I operate with a bridle main-sheet system with single purchase to the jammer just behind the centre-board case.  (No traveller)  I quite like the system except that the lack of purchase makes it pretty unmanageable in anything over a force 3. (I am not the Incredible Hulk).  

I am wondering how to put extra purchase in without buying a new mainsheet jammer.  Unfortunately I don't see any becket or other loop on the jammer to attach a second block, although there are two eyes on the boom, presuambly for two blocks (only one is present).  Could I screw a second block into the centre-board case, to the fore of the jammer?  Would this work?

I.e. How do I get more purchase in the fig 2 diagram in the attached link?

One thought, if you are using a system similar to the diagram, you could move the becket block from end of boom to bridle and put a double block on the end of the boom. This would give 3:1 instead of 2:1 and only cost the price of a new double block.
Need a longer mainsheet though I suspect (if you want to go downwind).

Hi mark, I know where your comming from, my arms ached all week after the training monday squall.
I find that a rachet block buy the main jammer helps reduce the pullback load, which is some of the battle. 
I have a spare if you want to try it.

DaveF

As you suggest, you could fit a block forward of the jammer together with another block on the boom - most of the centre sheeting arrangements are variations on this.  The fact that you are sheeting through the end of the boom, not just in the middle means that you are getting significant additional leverage anyway, and, yes it will involve a lot longer mainsheet, and less sensitive control.

This may be a daft laddie question, but is your main jammer block a ratchet one? And is it working properly? Makes a huge difference.  

Finally, a new jammer block is very expensive but I have just had to rebuild mine, and only needed to buy a basic block as a replacement and used all the other existing parts.  Much cheaper.

You should be OK if you put another single block on the centre board case, but make sure its securely fastened it will be under quite a lot of load and if you just screw down into the case I think they would pull out.

You could put a single block on a bridle across the thwart (width of the boat). This has an advantage as the forces are not straight up and down that a) less likely to pull the fixings out b) less downward force on the boom. Jury rigged something like this when Chambull's mainsheet hoop broke, and it worked pretty well.

Mark,

Burn it and fit a square-topped hoop and a big kicker.  3-1 on your mainsheet and a sexy harken traveller.  Someone on here is offering track and car for good money.

Alan,  I'm intrigued by your suggestion, although a bridle across the middle of the boat sounds like something extra to get tangled up.

Jon, Like your idea but the time value of money is high if I have to fit a full traveller system on my boat and get all the bits etc.

DaveF, No I do not have a ratchet block and I would welcome the opportunity to try one - thanks

MG3508, I like this idea - simple.  Would it run feely when sheeting out?

Bill, A block forward of the jammer was my initial thought, but I can't quite see how to do it without the ropes crossing at at least one point and resticting the flow of the sheet, what do you think?

5 good ideas in less in about an hour, choices choices...  This forum does work well - many thanks.

Should run ok - but would you can reduce the purchase in light airs to suit.
One correction to my original suggestion - the diagram you referenced is already 3:1 so by adding a double block to the boom you will get 4:1 - which is quite powerful for a mainsheet - but as you haven't got a ratchet it might be a good compromise.

Mark

I changed the mainsheet as you describe (split bridle long enough to enter the boom-end block when sheeted in, along the boom to block on boom above thwart, down to the mainsheet ratchet) but I left the round topped hoop in the boat.  Rather than running directly from ratchet to boom, the mainsheet also passes through a block attached to the top of the hoop and I think this helps.   Between the hoop and boom the mainsheet is much closer to the horizontal so your arm strength is spent pulling the boom towards the centreline rather than downwards.   It works without big arms and is useful to grab.

I tried the centre bridle because I wanted to be able to pull the block to windward and bring the boom onto the centreline.  Too fiddly, too much friction and I managed to loosen both ends of the thwart.  I switched to the arrangement described by Chris 3361, of a split main off the transom, simpler, and, with a ratchet block quite strong enough for my purposes.  However, if you wanted the extra purchase provided by a block on the thwart, the answer would be a double block on the boom, rather than two separate blocks.

Bill,

I think that is the answer.  If you remember the original diagram in the link, figure 1 is a split main and figure 2 is a single main running to a 2;1 at the end of the boom, a bit like a Laser main sheet system, except the pulley is fixed in the middle of the bridle rather than travelling.

What are the advantages of a split main?  My split-main is a bit of pain as it keeps twisting up.

The theory is that the split main allows you to centre the main without tightening the leech as the force is at approx 45 degrees rather than straight down, thus allowing your powerful kicker to do the fine adjustments to the main sail shape. On a laser (without a jib)you don't need to centre the main.

Many thanks.  Here is what I think I am going to do.

At the point where the main splits I am going to insert a single block fixed to the centre point in the bridle.  Then a single block with becket on the end of the boom will give me 2:1 when the end of the main sheet (where it used to split into two but is now cut) is attached to the becket with a stopper knot.

Then in the middle of the boom, replace the current single block with a single ratchet block, and continue to use my old non-ratchet mainsheet jammer.

This should give me 2:1 with a ratchet, where before I had a split main with 1:1 and no ratchet.

Sounds like a plan...

If you haven't got a ratchet block now then I stromgly recommend you try it with the same purchase and the ratchet block before you get into putting on extra purchases at the transom. The difference a good ratchet block makes is tremendous and there's a lot to be said for having half as much rope to pull in and out in every gust.

Mark I will put the ratchet in the post to you so you can try it.