Ditto what Keith says about leverage and mechanical advantage. All things being equal, mid-boom sheet gives you half or 1/3 as much mechancial advantage as end-of-boom sheeting.
Most C27's have 4:1 for end of boom sheeting, which is still a handful for most men when beating upwind in high wind, IMO. (I personally like 6:1 end-boom sheeting for short handed sailing upwind). For mid-boom sheeting, you'd want something significantly higher than 4:1. Because I'm no gorilla and I'm lazy, I chose a Harken System 332. (see the attached jpg) for my end of boom sheeting. It's a two speed (6:1 and 3:1) hexarachet system, with a continuous (loop) mainsheet. I use the 6:1 for trimming upwind, and the 3:1 for trimming downwind. 6:1 is a lot of sheet to haul when gybing so the 3:1 is perfect for that or for light wind trimming. (The only drawback is that it's a lot of sheet in the cockpit) The hexarachet feature increases the holding power of the block by about 10 times, so I don't need a winch to trim when beating upwind in heavy weather. I can trim with one hand and steer with the other upwind with ease. Making it easy really improves my sail handlig skills! ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Bill Smith already mentioned the importance of safe working loads... ditto on that. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ One more thing to consider in addition to safe working loads: bearing engineering The bearings on your old mainsheet blocks may be shot or may simply be the wrong kind for a mainsheet Different blocks are designed to handle different kinds of loads. Halyard blocks are designed to handle high static loads for a long time without deforming. Blocks that handle free running lines are designed to minimize friction and to run freely under load. They're engineered differently. For a mainsheet, you want a block designed for low friction under high dynamic loading. Below are some notes on block sprecification from my C27 deck layout page at http://www.blumhorst.com/catalina27/RiggingDesign.htm Fair winds, Judy B Safe Working Loads: There is a program on Harken's website that calculates the required strength of each piece of hardware, given the specifications for you boat. You can go to Harken's CompuSpec program, select your boat (in my case, a Catalina 27) and get a report on the loads on all the lines on the boat. The program will interactively recommend blocks and other hardware with the correct safe working load for specific applications (mainsheet, turning blocks, winches, etc). You can look up the specifications for each recommended block, to see how strong a block you'll need, based on the application and the angle of deflection of the line. Match the Bearings to the Application There is one more important thing to consider besides the safe working load of a block: that's the type of bearing. You can't always safely substitute one 500 pound block for another 500 pound block. Blocks that must survivelarge static loads require special bearings that won't deform under a constant load. Examples of these are blocks for the main halyard and jib halyard . Other blocks must have bearings that will release quickly and run friction free under heavy load, such as the mainsheet. Not all bearings are the same!!!! You may buy a cheaper block that's strong enough according to the required safe working load, but if the bearings don't perform properly for the application or won't hold up to a standing static load, you're just wasting your money ________________________________ From: "Sneddon, Keith - ES/IS" <[email protected]> To: "[email protected]" <[email protected]> Sent: Thursday, June 11, 2009 8:20:18 AM Subject: RE: [IC27A] Main sheet trim This all comes down to the physics. To trim your main, you have to pull against a force (the lateral force on the main) through a distance (the angle the boom moves). The product of force times distance is work. When you pull on the end of the boom, you are pulling the same angle to achieve proper flow, but the point you are pulling on is moving a lot farther than if you have mid-boom sheeting. To do the same amount of work, you don’t need to pull as hard. To put some rough numbers to it, if you pull at the mid span of the boom, you have to pull twice as hard for half the distance. To get back to a reasonable pull force, you can add purchase (8:1 vs. 4:1 pulley setup, for example) to trade back distance for force. Nothing is ever free, however, and each layer of purchase adds drag, which hurts in both directions (sheeting in and out). Big sheet tension hurts drag too, as the frictional force is directly proportional to the sheet tension. For this, and a bunch of other reasons, end boom sheeting is mechanically more efficient, but Sailboat Design, like life, is a series of compromises and imperfect solutions. There are a lot of practical advantages to mid-boom sheeting. Keith Sneddon #4760, “Are We There Yet?” ________________________________ From:ic...@yahoogroups. com [mailto: ic...@yahoogroups. com ] On Behalf Of Tim Millar Sent: Thursday, June 11, 2009 10:49 AM To: ic...@yahoogroups. com Subject: Re: [IC27A] Main sheet trim Jeff, I have the same original setup on our 1986 and agree that it can be difficult. Hope we can get an explanation and solution. Tim Sea Scout Ship 76 Spirit, #6254 On Jun 11, 2009, at 10:29 AM, jlenox225 wrote: I searched the forum and did not find any other posts regarding this issue. Does anyone else have trouble trimming and easing their main. I have mid boom sheeting to the original cabin top traveler. I find that the sheet binds up when I try to ease it under load and I need to assist it by feeding line from where the sheet enters the aft most block on the boom. When sailing to weather it seems like there is a significant load on the sheet when trimming which requires the use of the winch handle. I would even say that with the sails down, to fully sheet the boom amidships it feels a bit tight. It would seem that it binds somewhere in the turns from the winch to the turningblocks on deck to the 3 blocks which redirect the sheet to the traveler. I found that I had the same problem on my S2 27 which had a similar set up. It appears by looking at other C27s that this is the standard set up. I hope I explained this in a not too confusing manner. Thanks, Jeff ________________________________ This e-mail and any files transmitted with it may be proprietary and are intended solely for the use of the individual or entity to whom they are addressed. 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