Spreader and shroud loads- swept aft

[Brian Duff]

Hi Riggas

so, brions book has some great math for calculation shroud loads, but I am wondering about some slightly more complicated calculations for simpler rigging... I want to understand how to calculate loads seen by these rigs which carry a single upper swept aft and a single lower lead aft to same chainplate, with no backstays carried - but not full B&R rigging.

This is basically an attpt to further what idid at boatshow, by expirimentinf with my own westsail. I want the ability to carry a full headed mainsail which precludes standing backstays- I also intend to run the staysail to near masthead seeking more power in light winds through bigger mainsail and headsail, better running through twin long luffed headsails-

so, I am trying to wrap my head around combining the side and backstays loads and supporting all that not directly from the side of the rig.

Ps I do have a different mast section than most westsail, and understand that the math may prove I need completely different rig and compression structure, but where do I find said math?

Excuse my iPhone typing.

[Gashmore]

Good question. If you knew the headstay load you might be able to work it out with a little vector analysis. Unfortunately most of the procedures I have found cover transverse loads well but give short shrift to the head and backstay.

[P38]

Hi Brian,

I'm confronted the same problem, designing a modern version gaff rig for a 42 footer. The approach I took was to seperate the transverse and fore/aft loads and then sum them up.

For the transverse loads, there's lots of information available on calculating these. Brion's book addresses it. You start with the righting moment (typically at 30 degrees). You can use vector analysis to calculate the rig load required to heel the boat 30 degrees.

I started the fore/aft loads by picking the static tension I wanted on the forestay. The tension required to achieve a given forestay sag is calculable. You can then calculate the loads on your swept shrouds necessary to supply this tension to the forestay. You do this as a side view vector analysis.

You're going to get some big numbers for shroud tension necessary to keep the forestay taut. Remember that the load is shared between both shrouds.

The end of all this is that you have both transverse and fore/aft loads. These are added together to get the total load on the shrouds, ie the transverse load heeling the boat and the fore/aft load tensioning the forestay are both carried by the shrouds. Calc them seperately and add them together.

You're also going to have spreader loads, etc. But all these things can be seperated into two components, ie transverse and fore/aft, calculated, and then summed up.

Regards, Paul

[Brion Toss]

Excellent thus far. Just remember that those big upper shroud loads have to go somewhere. One place will be along the spreaders, which means the (aft-led) lower shrouds will pick up a big accumulated load, dealing with their own panel as well as the referred upper loads. You certainly won't want any forward-led lowers, as these would just add to the load on the aft lowers.
There will also be more compression on the mast than you have with a standing backstay -- angles.
I suspect you'll end up compromising the forestay height, playing it off against realistic shroud loads and mast compression loads.
Have you looked at a mongo backstay crane as a factor?
Fair leads,
Brion Toss

[Brian Duff]

All,
Thanks for the input - I was looking for that valueable bit - combine the normal backstay and upper shroud loads but work out all that plus comrpession, etc, for the real angles used.

I am really waiting to hear how the W32 up in Naptown sails with the turbo rags comming from Quantum, before I go to far into modifying mine - but I can do the math parts now anyway.

Lets say I do a 'John Holmes' style fixture on the masthead aft, how do I calculate the loads on that sucker. I guess I have to weld that piece in place, or get way overweight providing fasthening area

[WorkingOnaBboat]

Team Southbound is currently working on a set of ski's with which to retro-fit the wireless Westsail 32, so we can sail it!
Unfortunately the extraordinary amount of snow this season has forestalled the continuing rigging efforts on the WS!
We'll keep you updated . . . enjoy the heat down island!

-CP