Not sure I see any advantage other than the crew can pull from the midboom. From what I can tell from your description, purchase from the two triples plus single on the becket is 7:1 and the two tails do not add more.
Also, doesn't pulling a line from the midboom cause the boom to move towards you against the sail force, making you pulls less effective? I get this sometimes from the main traveller if it's out a bit (reaching in heavy air) and needing to sheet in the mainsheet after a gust...traveller comes up until near center before main sheet comes in.
I'm pretty sure a fully loaded main wont move with you pulling on a block at midboom. My Acat midboom doesnt move the boom when I pull on the main sheet. Keep in mind the load is traveling thru a block and not directly on the boom at the midboom point.
If you had the midboom farther back or it was light you might move the boom.
As far as what advantage you get, the only reason I think you would want to do this is to give the crew a cleaner "pull" and not have to worry about the helm getting in the way of the main.
On the Ht (or i20 or F18) the crew trims the main and if the driver is sitting on the boat, the sheet usually is over the drivers legs or his lap. So the midboom moves it away from the driver. Not really any advantage, and it complicates the tacks in my opinion.
O the Acat the advantage to midboom sheeting is that you can sit forward downwind and play the main, since thats your main control downwind its nice to be able to sit in the middle of the tramp and face forward.
I'm actually thinking about a way to sheet rear beam upwind and midboom downwind. I think tacking a midboom is harder than the rear beam sheeting.
Bill
