Shear legs on metal boats

Discussion in 'Metal Boat Building' started by welder/fitter, Feb 24, 2009.

  1. welder/fitter
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    welder/fitter Senior Member

    While I prefer a centreline keel to bilge keels, It would be of value to be able to "park" on a beach. Aside from bilge keels or twin rudders, shear legs seem to me to be the only way to achieve this. I'd be most interested in the thoughts of others regarding design/material considerations, footprint/mount/carrying/angle/support requirements, etc.

    While this is, perhaps, more of a design-related thread, I feel that the "experts" in steel design/construction are best found in this area of the forum. For discussion sake, I'd like to use an example of a boat of 40 to 50 feet (approx. 13 to 17 metres) LOD, with a displacement of 10 to 20 tons(Imp), a beam of 12 feet(3.7metres).

    Ideas?
    Mike
     
    Last edited: Feb 24, 2009
  2. waikikin
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    waikikin Senior Member

    Mike, maybe you could have dagger boards positioned like bilge keels & use them like the "spuds" on a barge, they would want pretty big bases though or biggish removable bases to spread the load particularily on a soft bottom. Regards from Jeff.
     
  3. welder/fitter
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    welder/fitter Senior Member

    Here are a few pathetic drawings of what I'm suggesting. The sheer legs would follow the contour of the sheer, when not in use(elevated position). When placing in standing(vertical) position, they would need to rotate approx. 180 degrees. Support struts would prevent forward/aft movement, locking into mounts on hull sides. Aesthetics aside, structural integrity of legs, struts, and all joints would be paramount. Of course, for ease of use & balance considerations, components would have to be 'relatively' light.

    The deck plan used("borrowed") is for a s.v. of: LOD 49'(15M), Beam 14'04"(4.4M), Displ.@DWL 34,500lbs(15,682kg), Draft 6'06"(2M), Freeboard@midships 4'06"(1.38M).

    I recall, while doodling, that I asked about this concept some time in the past. As I have been recovering from shoulder/bicep surgery, perhaps, I have too much time on my hands!
     

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    Last edited: Feb 25, 2009
  4. nofacey
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    nofacey New Member

    Why carry the legs? Set up for a standard size 4x4, and buy them when you need them - won't work crusining in the wild, but how often do you plan on using them? twice a year maybe?

    Norm
     
  5. welder/fitter
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    welder/fitter Senior Member

    Norm,
    My wife and I own several beaches in the Philippines & are planning to add cruises to our business. So, they'd be used often.
    Mike
     
  6. nofacey
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    nofacey New Member

    S'ok Mike - why not go pipe within a pipe, seal welding outer pipe to hull, inner pipe manually retracts upward and follows line of side stays. Pin in collar at deck level holds it in place - anything 4" or over should be stiff enough, especially if it's more vertical than angled.
     
  7. welder/fitter
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    welder/fitter Senior Member

    This slide-pole idea would certainly have the advantage of using straight pipes. My concerns would be weight aloft, stability of pipes when retracted, lateral support when extended(lowered), risk of snagging sails & sheets, aesthetics.

    The reasoning behind a curve in the legs is to follow the curvature of the hullsides and offer a greater angle of support. Obviously, straight legs would offer better strength, but they'd stick out from the sides of the hull when raised(based on my suggested method).

    It seems to me, that I need the advice of a structural engineer on this one. As my old man kicked off some time ago, the free advice is gone. Still, if any engineer reading this thread believes that they have what I'm looking for, I have no problem paying for the design work. I would, however, want a clear understanding of what they were going to design for me, before I'd be willing to commit to the creation of the design plans. IMHO, Ideas are easy, they seem to float through the air. It is the 'sound' engineering that makes the difference, the true understanding of the mechanical & physical properties of metals. However, ideas of other members are of great value. I am sure that there are a multitude of approaches which I have not considered. As a matter of fact, when my arm is healed & I'm back at work, I'll steal a few minutes from one of our engineers & present all suggestions made here.

    Hey, Jeff! Yeah, I've seen that done before & that's what started me thinking about this, again. Truth is, I've got "bilge keel envy";)
    I prefer a centreline keel, for a multitude of reasons, but would like to be able to pull up to beaches, set some struts, and wait for the ebb, all without a lot of messing around or stepping off deck. That's the great thing about this forum; a large group of men & women with boat design & construction ideas flowing through their brains, constantly.

    Mike
     
  8. Brent Swain
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    Brent Swain Member

    Sheerlegs

    With single keelers, I always weld a piece of 2 inch sch 40 ss pipe in at the chine ,projecting 8 inches inside the hull. I cap this with ss, then a gusset tying it into the topsides. It is flush with the hull outside , having no more drag than a thru hull.
    Then I weld in a couple of flush, half inch ss acorn nuts, 90 degrees to the sheerleg socket, a couple of feet away from the sheerleg socket, to bolt braces on after the tide is out, if I plan to stay a while.
    The sheerleg is inserted from a dinghy and tied to a stanchion with a piece of line, tied to a padeye near the top of the sheerleg. You can weld a couple of oval pieces of steel on the bottom with a couple of holes, to bolt a piece of wood on, to give you a wider base.
    When the tide is out , you can bolt the braces on to the sheerleg and hull. triangulating the sheerleg to the hull. A couple of pads welded lower down the sheerleg gives you something to bolt the braces on
    The advantage of this arrangement is the sheerlegs dont have to be as long, reaching only to the chine, and are easier to stow. The socket also holds them in place while the tide is going out.Aluminium sheerlegs can be used for short term dryout.
    Brent
     
  9. welder/fitter
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    welder/fitter Senior Member

    That's right, I remember now. The boat we were discussing on the other thread had the pipes & gussets tacked in, but the hull hadn't been cut through, yet. Just as well, I had to cut them loose when bringing uniformity to the shell plate. I'll have to give that method some serious consideration, if I can not come up with an "on-deck" alternative. Swells & breakers might make for an awkward situation, vis a vis, a tender(dinghy). Something to think on. Thanks.
     
  10. MikeJohns
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    MikeJohns Senior Member


    A structural engineer ( this term is universally misunderstood) is simply an engineer licensed to design the structure of static civil structures (mostly buildings and bridges ), both Civil and mechanical engineers can qualify as structural engineers but this is not what you want here.

    Any Prof. mechanical engineer (or similar ) will be happy to design to your loads but defining the static loads is easy, the hard part is making a call on the dynamic loads while the vessel is somewhere between hard-aground and floating with clearance. Usually its one pole aside stayed fore and aft and two attachment points one on the rubbing strake one on the bulwark top or similar. An arrangement whereby they can be adjusted ) usually pushed further down is a good idea as they can sink a foot into the sand. Sleeves bolted on and alloy tubes with adjustable pinned arrangement seems to work very well. The English are the masters of sheerlegs and many boats have them permanently rigged when the boat is on its mooring.

    Cheers
     
  11. welder/fitter
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    welder/fitter Senior Member

    Thanks, Mike. Yeah, by structural engineer, I was defining between structural & design engineers. My father was a metallurgical engineer, emphasis on structural over design, but had local shipyards as his clients, as well as land-based fab. shops, etc.

    One would think design eng. over structural for this, but - at least with the old man - dynamic loads were of as much importance as static, in his work. Mechanical eng. for the joints, etc., I suppose. My Dad may have been a different sort of monkey, as he was a journeyman boilermaker - in the shipyards - before going back to school.

    For myself, I'm having a hard enough time studying to go back into inspection - 'health & safety' is killing me! - & would rather pay someone to brain-calculate all of the forces acting on such a structure & the appropriate materials, shapes, etc. . Besides, my ego isn't big enough to convince me I'd do it right!

    I've seen the type of sheer legs you're referring to & with the exception of a rotating joint, that's what I was going for, though I hadn't considered the need for the support struts to be able to adjust as the legs are adjusted for ground anomalies. Thanks. My wife wants me to go buy another plastic boat so she can have her computer back!:eek: Oh well, maybe I'll sail plastic to the Philippines. That'll give me 4 or 5 months to think about stuff like this.:D
    Regards,
    Mike
     
  12. Brent Swain
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    Brent Swain Member

    If the bottom of the keel is narrow , or you are on flat solid ground ,the loads on sheerlegs are minimal , simply balancing the boat. On lumpier bottom the loads are greater, but still not all that much.
    Going off the sheer instead of the chine, makes for much bulkier gear.
    Brent
     
  13. welder/fitter
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    welder/fitter Senior Member

    Yeah, & weight that sits higher & at extents of beam, if carried in position, rather than attached when needed. I remember earlier discussions we had on this, in this forum, or another. The discussion moved from sheer legs to consideration of drag/turbulence caused by the consideration of below waterline sockets, if I recall correctly.

    At least one of our beaches is toward the end of convergent waterway & airflow area, leaving for some "heave & puke" sea state(small, closely bunched, confused chop), just offshore. I'm concerned that trying to install legs from a tender might prove to be a constant challenge, involving much bobbing & banging against the hullsides of the larger vessel. Being that we own the beach & have friends in the coast guard, there, it will be easy enough to gain permission to alter the seabed. I'd planned to arrange an anchored buoy & depth marker, anyway, for positioning when coming onto the beach. We do have a dock at one of the other beaches &, in the end, may find it needed at all beaches. I'd like to put this(docks) off as long as possible, however, to avoid the financial expenditure & to prevent further degradation of these pristine beaches.

    I don't even allow people to smoke on the beach, unless they are carrying a can or bottle, & that goes for myself, as well. Eventually, I amy have to install the odd breakwater to reduce erosion, but that's another headache, for another day.
    Mike
     
  14. Brent Swain
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    Brent Swain Member

    Nobody owns a beach in Canada . Everything below high tide line is public access.You have no jurisdiction over what anybody does on a BC beach.
    I once saw a fishboat on the Fraser River on a trailer that the owner had built with big tires, that let him haul his boat into his backyard when the season was over. Didn't need to be that elaborate, as he was only going 50 feet or so. No railway needed. Simple solution.
    The sockets are easy to find and use from a dingy. I wouldn't use the sheerlegs in a chop anyway, as the boat pounds when the tide is just right ( or just wrong)
    Brent
     

  15. welder/fitter
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    welder/fitter Senior Member

    Same with Philippines, most countries, I think.(High tide line)
    I think the sockets are a good alternative, have kept them in mind since we last spoke about them. Still, I am interested in exploring the sheer legs further. BTW, Do you know anything about a 39' Folkes "Two Jacks"? Apparently, it's for sale. A bit smaller than I'm interested in, but with the economy going "**** up", it might be a good time to go sailing.
     
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