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post #1 of 9 Old 04-16-2018 Thread Starter
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Tuning The Mast

My 1977 Mirage 24 was designed by C&C. An article on the C&C photo album web page about mast tuning explains the process without using a Loos gauge. Can a mast be tuned well without a gauge? I wouldn't torque keel bolts without a torque wrench.
-CH
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post #2 of 9 Old 04-17-2018
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Re: Tuning The Mast

The correct way of tuning the mast is without a gauge. The gauge can be used if you have the values supplied by the rigging producer but you do not need them. Tune your rig, go sailing and adjust the tensions when close hauled. You can also do it without sailing. It is impossible to apply more than enough tension with standard tools.
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Re: Tuning The Mast

the rigging keeps the mast where the mast should be to sail correctly. After you have the proper position of the mast during sailing then you can use a loos gauge to duplicate it if the rig is removed and re rigged. forestay length sets the mast rake fore and aft to balance the helm. backstay tension sets the forestay curve to match the designed foresail luff curve. the shrouds keep the mast in the center of the boat and straight. if you set the wires to do that when sailing the tension on the wires when not sailing will be correct because it does not matter what the tension is when not sailing because the wires are only there for sailing.
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Re: Tuning The Mast

The should be tight enough to prevent stressing the foot of the mast by allowing movement past its range of motion. I am seeing a lot of damage lately at the foot of the mast from too much play in the standing rigging with the majority from the forestay being left too loose to increase rake shearing the front rivet between the tube of the mast and the foot casting or causing the mast tube to fracture below the front rivet. A lot of tabernacle plates are being bent too from forcing masts too far back or leaving the shrouds too loose. A bit of common sense goes a long way here.

Boats are in a rental pool so we have a lot of tinkering by renters who feel they are set too tight, have the rake tuned wrong, etc and personalize things to match their misconceptions. Its been estimated we see about 5 times the normal wear and tear when the boats are in day rental instead of a long term lease or personally owned.
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Re: Tuning The Mast

I may have been doing in wrong for 40 years across 5 boats, but I've always done it by feel.

1st I center the mast with the uppers using a halyard to a known point on either side. Then I site up the mast to set the lowers. Forward and aft is a running adjustment on most boats anyway if you have an adjustable backstay. Otherwise, pretty tight to avoid head stay sag. How tight on everything static? I don't know, you just get a feel. How far can you move the stay out of column.

Then, with everything set static, I go sailing on both tacks. Site up the track. If it stays straight, I've got it. If not, I tweak. OK if the leewards get a bit of slack, but not so they are flopping around.

One hasn't fallen on my head yet. Maybe I've been lucky .
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Re: Tuning The Mast

Adding very little to overbored... Every boat wants a certain amount of rake, prebend and the like as well, and that is determined by the sailmaker, and how they cut the sails for the boat.

The gauge is just so you can repeat good settings. By all means you should be doing this without the gauge if you don't have numbers to get you back where you should be.

What I usually wind up doing is, backstay tension off... (and I only have uppers, caps, and lowers).... set in column first, hand tighten to take wobble out. Set rake by loosening forestay, and adjusting mast foot. My boat likes just a little prebend, so I take up to hand tight the backstay, and adjust with caps and lowers loose, until I get just the right prebend (about 1 inch), measured with the main halyard laid against the mast... eyeball for space the tight halyard tied at the goose deviates from the mast.

Now I bring up tension to the caps, alternating making sure the mast stays in column. I look at prebend again. I try to bring tension up alternating uppers and caps until I get where I think I need to be where a bit of heel won't make my lee shrouds wobble.

The last thing I set is my lowers. I want my lowers to set so that the mast cannot bend to far forward, and pretty much nothing else.

Backstay adjustment should be run min to max to see how much bend is applied.... given that mine is a masthead rig though the actuality of backstay tension is more about forestay sag, than mast bend.

MINI MOO, a 1983 Wavelength 24 - she's a fast cow!
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Re: Tuning The Mast

I found this paper from Selden to be really informative.

seldenmast(dot com)/files/1456145028/595-540-E.pdf
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Re: Tuning The Mast

In tuning your rig, the goal generally is to (1) make the mast erect laterally, (2) to adjust it's rake so that you have adequate weather helm, and (3) to tension the stays and shrouds so that the mast doesn't move or bend in a manner or to an extent that you don't want. A Loos gauge only helps you with #3, to adjust the tension.

With a typical masthead rig without a backstay adjuster, I begin by getting the mast erect, both laterally and fore-and-aft. Then I rake it aft slightly. For a 25' boat I'll rake the mast aft about 2". For a much bigger boat, I'll start with more rake, perhaps 5", just as an approximate starting point. I tension the forestay and backstay to eliminate forestay sag when at rest.

Next, I'll snug down all the stays and shrouds with an eye to keeping the mast straight and in column. If I have reliable information that suggests a prebend, I'll follow it. In snugging down the wires, I'll adjust the uppers an equal number of turns on each side. Then adjust the forward lowers an equal number of turns on each side. Then do the same with the aft lowers.

Lie on your back and sight up the mast frequently to be sure you aren't inducing an unwanted bend or curve in the mast.

Then sail the boat. Watch the forestay under load to see if there's too much sag under load. Feel the helm pressure to see if you have adequate weather helm. If there's too much weather helm, ease the backstay turnbuckle and tighten the forestay turnbuckle. Also watch the lee stays and shrouds. If they sag too much when closehauled and under load, tighten them a little, alternately, on each side, by an equal number of turns. Sight up the mast under load to be sure the mast doesn't have an unwanted bend.

Overtensioning the rig won't make the boat sail faster or point higher. It will only risk damaging your boat.

You don't need a Loos gauge to tune your rig, but it can be a useful aid.
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Re: Tuning The Mast

Thank you for great information!
Years of value in ten minutes.
-CH
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