Looking for information wing mast STEP

Discussion in 'Sailboats' started by Tcubed, Jul 11, 2013.

  1. Tcubed
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    Tcubed Boat Designer

    I've been searching the web and this site for what sytem the open 60's are using for stepping their fully rotating stayed wingmasts, but have not found much.
    Could someone please post some info on this, links, images etc.

    I am specifically wondering if they are using proper thrust bearings (itself articulated to maintain even bearing pressure despite the mast swaying through small angles) or if they just go with a cup and ball setup like on a small boat, and if so how does this affect the ease of adjusting the trim angle of the mast when under full load..

    I am not necessarily looking at just the open 60 solutions but monohull wing mast step combination in general as the shroud angles are smaller therefore the compression loads are higher than in the case of a multihull with wing mast and therefore the friction against rotating when under full load is also higher.

    Thanks.
     
  2. Olav
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    Olav arch. nav.

    Many years ago I read an article in a sailing magazine on Yves Parlier's "Aquitaine Innovations" (must have been around 1996 or 1997). Her mast step was a massive ball made of titanium on deck with a corresponding cup at the bottom of the mast.

    Maybe said article is still at my parent's house - I'll ask them to have a look.
     
  3. Tcubed
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    Tcubed Boat Designer

    Ok so that is at least one case of cup and ball system at large sizes.
    Interesting.
    Got to wonder if it is still turnable once some 180 KN of force comes down onto it..
     
  4. PAR
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    PAR Yacht Designer/Builder

    Eric would be a good member to answer this, but I don't think the ball and soket approach is a very good one. I'd think tapered bearings a better option, with a whole lot less friction, possibly on a stub, so you can bring lines down and the bearings, line exits, quadrants, etc. can all remain above the deck, so no leaks.
     
  5. Gary Baigent
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    Gary Baigent Senior Member

    Not a boat the same size as the 50's or 60's but Groucho is 11.3 x 11.3 m - and the mast bearing and base, ball and socket, turned from simple old wood and sheathed in glass, some grease between, has worked well for 30 odd years. You can cant it sideways and also lift it from horizontal just as long as the mast and the male bearing is packed up to align with the cup bearing; as you lift it pops into position. Cost to construct same, maybe a couple of dollars.
     
  6. Tcubed
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    Tcubed Boat Designer

    Par ; agree that this sounds like the best approach, however there has to be some provision for a little "swing" in the mast which could potentially thow the load entirely on a single cone bearing. Thus requiring a very small angle universal joint of some kind or another.

    Gary; can't beat ball and cup for simplicity and reliability.
    Quick estimate of turning force required
    F = f*T*l/L
    where f coefficient of friction , T mast vertical thrust, l friction moment cup and ball , L mast control moment.

    IF f = 0.1, T = 180 KN, l = 0.05 M, L = 0.5 M

    We get F = 1800 N

    Which is bloody stiff but certainly movable with some purchase and that coefficient of friction could certainly be improved, although one has to wonder how long heavy grease would last when the deck is being lashed in solid water, and adding constantly repacking the pivot with grease to the list of heavy weather chores is not too appealing, so maybe dry joint, but what really slippery materials can really hold up to that kind of non lubricated abuse??
     
  7. Eric Sponberg
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    Eric Sponberg Senior Member

    TCubed, Thanks for PAR for recommending my reply. I have used journal bearings, ball-and-socket bearings, and ball and roller bearings in all my free-standing mast designs. I have seen all these bearing types in other boats. On the ball-and-socket types, it appears that they are all custom-designed by the spar builders, usually out of cast stainless steel, and polished. They don't seem to wear down much at all. The latest one that I have seen was on a 50' production catamaran out of S. Africa.

    On my Deflt 25 design, I specified a stainless steel trailer ball hitch that is mounted onto the bottom of the mast, and this sits in a socket made of mahogany impregnated and coated with epoxy filled with graphite. In the bottom of the socket hole is a Delrin disk for additional low friction operation.

    On my latest mast designs which employ a stubmast that extends up through the deck and over which the wingmast sits, I have metal rolling-element bearings that I custom-design. The upper bearing has a single race of long rollers in journal orientation (side load) and it sits at the top of the stubmast which is about 10% of the mast height above the deck. The lower bearing has two races of rollers--the journal rollers (identical in size and orientation to the upper bearing) and the thrust rollers (smaller, in the thrust orienation).

    The stubmast and wingmast are really stiff, so I don't really need to account for any bending deflection between the bearings (there is only an extremely small amount). And while PAR's suggestion of tapered beaings makes sense, they are a lot harder to build with smooth enough races so that I steer away from that option. Not only do I have to size the rollers correctly for low loads per roller, but the bearings have to be designed so that they are easy to assemble and disassemble, yet trap the rollers so that they don't fall out in the process.

    For rollers, I specify 316 stainless steel. Ideally, for the bodies, I specify cast silicon bronze because of its electrolytic potential, its hardness, and its ease with which it can be cast and machined. Machining orthogonal bearing races (journal and thrust) is easy, machining cone races for tapered bearings is a lot harder. For aluminum boats, I can't use bronze because of the electrolytic incompatibility with aluminum, so I have specified cast or machined aluminum. Machining bearing bodies out of large aluminum ingots seems to be fairly easy to do, but the bearings have to be double hard coat anodized for long wear life. These are all dry bearings, and there are PTFE (Teflon) keeper rings and seals designed into the bearings to make sure the rollers stay aligned and can move freely.

    The reason for going to all this trouble is that no one makes good large size bearings that are suitable for masts, particularly for free-standing masts. The sources for large bearings in the US all are for high speed operation in industrial or automotive (non-marine) applications, and as such, they are not properly designed for high static loads, plus most use 400 series stainless steel which corrodes really fast in a marine environment. And no US manufacturers make large rudder bearings any more that could be adapted to masts. In Europe, there are a few rudder bearing manufacturers, but they really shy away from mast applications.

    For stayed rigs, where the amount of rotation is limited to a very small arc, the ball and socket system is really hard to beat for simplicity. The ball, incorporating an integral base plate for mounting, can be fastened either to the mast or to the deck--it's done both ways. Cups can be stainless steel or UHMW which would be my preference.

    I hope that helps.

    Eric
     
    Last edited: Jul 14, 2013
  8. Gary Baigent
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    Gary Baigent Senior Member

    Tcubed, one thing you don't want on a wing mast/soft sail rig, is for the mast to rotate too easily; you end up with a flapping mast when tacking, going nuts in a decent wind and seaway, ruining the wind flow onto sail and stopping the boat.
    In my experience a simple ball and socket bearing will always rotate and requires only a 2 to 1 rope/spanner control.
     
  9. garydierking
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    garydierking Senior Member

    On the Aikane X5 (60' cat, 85' wing mast) we had a simple ball and socket at the base.
    Gary
     
  10. Olav
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    Olav arch. nav.

    Ok, said article (from German "Yacht" magazine 21/1996) about "Aquitaine Innovations" was still around my parent's house.

    Here's a photo of the ball mast step.

    [​IMG]

    The holes left and right in that diamond shaped plate are for the attachment of the then (1996) new mast foot spreaders, the hole in the back is for the gooseneck.
     

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  11. Tcubed
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    Tcubed Boat Designer

    Thanks Eric for all that info!

    Thanks Olav for the picture.

    Baigent, yes, good point.

    So, in conclusion;
    Clearly there is no problem in using a cup and ball system on my design, which satisfies my default simplify everything approach. Complexity rapidly increases the odds of attrition.

    Eric you say «For stayed rigs, where the amount of rotation is limited to a very small arc» .. could you elaborate please? Or rather what do you mean by very small?
    It is very important to fully realize the performance potential of a wing mast to be able to rotate it to a fairly large angle as the apparent wind swings out to the beam to delay the onset of sail stall as far as possible of course. Now i realize with stays it can never be like the unlimited rotation of the unstayed rig but instead the limit will be dictated by the mast coming to rest against the lee shroud. As the lee shroud is slack and can actually be quite slack if the rig is set up rather relaxed, this maximum rotation angle could be as large as 75 maybe even 80 degrees, depending on the setback of the shrouds of course. This already is pretty good i think since past a certain apparent wind angle the sail will be stalled no matter what and at that point the angle of the mast stops being of much importance, at least when the sail is up. (it's a different problem if in a storm under bare poles)

    About the UHMWPE, this seems great as the coefficient of dynamic friction is only 0.12 hard to beat considering it is unlubricated.
    However i hesitate to trust a plastic under these kinds of pressures we're looking at (in the order of 100+ MPa).

    I pulled up some data on UHMWPE ;
    http://www.polytechindustrial.com/products/plastic-stock-shapes/uhmw-polyethylene
    http://www.machinist-materials.com/comparison_table_for_plastics.htm
    http://www.ewp.rpi.edu/hartford/~peetrm/FWM/Project/Reference%20Papers/infl%20counterface%20roughness%20on%20frict%20eng%20plastics%20bearings.pdf

    Not very conclusive data . The first says just 750 psi !
    The last reports testing the material under 20 MPa pressure, which is better but still far short of 100 MPa ..
    I'll have to spend a bit more time researching this unless someone can post some definitive physical properties , which would make me very happy of course.
    I'm concerned about the sort of fretting and burring wear this material could undergo if too near its mechanical limits.
    Lowering the pressure by increasing the bearing area would increase the moment of frictional inertia which could completely eliminate the low friction advantage over a higher friction material that is better able to withstand high pressures...

    Curious how it seems that the lower end of the diamonds are set to not rotate on that base plate, at least that is how i interpret that image.?
    I'm going for a single diamond to give the mast rigidity across the thin axis, and then that whole rigid mast/diamond unit rotates around the axis from center of ball at the bottom to a common attachment point on the leading edge for forestay and one single shroud each side. No point adding the compression force of the diamonds onto that pivot.

    I'll post a few design images later when it's a little more refined to (hopefully) entertain readers here.
     
  12. Eric Sponberg
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    Eric Sponberg Senior Member

    Tcubed--I got called away to Key West, then got stranded overnight, so sorry about being tardy in replying.

    Small Arc? I typcially have designed stayed rigs for ±45° or less. The shrouds just bind too much, and as Gary says, you can't keep the rig too loose or it will "flap" around too much.

    UHMW: I have designed UHMW sockets to half tensile strength for the live bearing strength at maximum load = 2900 psi = 20 MPa. On one rig design I did a few years ago on a wingmast rig for a 43' catamaran, this resulted in a bronze ball on deck of 4.5" diameter = 114 mm, with a corresponding socket of the same inside diameter, 1" (25mm wall thick. minimum) all around and completely encased in wood-epoxy blocking. The mast compression load was calculated at 14,391 lbs compression, which included the mast compression plus the halyard loads for main and jib. As far as I know, that rig is still working.

    Yes, lower diamond wires should be terminated on the mast, not on the deck or the baseplate.

    Eric
     
  13. Eric Sponberg
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    Eric Sponberg Senior Member

    Tcubed--I got called away to Key West, then got stranded overnight, so sorry about being tardy in replying.

    Small Arc? I typcially have designed stayed rigs for ±45° or less. The shrouds just bind too much, and as Gary says, you can't keep the rig too loose or it will "flap" around too much.

    UHMW: I have designed UHMW sockets to half tensile strength for the live bearing strength at maximum load = 2900 psi = 20 MPa. On one rig design I did a few years ago on a wingmast rig for a 43' catamaran, this resulted in a bronze ball on deck of 4.5" diameter = 114 mm, with a corresponding socket of the same inside diameter, 1" (25mm wall thick. minimum) all around and completely encased in wood-epoxy blocking. The mast compression load was calculated at 14,391 lbs compression, which included the mast compression plus the halyard loads for main and jib. As far as I know, that rig is still working.

    Yes, lower diamond wires should be terminated on the mast, not on the deck or the baseplate.

    Eric
     
  14. Tcubed
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    Tcubed Boat Designer

    As i had said here is the mast etc.
    This is a work in progress ie unfinished. I say this preempting people getting over-excited as too often happens on this forum..

    This design is conceived for high performance (200 mile days common enough) cruising with a certain specific amount of gear, not a multihhull, but with shoal draft, singlehandle-able and buildable in wood epoxy.

    Rough preliminary calculations ;
    The hull this is meant for is a ULDB centerboarder 9.15 M³ disp 19 M LOA, just over 3 M beam, 4000 kg internal ballast. Max righting moment if CG 0.1 M above LWL 55 000 NM at around 62 degrees.

    See basic render.
    Like the liquid metal terminator gesticulating in the cockpit?
    Got to have some fun in this, designing can get a bit tedious after a while..
    Say anyone have any tips on modelling a harbour furl??
    It would be nice to complete the "at anchor" render.


    So due to heeling/righting moment only we have compression force on the step of 36 000 N
    Add on 5 000 N due to peak pitch axis accelerations (assuming total rig weight 150 kg , rhino values for gyradius and Theta double dot ~= 1 r/s² for 1 g accelarations at bow since don't yet have hull gyradius data) and estimated mainsheet load 2000 N + jib tack attachment tension 2000 N
    For a total maximum mast downthrust of 45 000 N.

    I designed the cup to be 1 mM greater radius than ball because under stress the ball will tend towards an approximate oblate spheroid and the cup i expect will develop tension in the vertical axis on the concave face, causing in turn compression around the rim of the cup, thus tending to close (very slightly). These two strain effects tend thus to bind the mating surfaces. Unfortunately it is a problem too complex to solve analytically, so for the time being i'll just use some engineering common sense and give it a small amount of flex room. Some basic calculations again seem to indicate that with a slight amount of deformation the maximum average mating surfaces pressure to be around the 20 MPa mark which has been mentioned in the literature and i thus will accept as a tolerable amount for UHMWPE.

    Ball SS and cup UHMW btw.
    Ball on deck of course so no water retention.

    Other points; The wing mast section i created specifically for entirely external halyards, for reasons of reliability. The mast is entirely sealed and has some 530 liters volume which has a pretty dramatic effect on large angle righting curves..

    The scallop thus hides the halyards out of the air stream. Also it has a favorable effect on the pressure gradient so that mast and sail stall is delayed considerably. It also puts more shape into the side panel rigidifying it. It also has two spars running up inside.
    Halyards which come out of a hole prevent one from effectively swatting, which is a major problem i find.

    On my boat my mainsail is almost as big as this one and i use 3 to 1 on the peak and 2 to 1 on the throat purchases. By swatting i can set these main halyards up as tight as necessary without resorting to any winch.

    I have devised a way of setting a topsail on a wingmast, thus allowing one to use a gaff sail with quite a short mast (less weight and windage aloft) and still be able to get a lot of sail up when it gets calm.
    Topsails are remarkably useful sails as not only do they get into stronger wind further away from the sea surface, their great moment make them particularly effective roll dampers. Often boats suffer in oceanic calms unnescessarily simply due to insufficient damping and the boat does not let the sails fill in what would otherwise be quite enough wind to make reasonable headway. Furthermore , they permit the rest of the rig to be optimized for winds above 10 knts say, making for a much snugger and more seaworthy rig arrangement.


    Further points on wing masts;
    The CG should be as near as possible to the axis of rotation.
    The aerodynamic center, corrected for wind shear effects, should be just behind the axis of rotation.
    The axis of rotation should have minimal rake to avoid the mast wanting to "hang".

    Finally a calculation (quite complex but doable) should be done to determine whether there are any bad inertial-aerodynamic positive feedback loops.
    This kind of thing can bring the mast down (high frequency flutter) or knock the boat down (low frequency roll-mast tacking effect) in storms at anchor.
     

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  15. Tcubed
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    Tcubed Boat Designer

    Rig image without spinaker or flying jib.

    Cheers
     

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