Sizing Bridle Eyes

Discussion in 'Fiberglass and Composite Boat Building' started by fallguy, Mar 14, 2022.

  1. fallguy
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    fallguy Senior Member

    I need to install two 316ss eye bolts. They are for the bridle connection and are supported by a fixture that holds the netting beam.

    Boat weighs 10,000 pounds or so.

    The current hole is 3/8" which would support a bolt rated at 1200# vertical lift; the forces are a bit not vertical

    A 1" bolt is rated at 9350#
     
  2. Blueknarr
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    Blueknarr Senior Member

  3. Blueknarr
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    Blueknarr Senior Member

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  4. fallguy
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    fallguy Senior Member

    super cool, so I can spec a longer bridle and reduce the demand to 5200

    I don't have the bridle ordered, but I am about to and was going to 45/90 degrees, now gonna go further.

    thanks a whole bunch BK!
     
  5. fallguy
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    fallguy Senior Member

    Now. Suppose I go to the 30 degree and use 5200# as the design tension on the bridle.

    Where is the failure supposed to occur?

    Certainly not the eye. It looks like I need to probably go the full inch.

    3/4" is 6000 pounds, but that is a perfect vertical pull on the hook and it will be somewhat sideways to the pull...

    The bolts are $102 each in 316ss for the shorty and like $120 for next size up..
     
  6. fallguy
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    fallguy Senior Member

    I have bow eyes, but they are no way gonna handle those loads...they are like 3/8-16 and the sides are 4# foam only. I set them up for a reasonable lateral pull in towing, but nothing too much.
     
  7. The Wing Guy
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    The Wing Guy Junior Member

    There are many types of 4# foam, but none can support the pull that a 3/8 stainless bow eye could see. Is this bridle a lifting bridle or a towing bridle? If a towing bridle, is it for something towed by your boat (such as a sea anchor) or is it for use in towing your boat with another vessel? If it is lifting bridle, it should be engineered, because you don't want to kill a yard worker when the bolt pulls out because of inadequate backing plate size or material, inadequate structure in the area of the bolt, failure to anticipate the failure mode, load direction, etc. Re the bow eyes, "reasonable lateral pull in towing but nothing too much" can mean entirely different things to two different readers. Do you have a friend who is an engineer?
     
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  8. fallguy
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    fallguy Senior Member

    I appreciate the concern.

    I have two different places on the boat up front with eyes.

    On the bows, I have eyes that are set in light foam, but there is a good bit of glass tabbing and a half piece of 12" 316ss. The eyes are in rebored 4# foam that is about 6" deep and supported with a 30 sqin or so backing plate. We did some calcs and figure the boat could be towed or the hulls could be held to a semi truck bed with those. I never intended to use them for anchoring..

    The ANCHOR bridle eyes would be going through a 316ss fixture that is designed to support the netting beam. That fixture is set atop 12# density foam and has a plywood backer about 100 square inches. The forces from the anchor pull would work against the netting beam and the bracket.

    Here are some pictures. The zoomed out shows both the fixture for the netting beam and the bow eye with the butterflied tubing. The closer up shows the fixture. I was going to install the bridle eye in the forward or aft bottom bolt hole. Probably would drill it out for one inch eye.

    Comments are welcomed. The fixture is on top of 12#foam, but I never overbored the foam or anything.
     

    Attached Files:

  9. fallguy
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    fallguy Senior Member

    I think I will email Richard and see what he typically advises. This boat could see a hurricane, so want to be ready for the worst. The eyes might be useful for more than anchoring..
     
  10. The Wing Guy
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    The Wing Guy Junior Member

    She's a great looking ship!

    It's hard to judge scale from the pics, but if the netting support tube is 4" spinnaker pole tubing, then those four fasteners look quite small, say 5/16" or even 1/4" . A single eyebolt placed in one of those holes, suitable enlarged, would see primarily a cantilevered load, and to be on the safe side, you would want to consider the pull to be at 90 degrees from the bolt shank. That gives the fitting (provided it is shouldered) 25% of its straight line pull rating. So a one inch eyebolt rated at 10000 lb straight line pull becomes a 2500 lb eyebolt.

    The reasons for the down rating have their effects on the mounting surface too. The shank is now exerting a moment load on the structure to which it is mounted, so at one side of the shoulder, the core (and skin) is being crushed, and at the other, the shoulder is lifting clear of the surface. As the structure fails, the eye bolt will become more aligned with the load, as it crushes the core and the backing plate, and bends the stainless steel support for the netting support tube. This stainless support will have been weakened by drilling the larger hole.

    The stainless bracket for the netting support tube appears well-designed for its original intended purpose. The highest loads are likely to be from someone falling on the netting, in which case the fixture will try to slide toward the cabin, and those smallish fasteners will try to shear though the fiberglass hull skin. Depending upon the thickness of the core at that point, the inner skin may have little utility in resisting that shear, but the bearing area of the four fasteners on the outer skin is likely to be enough to resist the load, particularly if the netting is stretchy (which makes the geometry more favorable). In a perfect world, the core all around the fixture would have be replaced with wood or solid fiberglass, increasing the effective bearing area for the original intent, but also doing the same for the new intent.

    If you have a ready source for tig welding, a lip could be welded to the underside of the stainless bracket, to distribute the pulling load all along the length of the bracket. This lip would be tapered from back to front, with the front large enough for a hole that would accept the anchor bridle. In rough terms, this changes the lever arm resisting the pull (compared to an eye bolt) from approximately hull panel thickness to the spread between bolts. With a solid core, then the four bolts would be seeing mainly shear loading, so you would check for allowable fastener shear loadings and also check for bearing strength of the core/laminate structure.

    What's the load likely to be in a hurricane? Well, at least no more than the breaking strength of the lines used... or the resistance of whatever the boat is tied to.
     
    Last edited: Mar 15, 2022
  11. Barry
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    Barry Senior Member

    You have identified that the bending stresses in the eye will require you to purchase a very large single shanked eye bolt.
    Due to the stress concentration caused by the notch at the root of the thread, I would expect that failure would occur at the base of the thread or the material between the outside and backing plate.
    I have attached a very crude, out of scale sketch of the way that I would tackle the issue. The idea of reducing bending stresses in any of the bolts. You could have this built in stainless
     

    Attached Files:

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  12. fallguy
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    fallguy Senior Member

    Thanks Barry.

    This is certainly better than the weakness of the thread minors.

    Too bad I attached and painted the darn fixture!

    I can fab all that stuff in a day and probably have all the stock and only cost is welding guy.

    Thanks a bunch. Perhaps this thread will help someone else. I may need to add a few bolts to the fixture bottom as those are also only 3/8-16..

    I am taking the liberty of adding a screenshot of the sketch. I cannot access the top of the fixture,but the bottom will suffice.
    41B98C5D-0726-4F7F-9B55-29392CBFD377.png
     
  13. The Wing Guy
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    The Wing Guy Junior Member

    Funny... we had exactly the same thoughts at the same time. Alternatively, the the plate could have a bend at the lower edge, instead of a weld. A modern blacksmith could do this.
     
  14. fallguy
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    fallguy Senior Member

    ..some follow up, thanks to both you and Barry for setting my eye bolts straight! Makes a lot of sense and I was nervous about the 1" breaking tbh.

    If the boat is hurricane tied; it will be behind my concrete house bows to the house; 50 feet away and down 12 feet less surge; probably less force than a bad anchorage at 40 knots.

    About these forces on bolts and skin/core.

    The core is 12# density foam. The tube is 4.5" od, 1/4" thick 6061 black anodized. The bolts are 316ss, 3/8-16 thread. The backing inside is only 6mm okume plywood; largely to avoid crushing laminate. The skins are 900g triaxial laid correctly.

    The bolts have a tensile rating of 70,000 psi. If we use 50% for shear, 35,000 psi. Minor diameter is 0.0678 in^2. Rating all threads is 2373#. Rating partial thread or less thread or threads below the bracket is 2712#. Four of the last bolts on the bracket would be good for over 10000#.

    core..core is not as easy for me to calc because I don't know skin factor...

    core alone is corecell M200 with a shear of 428psi...effective area of the bolt, core only is about 0.5" • 0.59" (half circle distance) or 0.295sqin time 4 bolts is 1.18 sqin for a rating of only 505#...but not at all acccurate because there is a skin factor here... I'll see if anyone can venture a guess on shear rating for my core. I'll start a new thread and come back and fill in here, so please don't quote this part..
     

  15. The Wing Guy
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    The Wing Guy Junior Member

    Another thought is to buy or fabricate a larger version of one of these:
    Heavy Duty Tow Pad Eye - Suncor Stainless https://suncorstainless.com/steel-product/tow-pad-eye/
    It could be mounted below your netting support tube, and you could remove the core in something like a 6" diameter circle, replacing it with wood or g10. That way you would not have to change anything in the way the netting support tube is mounted, if you are happy with that.

    Personally, I like to remove the core (replacing it with wood, etc.) anywhere there is a bolt that could be tightened to a typical torque value by someone who did not realize that the core would be crushed upon doing so.

    6mm okoume is not adequate as a backing plate for any fastener bigger than about #10. It does not provide enough stiffness in bending to spread the load for larger fasteners.

    Where there is curvature to deal with, you can laminate a couple layers of 6mm ply together with epoxy putty and lightly mount them (just enough to bend to the curvature). Then after the cure, you can tighten things up, without having to worry that the edges of the backing plate will point load the skin and core, damaging it.

    BTW, I attached a really crude pic of what I had in mind (in earlier post), if you want to integrate the tow eye with the existing stainless bracket.
     

    Attached Files:

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