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Catalina/Capri 25/250 Sailor's Forums

General Sailing Forum

How do you know your mast is plumb?

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Voyager
Master Marine Consultant

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USA
5358 Posts

Initially Posted - 07/16/2012 : 15:28:04

This weekend while sailing in 10-12kt westerlies, as I was heeled over, I noticed that my shrouds on the port side seemed a little looser than those on starboard. I asked a friend how he knew his mast was plumb port to starboard and he said, "I use a bubble level" to which I immediately asked, "yeah but how do you know your boat is level, and not listing slightly due to the weight of the engine, fuel, watertanks and your batteries?"
He said, "first I check the level on the boat, then the mast"
I said, "yeah but...."
So another friend who was there suggested I use the halyard. "Take the shackle and hook it onto your port chainplate. Tie off the line to a cleat so that it's reasonably taught, then try it on the starboard chainplate".
I gave it a try and the difference between the two was about 1/2" which I thought was excessive.
Now I don't have a Loos Gauge but I did check the deflection of the individual shrouds, and they all seem about the same. I used a stick between them with a marked centerline and they all come out about the same.
I can loosen the starboard shrouds by a half turn and tighten the port shrouds by half a turn and remeasure.
Am I worried about nothing, or will this affect sailing and handling of the boat?

Bruce Ross
Passage ~ SR-FK ~ C25 #5032

Port Captain � Milford, CT

Edited by - on

Derek Crawford
Master Marine Consultant

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Response Posted - 07/16/2012 : 15:33:10

If your mast is tilted to one side, or if it has a "hook" at the top, you will be faster on one tack than on the other. Sight up the kerf and you can tell if there's a hook and use the halyard trick if it's tilted. If the leeward shrouds are not loose when going to weather, your rig is too tight.

Edited by - on

delliottg
Former Mainsheet C250 Tech Editor

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Response Posted - 07/16/2012 : 15:37:09

I usually use a 100' fiberglass tape measure. Attach it to your main halyard along with a messenger line (so you don't have to pull down on the tape to get the halyard back down). Run the tape up the mast, and then measure to the chainplates as you did before.

I'm hardly expert in tuning, but I'd think it can't hurt to take out the lean as you mentioned by loosening one side and tightening the same number of turns on the other. Remember as you loosen, you're only trying to remove half the error, the other half will be picked up as you tighten the other side.

Also remember to check your spreader angles, they should be bisecting the shroud so they're completely in compression with no side loads on them.

Edited by - on

Steve Milby
Past Commodore

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Response Posted - 07/16/2012 : 16:36:01

I think what is confounding you is the fact that the boat is floating in a fluid environment, and that you have always thought of "plumb" as being in the context of a terrestrial, earth environment. When you check the plumb of a wall, you know the ground under it isn't going to move, but that's not true on a boat.

The answer is that, when talking about the mast on a boat, what you are concerned with is not precisely that it be "plumb," in relation to the earth. The mast must be erect, consistent with the architecture of the boat. In relation to the hull of the boat, you don't want the mast to lean to one side or the other.

If the mast is oriented correctly in relation to the architecture of the hull, and, if the hull is oriented correctly in relation to the earth (i.e., not heeling one way or the other), then the boat should theoretically sail equally well on either tack.

By using the jib halyard to determine that the distance between the top of the mast and the chainplate on each side is the same, you have determined that the mast is not leaning one way or the other in relation to the architecture of the boat. Using a bubble level to check the plumb of the mast doesn't help, because it assumes that the boat isn't listing. If the boat is listing, and you check the plumb of the mast with a bubble level, the mast could be leaning waaaay the heck to one side.

Edited by - on

John Russell
Master Marine Consultant

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Response Posted - 07/16/2012 : 17:39:07

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Voyager
 This weekend while sailing in 10-12kt westerlies, as I was heeled over, I noticed that my shrouds on the port side seemed a little looser than those on starboard. <hr height="1" noshade id="quote"></blockquote id="quote">Where was the wind? The leeward shrouds should be a little loose. I see how this would prompt your question but, I think it's the wrong question. When you're sitting quietly at your slip, are the port shorouds still looser than starboard?

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Joe Diver
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Response Posted - 07/16/2012 : 17:53:27

I've used the halyard method swung around different points.....

But really just use my micrometer bionic eyes. From the front, on the dock, I position myself directly in front of the forestay and align my site so that the bottom of the mast is centered on the cable. I then carefully sight up the cable to the top of the mast. As long as no part of the mast bows out, either side, the mast is true (enough, the tensioned forestay will be a straight line).

From the side I can sight up along the luff of the main, and by the wrinkles, or lack thereof, and the position of the pocket......tell if the mast is bent back or forth.

Yes, I realize that I'm "eyeballing" it, and careful measurements, tensions and tuning would squeeze out another 3/4 knot.....but for my uses and reasonably critical eye....putzing back and forth across the lake.....I'm good.

Edited by - Joe Diver on 07/17/2012 07:30:44

Stinkpotter
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Djibouti
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Response Posted - 07/16/2012 : 18:51:22

You can discover any list with a carpenter's level on a stick across the cockpit coamings. The halyard-chainplate method will tell you if the mast is leaning relative to the hull--no tape measure is needed. If you think about the geometry, 1/2" is negligible, and probably trumped by as much as 1/4" of list. More significant is the fore-aft rake, which affects your helm. A weighted object (like a wrench) on the halyard shackle can give you that measurement--I think I had about 5-6" aft rake, which gave me just the light weather helm I wanted.

Will all of this affect handling? Fore-aft rake will... A side-to-side imbalance, as Steve said, might make you a fraction of a knot faster on one tack than the other. (But half an inch? I don't think so.)

Also, my leeward uppers generally went slack a little, although I'm sure they didn't wave around in the breeze like Derek's--his racing tune is not what most cruisers choose.

Edited by - Stinkpotter on 07/16/2012 19:28:44

Dave5041
Former Mainsheet Editor

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Response Posted - 07/16/2012 : 19:10:37

I agree with Dave in that it probably won't make a significant difference at 1/2", but it is so easy to correct You're on the right track with loosening a little and tightening the same amount and re-measure.

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Voyager
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Response Posted - 07/16/2012 : 19:53:44

The mast is true and is not hooked port to starboard. That's a relief. The spreaders are good. It may be that I'll have to loosen the lowers first, then tighten the uppers moderately and equidistant using the halyard as a guide, then tighten down the lowers, making sure to keep track of the turns. I realize it is possible to bow out the mast by overtightening one side of the lowers more than the opposite side. Perhaps this is where I'll use the tape measure - on the lowers.

Edited by - on

Steve Milby
Past Commodore

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Response Posted - 07/17/2012 : 05:10:11

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Stinkpotter
 
Also, my leeward uppers generally went slack a little, although I'm sure they didn't wave around in the breeze like Derek's--his racing tune is not what most cruisers choose.
<hr height="1" noshade id="quote"></blockquote id="quote">
I wrote a detailled discussion of rig tuning that is posted in a long article in "racing tips." In that article, I explain the purpose that each stay is intended to serve. Some stays are intended to prevent the mast from bending in a certain direction, and some are designed to permit it to bend in a desired direction while limiting it's ability to bend too far. If you'll read that material, you'll understand which stays should be slightly more taut, and which should be slightly relaxed, and you'll understand why.

You shouldn't be intimidated by rig tuning. It looks complicated, but it really isn't. It's common sense. If the mast is erect, and straight, and the stays are snugged down all around to limit the mast's ability to flop from one side to the other when you tack or gybe, the boat will sail well and will be reasonably well tuned. The only exception to that overall formula is that the mast should be leaning aft very slightly. (That lean is what creates weather helm, so that, if you let go of the tiller, the boat will automatically head up to windward.) You can increase the amount of weather helm by leaning the mast further aft, and decrease it by not allowing it to lean as far.

You can go beyond that basic formula, and fine tune the boat for optimum performance for racing, or for sailing in different conditions, but the average cruiser and casual racer can get by very nicely with a simplified rig tuning.

With regard to the question of tension, when I raced my C25, all the shrouds and stays were snugged down without being bar-tight. I wanted them to be just tight enough to prevent the rig from swaying or moving around, but no tighter, because overly tight stays can stress the decks and chainplates unnecessarily. The only shrouds or stays that were slackened were the forestay and backstay (when the backstay adjuster was slackened) and the aft lowers. When sailing in a chop, you don't want the rig flopping back and forth loosely, due to overly loose shrouds or stays. If the mast is permitted to move too much, then it's hard on all the fittings when they come up short and snap the mast to a sudden stop. Limit the movement of the mast, but don't make everything overly tight.

Edited by - Steve Milby on 07/17/2012 05:19:52

awetmore
Master Marine Consultant

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Response Posted - 07/17/2012 : 08:36:09

Steve: I just wanted to thank you for that article, I've referred to it a number of times since purchasing my boat.

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redviking
Master Marine Consultant

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Response Posted - 07/17/2012 : 08:56:20

Steve, you really should write a good how to book... In so few words, a skill I am not capable of, you provided the best advice ever... My only addition if I missed it is that everyone should lube the rigging annually and work the turn buckles back and forth. The threads are the weakest link and the most easily hidden problem. Especially those barrel type...

sten

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dmpilc
Master Marine Consultant

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Response Posted - 07/20/2012 : 08:36:23

For checking side-to-side lean, you could do what a lot of smaller boat sailors do. Let out your main halyard and cleat it. Tie a line to the halyard and extend it to one of the corners on the transom taking out all of the slack. Holding that spot on the line, carry it over to the same position on the other corner of the transom. It should measure close to the same on both sides.

Edited by - dmpilc on 07/20/2012 08:37:17

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