Mast pumping

[Clay Greene]

We have what I believe to be a very annoying mast pumping problem and I am looking for some suggestions on how we can improve it.  I've read through the other posts and the tech wiki on this subject but I am hoping for some more targeted advice from others who have have dealt with this issue. 

The problem only seems to occur when we are docked in our slip.  We face west and the issue arises mainly when we have a north wind over 10 knots.  When it occurs, you can see the mast moving forward and aft approximately an inch or so.  It does not move at the partners (we are keel-stepped) or at the masthead.  The movement is mainly above the spreaders.  It looks like a guitar string that is tight on both ends and is vibrating at the middle.  You can feel the movement throughout the boat but it is worst in the v-berth.  We've never experienced it sailing, fortunately. 

We have a Loos tension gauge so I am pretty confident that the tension on the upper and lower stays are all approximately the same, in the range of 10 percent of the breaking strength of the wire.  The mast is straight fore and aft and port and starboard. 

The one remaining tuning option I have considered is to increase the backstay tension.  I have taken out about as much tension I can at the turnbuckle and it is still less than ten percent of the breaking strength and a little floppy.  I'll need to get at the forestay turnbuckle and see if I can take out some of the slack that way. 

Any other ideas would be much appreciated.  Thanks!

[Ken Juul]

I think your tensions are too low.  Link to a previous thread

http://c34.org/bbs/index.php/topic,5932.0.html

[Clay Greene]

Thanks, Ken.  I did review that post previously.  On the other hand, the Loos website indicates that 10 to 12 percent of breaking strength is appropriate shroud tension.  They recommend 15 percent of breaking strength for forestay tension (which we can't measure because of the foil for the furler) and that the backstay tension will be less because of the angle from the mast. 

The C34 Mark I manual states that "the upper shrouds should be firm but not bar tight. A 50 pound push should deflect the upper shroud about 1 inch at shoulder height."  The manual goes on to say that the lower shrouds should be looser than the upper shrouds and "[w]hile at dock, they should have no slack, but no tension either.  No lower shroud, when pushed, should deflect the mast more than any other shroud when pushed equally hard."

10 percent of breaking strength for shroud tension is far greater than what the C34 manual describes.  Our upper shrouds are in fact "bar tight."  Our lowers have plenty of tension and the leeward shrouds do not go slack when we are going upwind, as I have seen when racing on other boats.  As a practical matter, I don't have much room left in the turnbuckles to make the shroud or backstay tension any tighter. 

So, I might think the same thing based on what I have described but I don't think it is inadequate shroud tension that is causing the issue.  I am tempted to wait until the next day we are having the problem and start slackening the shround tension to see what happens then. 

[tonywright]

Pumping is generally caused by a lack of prebend in the mast (non-furling variety of course).

You should set up the mast so that it has a very small amount of prebend. This means tightening the fore lowers before the aft lowers, and even leaving them with a little more tension. The prebend causes the mast to keep in column, and will actually give you a better sail shape with the main.  Allowing the mast to pump could cause premature failure, according to some literature that I have seen (causes fatigue). Too much pre-bend can cause the same problem - I once saw an ocean racing boat that seemed to have far too much pre-bend. Sure enough, on the next leg the mast came down.

If in doubt, ask an experienced rigger to help.

Tony

[Ron Hill]

Clay : Here are the LOOS numbers my rig is tuned to:
Fwd lowers - 14.0 (1/4" wire) about 15%
Uppers - 25.0  (5/16" wire) about 11.5%
Aft lowers - 12.5 (1/4" wire) about 14%

I always have had tighter fwd lowers to hold the mast in column with a 150% Genoa.  No problem with mast pumping !!

The LOOS gage instructions says that masthead rigs are designed for a wire breaking of between 10 and 12% which it says is "adequate"  

A few thoughts.

[Stu Jackson]

There is this:  http://www.c34.org/faq-pages/faq-mast-pumping.html

And a search on the phrase works, too.

[Clay Greene]

Ron, thanks for sending the Loos numbers for your rig - given the similarities of our two boats, that is very helpful.  My uppers are the same as yours but the lowers are less and even so the greater tension in the forward lowers is an interesting idea.  I'll make the adjustments and see what happens. 

[Paulus]

Had the same problem for the past 2 yrs.   Took the mast down last fall and this spring after reading everything I could find, I tightened the the forestay inside of the furling.  Presently in the north channel and my problem seems to be gone.
The info came mostly from the C34 forum.
Paul