Spreader Orientation

[brawny84]

Hi,

I am designing a deck stepped aluminum mast and am trying to analyze the forces and failure modes for the mast due to rigging. My main question is: is there any formula or qualitative method to determine the optimum angle that the spreaders are to make with the mast? I am aware that the angle should be slightly off of 90 degrees. I think I need this information in order to determine the length of my spreaders, the length of the shrouds and the compressive force on the mast. If I'm wrong about this, could you help clarify the process for me?

Thanks.

Tom


PS I did a search on the forums. I was unable to find any material that answered my question, and gave up looking after 3 pages. So, I apologize if my question is redundant and there actually IS the answer somewhere. I did try...

[Robert Gainer]

The spreader should bisect the upper shroud; the angle of the shroud to the spreader above and below the spreader must be equal. This keeps the spreader from trying to slide on the wire.

[Zewe]

Also, the spreader tips will sweep forward or aft to meet the shrouds. Viewed from above, the shrouds appear to be a straight line, and the spreaders reach from the mast to contact this line.

[yokebutt]

Mr Gainer is absolutely correct about the vertical intersection of spreader and shroud, if you're concerned about the fore-and-aft sweep, then you're looking at a few trade-offs, each with it's own benefits and detriments.

Yokebutt.

[brawny84]

When you say that the "spreader should bisect the upper shroud," I'm not sure what you mean by upper shroud. Do you mean that the intersection of the spreader with the shroud should occur at the midpoint of the shroud?
I've read also that in a single spreader situation, the placement should be approximately 2/3 the length of the mast. Are these two guidelines essentially two different means to the same end? I assume the latter is concerned mainly with preventing failure due to harmonic resonance.

Am I off the deep end here? There seem to be so many variables that come into play. I think some of them can be designed to and others just need to be covered by general assumptions and FS's.

[dionysis]

brawny84,

Read this file and have a look at the last diagram. The file is from Classic Marine.

[brawny84]

thank you. This is very helpful!

[dionysis]

See if you can borrow or buy a copy of Skene's "Elements of Yacht Design", a classic.