Tuning The Rigging On Fractional Rigged Sailboats

Posted By: Bruce Whitmore
Date: 5/25/00 11:51a.m.

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Tuning Fractional Rigged Trailerable Sailboats A Primer By Bruce Whitmore I have a MacGregor 26x, and can't tell you how much the proper tune helped my boat this year. I have done some reading on the subject, watched Brion Toss's video (http://www.briontoss.com) and have generally become familiar with the subject. While a number of the comments here will be specific to my MacGregor 26x, the basic comments will apply to most trailerable fractional rigged sailboats. The procedure is similar for masthead rigs, but not quite the same, as the spreaders may not provide a bending force on the mast, instead relying on the backstay to provide the mast bend. One question that is commonly asked is ăWhy bother?ä ăStainless rigging doesnât stretch, right?ä and finally, ăGee, it looks tight ö it must be OK, right?ä Well, stainless cables DO stretch. Not much once they reach a reasonable tension, but nonetheless, stretch does occur. Also keep in mind that there is some initial stretch that occurs because of the cables are made of stainless wires that are wrapped around each other - in other words, initial tension removes the "slop" from with wire itself and the wire bundling. Not to mention, on many smaller boats, the rigging is attached to the mast and chainplates using simple loops, thimbles and crimp connectors, meaning you need to get the slop out of the connections before the rigging will be tensioned correctly. The initial tension required to take most of the stretch out of the wire is about, IMHO, 5% of the breaking strength of the wire, or in the case of my 26x, about 165 lbs. of tension. You will be probably be surprised at just how much tension that really is. I suggest, however, that you get a gauge to do this - it is extremely common for people with regular turnbuckles to over-tension the rigging. It is also very common to under-tension the rig as well. Be aware that overtensioning is bad (here's a case where more is NOT better) but at the same time, undertensioning is bad too (and maybe worse). If rigging is too tight, you will wear out your rig faster, and cause some nasty mast compression forces to build up. On deck-stepped masts, you can even do damage to your deck, especially if there is no compression post inside the cabin to support the mast compression forces. On the other side, if the rigging is loose, the mast will slop side-to-side, from waves, sudden bursts of wind, etc., causing tensions to suddenly rise in the rig. This is called ăshock loadingä. These shock loads can end up being way above the designed loads, either weakening the rig or breaking it outright. In addition, a loose rig will cause the mast to tip off leeward, increasing the wind force downward (think about it for a moment, it will make sense) and therefore increasing heel. As the heel increases, the weather helm increases. In the end, you have a boat that doesn't want to point. As for doing the job, plan on a couple hours worth of tinkering, especially if you don't have true turnbuckles on the stays (I just have those slot adjuster thingies that MacGregor uses ö this makes the job kind of a pain). Once youâre done, however, you shouldnât need to do it again ö assuming you mark the turnbuckles and adjusters for the correct position. First, check your owners manual and try to find out as much as possible about your particular boat. While these comments are guidelines, each boat can be a little different, so relying on any one source of information (even the manual, if it is vague or unspecific about tension specifications) can be dangerous. For those of you with MacGregor 26xs, I have found that the mast rake should be about 2 degrees and the mast bend should be about 1-2" if you have an adjustable backstay. You will probably not that this contradicts the manual ö its just what I found works for me, reducing weather helm and increasing pointing ability. Now, with the mast down, measure the shrouds for equal length. Adjust the shrouds to equal length if they are not there already. Another option is to measure the shrouds with a tape measure with the mast up. Raise the tape measure on a halyard and measure to each chainplate for equal length. If not equal length, adjust the shrouds accordingly. This is your starting point for a straight mast. Now get your mast stepped with all shrouds loose, including the backstay (for now). Adjust the forestay to get the amount of mast rake you have in mind. Moving mast rake forward will reduce weather helm or increase lee helm. Moving the mast rake rearward will increase weather helm or reduce lee helm. In the end, you want a little weather helm. Once adjusted, make a mark on the forestay turnbuckle so you can re-set it to that rake consistently. Now remember, rake is the angle of the mast to the WATER, not to the deck or the bootstripe, unless your bootstripe or waterline is truly level with the water. With the MacGregor 26x, so many of us have different motors (including some up to a whopping 90 hp!) that loading of the boat can differ substantially, effectively changing the mast rake, even though the angle to the boot stripe is identical. Therefore sticking to the ăspecsä may not get you the handling you want. Once you have the mast rake set for the desired angle, tighten the upper shrouds EQUALLY ON EACH SIDE (in SMALL increment) and tighten the backstay to induce mast bend to the desired amount. Some people recommend tightening the backstay to get the bend, then tighten the upper shrouds to maintain that bend, but either way, you want to tighten just enough to get the bend you want. With fractional rigged boats with backswept spreaders, the upper shrouds induce the mast bend. The lower shrouds limit the maximum bend, and keep the mast from ăpumpingä over waves and in puffs. As you tighten the shrouds and backstay, sight up the mast to make sure it is straight laterally (side-to-side) and doesn't tip off one way or the other. Do this often (after each adjustment from this point forward). Also, double check your mast bend, and make sure it is a nice smooth bend, with no s-shapes, etc. Keep in mind that on a fractional rigged boat, an adjustable backstay will only bend the top of the mast, so don't rely on it for all of your mast bend under sail. Instead, induce some bend using the rigging, and use the adjustable backstay to get a little additional bend when needed. I have heard it is possible to actually bend (permanently) the mast if you give the adjustable backstay a gorilla pull. At this point, you may be having to back off the forestay turnbuckle, and maybe even releasing the forestay entirely to make the adjustments on the shrouds if you have those slot/hole adjuster thingies (love those nautical terms!) rather than true turnbuckles. Once you have the desired mast bend, tighten the lower stays to hold correct amount of bend in place and stop mast "pumping" in heavier seas. Ideally, you will keep adjusting first the upper shrouds, then the lower shrouds. You will keep going until you have tension on your upper stays of about 15% of the breaking strength of the shrouds (check a West Marine or other catalog for specs on wire) and about 10% of the breaking strength of the shrouds on the lowers. All this while maintaining the proper mast bend, rake, and lateral alignment of course. Remember, once you've got it right, its done - no more messing with it. So if you can borrow a gauge, rather than buying one, so much the better. In closing, I will tell you that with our rig undertensioned, I found one of our thimbles on the lower shrouds cracked all the way through from shock loading ö obviously not a good thing· Keep in mind that the MacGregor manual and many other trailerable sailboat manuals are extremely vague on the issue of correct rig tension. I have asked a master rigger about the correct tensions, and it seems that MOST builders design their boats and rigging to be tensioned according to the "rules". They are: tension should be 10% of breaking strength of the wire on the lower shrouds, 15% on intermediate shrouds (if you have them) and on taller rigs, between 15-20% on the upper shrouds (on a short rig like a 26x, I took this to mean 15% on the upper shrouds, and this is supported in other reading I have done). I do NOT know for certain that the 26x (or any other boat) is "built" for this tension, but when you read the information available, it seems most manuals seem to support these conclusions. Any way about it, if your rig is undertensioned to the point that you get "shock loading" over waves and in puffs, this results in much higher temporary loads than would occur if it were properly tuned in the first place. Think of it this way - you're pulling a car out of a ditch - if the rope is loose and you get to 10 mph before you hit the end of the rope, BANG! Something's going to break ö it might be the rope, it might be the car - the same thing applies here. By the way a 10% load on the shrouds will make a very low "musical note" when whacked. Were talking about as low as the lowest piano note, maybe lower. 15% makes a slightly higher note, but not much. In the MacGregor 26x manual, I think there's a comment about the rigging not being "so tight as to play music", and I think this would qualify. Once you have all this done, you've completed your dockside tune. Go sailing. The lee shrouds should loosen a little in 15 - 20 knot winds, but not go slack. The top of the mast shouldnât fall off to weather, and should look straight in these winds. It is possible that the top might point toward the wind in very light winds, but that is OK, as long as the mast is straight in 15 knot winds. The boat should point well. It should have a little weather helm (my 26x has none - the steering with a big motor attached is too stiff to feel weather helm). The mast should not "pump" forward when hitting waves. The boat should not heel excessively, and it should generally be well behaved. Is "close" good enough? It all depends on how "close" is close (no, we are not talking about "what the definition of "is" is). If you asked me is 5% tension close enough for most trailer-sailors, I'd probably say yes. But then again, I'm not an expert. I would say, however, that if your mast is straight laterally, if it has the correct mast bend, if the lee wires do not go floppy on you in a close reach in 15-20 knot winds, you're rig is probably tuned acceptably UNLESS the rig is over tensioned. As a test, ăwhackä on your wires to make sure youâre getting a LOW note, that the lowers are a slightly lower note than the uppers, and that they are the same ănoteä on each side. Lots of ifs, I know. This completes the primer on rig tuning. Lots to know, but lots to gain. In closing, racers will generally keep their rigs tighter than cruisers, but may also get a correspondingly shorter life from their rig. Each boat is different, so study up. Finally, talk to a rigger if you can find one, or get a rig tuning video. The tuning of the rig will have more impact on your speed and handling under sail than anything else except sail trim, and great sail trim still wonât offset the affects of a bad rig tune. As always, I hope this helps you enjoy your sailing to its fullest. Fair winds.