Has anybody got any ideas as to why this keeps occurring and how to stop it?

You just need a block on the boom that doesnt swivel.

Could it not be that the winches are causing it? I think that the self tailers may be putting in a twist. I have been doing a bit of canyoning and abseiling and there is a fair bit if study on how putting rope through a fitting can make a twist occur. I would try it without the self tailers and see if it stops. If so, you may have to feed the rope in with an opposite twist as you ease the sheet.

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Kankama said..
Could it not be that the winches are causing it? I think that the self tailers may be putting in a twist. I have been doing a bit of canyoning and abseiling and there is a fair bit if study on how putting rope through a fitting can make a twist occur. I would try it without the self tailers and see if it stops. If so, you may have to feed the rope in with an opposite twist as you ease the sheet.

If you take off the mainsheet and lay it down the deck, coloured fleck(or even the weave) in the rope will help show if it is twisted, the pattern may appear to spiral one way or the other - roll in the opposite direction required till it all 'lines up' straight again, and re- feed.
Having said that, i think Stray is on the money - a non-swivelling block with an appropriate shackle.

Some blocks have a small grub screw on the swivel part and you can lock it in place.

Thank you so much for those replies. Once again it's great to have so much experience here particularly for those of us who don't have a lot of sailing buddies.

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tarquin1 said..
Some blocks have a small grub screw on the swivel part and you can lock it in place.

Thanks, upon close inspection it turns out that my newish Selden blocks have a little slide gate which is not immediately obvious which simply needs pushing in to lock the swivel. I slipped this across on all three blocks today but didn't get a chance to go for a sail to see if the problem is rectified.

I had the same problem driving me nuts. Locked the boom block rotation and now all is well.

The twist is caused by the winch. When you winch on you will have about four wraps on the winch, but as you ease you usually take a couple of wraps off. Each time you do that if feeds those deleted wraps back into the rope. This is a problem on big boats where you need to use winches 100% of the time. We need to untwist most of our sheets after every race.