Rigging loads

[30boat]

I wonder what the loads are on the cap shrouds of a monohull ,relative to its displacement,when sailing.These loads must be proportional to heeling the limit being when the mast is horizontal .And what about shock loads due to gusts.Variants would be hull shape,ballast,draft, mast height etc.Can they be calculated with any accuracy?

 

[trapezeartist]

The Selden Rigging Guide suggests 20% of breaking load as a static load (25% absolute max). The weight of the mast when the boat is heeling would be a negligible increase, I would have thought. Extra loading on the windward shroud due to wind pressure would be significant but it would be a wild guess to try to put a figure on it. How about another 10% of breaking load? Finally there are the dynamic loads due to the rig flailing around as the boat bounces over the waves: these will be quite small for a rig that is firmly held but could be quite significant if the rig is slack and the mast can flop around.

 

[VicS]

The Selden Rigging Guide suggests 20% of breaking load as a static load (25% absolute max). The weight of the mast when the boat is heeling would be a negligible increase, I would have thought. Extra loading on the windward shroud due to wind pressure would be significant but it would be a wild guess to try to put a figure on it. How about another 10% of breaking load? Finally there are the dynamic loads due to the rig flailing around as the boat bounces over the waves: these will be quite small for a rig that is firmly held but could be quite significant if the rig is slack and the mast can flop around.

I though the recommended tension was 15% at least for a mast head rig.

It all in here http://www.seldenmast.com/_download.cfm?id=6740&download=2956550&filename=595-540-E.pdf anyway

 

[William_H]

For the cap shrouds it is very hard to calculate because the wires are pulling to heel the boat against the stiffness of the hull. This depends on weight and hull shape. I would suggest you get a spring balance type weighing device and put it at the end of main halyard. Measure the load it takes to pull the boat over. It will probably be light at first then become difficult around 45 degrees then I am guessing at less load to hold the mast down near 90 degrees.
From trigonometry you then calculate the load on the shrouds ie the shrouds will have more load being at a smaller base ie mast to chainplate distance than the distance you were pulling from to heel the boat.
Having found the max shroud load you can calculate or guess the end pressure of the spreaders. This assuming that spreaders are on a typical mast head rig where it is purely end pressure.
Swept aft spreaders will have to be a lot stronger to handle the pressure pushing the tip forward.
Then there is the forestay load. This can be quite small if the jib and halyard take the load but the jib itself can exert a load by trying to make the forestay curve. Add to this the tension of the backstay trying to make the forestay straight. The backstay of course has a tension not so much a sailing load as the main sail and sheet will provide support.
I don't know how you could find the loads on the inner shrouds. They need to be as strong as the cap shrouds and they do get very tight on the windward side when beating or reaching. certainly it is goodbye mast if they fail. I guess a tension guage would be the easiest way.
So when you find the loads you then decide what strength you need and you will probably want to double it to be safe. Or just use rigging same as similar boats. There seems to be a settled impirecle standard that has evolved that is safe. good luck olewill

 

[BAtoo]

I'm always amazed at how much effort is needed to careen a boat. On my 19ft Squib (680kg 50% ballast ratio) its the devils own job to get any decent heel on even with someone on the side to induce some heel and a fairly well angled pull on the main halyard to an adjacent gantry.
I suppose we might get 100kg load on, so its around 15% of the displacement.
Might measure it next time if I remember.

 

[lw395]

With a properly set up rig, the leeward shroud loses nearly all its tension as the boat is fully powered up. This tension is, roughly speaking, transferred to the windward shroud, so the max design tension is around twice the preload.
For a lot of boats, there are stability curves published, which will give you the righting moment at various angles of heel. You can work out the total/net rig force from that. But with a multi-wire rig, how the loads are shared between caps and lowers etc is more complex.
On a race boat, you have to add the effects of all those fatties on the weather rail.
Dynamic effects obviously depend on how big the seas are etc.
I think Larrson and Eliasson is the best book I have for this subject?