Mast pumping bav47 cruiser

[UP]

Hello,

I stepped the mast on my bav47 cruiser (2001) this spring. For the first month I was doing other works on the boat and the mast was pretty much hand-tightened, or slightly more. No pumping evident. I then took to the Selden "folding ruler" guide, as my cap shroud diameter is 12 mm and there is no good tool available for gaging tension, and tuned my mast. I was sleeping on the boat one night and the wind gusted to 45 kn and I woke up to an "earthquake" in my cabin. To specify - the mast is bare presently, no rodkicker, no boom, no sails on the mast or forestay Furlex. So, my mast is pumping and I want to do something about it and re-tune my mast.

I know there is a thread here in the forums with some very good input, however I want to ask advice on some specifics.

Firstly, I have a fractional rig, two spreaders, baby stay, one set of lower shrouds. Cap shrouds are linked at the first spreaders, so not continuous.

Secondly, what I learned from my first tuning is that do not use a plastic folding ruler! The thermal coeficient of expansion of plastics is in general greater than that of metals. During my tuning I noticed the distance between the shroud terminal and my ruler tip changing as the sun angle changed relative to the ruler. I find this should be mentioned in the Selden manual, as ... it is quite important when tuning from 0-3 millimeters

Questions:
1) My backstay is tensioned with a winch handle and the lower part of the stay is attached to the stern in two places, forming a "V" shape. From the tip of the V, via a pulley, the stay runs to the top of the mast. What tension should I have the backstay when I start the tuning process? Should I already have a slight pre-bend in place with the backstay? I do not want to risk mast inversion. The Selden guide does not say anything about this to my understanding. The V joint to the pulley is rather high up and it would be nontrivial to attach a 2 m ruler to the backstay beyond this V joining point;
2) The babystay - how tight and when do I tighten this in the "folding ruler" method? It obviously cannot bear the same weight as the side stays, as the babystay chainplate is ... considerably weaker;
3) Do I tune the mast with boom on, sails on or with a bare mast as I have presently? Having sails and boom on in my consideration would act as oscillation dampeners. I do not know if this is desired or not. The Selden manual does not say anything about this to my understanding.

Thanks!

[Yngmar]

The pumping is caused by vortex shedding. When the frequency of shedding hits the resonant frequency of the tensioned mast, you get pumping. Selden published a document about this, a copy can be found here: https://jbhsailing.com/wp-content/uploads/2019/02/mastevibrationer-selden.pdf

They sold a spiral wind guide (as you can see on industrial chimneys), or you can just spiral-wrap a halyard when this happens.

Changing the resonant frequency by tensioning or loosening the backstay also fixes it, although it just shifts the resonant frequency and the wind speed may change and find it again.

About tensioning the rig, use a wooden folding rule. There is no risk of inversion because the shrouds are swept aft and you tighten the (top) shrouds first, so they pull the mast top aft. Following the guide works fine. It does tell you what to do with the backstay, although with the V setup you'll have to half the backstay tension.

There's a babystay on the 47 Cruiser? From original?

You can tune with boom and sails on, but you must loosen everything that holds tension there, that means halyards must be slack, topping lift, kicker and mainsheet loose.

Another thing I found doing this on a friends boat: Keep a note on tension (measured stretch) per turn. Once you get diminishing results (less stretch per turnbuckle turn), stop - at this point you're no longer tensioning the rig, you're just pulling the hull up or the mast down into deck/keel. Not all hulls are rigid enough to support the suggested tension and IMHO the Selden guide is a bit on the too tense side of things.

Under sail, keep an eye out for slack/movement on the leeward shroud pins - if that happens, things are too loose. That's how the seasoned rigger did it, with no numbers, rulers or gauges. Go sail under full canvas in a stiff breeze, if it goes slack, tighten shrouds up a couple turns, then re-straighten the mast (lowers) back in port

[tiger79]

Selden used to offer a flexible strip, about 150mm wide, which could be hoisted in the mast groove.  It worked by disturbing the vortex detaching.  I had one provided with a Westerly yacht in the 1980s and it worked.

[Mirror45184]

Hi UP,
The back stay should be tensioned to around half maximum tension. this will provide a minor bend when the uppers are tensioned that will remain when the back stay is released. Although the Selden un tapered section is a tree trunk which does not bend much!

I would not recommend "going for a sail in a stiff breeze" to test the rig tension. With the rig under full load and vessel heeled to around 20 - 30 degrees the leeward shrouds should be slack. Not extremely loose, but not bone tight. It would pay to get a rigger to assist with the first set up and watch closely!

Cheers
Mark
SV Synergy
B40 Cruiser 2009

[Yngmar]

No part of the rig should ever be slack. A slack rig will cycle through loading and unloading parts and rapidly wear out pins, toggles, etc. This is the primary cause of early rig failures. That is pretty much the first thing the Selden guide points out.

[tiger79]

Rig tension is incredibly important.  As Ymgmar has said, insufficient tension causes premature failure.  A well-known rigger once told me that boatowners almost never tension their rigs sufficiently.  The answer is to pay a professional rigger to set up your rig properly any time the mast is disturbed.

[UP]

Hi,

Thank you for all the replies and suggestions. I have also received a reply from Selden. Here is their answer:

• "Mast pumping can occur even if the mast is correctly tensioned. If disturbing the wind flow around the mast has to be stopped." - general reply to my inquiry.
  
• "it doesn’t matter if the mast is bare or not." - referring to mast being bare or not for tuning, eg doesent matter if you have sails on.
• "Hand tight (=not tight at all)" - referring to the tightness of the backstay when starting the tune.
• "When you have tightened your lateral shrouds (V1’s) you give the mast a slight positive curve by adjusting lower shrouds and babystay." - referring to my question on how to tune the babystay.

@yngmar the babystay definitely looks original. Thanks for the wooden ruler tip, I have bought one now and will start tensioning today. The nigt promises gusting winds so it'll be a good test.

As for the recommendation of paying someone - it became evident quickly that there are plenty of people ready to take my money However I'm not interested in paying a "black box" annually to tune my rig. Their market is mainly the racing folk and I'm a cruiser. Secondly, I plan on going on a longer trip with the boat and I want to learn the skill. Thirdly, I'm a physicist by education and this task does not scare me at all There are not too many points of adjustment here and I'm looking to apply the 20/80 principle good enough for cruising. Thanks for the suggestion though and I respect the input.

PS 1 - in fact I think I have presently over-tensioned my rig, as I had the backstay on pretty good and the plastic ruler elongated with heat. I'm a pretty big and strong guy and I definitely was putting in a ton of work. I think the combination of these factors led to over-tensioning.

PS 2 - I would avoid slack shrouds at all cost, as Yngmar said, as metal becomes brittle (cold forged) when it is exposed to repetitive blows. Pre-tension at any sailing angle for any stay or shroud for this size of boat is crucial.

[UP]

Hello!

I took all the advice given here and by Selden and re-tuned the mast. The two main differences being:

• using a wooden ruler
• having all stays and shrouds really loose

What I did wrong last time was that I had the backstay pretty tight when tuning. Now I had it loose and swinging about. What also made a difference to getting rid of pumping was giving more tension to the babystay. This dampened the oscillating of the mast considerably. Adding sails to the rig, especially forestay, seems to have gotten rid of the pump completely. Lets see how she handles under sail.