Does an Alloy Toe Rail Add to a Hull's Stiffness?

Discussion in 'Boat Design' started by Southern Cross, Aug 12, 2013.

  1. Southern Cross
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    Southern Cross Senior Member

  2. PAR
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    PAR Yacht Designer/Builder

    The aluminum extruded toe rail doesn't add notably to the hull's longitudinal stiffness. If it was stainless or bronze, this would be a different matter. It does serve as a load bearer for the flange joint fasteners, on certain types of hull/deck cap arrangements. If the flange is bonded well, the toe rail just carries the sheet loads, or whatever else you tie to it.
     
  3. Southern Cross
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    Southern Cross Senior Member

    The debate continues as some say the toe rail makes the hull more stiff and others say it doesn't. It's the same response I'm getting from other owners and professional yards.

    So, to be safe, I'll do both, glass and reattach the rail.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    It is a simple enough calculation.

    If you have, for example, a hull which can be idealised as a box. And let’s say that box is 3.0m wide and 2.0m deep, and lets say the hull thickness is 4mm. Which no other scantlings, what is the modulus of this "box hull"?

    It is 29,174cm^3.

    Ok, what is the modulus of say a 50x6FB toe rail? it is 2.5cm^3.

    Thus just look at the 2 numbers it should be very obvious that is doesn't really add any stiffness to the main hull, when subjected to wave loadings, as PAR noted. The difference being a factor of 11,700 times!!

    It may affect the local stiffness/strength, but that depends upon how the deck and the sides and any cabin is arranged. But still again, it shouldn't be much at all, as the stiffness of a 50x6mm FB toe rail, is minor! It is more for load bearings like noted by PAR.
     
  5. Petros
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    I posted this on the other thread that might ad some relavaent information, I state this as an engineer that has done similar calculations, it may make a difference if the hull itself is flexible and is prone to buckling.

    "if the hull is known to be a flexy flyer anyway, I would think any metal added to the structure would assist in stiffness. Fiberglass has about half the stiffness to weight ratio as wood and aluminum, so adding metal into the structure would give a large benefit. It would be required however that the metal be fastened to the structure in such a way to both take and transmit loads to the composite hull uniformly and without stress concentrations, or else it will shorten the life of the hull as the fasteners work their way out of the hull material each time the hull flexes."

    "It is a very complex calculation to determine the number of fasteners required to prevent premature failure if the stiffer material is actually going to be carrying some of the structural loads."

    The real problem is that when you put a strong stiff material (metal) against a flexible material (fiberglass) than you do not stiffen it much unless you have a lot of fasteners to both distribute the loads between the two materials to prevent stress concentrations, and to prevent it from buckling. On a hollow shell structure, failure occurs when the skin buckles, if the metal is added to the hull in a way to reduce the risk of buckling than it will make the hull stronger. If it is added in a place that is not prone to buckling, than it will do nothing to strengthen it.
     
  6. PAR
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    Agree Petros, though speaking generally about the most common aluminum toe rail extrusions, they offer next to nothing, though as pointed out by John, some localized reinforcement could be expected, if every hole is used. I'll assume this is the typical 4" on center (usually 1/4" FH fasteners)

    [​IMG]

    There are several types, some much better than others in regard to stiffness, but most are 6061 T-6.

    One of my favorites:

    [​IMG]

    stiff, but requires special fasteners and these are on 6" centers..
     
  7. Southern Cross
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    Southern Cross Senior Member

    Thank you all ...

    The real issue with the boat and stiffness has come from overloading the rails and mast tension. The quick fix has been to reinforce the mast step (was just a 2 x 4 that at sat in the bilge and subsequently became waterlogged and failed), add a jock strap and a "beam of destiny". See attached picture.

    It's really been an issue of performance not safety as their have never been any reports of a catastrohic failure to the hull. Hard core buoy racers have put a lot of stress on the boats over the years.

    The toe rail that came off the boat did not have a lip as it was bedded over the joint which is a few inches inside the hull edge.
     

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  8. Petros
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    Petros Senior Member

    You are correct, that would not add much in the way of stiffness. It was perhaps 3" on a side with 1/2" thick legs, than maybe.

    It might be an interesting way to make a hull lighter, use heavy gauge toe-rails, and perhaps a metal rib down the keel line, to make the hull act like a truss or "tri-beam" (like an I beam but with three cords instead of two) and the skin of the hull acts like the web between them to take the shear loads. It would be a very interesting, and complicated design study.
     
  9. Southern Cross
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    Southern Cross Senior Member

    A sketch of the joint ...
     

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  10. Petros
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    Petros Senior Member

    Not likely that would add much stiffness if that is drawn at an approximate scale. How far is the rail from the corner of where the deck/hull corner is on the outermost part of the hull, looks like it could be several inches.
     
  11. Southern Cross
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    Southern Cross Senior Member

    It's about 3" from the corner to the gap between the deck and hull. The 1" x 1" toe rail sits over the gap.
     

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  12. Southern Cross
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    Southern Cross Senior Member

    Pardon me. I spelt "adhesive" wrong.
     
  13. Petros
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    that will make no difference in hull stiffness in my engineering opinion on whether you retain the toe rail or not. a more difficult problem it seems to me is how do you get a good clamp-up between the hull and deck, distributing the fastener loads over the joint, and not get eventual leaking at this joint. using a flat metal strap would replace this function of the toe rail, or adding more fasteners. I am not a expert on adhesives, so I would not know if there was a way to continuously bond this joint with structural adhesive that can replace the function of the fasteners, but it seems like a risky thing to do if a reliable bond can not be made.
     
  14. Southern Cross
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    Southern Cross Senior Member

    Here is an O30 with the metal tab like you suggest...
     

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  15. Southern Cross
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    Southern Cross Senior Member

    If I was really die hard I would dig out the old bond and glue the joint with 5200. But that's beyond what I have the time, expense and facilities for.

    I'm thinking that 3 progressively wider tapes of structural eglass with epoxy resin will hold the top part of the joint pretty well. The tricky part is underneath with that void under the lip in the hull.
     
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