Beneteau 361 mast shake

[bshock]

Hi folks,

I've just got back from (fine) tuning my mast and putting boom/sails on, so all ready to go sailing. Before I put the boom on, I had the mast pretty well tuned, with the cap shrouds and aft lowers tensioned to 15% as instructed in the Seldon manual (I could go up to 20% if necessary).

After I put the boom and sails on, I noticed the mast would frequently "shake" in the wind (about 10 knots on the aft quarter) My boat has always done this, and it's always irritated me. So I looked up at the mast when it started shaking next and it appeared to be shaking mostly where the after lowers connect to the mast at the upper spreaders. I decided to try an experiment and tensioned the after lowers to 20%, and that seemed to help a lot. The "shaking" does still happen, but much more infrequently.

Am I making a mountain out of a mole hill? Do other B361 owners notice the same thing?

 

[NYSail]

I had this same thing happen with my old boat..... wind would be just right and mast would "pump".

 

[jssailem]

Bshock. Do you have a radar reflector on the Aft Stay?

 

[bshock]

NYSail, I kind of thought the direction of the wind might have had something to do with it. Jssailem, I do not have a radar reflector, or anything else for that matter, on the aft stays.

 

[jssailem]

I never had mast shake till I located the radar reflector on the backstay. You are likely correct that the wind is setting up a harmonic making the stay/mast vibrate.
I minimized my vibration by tightening the adjustable back stay.

 

[Jackdaw]

If you did so, you would not be the first person to add a babystay to such a boat to cure such a problem.

 

[bshock]

Thanks Jack, the B361 already comes with a baby stay, and I've tightened it down pretty well. My loos gauge won't give me a reading on it since the diameter is so small. I DO have the smaller size loos gauge, so I'll take that up next time to the boat and make sure it's tensioned appropriately. It connects to the mast about a foot above the lower set of spreaders.

Jssailem, I'll try tightening up on the twin backstays, buy my concern there is creating too much bend in the mast. As it is, I think I'm at about six to eight inches, using the weight-on-the-halyard method of checking mast bend.

 

[NYSail]

My old boat (Pearson 36-2) had a baby stay as well..... mast just lived to pump in the right conditions on the dock. When on the hook or mooring never had pumping as always pointing.

 

[mcrow33]

The Selden website has a write up about vibrations. Link below
http://www.seldenmast.com/files/1380900733/595-561-E.pdf

 

[Skipper]

Had the shakes. Mast vertical -not sailing- wind out of the sw when docked west, and over 15 knots.

Tuned the rig a little tight and it mostly quit, like yours.
Keel stepped mast.

I did find that, prior to tuning and even now on rare occasion when needed, I could easily play with the mast vertical tension by attaching the main halyard to the aft of the boom, drawing the mainsheet tight and even winching a little. This subdued the pumping noticeably, and didn't require pulling pins or monkeying the rig tune.

In the end, it's harmonics. Hard to prevent but are options for control.

 

[bshock]

Thanks everyone! Skipper, I hadn't thought of trying that trick with the main halyard. I'll give that a try!