Help on shear rating calc

Hi Dan,
I did not know, until seeing another post, that the stainless plate with the 4 holes is only 1/8". That means Barry's design is much better than trying to weld something to the existing plate. For a boat the size of yours, I'd use 1/4" plate for Barry's fixture. The foam makes virtually no contribution to the strength of the system. As rxcomposites says, the core needs to be replaced with solid fiberglass to get a reasonable shear value for the assembly with three bolts.

The netting tube, with its 1/4 in wall, would provide adequate compressive buckling strength for a bridle system, but it is not fixed adequately to the existing bracket for that purpose. The compression would need to be fed more directly into the tube, such as by inserting pieces of 3/8 x 2" aluminum bar stock through the tube diameter, and welding them it into place, ideally with them angled toward the center by 30 degrees to match the bridle pull angle. Then, anchoring forces would pull the net tube forward, and the existing brackets are fairly good for that direction of force, but they are too light, and they need to be bolted to a structure that will adequately resist the forward pull.

"Ad hoc" makes the good point that you want something other than the hull to fail. Suppose you set a limit of 3000 lb per side, and make the bridle 30 degrees. That would be a combined straight line pull of 6000 x cos 30, or 5196#. The angle between the bridle rope pull and the hull would be greater than 30 degrees, so there would be more pull out force on the bolts than just the 30 degree pull would suggest. If we assume that the bow converges at 15 degrees per side, then the pull would be 45 degrees to normal vs the skin. The front corner bolt would take most of the pullout force with Barry's bracket as drawn, but moving the load hole back would help share the pullout load between the two lower bolts. The pullout load would be about 70% of line pull, so 2100 lb. It that were split equally between the lower holes, then 1050lb per hole.

The shear load, also 2100 lb, would be shared by three holes, so 700# per hole. With a solid core near the bolt hole, both inner and outer skins could receive the shear load. If the skins are .040" thick, and can accept 10,000 psi shear, then your solid areas around each bolt would only need to be (with a safety factor of 2) : 1400/800 or 1.75 in across. Careful tapering could make a flush skin repair, or a circle 2.5" across could be applied above the current surfaces to provide load transfer through a lap.

Check my figures, of course.

You're getting close!

Ken

 

I want to know the strength of my current hull configuration.

If I were going to start decoring my hull to make it solid inside, I would not do the work at the plate because access is pretty hard. I'd just move the thing back.

I am traveling and can't really draw anything in the car. And have an email into Richard for advice.

I am really more confused than before about the strength of the system. Barry's drawing made a lot of sense, but I'd rather it not shear the hull, so wanted to understand the strength of my existing setup.

Can probably make the shackle the weak point if I understand the hull strength..of what I have now.

Decoring and making a solid core 'spot' of doesn't sound right to me either..because that would also need to be rated and the surrounding hull would be a factor.

Another thing I wondered about is making the eye from aluminum. That way if we had too wicked a force from the anchor; the metal could bend..

I must admit uncertainty.

 

Great. That was your initial analysis and my understanding too. It is a bridle with a spreader bar (netting bar).

That reduces lateral load and transfer some loads to vertical (bolt pullout). Following AH advice, make it fail before the hull fails. Makes sense because what is the use of a padeye with its attachments intact but with a hole in the hull. We can reduce safety factor down to 1.75 or even less as the shear of the bolt is 60% of tensile strength. Even with laminate, F.S. of 2 would mean 50% of ultimate and have a small margin of safety. We can go 60%. Take note. All these shear calculations is with the assumptions that the bolts are finger tight. Tightening to specs will create friction between the plates and the laminate increasing strength.

Now we have two threads, Where to answer?

 

Yeah, I know two threads happened. My fault. Maybe the moderator can close this one and we make a reference to it on the other?

 

Sizing Bridle Eyes https://www.boatdesign.net/threads/sizing-bridle-eyes.66743/