Spreader position

Home Design Build Race Links Reports Other Topics


A very small question concerns the "correct" position of the spreaders up the mast.

If a loaded beam is set across two supports, other things being equal, it will exhibit maximum deflection halfway between the supports.  On the other hand, if the same beam is set across one support, but the other support is a clamp (so the beam is cantilevered), than maximum deflection occurs about 41.5% along the beam from the support, and about 58.5% along the beam from the clamp.

Spreaders are meant to control athwartships mast bend, amongst other things, and act to control mast sideways bend against two mast supports -- the mast step, and the shroud attachment at the hounds.  If the mast step is on deck, and is not clamped, then the spreaders could do with being halfway along the mast, that is, halfway between the mast step and the shroud attachment.  On the other hand, if the mast is clamped at deck level, as happens with the Ikon or Italiko if a suitable deck fitting is used, then the spreaders could do with being around 60% above the deck clamp, or around 40% down from the shroud attachment.

Don Martin has made a couple of comments on the above.

Your load appraisal is fine as far as it goes but you have not shown the transverse cantilever loading on the top section of the spar. This loading actually has the effect of stiffening the lower section of the spar.

Don is quite right. I'm guessing, however, that just considering transverse loading, the top section of the spar (above the shroud attachment hounds) only provides a modest change to the general idea of where to put the spreaders when your mast is deck- or keel-stepped.

I believe that the spreaders are very important in controlling fore and aft mast bend. I think the choice of spreader height may also be influenced by the requirements of fore and aft bend control.

Again, Don is right. Thing is, I've not been able to model fore and aft mast bend adequately yet, and so can't say where the spreaders "should" go, except that I'd guess it is somewhere between 50% and 60% down the mast, somewhere between the "effective height" of the backstay attachment and the mast step. (See the "effective height" page for the backstay crane.)

In closing, I think that transverse mast bend is an extremely important issue for an IOM. It is really the only dynamic response that we are able to harness when the boats encounter a puff. Too little transverse bend and the boat is very hard to control in rough and puffy conditions. Too much transverse bend and the boat will not accelerate and point when she encounters a moderate puff.

Absolutely!

2005-12-18


2011 Lester Gilbert