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A manufacturer as Seldéns may very well have rather good control on their parts, mast, wires and so on. Then it is easy to recommend 20% of max load.

But there are some other weak points:
- turnbuckles. These gall very easy when made in stainless.
- hull and chainplates (I have seen many destroyed chainplates just due to high rigg tensions).

The 20% may be for those racing, wanting to get optimal performance and pay the price for it. Cruising - is a fraction of a knot important? Lowering stress somewhat (to, say, 10%) will decrease stress on the hull considerably.

Then, one must think some:
On a MH rigg, the aft stay is used to get high tension in the forestay. Generally, the mast should be stright, it is often also dimensioned for that. To induce a bend in a heavy dimensioned mast requires a lot of tension.
From stress point of veiw the MH mast is superior. All forces ar handled.
Then .. those who starts adjusting (during sailing) aft stay what is the expected result? One things is for sure: the mast top will move back & down. This the side rigg will be more slack ....
The exclusive alternative is to use hydralics to pump up the mast. Works better, but this is expensive (and of course increase all stress).

A partial rigg is -principally- designed för being trimmed. Thinner cross section, in order to be able to bend. With the 9/10ths one never knows, these could be a MH is disguise.
Most of us use to trim the aft stay on thse partial mastas. With about the same result as with a MH: mast top is moved back & down. Side rigg tension is decreased, and mast topp is also moving to lee, spilling some wind from the top.
If this behaviour is decired, then there is no need to have very high tension in the side rigg - and certainly not in the fore & aft stays.

/J
 
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