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Catalina/Capri 25/250 Sailor's Forums

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Backstay/Shroud Tension While On-The-Hard

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jerlim
Master Marine Consultant

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USA
1484 Posts

Initially Posted - 11/12/2016 : 13:38:17

Does anyone adjust the tension, when storing on the hard with the mast up?

Jerry

glivs
Admiral

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836 Posts

Response Posted - 11/12/2016 : 17:02:55

We relax our split backstay and loosen the lower shrouds fore and aft 4 full turns. If you have internal halyards, I'd also recommend attaching messenger lines, pulling them to near the top of the mast and then wrap the messengers around the mast to keep them from flailing all winter.

Gerry Livingston, Malletts Bay, VT
"Great Escape" 1989 C-25 SR/WK #5972

Steve Milby
Past Commodore

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Response Posted - 11/12/2016 : 17:39:06

The backstay on my Cal 25 is adjustable. I put moderate tension on it - not cranked down hard, but reasonably taut, as it would be when sailing to windward. The backstay on my C&C 35 is not an adjustable backstay, and I leave it tensioned normally. When the rig is properly tuned, the tension on it won't hurt the boat. In fact, it has much less tension on it when the sails are off it than it has when under full sail in strong wind. Normal rig tension prevents the rig from moving or from "pumping" in the wind. At best, loosening the rig doesn't help, and, at worst, it allows the mast to move when the rig has been designed to keep it from moving. Loosening it by a couple turns probably won't hurt anything, but I think it's best to leave it as is during the off season.

Steve Milby J/24 "Captiva Wind"
previously C&C 35, Cal 25, C25 TR/FK, C22
Past Commodore

Stinkpotter
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Djibouti
9080 Posts

Response Posted - 11/12/2016 : 21:45:02

I left mine as-is on the hard when I didn't have the mast taken down. But I can see some reasons for de-tensioning... If your rig tension induces a slight bend in the mast, high winds can turn that into a harmonic motion called "mast pumping", which can shake the whole boat. If you've experienced it on a windy day or night, you know. Loosening everything probably reduces or eliminates that effect. But I wouldn't loosen things to the point that the rig moves around enough to create shock loads on terminals as the mast moves in wind gusts. I'd keep it "snug".

Dave Bristle
Association "Port Captain" for Mystic/Stonington CT
PO of 1985 C-25 SR/FK #5032 Passage, USCG "sixpack" (expired),
Now on Eastern 27 $+!nkp*+ Sarge

Edited by - Stinkpotter on 11/12/2016 21:50:06

Steve Milby
Past Commodore

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Response Posted - 11/13/2016 : 04:56:18

Loosening the stays promotes mast pumping because it reduces the support that the stays provide the mast. Here's a link to a good discussion of mast pumping, it's potential harm and it's remedies.

http://www.sailnet.com/forums/general-discussion-sailing-related/193698-pumping-mast.html

Steve Milby J/24 "Captiva Wind"
previously C&C 35, Cal 25, C25 TR/FK, C22
Past Commodore

glivs
Admiral

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Response Posted - 11/13/2016 : 07:23:30

Interesting. Looking back at my original post I see there is little context for my use of "relaxed" or "loosened". To try again...everything is snug, the upper shrouds are tight, the lowers are not as tight as during the sailing season but still tensioned, and the backstay is positioned as it is in light winds, i.e. little aft rake. I remember reading at one time why lessening some of the tension was advised but I don't recall the argument...will have to do some searching.

Gerry Livingston, Malletts Bay, VT
"Great Escape" 1989 C-25 SR/WK #5972

Stinkpotter
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Djibouti
9080 Posts

Response Posted - 11/13/2016 : 09:18:58

quote:
Originally posted by Steve Milby

Loosening the stays promotes mast pumping because it reduces the support that the stays provide the mast. Here's a link...

What I see in that discussion is that to eliminate pumping you need to change something that will alter the harmonic frequency of the mast. It mentions one thing I did when storing mast-up, as advised by a rigger: I wrapped the halyards around the mast in a spiral (below the spreaders) to disturb the air flow that otherwise, at some particular speed, can generate the harmonic motion. Just about at the spreaders, I ran each halyard outside of a shroud to prevent slapping on the upper part of the mast. I also recall him advising to loosen the upper shrouds and stays a few turns, but not the lowers. (The geometry of the C-25's double lower shrouds probably is an advantage for reducing pumping compared to the single lowers of the C-250.) Then I just had to remember the number of turns to return the rig to the proper tune in the spring.

I've experienced pumping once or twice while on the my C-25 at the dock on windy days--a rythmic, sub-sonic "oil can" effect that shakes the boat, and no doubt stresses the swages, chainplates, etc. My rig was pretty tight--unlike Derek Crawford's, very little or no sag in the upper shrouds at any point of sail, and no visible bend. I always suspected that contributed to the pumping. If the rig is capable of bending the mast (fractional rigs are better for that, but double lower shrouds help), I can see how bending would prevent the harmonic in most wind conditions, by pre-stressing it. Years ago I learned about a similar technique in civil engineering. But it's kind of mysterious stuff!

Dave Bristle
Association "Port Captain" for Mystic/Stonington CT
PO of 1985 C-25 SR/FK #5032 Passage, USCG "sixpack" (expired),
Now on Eastern 27 $+!nkp*+ Sarge

Edited by - Stinkpotter on 11/13/2016 09:22:24

Steve Milby
Past Commodore

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5902 Posts

Response Posted - 11/13/2016 : 09:58:10

quote:
Originally posted by Stinkpotter
What I see in that discussion is that to eliminate pumping you need to change something that will alter the harmonic frequency of the mast.

Correct. That's one way of remedying a mast that is pumping. In the discussion, however, "Faster" describes the cause of mast pumping in this manner: "Mast pumping is the middle section of the mast having enough flexibility and lack of support such that it can start to move around..." Easing the stays allows the middle section of the mast more freedom and flexibility to move. The purpose of the upper shrouds is to hold the mast erect. The purpose of the lowers is to hold the middle of the mast inline, so that the middle can't bow either to windward, or to leeward, or forward or aft. Any ability of the mast to flex contributes to mast pumping.

Steve Milby J/24 "Captiva Wind"
previously C&C 35, Cal 25, C25 TR/FK, C22
Past Commodore

Edited by - Steve Milby on 11/13/2016 09:59:49

Stinkpotter
Master Marine Consultant

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Djibouti
9080 Posts

Response Posted - 11/13/2016 : 11:55:04

quote:
Originally posted by Steve Milby

...In the discussion, however, "Faster" describes the cause of mast pumping in this manner: "Mast pumping is the middle section of the mast having enough flexibility and lack of support such that it can start to move around..."

The "pumping" I'm referring to is not simply "moving around in a seaway" (which I might call "flopping.") Mast pumping is a harmonic motion, induced by wind, that is inherent in the rig. Similar harmonic motions are slapping halyards and singing shrouds. Tighten a halyard, and it slaps faster and harder.

On an extremely tight rig, it's possible you'll hear a loud, low-pitched hum from the mast at a particular wind speed, while on a less tight rig, it could be more of a sub-sonic vibration caused by a different wind speed. Both can be surprising and disturbing. Neither is good for the rig. Changing the tension of the stays changes the frequency of the harmonic, but doesn't make it go away. As long as the frequency is one that is not induced by the wind conditions, you don't have pumping.

The boat being stationary, as in a slip or on the hard, increases the likelihood of pumping because it allows the harmonic motion to build up undisturbed by random forces. Having sails up and filled, or substantially bending the mast, pre-stresses it in one direction, making it harder to generate the harmonic motion. Spiraling a halyard around the mast apparently disturbs the air flow around the mast enough to help prevent the harmonic motion from building--although I didn't hang around the boatyard long enough to verify that theory.

Dave Bristle
Association "Port Captain" for Mystic/Stonington CT
PO of 1985 C-25 SR/FK #5032 Passage, USCG "sixpack" (expired),
Now on Eastern 27 $+!nkp*+ Sarge

Edited by - Stinkpotter on 11/13/2016 11:57:14

Steve Milby
Past Commodore

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5902 Posts

Response Posted - 11/13/2016 : 14:06:16

Mast pumping can happen either in a seaway or in the slip. The flexing of the mid-section of the mast can be caused either by the boat's motion in the seaway, or by vibrations set up by strong winds in the slip. In either case, the effect is the same. Here's a discussion of it on the Catalina 36/375 forum. Note that the OP in the C36 thread eventually corrected the problem by tuning the rig.https://www.catalina36.org/forum/technical-discussion/mast-pumping

If a boat's mast is pumping, the correct remedy depends on what's causing it to pump, and the way in which the mast stays are arranged.

It isn't the same as a slapping halliard. The tension on a halliard is applied at each end. There's nothing to restrain it in the middle, unless you attach a bungee to the middle of the halliard and pull it aside and attach the other end of the bungee to a shroud. In effect, that's what the four lower stays do on the mast. They immobilize the mid-section of the mast.

What we're talking about on this forum is the C25, not other boats with different types of standing rigging. If a C25 mast is pumping while the boat is in it's slip, you might be able to quiet it down for tonight by wrapping a halliard around it, but that won't correct the underlying cause of the mast pumping. Tomorrow night it will have the same problem.

Some boats are more susceptible to it than others, because of the way the mast is supported. There's really no reason why a C25 mast should pump, because it is supported by a forestay, a backstay, two upper shrouds and four lowers. When that rig is properly tuned, it reduces mast flexing to such an extent, especially in the mid-section, that it simply can't flex enough to become problematic. When the four lowers are pulling against the middle of the mast from four different directions, and the adjustment gives the mast a little pre-bend, the mid-section of the mast is essentially immobilized. Thus, the way to remedy mast pumping on a C25, and in fact on most cruising sailboats, is to tune the rig correctly.

What makes this subject especially confusing is that many of us are talking about apples while others think we're talking about oranges. A racer tunes his rig for speed and pointing ability. He doesn't really care much about mast pumping, as long as the rig stays upright and the boat is fast. For general sailing, however, we do care about it, and that's what my comments are aimed at.

Steve Milby J/24 "Captiva Wind"
previously C&C 35, Cal 25, C25 TR/FK, C22
Past Commodore

Edited by - Steve Milby on 11/13/2016 14:09:42

GaryB
Master Marine Consultant

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4304 Posts

Response Posted - 11/13/2016 : 14:26:27

I'd at least re-adjust it while it's sitting on the stands or a trailer as the boat changes shape when it's not in the water. The support to the hull is not the same.

Association Member

GaryB
Andiamo
'89 SR/WK #5862
Kemah,TX

Stinkpotter
Master Marine Consultant

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Djibouti
9080 Posts

Response Posted - 11/14/2016 : 13:24:22

quote:
Originally posted by Steve Milby

...It isn't the same as a slapping halliard. The tension on a halliard is applied at each end. There's nothing to restrain it in the middle, unless you attach a bungee to the middle of the halliard and pull it aside and attach the other end of the bungee to a shroud. In effect, that's what the four lower stays do on the mast. They immobilize the mid-section of the mast...

I used "halyard slapping" as another example of a harmonic motion where the tension on the halyard determines its "resonant frequency". A discussion on the Selden Mast forum gets much more technical--going into standing wave-forms in the mast, wavelengths relative to the mast height, and changing the compression force of the rig to change the resonant frequency and eliminate pumping.

Along that line, I doubt that the lower shroud tension alone will eliminate harmonic pumping in most cases, unless those shrouds run a lot closer to perpendicular to the mast. At their acute angle, their geometry doesn't create the rigidity that would prevent the very small vibration motion of the mast. However, they can alter the frequency by being used to put some prebend in the mast, which should significantly increase its rigidity. That has sometimes been found to eliminate pumping. That's probably more effective with the C-25's double lowers than with C-250's single lowers, where I presume some bend can be induced by tightening the upper shrouds enough to push forward on the swept spreaders. (I don't know if the spreader angle is sufficient to bend that mast.)

Sarge out.

Dave Bristle
Association "Port Captain" for Mystic/Stonington CT
PO of 1985 C-25 SR/FK #5032 Passage, USCG "sixpack" (expired),
Now on Eastern 27 $+!nkp*+ Sarge

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