Rig tension

G

Guardamar

Active Member
Joined
8 Feb 2010
Messages
42
Location
NE England
www.randpsystems.co.uk
I usually sail a dinghy and have reasonable experience to set up the rig tension. But my 26 ft keel boat is another matter. It has a fractional rig, forestay, caps, lowers and backstay.

Without backstay tension, and with reasonable tension in caps and lowers, the mast minorly inverts till I put on backstay tension. The exit point for the forestay is above that of the caps, but only just, and the spreaders are more or less in line with the caps.

I have tried slack lowers, but read that is bad. Ideally I would like to change the spreader angle a little, but that does not seem easy. It is a comparatively new Seldon mast - perhaps 10 years, and to me appears like a tree trunk.

Any thoughts as to how I can loose power?

I do use a backstay a lot, and have 16 : 1 purchase.

thanks
 
Its not clear from what you say how much sweep back you actually have on the spreaders. If you have a fractional rig then the spreaders should rake backwards sufficiently so the caps provide the forestay tension. The tension on the caps also pushes the mast forward at the spreaders due to the rake. The caps need to be balanced with the lowers as they will affect the pre-bend. They should be on chainplates located aft of the mast. The lowers should be in line with the base of the mast. If the lowers go to chain plates too far behind the mast then they will try to pull the centre of the mast back. Are the shrouds going to the right chainplates?

The rake of spreaders is not usually adjustable without changing the mast fittings. I guess you can get away with limited rake provided you always have enough backstay tension. Perhaps your rig is designed to rely on that as having spreaders with less rake allows the boom to go further forward before the sail rubs on the spreaders or hits the shrouds.
 
Mast prebend

Yes the mast should have some middle forward bend with no backstay tension.
This is usually provided by pressure on the tips of the spreaders from the cap shrouds. This push forward is then counteracted by the lower stays also pulling backwards from chain plates set aft of abeam the mast.
You may have the chain plates swapped over as said. If that is OK then loosen lower stays. They may simply be too tight pulling the middle back.
The dimension of the cap shroud aft of abeam the mast provides support for the mast to stop it falling forward if backstay is loose.
On my little fractional rig the same chain plates are used for cap and lower stays. If the cap stays are further forward they will exert more middle forward of the mast via the spreader tips.
Do make sure the spreader bases are robust in pushing the tip aft. I had a mast failure when rivets corroded in spreader base allowing spreader to swing forward to in line. This caused the middle of the mast to move aft and the whole crumpled. (in milli seconds).
Unless the mast is really tree trunk dimensions it will need that middle of mast support in fore and aft direction. good luck olewill
 
Mast Rake

Had another thought on this problem of not enough push forward by the spreaders. If you have raked the mast aft at the top by a lot this will possibly move the cap shrouds more into line with spreader tip and chain plate so reduce forward push. If you shorten forestay and pull mast top forward. I reckon vertical is right then you should get more push f0rward of middle of mast. just a thought olewill
 
William_H said:
Had another thought on this problem of not enough push forward by the spreaders. If you have raked the mast aft at the top by a lot this will possibly move the cap shrouds more into line with spreader tip and chain plate so reduce forward push. If you shorten forestay and pull mast top forward. I reckon vertical is right then you should get more push f0rward of middle of mast. just a thought olewill
Click to expand...

You could be onto it with that suggestion; on the pictures here:- http://www.western-horizon.co.uk/archive_boat.php?boat_id=88# the spreaders seem to be well raked aft.

Would look at a vertical mast or maybe raked aft by a little - say 6 inches max.
 
Thanks guys.

Looks like the problem is as I suspected, the spreaders are just not swept back sufficiently by the rigger who originally set up the mast (before my time with the boat). If you removed the caps from the spreaders, the spreaders would just about be in line with the caps, or marginally forwards. I'm not yet sure how to swing back the spreaders a little.

The mast is more or less vertical, and I still have weather helm, but don't want to put it forwards of vertical.

By the way, slow response as I don't have easy access to internet.
 
Guardamar said:
If you removed the caps from the spreaders, the spreaders would just about be in line with the caps, or marginally forwards.
Click to expand...
Mine are in line (Jaguar 21 fractional rig). There isn't really anything to adjust because until I tension the shrouds the spreaders are quite loose. When the mast is being raised the spreaders are only really held in place by the ties to the shrouds. They aren't solidly bolted to the mast like I would have expected. The bases are obviously fixed in place but they are (intentionally as far as I can see) "wobbly".
My lowers are only set tight enough to stop them rattling. According to the manual "the final tension on these lowers should only be slight."
 
I have the same boat as Guardamar but an earlier model. When I bought my new mast, Z Spars said that the original spreaders weren't angled far enough aft. However, I still have trouble inducing pre-bend in my new mast. I wonder if tying a loop of rope halfway up the mast and pulling it forwards whilst tightening the cap shrouds would work?
 
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