Mainsail roach & backstay

[charles_reed]

Good lateral thinking Nigel - the boom position is, in fact 30mm higher than on the original mast and there is more than adequate clearance for my 1842mm height.

However:

Lowering the boom would require changing/shortening the gas-strut and/or re-fixing its lower cleat farther down the mast. That would require drilling out and redrilling 10 large monel rivets and 3 bolts on the gooseneck and 8 rivets on the vang fixing. These are large rivets, requiring a pantograph or power riveter. Whilst I've got a breast-operation riveter doing those rivets is right at the limit of its capacity.
Additionally, the FMEA we did on the original fracture suggested that the juxtaposition of the vang fixing to the restraint of the coachroof cap was a significant factor in the work-hardening and subsequent fatigue-fracture of the original mast.

For this reason it was one of the first solutions to be evaluated and rejected.

 

[charles_reed]

Yes this was the Capstick favourite - but a few calcs showed you had to have an enormous structure to effectively change the angle of the backstay, with equally enormous fixings to distribute the load.
Now Capstick trained as a draftsman and should have known better - as a stern extension alone we had to add on <1.5m.

 

[Guest]

I have a mainsail on 37 m2 with similar roach. I also have noticed the difficulties in tacking, especially in light winds. I have solved it very simple. A soft GRP batten, approx 50-60 cm long, is bolted at the masttop 90 degrees out from the mast. The backstay runs through a small block in the loose end of the GRP batten. In light winds I let the backstay loose and the mainsail runs free when tacking. In stronger winds when I tighten the backstay the batten bends and the mainail hits the backstay but without any problems at all to "jump over". My mainsail is a Technora-Kevlar laminate and I can not see any wear on the aft luff caused by the backstay. I have had this solution for a few years and it is quite common on racing yachts in Sweden. BR from Sweden /// Patrik

 

[charles_reed]

Thanks Patrick - sounds like a similar system to the Melges and Farr 40 systems. Could you let me know the sweep-back angle on your spreaders?

 

[charles_reed]

Thanks Patrik - sounds like a similar system to the Melges and Farr 40 systems. Could you let me know the sweep-back angle on your spreaders?

 

[spark]

You've probably thought of this and discarded it but....

How about shortening the offending batten (#2?) by 6 inches or so at the forward end? This combined with something slippery/rolly on the relevant bit of the backstay might be enough to allow the batten to flip past on both tack and gybe.

You could buy a shorter batten and stitch a bit of elastic into the pocket. If it didn't work it would be easy enough to revert to fully-battened and then go for one of the more expensive options.

 

[claymore]

I think we are on to something here - perhaps whoever designed your boat and rig Knew what they were doing and you shouldn't have messed with the original set up.

regards
Claymore
/forums/images/icons/smile.gif

 

[charles_reed]

I had thought of trying that out, but the overlap @ No 2 batten is far too great (>150mm) - it would also require far more slack on the backstay - and I reckon it's got a very low probability of success.

 

[Guest]

My three pairs of spreaders are not swept at all. And I have a VERY thin (ie light) mast profile. On other words, gybes in strong winds can be interesting... I have two pair of running backsstays (right words???) for keeping the forstay tight. I only use the aftstay for keeping control of the rake of the mast. I have understood that you have the spreaders sweept back and that means that your sidestays (definately wrong word...) keep the mast in place and also keep the forestay tight BR Patrik

 

[charles_reed]

My 22 Degree swept back spreaders means the mast is "almost" self-supporting so the backstay is there only to act as a check when running before the wind and (more) to tension the forestay and bend the mast to change sail shape.

The original 135x88mm section developed a remarkable 450 degree circumferential fatigue crack, without falling down, off Cabo Gato after an unscheduled (Port San José untenable due to swell and Cala San Pedro full of tunny nets) overnight sail in F6-7 headwinds, due cyclic overloads.

The FMEA pinpointed:
a) Contiguity of lower gas-strut fixing to thro' deck cap.
b) Unbalanced couple of double-reefed main and roller genoa with high backstay pressure
c) Inadequate cross-sectional area.