Building a New Rig! What to use for Tube?

Discussion in 'Boat Design' started by Samphire, Sep 17, 2012.

  1. Samphire
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    Samphire New Member

    Hi all, opinions and advice are welcome and appreciated:

    I am re-rigging a 52', 60,000 lb boat with a schooner rig and switching to a cutter rig. The current schooner rig weighs around 820 lbs excluding furling sails, and is around 40 ft off the deck max height. The boat is very stiff and could, I believe, handle the same amount of weight higher up. The reason for re-rigging is to increase sail area.

    I want an all-welded mast (no fastenings) and have been thinking I'll use 8'' sched 40 6061 aluminum pipe (8.6'' o.d.) for the tube. This weighs in at just under 10 lbs per foot. It's wall thickness is around 1/3'', plenty thick enough for tangs, spreaders gooseneck, etc to be welded on using appropriate doublers. I will have the services of a qualified naval architect to draw up the plans for the rig and work out loads.

    My questions are:

    Can I (and should I) consider using 8'' sched 20 pipe, which is 22% lighter than the sched 40, weighing in at around 7 lbs/ft? I'm sure we'll be working up a double spreader rig as it will probably be 65-70 off the deck, so concerns about over-long unsupported panels in the spar should be mitigated. It has a 1/4'' wall and I'm not a welder and don't know if it could be made as robust as the heavier walled sched 40.

    By that same token, might it not be better to use 10'' sched 20, which weighs the same 10 lbs/foot as the 8'' sched 40, and have the added stiffness of a larger section if it's 1/4'' wall thickness is thick enough for strong welds? Yes, I know it's 10.75'' o.d. is pretty darned big, even for a big boat, but see my next question.

    What about having the lengths of whichever pipe we choose pressed into oval sections for added fore-aft stiffness and reduced windage? It seems to me that this would allow me to mimick the beefy oval section extrusions of older cruising boats, but I am unsure if there would be any weakening of the aluminum from the pressing. I would have an experienced shop put the 20-24 foot lengths into a brake press to do this.

    And having looked at all of this I do find myself wondering if I might come across an appropriate extrusion at a marine salvage somewhere. If it were in good condition and *heavy enough to be welded* I would consider using it for it's superior-to-round-tube stiffness:weight ratio. But are mast extrusions made from aluminum which can accept welding, i.e. 6061? I'm aware that I might have to heat treat my welds if the tube had been heat treated.

    Let me be clear before folks clamor to point out the calculations which we must do on the boat itself to determine her VCG to see just how much weight we should allow up there: what I'm specifically looking for from the forum is a theoretical comparison of the pros vs cons of

    -8'' sched 40 vs 8'' sched 20
    -either of those vs 10'' sched 20
    -round tube vs pressing into oval tube
    -any of the above vs purpose built mast extrusion, new or used

    Thanks in advance for your considered thoughts!
     
  2. gonzo
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    gonzo Senior Member

    Pressing a round pipe to make it oval will put a lot of stresses in it and most likely weaken it. The wall thickness necessary is dependent on the design of the standing rigging, so it is not really possible to give you a relevant answer.
     
  3. PAR
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    PAR Yacht Designer/Builder

    On a rig of these dimensions, you should strongly consider having the spec's drawn up professionally. Making guesses about section, wall thickness, etc. will just result in a tangled mess on the deck, after it fails.
     
  4. Samphire
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    Samphire New Member

    Hi guys, thanks for that.

    Gonzo:
    -I don't know if pressing a round pipe into an oval shape will weaken it or not: do you? Clearly it will give it more stiffness in the fore-and-aft direction.
    -Your point about the standing rigging is well taken, however it will be sized along with the spar. In the end it will come down to the VCG of the hull, how much sail I want to carry, and what safety margin I want (big). My question is: will an 8'' sched 40 pipe have a similar moment of inertia as a 10'' sched 20 pipe?

    PAR:
    -Please note that I am engaging a professional marine architect to draw the plans, work up loads, size rigging, spar, tangs, etc. At the end of the day his word is law to me as I trust his expertise. I'm doing this research to explore how close I can get to the strength and weight characteristics of purpose-built mast extrusions using industrial aluminum pipe. Please take for example this extrusion

    http://www.rigrite.com/Spars/Kenyon_Spars/78120-cruising.html

    which matches the size of my vessel well. It is 12X8 inches with a wall thickness of .225'' and a weight of 8.4 lbs/ft. It is also 6061-T6 aluminum.

    Now please see this link and look for the 10'' sched 20.

    http://www.transamericanmetals.com/images/stories/tamproducts/pipe%20chart%20front.pdf

    As you can see the pipe has a .25'' wall, and weighs 9.7 lbs/ft. It's also 175 bucks for a 24 foot length... a smidge cheaper than mast section.

    See where I'm going with this? If I can safely squish the tube into an oval, or even if I leave it round, I'm getting into the same ballpark of weight (actually a little heavier) and strength (potentially stronger) for a fraction of the cost of the piece of gear that has the word "sailboat" on it.

    I'm no revolutionary. People have been successfully fabricating masts out of steel and aluminum pipe and tubing for generations, and I fully intend to do the same. I'm just trying to finesse my way into a spar which will retain the many advantages of industrial pipe construction (low cost, monolithic fastener-free construction, extremely low maintenance and high level of durability) while keeping my weight and stiffness characteristics as favorable as possible.

    Thoughts?
     
    Last edited: Sep 17, 2012
  5. gonzo
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    gonzo Senior Member

    Deforming T6 aluminum will create stress cracks. It will be too much to make a round tube into an oval. Is there any reason you don't want to use the existing masts? You could extend one or make one from the two.
     
  6. messabout
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    messabout Senior Member

    Samphire; I doubt that pipe uses 6061 alloy. All the pipe that I have worked with was pretty soft and was probably one of the 5000 series alloys. You can get tubing, as opposed to pipe, with the dimensions you want. Tubing can be specified as 6061 alloy. Standard joint length is either 20 or 24 feet with some sizes at 12 feet. In any case you will be looking at several splices or butt joints to get the height you need. I have a sneaking suspicion that this is a bad idea.
     
  7. Petros
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    Petros Senior Member

    you can deform a round without losing strength if you have the tube annealed (or just buy the tube in the annealed "zero" condition), deform it and than have it heat treated after deforming. That will prevent micro surface cracks and relieve any internal stress from the deformation. Not sure that is going to be less costly but it can be done. It is better to feed it between two rollers to ovalize it rather than press it in sections.

    As for strength: the larger diameter will be stiffer and stronger for the same weight, but it just depends on the spreader/shroud arrangement you have. Of course if you squash it oval than you have an unknown shape and strength will be subject your final configuration. Area properties can be developed if you can get accurate measurements of the new shape.

    I would think you could save more money simply by looking around for a used/salvaged mast from something a bit larger than you need, and than cut it down to suit your project. IT would be likely much less costly and stronger with much fewer unknowns.
     
  8. pdwiley
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    pdwiley Senior Member

    The boat I'm building has steel tube masts as aluminium tube was impossible to come by locally. Full designer approval; he has steel masts in his personal vessel. In principle there's nothing wrong with what you want to do.

    However, I agree that pressing the tube/pipe into an oval shape is going to stress it and may well cause it to crack. I would not do this, personally, with aluminium.

    As for the mass aloft, mass is mass. A bigger diameter tube with thinner wall has the same inertia but greater windage. I've seen a big Colin Archer type ferro hull with 10" steel masts and they didn't look ridiculous in size. However the owner was an engineer and had the masts made to his specification with taper.

    How do you plan on attaching the sail track? If you weld it to the mast, the T6 heat treatment is now nonexistent. If you bolt it, there goes your wish for fastener free construction.

    PDW
     
  9. waikikin
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    waikikin Senior Member

    What I've been told by riggers & a mast Engineer that the only places to weld an aluminium mast is at the head & heel. Might be worth investigating. Regards from Jeff.
     
  10. Eric Sponberg
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    Eric Sponberg Senior Member

    Samphire, you are second-guessing your naval architect. His job is to determine the appropriate size of section for the mast, and you said that you consider "his word is law." The design effort starts with determining the loads, working with different rigging and spreader arrangments, and coming up with the required moments of inertia for the mast in each panel between spreaders. The longer the panels, the higher the moments of inertia will be required. So you can't determine anything about what to buy until your naval architect tells you what those moments of inertia are.

    I am sure you will find that pipe sections are going to be quite inappropriate for your mast. First, because they are round--round is a bad aerodynamic shape for a mast--and second, because you will likely find that standard mast sections are lighter than pipe sections, which is beneficial to your boat--less weight up high for better stability.

    My recommendation is that you get the engineering results from your naval architect first, then go shopping for a suitable mast section of the appropriate size based on the required moments of inertia.

    I hope that helps.

    Eric
     
  11. Petros
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    Petros Senior Member

    If getting the right size mast is costly and difficult in your area, how about have Eric Sponburg design you a composite mast? That can be made locally from raw materials and minimal tooling.
     
  12. Samphire
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    Samphire New Member

    Thanks all for the helpful input!

    Messabout:
    Yes you can buy 6061 pipe and tube. Like, everywhere. I'd go to cruisersforum if I were looking for sneaking suspicions. I come here to get input from experienced amateurs and helpful professionals. Tons on people have successfully fabricated masts out of pipe and tube.

    Petros:
    Good info about annealing and heat treating. Your points are well taken. I will stop barking up the "ovalling" tree. Too many unknowns (and costs).

    pdwiley:
    Interesting to hear your point of view on this, given that you're doing something similar. My last boat had a steel pipe mast (5'' sched 20 I believe) and it worked great. Didn't seem to throw too much weight aloft anyhow. Mind you the vessel had been designed for a solid wood spar, and the weight would have been comparable. My only complaint: hard to keep paint on the bugger!

    So I'm to take it you don't think the larger, thinner walled pipe would be stiffer? No advantage over the thinner, heavier walled stuff? Now I'm no engineer, but I would have thought that the wider section would be more resistant to being pushed out of column. Is this incorrect? As for the sail track, my designer specifies that the sails be laced on, back and forth. Says it makes the sail easy to raise and douse. I must admit I don't quite understand how this would work on a non-gaff rig, with stays and spreaders and stuff on the way up... I can tell you that on my Buehler cutter the mast had an external track. I don't even think it was purpose-built to be sail track. It was stainless flatbar welded to intermittent squares which raised the track 1/2'' away from the mast and the whole thing was machine screwed into the mast through the little squares. Worked great! Would grab and bind the cars a little occasionally, but if I'd know enough back then to squirt some sailube on there I don't think it would have jammed. Only thing: under the track was one of the 2 main places where rust would develop. There and where the mast steps were welded on. Y'know I once saw one of these masts done out of stainless pipe! Whatever drawbacks it may have had, I didn't see any rust on it! All's I can say having had a steel mast which was, let's see... 10 years old when I got it: sandblast that baby, and don't skimp on the epoxy!

    Eric Sponberg: Ah! I've been admonished! Of course you are entirely correct that I should just let my guy do his job, and that is what I fully intend to do. My aim in this forum discussion is to educate myself so that I may understand and participate in the decisions which will be made as the new rig takes shape. In fact, I have already learned quite a few things from the helpful and knowledgeable folks who've taken the time to answer my questions. I am by nature very curious, and my obsession with sailboats is more like a sickness than anything. I'm taking a lot of joy in trying to understand all the many variables involved in making this rig.

    Now you're the pro and I sure ain't, but you are so quick to assume that pipe sections are unsuitable. First of all the weight differential could be very minimal, like 1 extra pound per foot, only half of that having any real lever on the boat by being high up, and the gains to be had from monolithic all-welded construction are considerable. My boat's a big heavy cruiser, not a racer. I don't know how much extra windage a round profile will create. Many boats are rigged with solid wood, laminated wood, and steel or aluminum pipe. And it's my understanding a boat's motion does not improve when it loses its rig, so less weight is not always better. But yes, first comes the engineering, then comes the shopping! And I promise to stay out of my architect's way.

    Thanks everyone for weighing in! This is a great forum!
     
  13. pdwiley
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    pdwiley Senior Member

    Euler's Formula is what you need to look at - your designer will for sure.

    Bigger diameter, thinner wall, stiffer for the same mass, yes. More windage. If you don't care, fine, as long as you know.

    A laced on sail & no sail track, couple other issues eliminated, all to the good. Mine is a junk rig so I have no issues either.

    I'm using galvanised steel so hopefully rust day will take longer to arrive. Epoxy on top of course.

    The designer of mine said that I could use 4" Sched 10 steel in place of 5" Sched 40 6061-T6 ally. It will flex more but within his acceptable limits and as both he & his son have their personal boats rigged like this, I think it's good enough for me. I couldn't find the ally pipe anywhere locally or I probably would have gone that way.

    PDW
     
  14. MikeJohns
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    MikeJohns Senior Member

    Alloy RHS if you can find a suitable section is a better choice for a conventional stayed mast than tube, it has a more suitable section shape and a much better aerodynamic shape. It’s similar to a box section timber mast. But your suppliers may not have large enough sections. I’ve used steel RHS before very successfully for similar weight to alloy and a smaller section .


    I’d like to add to the above posts that you need to understand that both roll gyradius and metacentric height are important. A boat can be rendered uncomfortable and even unsafe by removing weight aloft that was factored into the design.

    Sometimes a boat that has been re-rigged adversely with light spars can be made a lot more comfortable and considerably safer by adding heavier masts. The roll period can tell you a lot about your boat and how stiff it is. A reasonable rule of thumb for displacement vessels is that the roll period in seconds is designed to be equal to the max beam in meters.

    GZ drives the rig design but GM should be carefully considered in an offshore craft and GZ and GM are closely interrelated.

    A high roll inertia is very desirable in beam on wave impact. Whether the pitch inertia is an issue depends on the heading and on the sectional shape of the hull fore-n-aft and the current distribution of mass in the hull. A heavy mast has far less affect on pitch gyradius on a medium or heavy displacement vessel with full ends and lots of reserve buoyancy, but pitch gyradius is very important on a lightweight fine bowed performance designs.

    I’d run these issues past your chosen mast designer to see what level of understanding they have.
     

  15. Skyak
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    Skyak Senior Member

    While we are talking rigging with an open mind, I have a couple of questions.

    What ever happened to X rigging, as a concept? In X rigging the stay goes from the deck, through the mast and to the far tip of the spreader on the other side of the mast (as opposed to just going to the mast at the base of the spreader). The benefit comes from the reduced angle, resulting in lower stress and improved aerodynamics (less drag and better sheeting angles). X rigging was one of the big innovations of late AC monohulls, but it never made it to general use. Why not?

    The other thing that has always bothered me as an engineer, there is so much emphasis on reducing weight, but then all these rigs use a constant section top to bottom -get a clue! The stress at the boom is the maximum and the stress at the top is diddly. And aerodynamics are the kicker, the big fat mast section at the top completely screws the aerodynamics.

    It just seems to me that sail rigs spend so much on materials and construction cost when a little sound engineering would yield far better results.
     
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