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September 2008

On Thursday, 18 September I left my Raptor with Solway Dory for the day to have some of the pending jobs done. They had looked at EddieVB's modifications on my website but recommended a different approach for leech tensioning. We decided that it is preferable to use two different lines for outhaul and downhaul respectively, rather than to use a single line for both functions. Individual lines should allow more flexibility trimming the sail and quicker setup. EddieVB had sent me further photos and comments on his rigging and it turned out that he required one line less than I had thought, which made it a more attractive solution to me. His boom track car actually only needs one rope, used to pull it forward. The outhaul line from his sail clew, which passes through a block on the car to a fixed turning block at the aft end/starboard side of the boom, will always try to pull as straight as possible when it is under load. This will result in the car moving aft, due to the angle of the boom, so no additional car line is required to move the car aft. EddieVB does use a shock cord attached to the rear of the car, passing through a second fixed turning block, this time at the aft end/port side of the boom and then forward to a point by the furling line cheek block, where it is attached to the boom. However, this shock cord is only needed to pull the car aft when the outhaul line is not under tension. So only one additional cleat is required in front of his cockpit, not the two that I originally thought.

EddieVB's boom track and car solution looked good to me, but, since it is easy to reach the boom from the Raptor's seat, I asked DaveS if there was a way to move the car and lock it in position directly by hand (for simplicity), rather than by using a line. He suggested that we use a prussic loop to attach the block for the leech tensioning line to the boom (Solway Dory use this knot on their own boats), thus eliminating any need for boom track and car. The prussic loop can be slid along the boom until the block lies directly under the sail clew once the leech tensioning line is released, and the line then retensioned, at which time the prussic loop "locks" the block in position, preventing further movement. I thought this sounded like an elegant solution.

We decided against shortening the boom length, since I was concerned that a more acute angle of pull on the outhaul line might make unfurling the sail more difficult. However, we decided to move the boom fairlead 6" forward and to move the mainsheet block eye strap to the end of the boom, to allow a greater range over which the prussic loop could be moved (ensuring optimal position for the block within a range of sail reefing settings). DaveP came up with a simple boom fairlead solution for the three lines (mainsheet, outhaul and leech tensioning), using a strap riveted to the top of the boom. Solway Dory used 2-inch (50.8mm) OD aluminium tube for my new boom, with an ID of about 46.8mm. This was slightly too large to fit snugly over the mast step post (with OD of 44.3mm) so they had to rivet an additional tube inside the outer tube, with nylon sleeving around the inner tube to ensure a tight fit inside the outer boom tube (there's a 4.7mm gap between the two tubes). They used 1" aluminium tubing for the inner tube. Again, this was slightly too large to fit snugly inside the mast step post (which has an ID of 38mm), so they had to machine the end of the inner tube down to an OD of 37.5mm (about 1.48 inches). They also machined a notch in the top of the outer boom tube, to engage with the mast step bracing and to provide some resistance to rotation of the boom (although there should be no tendency for the boom to rotate). The inner tube protrudes 62mm from the inner tube. DaveS conducted a quick test with a spring balance on the old boom prior to installing the new one. With a downwards pull of 10 lbs on the end of the boom, the old (carbon fiber) boom deflected down by 6". By contrast, the same pull only moved the new boom down by 3", so the additional rigidity of the new boom is obvious.

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