Pumping Mast

[Robbb]

Good day Brian,

We spoke earlier this year about my haunting mast pumping problem. Forespar built the rig but the standing rigging was supplied by a local rigging company. My spar pumps fore and aft in the most moderate of conditions. In a one foot chop the upper two panels pump and is over all unstable in heaver conditions. In true off shore conditions I would expect the mast would be in danger of failing if not certainly wearing its self out by the time a passage was completed.

The boat is a mast head sloop designed by Bob Perry, model Passport 470. The tube section is Forespars section 5890 with moments of Iyy 54.68 and Ixx of 22.19. The rig design uses discontigious wires and two aft swept spreaders at nominally at 26 degrees. Single aft lower and V1 is attach to a single chain plate.

The wire is 1 X 19 ( 316 ) 7/16 in on V1,D1 and 3/8 everywhere else. Spreader dihedral design is 3 degrees up on the lowers and 5 up on the upper spreaders. The boat is equipped with an removable inner stay and runners for heavy conditions. A hydraulic back stay adjuster is installed. When the inner stay is set up hard the pumping generaly eliminated. The runners alone have minimal effect. The boat is main sail driven with a small fore triangle so running the inner stay in all but heavy conditions is highly undesirable.

Forespar had recently increased the length of the lower and upper spreaders by nominally 8 inches and 2 inches respectively and installed a turnbuckle on the V2/D3 wire for additional adjustment but this action had not solved the problem.

Visually the rig looks good. Spreader dihedrals are up, a nice prebend with the exception of the top panel being a bit flat and over all is laterally stiff. A pull on a V1 wire will cause a rotation about the mast tube axis. A tug on a D1 wire will deflect the mast and actually set up a slight oscillation fore and aft. This has been my dock side tuning ruler in that if I can make it oscillate at the dock then it will do the same in any sea way. So far this has proven to be true.

Additional wire tension does not improve the stability but instead will distort the hull to the point where doors will no longer close. I have concluded that there is either a geometry problem ( less likely ), too much wire stretch in the 3/8 in V2/D3 wire or simply the tube moments are too low for its height of mast ( 66+ feet including the burry) and weight of boat. ( 32,000# )

Do you or any of your spar talk members have an opinion as where to take this exercize next. Three experienced riggers have worked on the rig and all have concluded the mast section is undersized and without additional stiffners or a new tube,tuning will not solve the problem. I wondering if optomialy stayed, stability might be acceptable. The other suggestions seem extreem unless indeed this is all about low moments.

Robb

You mentioned tightening the rig to the point the doors won't open/close, have you already tried setting the rig up with signifigantly less compression ?
Perhaps the hydraulicly induced backstay loads and swept spreaders with tight, tight caps are trying to pump the rig with their tension/compression ?

Or maybe not...

Good Luck !

[Renoir]

A first analysis to do for your problem is to view your mast as a long cylinder. Draw your mast according to scale including the mast step, the deck partners, and any fore and aft rigging attachments, ignoring any lateral rigging attachments (this first step is in assuming that your real problem is as you describe in only a fore and aft vibration mode).

Using dividers mark 1/2, 1/4, 1/8 of your mast length for both the length conditions for mast head to deck and masthead to base step. Your mast pumping is not only from a resonant multiple of such even fractional denominators as noted it is also caused from a reflection of such multiples against the deck and/or step which causes standing waves in the mast.

If your fore and aft rigging attachments (like your lowers and running backs) are placed at one of these even fractional multiples then there is NOTHING that you can do with the rigging tension at those points to stop the pumping. In addition, there is nothing that you can do with the headstay and backstay tension to stop the pumping. ONLY attachments located off of the fractional multiples can damp the pumping unless you can make the mast cross-section full of damping material, not normally a viable mechanical solution.

[Renoir]

Your mast pumping, because it is so objectionable, is probably due to a standing wave caused by a 1/2 wavelength multiple and not a 1/8 wavelength multiple (which makes a higher frequency of standing wave with less of an amplitude of mast excursions as it "nooldes" in waves). The 1/2 wavelength multiple will likely be a length of 1/2 of the distance from the masthead to deck or 1/2 the distance from the masthead to the end of the mast at the step.

Originally Posted by Renoir

A first analysis to do for your problem is to view your mast as a long cylinder. Draw your mast according to scale including the mast step, the deck partners, and any fore and aft rigging attachments, ignoring any lateral rigging attachments (this first step is in assuming that your real problem is as you describe in only a fore and aft vibration mode).

Using dividers mark 1/2, 1/4, 1/8 of your mast length for both the length conditions for mast head to deck and masthead to base step. Your mast pumping is not only from a resonant multiple of such even fractional denominators as noted it is also caused from a reflection of such multiples against the deck and/or step which causes standing waves in the mast.

If your fore and aft rigging attachments (like your lowers and running backs) are placed at one of these even fractional multiples then there is NOTHING that you can do with the rigging tension at those points to stop the pumping. In addition, there is nothing that you can do with the headstay and backstay tension to stop the pumping. ONLY attachments located off of the fractional multiples can damp the pumping unless you can make the mast cross-section full of damping material, not normally a viable mechanical solution.

Thanks for your feed backand sorry to respond so late. I spent some time looking for signs that the geomerty might be inducing a harmonic. The only thing that jumped out at me was the staying angle of the foresatay verses the back stay. My mast is very far forward in the boat. The mast crain is symetrical in that the toggel pins are equal distant from the center of the mast tube.

If I project the fore and backstay above the mast the two imaginar lines intersect forward of the center axis of the mast. I'm now wondering if this fact could be inducing the mast pumping.

Thanks again for your insight.
Robb

[Renoir]

No, what you describe is merely a torque couple which could only introduce a dynamic perturbation to what is already a mechanical resonant mast. It is the resonance which allows standing waves to cause vibration excursions at specific locations of the mast.

There are various modes of vibration and we are addressing only that mode which can exist in a fore-aft plane of your mast because you did not mention a standing wave problem in a lateral plane which could also coexist with a standing wave in the other plane.

I reiterate that if you have a first order standing wave that vibrates at a frequency which generates one wavelength corresponding to the length of the mast from head to deck (for example) then there will be zero excrusion nodes at the head, halfway down and at the deck. Any attachments to those points can do NOTHING to minimize the vibration in that mode. You can, therefore, play with using highmodulus line attaching the line doubled to a spot right at the maximum excursion of mechanical movement (say a quarter of the way up and/or 3/4 way up) and put tension on the line as far back as possible on the deck to gain a good angle. Check with others on this board for a good way to temporarily keep the line from slipping down the mast when you try this test to see if you can either dampen the standing wave or change its frequency along with a less objectionable motion.

Once you find at least a test solution come back to the riggers on this forum for advice on a real fix. Another test is to add a mass at one of those test points of a hundred pounds or so yet, obviously, that is not a practical solution.

Following your suggestions and have noticed a number of interesting associations. If I analyze the mast from the D3 tang ( top wire conection ) to the mast step the upper spreader root is precisely at 3/4 of the distance from the mast step. 48'1" on the Forespar drawing and 48' 0.75" by calculation.

If I calculate from the partners to the mast head I get 28' 11" as half the distance. The Forespar drawing locates the lower spreaders at 29' 8". A difference of 9 " to the spreader root. Could this be close enough to be a concirn ? The D1 tang is at 29' 4", a mear 5 " off from this mid way point.

Both of these factors are telling me the harmonic nodes you mentioned could be at play here. I'm assuming the 28 deg swept spreaders are at play here as they do provide fore and aft staying in the swept spreader ( 28 deg ) rig design.

Thank you for your comments in advance.

Robb

Hi there,

You mentioned that "When the inner stay is set up hard the pumping generaly eliminated" but that it is undesirable to rig it because of the size of the foretriangle. Does this inner stay interfere with tacking? Why is it undesirable? Is rigging it worse than this pumping?
I have found that most rigs with inline (i.e. single) lowers need some sort of inner forestay to keep the mast from moving about in anything but a light wind (basically, it does the job that forward lowers would do in a more conventinally rigged boat). All this talk of harmonics and waves and whatnot is interesting but seems unlikley to help you go sailing, while rigging your existing inner forestay seems to stop the pumping. If that works, go with it, and try to work around the inconvenience of another stay up forward. If it is an issue of tacking a genoa through that narrow slot, eliminate shackles or knots from your sheets and place some plastic tubing on your inner stay to help things slide. Or set up a runner-like stay that you can release and re-set with each tack - you can use a bungy-cord to retract it automatically when released. The fact that your rig has runners and an inner stay indicates that someone thought them nessessary for some conditions - use them! The window of conditions these were added for may simply encompass a wider range of windspeeds than anyone thought.

Thanks for your comments. The rig design, with its significantly swept spreaders, is already a compromise in order to achieve a stiffer mast. With the mast set so far forward, the slot is miniscule and tacking any sail through it would be highly undesirable from a number of perspectives. The inner stay is there to fly heavy weather sails and to oppose its pull with the runners. The rig is ì runner independent ì according to the manufacture.

A Baby stay would be more acceptable but at days end, I believe this is mainly about the mast sections lack of stiffness and perhaps harmonics due to poorly chosen wire fixing locations.

I agree that the lack of forward lowers is contributing to the instability. I would wonder why any designer would not use split lowers except to save cost or for interior space issues associated with a second chain plate.

If the observed motion was only in higher sea conditions my concerns would be elevated. Seeing the mast pumping away in a one foot chop tells me there is something seriously wrong with this rig.

[Renoir]

Mast pumping IS caused by harmonic standing waves. The only question is how to provide an attachment which will alter the harmonic to a value which will not be sustained by existing geometry.

Because your inner stay stops the standing waves I will bet that the attachment geometry is altering your 1/4 or 1/2 wave vibrations. If the runners are directly opposite the inner stay attachment at the aft side of the mast section (or very close to it) they also alone should stop the vibration if they are set up with a sufficiently low modulus of elasticity and a high enough tension. Note mentally that the runners form two vectors which you can use to calculate literally just how much tension each one must have in order to equal the total tension put on the mast by the inner stay when is is attached an successful at stopping the vibration. Play with that.