7/8 rig tunig

[30boat]

I've been tuning my Fulmar's rig.Acording to the book I have(can't remember the name right now) I should tension the cap shrouds to 20% breaking strentgh and then tension the lowers until the required prebend is achieved.The book also says that on this type of rig the cap shrouds need to be fairly tensioned and if necessary one can go up to 25% breaking strength.I'm using the elongation over a 2 meter length method to check the tension.Every 0.5 mil equals 5% br.strength.The problem is that my boat has a thinner mast than is normal for Fulmars and when I do it this way I get a little too much prebend.
Any suggestions on how to correct this?
Thanks.

<hr width=100% size=1>

 

[William_H]

I am very doubtful about the figure 20% breaking strain of the wire as a static tension. It seems to disregard the other elements in the loop. ie the hull stiffness and especially the mast stiffness. (what would happen if you had thicker stay wires? I would suggest much less than 20% perhaps 5%. It is the effect on the mast that you want not just tension.
I would suggest with the intermediate shrouds loose the tension the mains until you get some significant bend say 150 mm at the middle for a mast 7 metres to the cap shrouds with back stay loose. This amount would depend on how far aft of the mast the spreader tips are as obviously you can't bend the mast to a point beyond where shrouds are straight. Don't go that far. Perhaps half of the distance the tips are aft of the mast (ensure mast is vertical to boat) Pull this prebend out with tension on the intermediates until the mast is straight both fore and aft and sidewayus. You should then be able to pull in a usefull amount of bend with the backstay at least 100 mm. I reckon this should be a good standing ternsion

<hr width=100% size=1>

 

[davidwf]

Have a look at

<A target="_blank" HREF=http://www.zsparsuk.com/rigtune.htm>http://www.zsparsuk.com/rigtune.htm</A>


<hr width=100% size=1>

 

[30boat]

Thanks for your reply.There are however several problems here.This rig does need tension.Last year I did a 1000 nm return trip with about 15% br.strength on the rig and I coud see the mast step grinding on the mast heel it was moving so much.Conditions where too rough to adjust anything so we just pressed on under reduced canvas.Once we arrived I increased tension a bit but still got movement at the base of the mast and the leeward shrouds would still sag noticeably wich is something you can't have with a fractional rig.The structure of boat is fine, it's been thorougly checked and was even heavily reinforced at the keel area,a known week spot with Westerlys.
The book I'm using is "Ilustrated rig tuning" by Ivar Dedekam and he is quite clear about tensioning the rig to at least 20% br.strength for fractional aftswept types.


<hr width=100% size=1>

 

[Jollyswagman]

You're correct with what you say, fractional rigs should be set up with arround 20% of the break load of the wire. In your last post you said you could see'hear the mast step grinding when under heavy sailing conditions, check that you in fact have a compatable heel plug/step I have seen instances where the boat builder has fitted the wrong step for the mast.
You should be able to correct the over `pre-bend' by tightening the aft lower shrouds which should bring the mast back into colum.

Under most sailing conditions you are correct in stating that the lee shrouds should not go slack. This is a recipe for loosing your mast, what happens is when the lee shroud slackens the mast can then pivot in an arc round the weather shroud as it goes forward the prebend increases, this has the effect of lowering the hounds height (liken it to pushing on a bamboo stick) by doing this the rig tension is then reduced further & if not prevented can lead to failure of the spar.

I would suggest beg borrow or buy a rig tension meter which will give you more accurate trension readings than using the elongation method.

<hr width=100% size=1>

 

[30boat]

I have replaced the mast step recently and the new one is exactly the same as the old wich I presume was the original.These Kemp masts have very little clearance between the middle of the mast heel and the step wich probably accounts for the grinding as there was a bit of sideways movement.
I have also checked the rigging tension with a gauge that I already own and can't remember the numbers right now but they where the same for both sides.I'll check things more carefuly this weekend.
Thanks

<hr width=100% size=1>

 

[Jollyswagman]

There should be no sideways or rotational play between the heel plug & the step, & therefore no ginding noises, check with your Selden agent that the step & hell casting part numbers are compatible. I've seen where the boatbuilder has fitted the incorrect step for the heel plug casting.

<hr width=100% size=1>

 

[30boat]

Yes I will do that.
Thanks for the advice.

<hr width=100% size=1>