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Self build a sailboat trailer | Tightening and Tuning Your Rig |
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Nick Adamson had dreamed about competing in the Azores and Back (AZAB) Race for a long time. And at last, after much hard work, this was the year he would do it. But, just eight weeks before the start, he noticed a problem with Rebel, his S&S 34 ¬namely, a dramatic performance differ¬ence from one tack to the other. Actually, he'd suspected as much last year, but possibly the imminence of the AZAB start now made him more sensi¬tive. He felt that he was sailing over five degrees lower on starboard tack, and maybe up to one knot slower. It sounded like a job for the Sail Clinic to him. We agreed! When we arrived, it was cold, with fifteen knot southerly - typical Sail Clinic weather! Our first job was to check Rebel's mast to see if there were any obvious asymmetries. The rigging itself seemed much too slack - some¬thing we would check later under sail. And there was also some movement of the mast at deck level. This is the first time the Sail Clinic has seen a keel¬ stepped mast (see drawing). Nick said, "There are chocks holding it in place. They fell out last season, but I replaced them when the boat went back in the water. “Clearly, the chock¬ing system wasn't doing much of a job.That alone could be a serious long ¬term problem, not just for perfor¬mance, but also for security of the mast. Over a period of time, excessive movement of the mast at deck level can substatially increase fatigue in the metal. Curing the problem would both make the mast safer for the race, and also help the speed and sail trim. However, it was unlikely that this was the cause of the uneven performance from tack to tack. For that we needed to check the rig more precisely. We measured from the centre of the mast to the edge of the boat on both sides (see photo, left). This established that the mast was slightly set to port, by about an inch. But without a means of securing the mast at deck level, there was little we could do about it. Next, we checked that the shrouds were set to the same length, just as we did for the Hunter 245 in PEG 378 ¬and found a small difference top of the mast was leaning slightly to port. So we took up five turns on the starboard cap shroud to get it straight. This also tightened the rigging overall. We still suspected that the rigging was too loose, but decided not to make any more changes until we'd seen Rebel sailing. These two things could be the cause of the performance difference, but there was one other potential culprit that we needed to check before we went afloat. That was the instru¬ment calibration (see left). For if the instruments were reading inaccurately, and then the performance difference could be just an illusion! It was time to go sailing, and once out of Poole Harbor, we asked Nick to sail Rebel with his normal trim. He obviously knew what he was doing, and apart from pulling in the headsail a little tighter, we found that the initial set-up was good. We sailed the boat on both port and starboard tacks with the trim as equal as we could make it. On starboard we were sailing at about 5.75 knots and at an angle of 45° to the wind, with about 18 knots of wind across the deck. We tacked Rebel over, and on port we were sailing at about 6.0 knots and 35° to the wind, with the same 18 knots of wind. It's important that you make this comparison in the same wind speed, as the speed and sailing angle will vary considerably as the wind goes up and down. So, there was a problem, but not as bad as Nick had reported. Perhaps our changes to the rig had made a small difference. Could the rest be due to Rebel's rig and sails? The trailing log confirmed the speeds (see photo left), so we were definitely going faster on port than starboard. But the sea state was quite different from one tack to the other. On starboard Rebel was slowing as she hit the waves. But on port tack, we were able to sail more smoothly across them. David and I both felt that this could easily explain the speed dif¬ference. What about the wind angle? This could also be explained by the instrument error we'd found during our calibration routine (see below). If we added the 6° correction to port tack we got 41°. And if we subtracted 6° from the starboard tack reading we got 39° ¬only a couple of degrees out from one tack to the other. So the performance difference seemed an illusion - sus¬tained by instrument error and sea state. Nevertheless, there were some real improvements that could be made to Rebel's performance before the AZAB. |
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