A pumping mast

[flip]

Hi all, I have a question about the mast and the pumping motion (forward and aft) when the wind blows from the side. Last summer while tied in the slip and the wind was blowing 20 mph from the side of the boat, my mast would oscillate forward and back maybe 2 times a second. The greatest movement was at the spreaders while the top of the mast stayed stationary as did the bottom of the mast at the cabin top.An exagerated example of how it was bending would be what a pole vaulters pole does.
I have also observed the same thing on the boat tied in the slip next to me which is a C&C 27. My boat is an Oday 30.
Has anyone else had this kind of thing going on and if so, what was the cure. I suspect it is a tention issue with the standing rigging but do not know the fix.
Thanks for any thoughts and advice.
Phil Mac

 

[RTB]

Hi Phil,

I notice the same thing on my boat. I have no idea why this happens, and have read other threads regarding mast pumping with no conclusive conclusion. I have a furling headsail, and wonder if it's movement in windy conditions might transfer to the mast and rigging?

 

[rpell1@msn.com]

Hey guys, While on vacation in the Carribean a few years ago, I had a conversation with an America Cup Racer about this very subject. He said he occassionally experiences this phenomenon and simply cranks down on the backstay and slightly tightens the upper and lower shrowds. I tried it and it seems to work for me as well. It's worth a try.

 

[alexco38]

On my CS30, there is a baby stay mid way up the front of the mast designed to reduce such pumping action. You can achieve the same with a mid-mast spinnaker pole topping lift line. Just hook it to the bow and crank it tight. It'll stop the oscillate.

If you want to sail with it than you'll have to put in a baby stay.

 

[sandpiper10471]

I suggest tightening your forward and aft lowers. Fore should be about 10 % tighter than the aft. And you are probably right - the oscillations begin due to windage on the rolled genoa.

 

[Ted]

Do you have double lower shrouds? If you do, try inducing some prebend if you don't already have any. Loosen the aft lowers then tension the forward lowers to induce some mast bend. Then tension the aft lowers to your specs. This should work and is similar to using a baby stay when a rig has only single lowers.

 

[RTB]

I'm not the OP, but thanks for the pointers.

Ralph

 

[flip]

Thanks for your input. I do have a roller furler on the bow and maybe that contributes to the pumping. Well, now I have some things to try. Thanks

Phil Mac

 

[David in Sandusky]

Tune the rig

I don't know the Oday 30, but from the pictures on this website, it looks like a standard masthead rig with two lowers, and no tensioner on the backstay.

I'd suggest that keeping your mast in column is the way to go. Get a Loos guage, and tune the rig. There are procedures in the archive. I bet that will eliminate the pumping.

If your rig is not designed for it, prebending the mast is a problem, not a solution, IMHO, of course.

 

[Snotter]

I'd be careful adjusting things too tight during this time of the year. In cool weather, the mast shrinks more than the stays. Perhaps that is why you are noticing it more at this time.

 

[RichH]

The standard ODay 30 is a masthead rig with lower fore/aft stays and single spreader set.
The correct way to tune this mast is to induce 3/4" forward bow (prebend). The forward bow will radically change the stiffness of the mast and will radically shift the natural harmonic frequency at which the mast 'pumps'.

FWIW - The mainsail most probably was cut for a standard mast with pre-bend so adding prebend will also 'slightly' depower the mainsail and bring it to its 'as designed' shape when sailing. Otherwise the mainsail will ALWAYS be slightly 'powered-up' (good for sailing into heavy chop but SLOW and with slightly more heeling).

 

[jibes138]

One of the worst aerodynamic shapes is a cylinder. Separation of the airstream around a shape like the shrouds and stays causes an amazing amount of drag. Add in the shape of the mast and the drag associated with that and you end up with an aerodynamic nightmare that all added up probably results in all kinds of harmonics that transmit to the mast. My O'Day 35 has the same problem and it is very annoying when sleeping. I've tried tuning, bending mast, etc. can't seem to cure it. A baby stay would likely cure it.
If you want to see the drag on a cylindrical object make a fist and stick your arm out the window of a car moving at 25 miles an hour.

 

[Sparman USA]

Hi Phil, I have been building masts and rigging for 25 year and have come across this many times. What you have is a combination of rig tune and vortex...sounds more high tech than it is!! try hoisting a small fender up the mast about half way which will disrupt the air flow around the mast, you should only need to do this at the dock as when you are sailing the mast will load up with compression and should not pump. On tuning you need to induce some prebend to stabilize the middle of the mast, most masts need a couple of inch of bend to stop them pumping. Keep your shrouds to about 15% of the breaking load of the wire as a bench mark...
If you need a rigger let me know!!!!
Hope this helps..