (life's work) and I find it really challenging and fun to investigate all the options because there are so many overlapping issues when you consider diamonds, masthead spinnakers, IMS rule big mast issues discussed previously, runnerless rigs with highly loaded backstays and so on.