Rotating mast Forestay tension - opinions

[bob the builder]

hi guys.

how much tension would you put on the forestay?
shrouds?

rotating mast stay attachment


if the mast is rotating?





onya,
mal.

[bob the builder]

let me start you all off.

the Gemini cat recommends 500Kg on each stay for a fixed mast.

i'm thinking of 100Kg for a rotating mast

[gggGuest]

Whack on as much as you need to get the jib to set right, and if it's too much for the mast to rotate back it off a bit and get the jib recut... On dinghies with the boom on the mast I've always found that the inward push from the kicking strap forcing the mast to over rotate vastly overrides any tendency from the rig tension to stop the mast rotating, so in practice its never been a consideration for me.

[bob the builder]

ok, can anybody tell me what they recommend?

if your catamaran is the same size as a gemini, surely you don't put 1500kg all up on a rotating mast?

i'm thinking 300Kg compression all up

[gggGuest]

500kg doesn't sound like a huge amount to me. Suck it and see.

[bob the builder]

interestingly, old books recommend

"Set no tighter than necessary to keep your mast straight", which implies a fraction of modern rigging tension.

what i'm currently considering.

(rather than doing it the set mast current fashion, of 20% breaking strain, roughly as above, 970 pounds tension for a single wire

i think i'll go up a size in wire, and have the absolute minimum i can get away with.

i've seen real damage from high tension on old cats. nose permanently bent/lifted 5"

couple of tons permanent load is like having 2 family cars sitting in the middle of your catamaran for 30 years.

no wonder they deform so badly over time. bouncing up and down with this REAL weight sitting on them.

[bob the builder]

also just read Cruising Catamaran Communique, and the old fossil who wrote that also recommended using the least amount of tension.

weird how different some of these old surveyors are from current production cat recommendations. (handbooks etc for rig tunning)

so unless somebody says otherwise, i'll be going up a size or two on the wire stays, and using least tension, hand tension on the leeward shroud set while under sail

any opinions?

[Gary Baigent]

Bob, keep that tension down - and if your headsail luff sags, put on some runners attached to mast sides at the hounds, in Spectra or similar and have them cleated by your helm position. Problem over.

[Grant Nelson]

I am not an expert in this area, but have been reading a lot about rigs the last weeks... here are my thoughts

Just out of curiousity, how did you come to your current stay, shroud and mast size? It sounds a bit like you guessed, or is this the rig you have been using for 13 years, and you only want to make it rotating?

If the later, you should be aware that the mast, because it has more degrees of freedom of movement, will be weaker, as far as I understand the rules of Euler. Just increasing the size of the shrouds will not solve that problem.

That aside, Of course you never want to (pre)tension your forestay more than your shrouds will be able to support and this will be much less than the breaking strength of the shrouds, since you have the leverage differences due to the low profile angle of the shourds in comparison to the forestay...

and in fact you will tenstion a lot less, since this pretensioning will only be added to the force from the sails. Thus, it does not add any strength to your rig to tension the stay (or shrouds) above what prevents any undo bending, movement and, as others point out, to keep the luff of the jib relatively straight (you will never get it fully straight). The less the better, which is in line with what others recommend.

Also, i would guess that the force of the main sail will provide some auto tensioning to the forestay so you do not need to pre-tension upfront for max jib sail forces..

But again, if you have running backstays, then this is a moot point, since you will tension the forestay using these while sailing. You would then only need to be sure you will not exceed the breaking strength of the backstays (which are always smaller than the forestay) or pull your mast down throught the bottom of your boat (or bend the boat)...