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C&C 27 Association – Tuning The Rig, Mks I to IV
Whether you race or cruise, it's worth taking some time over rig set-up. The boat will be quicker and easier to sail. If you race, you may want to use a tension gauge to check your settings, but a tension gauge is only a time-saver, not a necessity, because the whole point is to get the mast to behave a certain way on the water – basically to be straight and centred over the boat, with just enough rake to eliminate weather helm in your average sailing conditions – and this state can be achieved by trial and error on the water.
The following process description was submitted to the Forum by Bruce of Moonstruck, a Mk III in Chicago (the process will be slightly different on Mk I and II's because their lowers are in-line). The only tool mentioned is a tape measure, but even that isn't necessary – the site admin hooks a collapsible boathook into the main halyard, adjusting that to just touch the toerail on one side, then adjusting out any variation on the other side, until the masthead is amidships. Bruce also mentions tuning loads of percentages of wire breaking strength. The average sailor won't have a means of hitting these numbers precisely, but should think of the 10% number as "really, really tight" and the 15% number as "really, really, really ridiculously tight" (it's a log scale).
Running your turnbuckles up this tight cannot possibly damage anything that isn't already looking for an opportunity to break, but it can cause galling of the turnbuckles if they're not greased or oiled when you work (saltwater sailors may prefer anhydrous lanolin). Slather on lots, then wipe away the excess when you're finished.
Before you begin, you may want to ensure that the channel that forms the mast base is centred in the boat, says Clare Jordan of Aragorn. He discovered quite by accident that his mast base is a fraction of an inch out. The process he used is described at the end of the main tuning instructions. This only has to be done once if you hang on to the shim pieces, or at least their dimensions.
Another area to watch is spreader movement. If spreaders are free to move fore and aft and particularly if the degree of play is different on one side, the shroud tension will vary and it will vary differently on one tack than the other. Replace the rubber hose as described and shown here.
Basic rig tuning
"First, the 7/32" wire you have has a breaking strength of somewhere around 6,300 lbs. You will want to tune the upper shrouds to about 15% of their breaking strength, or 945 lbs. This is just a starting point, as you will then sail the boat and see if you need more tension. For the lowers, the starting point is 10% of breaking strength, thus about 630 lbs of tension, though on my 27 Mk III, I ended up at about 700 lbs + of tension to keep them from getting floppy at about 15 – 20 degrees of heel.
As for rake, you will want to mess around with this until such time as you get a little bit of weather helm under normal sailing conditions. We are talking small amounts of weather helm here, as in the boat just wants to turn up a little bit (and slowly) into the wind at say, 10 – 15 degrees of heel.
As for the process, here's how it all works.
1. Hoist the end of a tape measure partway up the mast. It doesn't matter how far. Measure over to the toerail (I measure to an acorn nut on the toerail so I know I'm measuring to the same spot side-to-side). Check the measurement on the other side as well. Center the mast so the measurements are the same. While doing this, tension the uppers somewhat, but not all the way. Say... about 350 lbs of tension – not enough to induce bend, but enough to hold the mast solidly in place. By the way, do all this tuning with the backstay adjuster loose, and the boom not tensioned against the topping lift.
2. With your rake set, begin tensioning the lowers. Sight up the mast slot to make sure the mast does not bend in the middle one way or the other. Offset any bend by tensioning the lower stays. Get it straight. One way to tell is to hook the main halyard at the gooseneck, and tighten it relatively firmly. The tension on the halyard will keep it straight enough to sight up the mast & look for bend. Then look at the side of the mast, and induce a small amount of bend in the mast. The small amount is say, no more than 1/2 the width of the mast. That bend should go forward, and not aft. Accomplish this by tightening the forward lowers evenly (and watching for inducing lateral bend!). Once the amount of bend is right, then tension the lowers (again watching for bend) to "freeze" the amount of bend in place. Then tension the lowers evenly to get to the 630 lbs of tension.
3. Tension the uppers to 15% of breaking strength.
4. Go sailing. Get out in enough wind to heel the boat about 15 degrees. Watch to see if the shrouds go loose. They should get a little looser, but not be shock-loading over waves. If you see the shrouds going "boing", they're shock-loading. Sight up the mast, and see if the masthead is tipping off one way or the other laterally. This is should not happen, and should be adjusted out.
Once you get this done, you should be finished. Mark the spots on the turnbuckles with rigging tape so that next time you step the mast, you can tension right to these spots and be close (if not right on)."
– Bruce, C&C 27 Mk III, Moonstruck, Chicago
Centring the mast base
"I found that the two chainplates for the upper shrouds measure equidistant from the toe rails so, at least on my boat, the rig is symmetrical with the hull but the aluminium base plate for the mast must have been installed a bit to the starboard side of the boat's centreline.
To compensate, I centred the top of the mast over the boat, vertically straight and with moderate tension on the shrouds, then measured from each side of the mast (at its base) to the inside of the upper shrouds on both sides, at the same level above the deck (approximately horizontal with the mast base), to see if the base of the mast is centred between the shrouds and therefore directly under the top of the mast. To be centred between the shrouds, the base of my mast has to be placed within 1/8" of the port side of the aluminium base plate."
Clare Jordan, Aragorn, Brockville
[Some fleets take a slightly different approach. In the J/35 fleet, people measure from the foot of the keel to the toerail to establish if the keel is centred and straight on the hull. If not, the difference between port and starboard becomes a corrector for the masthead measurement, with the object being to get the keel and the rig on precisely the same plane so that the boat will perform equally on either tack. This makes sense on a symmetrical hull with high-performance foils and rig like a J/35's, but probably is not applicable to a slack-bilged hull form with relatively low-performance foils and rig like the C&C 27's.]