'Pumping" mast

[Pennefather]

\'Pumping\" mast

We have just replaced our standing rigging...the boat sails beautifully...on the mooring though the mast 'pumps' at the slightest breeze..it seems to be the lower section....is it a tuning problem?...we have put her through her paces then retensioned the rigging...any thoughts?

 

[boatmike]

Re: \'Pumping\" mast

Define "Pumping"

 

[snowleopard]

Re: \'Pumping\" mast

it would also help if you told us what stays you have, i.e. where they join the mast and whether they come from ahead, behind or abeam the mast. i presume you mean the mast bends, it's probably down to the arrangement of the lowers and/or babystay or you may have managed to get an S-shaped pre-bend.

 

[Lakesailor]

Re: \'Pumping\" mast

Over on Scuttlebut Tigawave offered this link Rigging Tensioning which deals with masthead single spreader rigs and addresses the issue of pumping (and panting - but we don't want to go there).

 

[Salty John]

Re: \'Pumping\" mast

If the mast pumps now and it didn't before, it is down to tuning. At a guess I would say your lowers are undertensioned. If you have a bendy spar it will help to have a little pre-bend induced. Get yourself a Loos & Co gauge to set up your rigging scientifically.
If the mast pumping is only on the mooring this can be a phenomenon caused by air flow over the surface of the mast. It is suprisingly common and is named for some scientist who identified these vorteces. The solution is to disturb the air flow by wrapping your halyards tightly around the mast. Sounds daft, but it works!

 

[peterb]

Re: \'Pumping\" mast

You're right, it works. Known as "Karman vortices" if I remember correctly. Used to be a problem with factory chimneys, which is why many of them now have spiral spoilers.

 

[Steve_N]

Re: \'Pumping\" mast

The solution is to disturb the air flow by wrapping your halyards tightly around the mast. Sounds daft, but it works!

That is a great idea which I look forward to trying: on every boat I have owned the mast has been prone to 'pumping' when berthed with the wind in the right direction i.e. just ahead of the beam. Really annoying as the movement can be felt right through the boat and often disturbs sleep. I'm sure it's down to the mast section: a bit like a wing, and it tries to fly forwards when the wind direction is right..

 

[snowleopard]

Re: \'Pumping\" mast

interresting thought - i wonder if it would work to stop the howling noise from some in-mast furlers - has anybody tried it?

 

[peterb]

Re: \'Pumping\" mast

Many years ago I looked over a Nicholson 476 at Southampton Boat Show. On the mast it had a halyard marked "Main flute stopper". I asked the salesman what it was, and he explained that there was a strip of canvas that could be hoisted to close the slot in the mast and stop the mast "fluting".

And don't forget that SBS is in September, not April.

 

[Pennefather]

Re: \'Pumping\" mast

Thanks to all for their advice.....the halyards worked.

 

[Ships_Cat]

Re: \'Pumping\" mast

Even though your problem has been solved, it is a common one.

So just to add to what has been said, in case useful to others, if you have a foil on the forestay (either for streamlining or furling reasons) and have no sail on it then that can also induce pumping of the whole rig. Turning the foil so that it is at right angles to the wind direction stalls it so that the pumping stops (we are fortunate in that the wind always blows over our berth from opposite directions so no need to keep adjusting it but in heavy winds it will violently pump the rig of our 12 ton boat if it is not set up such). Another approach is to use a "pennant" that runs up in the groove with a line taken back to the mast from that so that the natural frequency of the foil/forestay combination is damped (sailmakers make these up).

Another inducer of pumping I have come across is not associated with the rig at all, even though it seems like it. If the boat is in a marina berth and has mooring lines from both port and starboard sides at both bow and stern (so it is restrained in all directions and the boat does not lie against the berth with fenders - some marina berths, especially in windy places are set up like this). In this case if the mooring lines are set up without considerable slack it is possible for a resonant system to be set up with the applied force being from the wind and the restraints being the four mooring lines. I have experienced a 10 tonne boat "pumping" quite violently, seemingly from the rig, in only around 45 knots of wind - it stopped immediately when the resonant frequency due to the constraints was changed by giving more slack to the lines.

John