I see atleast five advantages to this system (www.layline.com/IIf/Boats/505/505-mainsheet.htm) not the least of which is a better sheeting angle from the wire and an improved "feel" for the main...
i'm concerned about screwing this gizmo to my boom and compromising it's reliability. do any of you have thoughts on the subject ?
I will tell you a few things that I dislike about the idea of boom sheeting.
I need to pull my boom down toward my rear cross bar, not toward the windward hull.
I like to pull in line with my feet so that it actually makes my footing more secure. With a boom sheet, I'd be pulling my body toward the center of the boat.
If purchases are run along the boom, this increases compression load on the boom, so bumping into the boom could be more of a problem than if you were not using sheeting purchases along the boom.
Gary , I'll see this objection and raise you : 1. cleaner tramp (compared to the boom to tramp system i'm using now ; which is superior to the ubiquitous and outdated block to traveller setup I.M.O ). 2. more reliably uncleated (in emergencies) , because you don't have to "shoot" the sheet to uncleat ; cleat is uncleated parallel to boom. 3. Way better feel in light air 4. better sheet positioning for crew....
you are not on your own on boom sheeting for cats, Tim has been doing it on Mossie for a while, he sailed 4.9ers a bit and just prefers it. He hadn't found this fitting though and has tried a number of different cleating solutions and blocks.
I saw a system that Wouter had posted here a month ago or so, with both mid boom and regular sheeting. He said the F18 sailors are doing it by setting up the main sheet as normal but instead of tieing off the tail, you run it through a block at the end of the boom then forward to the mid boom block/cleat you have shown in the attachment. The system he talked about used a 2-1 set up which limits the amount of mid boom adjustments to 1/4 of your boom length, but if you don't use the 2-1 block, you could sheet from either end equally. That way, in light air you could use the mid, or use the regular for those "back of the boat" days. The "pull" of the boom would still be to the rear beam in either case.
Steve, I did that on my f18HT! It has a very low boom. I was crewing and on one tack I didn't duck low enough, when I went under the boom the outhaul block (near the forward boom, hung down just like the one pictured above) tore the back of my life jacket wide open! I had to buy a new jacket, the rip was so big that most of the foam flew overboard! It's so much easier to drive...
PK- My Taipan came with midboom sheeting from Aus (it was Greg Goodall's personal demo.). Jim Boyer and Greg swear by midboom sheeting and they do NOT use cleats!! I tried it when I received my boat but ended up looking like a rookie blowing every tack in the big breeze/waves of the Great Lakes so switched to conventional/rear. I did not realize at the time (since mine and the one Jim were sailing at the regatta were the only Taipans I had seen in person) that the booms from AHPC are different for midboom vs rear sheeting. My boom is quite a bit larger in diameter and heavier due to the bending moment induced by midboom sheeting. I think the ideal arrangement (for me) is the one Wouter mentioned- so I am modifying my boat by adding an autoratchet block with becket and cleat at midboom and a small carbo block at the mainsheet tail at top block. One of the biggest advantages I see with this system is the ability to double your purchase for upwind work in breeze and ability to sheet going wild facing forward. And you can even decrease the purchase of the "main" blocks at the rear to save weight and speed for gross trim. Main disadvantage of course is you only have so much adjustment. But you can "blow" the two part system and not worry about cleating because block will act as a stop on top block. The Marstrom "A" cats use a mainsheet system inside the boom with only a single block at the traveler car. IMO running the main down to a block on the tramp only confuses me and makes the job of tacking/jibing that much more complicated. I marvelled at how Jim made it look easy. I always ended up with the line wrapped around me or something else!
When mentioned my intend to try a mid boom sheeting setup a befriended sailors said that I should put the block at the middle of the foot or farther forward. He said that one would pull the boom too much towards himself if the block was further back. Apparently he tried this setup himself as well. Apart from that he was pretty ambiquious about which setup is better. He seemed to see advantages and disadvantages of both. Note that we are talking about the boom sheeting setup here and not the centre sheeting setup where the ratchet block is fitted to the trampoline and a vertical stage from tramp to boom is present. This latter setup looks clumsy to me and I will get in to trouble with my crew with that setup. The boom sheeting setup leaves enough open space for me crew.
There are a few other considerations I have for this mid boom system.
-1- I'm considering it because it allows my crew to sheet the mainsail much better while keeping the rear of the trampoline clear. At this time (rear sheeting) the sheet is nearly running over my legs rubbing against it. In a blow my crew can not stand to the rear of me as then the sheet will have to go through my upper body. With the mid boom sheeting I expect this to improof allow us to drive the boat harder.
-2- With the double ended setup , I as the skipper can still work the mainsail just as much as the crew can. I think this to be important for downwind legs when the crew is busy with the spinnaker. Also as a safetly will this be important especially if the crews end of the mainsheet is boosted by another 2:1 stage which will limit the play.
-3- I'm looking to boost the crews end of the mainsheet with a 2:1 so that when singlehanding I can sheet it continiously with one hand and arm and still retain enough strength for holding and working the spi sheet on the downwind legs under spi. I have a double handed main and I seems that I need to trim it continiously to really make it go singlehandedly.
-4- I feel that I can wildthing alot better with the mid boom setup if I ever need to do that. Like singlehanding on a reach and such.
-5- I sailed with a mid boom sheeting setup on the 49-er skiffs and I works okay apart from not having a cleat on the boat.
Also note how a boosting stage at the crews end of the mainsheet will pull the boom less to the luff side of the boat. The load ont he sheet line determines how much you pull the boom towards the luff part of the boat. I reduce this sheet load by a factor of 2 by adding a booster stage then the boom will move less by a factor of 2 as well and then by far most of the effect will be to pull the boom down.
Wouter
Wouter Hijink Formula 16 NED 243 (one-off; homebuild) The Netherlands
I marvelled at how Jim made it look easy. I always ended up with the line wrapped around me or something else! 
