Stay and Spreader Questions

[pda1964]

Hi,

Newby here. I have a Jeanneau Sun FIzz that was demasted just below the first set of spreaders. I have sourced a replacement NG-70 profile from another boat (apparently this was a popular profile) but the location for the four support stays (lowest 4 stays) and the spreaders are different. It's the same two spreader set set up, just about 2 feet lower with much longer spreaders. I was planning on re-using my spreaders and just relocating the brackets to match my old mast, but that will leave the 4 support stays about 2 feet lower than the spreaders. On my old mast, the lower stays attached just below the first set of spreaders. Would this be a problem?

Thanks,

Paul

[Brion Toss]

Hello,
Yes, a definite problem. Spreader thrust from the weather upper will be working on a rather long lever arm. On the other hand, if the mast section is right for a lower spreader location -- think unsupported length -- then raising the shroud attachment point might increase buckling vulnerability to an unacceptable degree. Not really likely, but definitely worth checking on. This is your opportunity to run a stability test and some mast scantling numbers. If the section checks out okay with the higher attachment, just move the lowers up to the spreaders.
Note that all of the above assumes that the old spreader location was correct in the first place. No guarantee here; it might even have been a contributing factor to the dismasting. But again, those mast moment numbers will reveal what is needed.
Fair leads,
Brion Toss

[pda1964]

Thanks for the reply! I guess I should explain that I am a complete newby as I have no idea what calculations you are referencing. Can you point me in a direction to look?
I am a mechanical engineer but haven't seen column buckling problems since college 30 years ago! And those were mainly static structural loads. Willing to learn it again but not sure what to look for. Please note that the old mast broke on the previous owner when the back stay snapped in high winds. The mast lasted 30 years so I would guess if I matched its rigging set up it would work again. But I'd rather not have to put more holes in it if it isn't necessary.

Thanks!

Paul

[Brion Toss]

Hi again,
As an engineer, you might enjoy/comprehend Larsson & Eliasson's Principles of Yacht Design. A much-simplified version can be found in the Rigger's Apprentice. There is really a rather slim chance that the original dimensions were ideal, and a very good chance that 30 years of not failing was the result of overbuilding, or of good luck; now is your chance to see what should be there. You might find, for instance, that the spreaders can be shorter, allowing better Genoa sheeting, or that the rig can be lighter, etc.
Fair leads,
Brion Toss

[pda1964]

Got the rigger's apprentice. Great book! Able to work through the equations and the mast and rigging are well within the design parameters for the expected loads. Whew! That's a relief.