Masthead rig set-up

[MCW]

Are there any riggers out there whose brains I can pick?
What is the correct way to setup and tension a masthead rig on a yacht. The vessel in question is a Mirage 29 tall rig with boyce spars, fwd and rear lowers. The problem I have is that when I tension the backstay to get the forestay tight the mast takes up quite a pronounced bend although the lowers are reasonably tight. Any advice would be appreciated.

 

[Chris_Stannard]

You should make sure the mast is vertical to the boat and has about 2 degrees rake aft. To ensure the mast is straight use the main halyard. you then set the mast up in column starting with the lowers. When you think you have got it right the mast should be in column which you judge by lying on your back and looking up the main track. Then take the boat out in a stiff breeze and repeat to make sure the mast is in column on both tacks. There is a slim volume on Rig tuning which might help you. Alternativelyif you are around the Hamble look up Geoff Merritt in the phone book, I think it is a Burseldon number, and ask him to do it for you. Tthat was my method, and it saved a lot of time and frustration although it cost a few quid, it was not much compared with the cost of racing the boat.
When you wind on tension to the forestay you are bound to bend the mast, it is designed for that and the cut of your main should allow for it. If you have any worries you should ask the mast manufacturer how much tension you can safely wind on and how it should be measured. Selden certainly provide this advice.

Chris Stannard

 

[dickh]

I always thought on a masthead rig the mast must be absolutely straight, set up the forestay. backstay and m/hd shroulds as tight as you can with a spanner and adjust the lowers equally so the mast is straight. Check when on the water going to windward, iff the lower shrouds are slack tighten and repeat on the other tack. You cannot usually overtighten the shrouds if you only use hand tools.
Or go to an experienced rigger...

dickh
I'd rather be sailing...

 

[vyv_cox]

There is a quite good description at http://204.220.138.252/newspics/charts/911masthead.pdf
It is quite easy to do it yourself by following this procedure.

In your case it sounds as though you have the lowers, especially forward ones, quite a lot tighter than the forestay. When you tighten the backstay the centre of the mast is staying where it is, whereas the top is being pulled back. If you follow the procedure you will tighten cap shrouds and stays first, then take up the lowers, thus avoiding this problem. You should have a small amount of pre-bend but this should not change with wind strength.

 

[qsiv]

Perhaps 2 degrees is a steep rake for a masthead rig - certainly that would be normal for a dractional rig, but MH usually a bit more upright.

I'm assuming that the mast is deck stepped, with inline spreaders ..

If the mastheight was 14m then the distance aft along the boom for the main halyard to hang is about 20 mm (for 1 degree rake). Adjust forestay to set this - the back stay will only tension the forestay - not adjust the asft rake on a MH rig.

Providing you dont use extenders on spaners/screwdrivers used to wind up the rigging screws you are unlikely to put excessive loads in the standing rigging.

Ensure cap shrouds are tightened, then set forward lowers to induce the prebend required in the mast - perhaps about 15mm - ensuring that you keep the rig vertical at all times. Now you can tension the aft lowers to stop the rig bending further forward. The purpose of pre-bend is two fold - firstly most mains are cut anticipating some bend - if the bend isnt there the max draft in the sail will be too far aft - not fast and weather helm. Secondly (if a spinnaker is flown) without prebend you run a real risk of inducing inversion (when the mast bows aft) which can quickly lead to mast failure.

Finally - as all others say - go for a sail and check that nothing goes slack - slack rigging will cause a modern rig to fail quickly due to the shock loads imposed as the rig pants. If you do have anything slack, halve the slack, tack and repeat on the other side. Keep looking aloft - the rig must be straight laterally and should have a little pre bend fore/aft. Look up the forestay - you shouldnt have any sag (in moderate breezes anyway) and remove this by tighteneing the backstay. Any sag will reduce your pointing ability. You will need to adjust halyard tension to control draft position in the headsail - less tension implies rounder entry, less pointing, more power, more tension tesnion gives flatter entry, higher pointing, less power.