Sizing Bridle Eyes

This bridle, whether used for towing or as forward stay rigging is no different from a lifting sling with spreader bar. It will be limited by the buckling strength of the forward crossbeam.

Although it will never happen, unless one of the bridle broke, you want to calculate the holding power and fixity of the towing pad eye?

Before we go into laminate details, give us the length, distance between centers, separation angle of the bridle, and total displacement of your cat.

 

No bridle, yet, so angle is anything from 30-90.

Distance between front beam is like 11 feet. Hull center to center is 12'6". Loaded waterline of hull is 9.504m.

Displacement is about 8650#. Scary low and a bit nervous about my weight. I digress.

Kinda tired. Signing off for now.

 

I have never used a "snubber" but have seen them on boats tied up when the winter storms hit. Others may have some experience. But perhaps you might consider not putting them on the bridle section from the bow to the tow point as they may cause the boat to dart around, when they absorb energy and release it. You could make up the bridle with a fixed center point and then say a 6 foot section from the center point to some type of loop to which you hook the tow/anchor line.
This might keep the shock absorber more in line with the centerline of the boat.

 

And to save the bending cost, a short length of angle iron with the bend already there would also work.
Fall guy, is there any chance to get a third bolt in play to reduce the bracket trying to rotate around a line that is between the centerline of the two bolts.? It would stabilize the plate.

 

A quick check of the laminate required to meet your specs would be to find the shear strength of the laminate. Okoume was ruled out as it has only 2 N/mm2 of shear strength. WR has 82 N/mm2, more if cross plied at 0/45/90/-45 as shear strength increases. Shear strength can be further increased with an insertion of stainless steel washer.

If we were to push laterally a bolt through a laminate, that would be Bearing Stress failure. That is the cross sectional area of the bolt pushing against the laminate section. Assume 4x 20mm dia bolt will suffice to suspend an 8,650 lb (38.475 N) boat.
The standard formula would be Shear= Force/Area. To use the standard mechanical engineering formula, max shear would be (3/2)*(F/A). The allowable shear for a WR laminate with a F.S. of 3 is 27.3 N/mm2. With 20 mm dia bolt, 27 mm deep x 4, we get an area of 2,120 mm2 bearing area. Using the formula, we get 27.2 N/mm2 of shear, slightly less than the allowable of 27.3.

27 mm thick of laminate is too much. The attachment will not break but the nearby structure of your boat will. Of course, you can increase the number of bolts to increase bearing area to reduce laminate thickness but the point is you have too much expectation. Nobody designs a towing padeye so that the whole weight of the boat can be suspended in one pad eye alone. At most, even in rigging, it will be in the vicinity of 1/4 the weight of the boat, divide further by 2 if it is a bridle.

A more intensive analysis is shown in fig 2, will be configured. This is a Lap Joint failure analysis. Forces are greater because the pad eye center will be at a distance away from the outermost edge of the laminate. This is a true inplane shear. As the bolt rotates, it pushes the top of the laminate and pulls the lower laminate. The allowable shear would be even less as the the bondline/glue line (weakest part) is epoxy and it is in the circa 15 N/mm2 of shear strength.

 

Okay. This is confusing, but helpful. The okume 6mm is only a backer.

I understand how the forward edge would pull and the back edges would follow.

I understand how the bondline can fail.

The hull is foam sandwich

bottom to top, or outside to in
1/8" 316 ss plate
Triaxial 850g
Corecell m200
Triaxial 850g
6mm okume backer unbonded

The only bolts now are 4 bolts 10mm each.

I am not quite sure how to interpret the bolt needs, or even what my current strength/limits are...

The fixture is made with 1/8" 316ss plate. If there were same on the back side, then I don't quite understand the lap shear bit.

 

In the Excel illustrauion, Fig 1, If you push a bolt or a cluster of bolt axially along the laminate you get a bearing failure. This is just to test the shear stress of the laminate. Not going to happen in real life.

In fig 2, this is your configuration. You are pulling the laminate from the center of the padeye to a distance away from the laminate. This creates deformation in the laminate.

In the spreadsheet, I played around with Fig 1. With a 10 mm bolt and 12 mm thickness of laminate, I get a bearing stress of 27.8 N against the allowable 27.3. This with a 2,000 lbs. (6,898 N) of force and F.S. of 3. Seems reasonable with 2,000 lbs being nearly a quarter of your boat displacement.

Note that I am using a solid laminate. Plastic core or okuome core is too weak.

That is why I uploaded the spreadsheet so you can play around with it.

 

If you have 2 pieces of laminate and you pull one side away and the opposite side is pulled in the opposite direction. You get a shear.

1. If your laminate is weak, weaker than the bolt, the laminate will shear along its width.
2. If you make the laminate slightly stronger, you will shear the laminate along the cross section area of the bolt. Bearing shear.
3. If your laminate is strong and the bolt is weak, It will shear the bolt (as if it is not there), the laminate will fail along it's bond (the weakest point), near the neutral axis where the greatest shear stress occurs. This is lap joint failure or Mode II failure. Shear along the bond.

 

What is my existing hull and skin rating with 850-900g triax each side and m200 core and epoxy? And I understand how it can be different, but based on a pull on Barrys drawing. Or since I may decore and build a new spot, a drawing of a beefier setup with more bolts.

If I remove a chunk of core firther back and not involving the bracket and make my core solid to about 14mm, how would I build it so the existing core of M80 and the solid core are not a new weak point?

I think I wil always buoy my anchor and make the shackles the weak point. But the hull canot be too weak..

 

A few things to be aware of:
Many foam core materials do not reach their compressive strength rating until they have been compressed to 90% of their original thickness. So you cannot usually just add up skin and core contributions because the skin fails before the foam can make any contribution.

If your skins are .040 thick, then each bolt bears on .040 x .375 = .015 sq in. If the bolt is not perfectly fitted to the holes in the fixture and laminate, almost all the stress will be at the hole on the panel side of the fixture, so my inclination is to consider only that skin. If the skin can withstand a bolt bearing stress of 20000 psi, then each hole could support a shear of 300 lb. That figure could be doubled if the fixture is thick enough and the holes snug enough to ensure that both skins come into play.

Friction between the fixture and skin could help substantially, but developing friction relies on the core compressive resistance. That is one reason that the core is always removed at critical fasteners. Another is that the bolt bearing area is then much greater than just the skin thickness.

Be aware that the actual loading on the fixture fastenings could be a roughly 50-50 mix of shear and pullout (reasoning in post in the other thread).

I've used a shop crane (with a digital strain gauge) to test various specimens. You could test shear this way, especially if the forces are a few hundred pounds. For forces more than perhaps 30% of the shop cranes rating, you would need to be especially careful to avoid the consequences of springiness when the sample fails. (An instrumented hydraulic press avoids most of the springiness issues.)

 

Most boat designers do not include detail drawings. Class rules do because build details are included in the rules.

So most designers leave it to the skill of the builder and sound shop practice of the builder/shop.

Foam cores with highly stressed areas are locally substituted with a higher strength core. Foam cores are to handle evenly distributed loads. Areas which needs to be bolted (point load) are transitioned into single skin thickness (solid fiberglass) + a little bit extra to compensate but not too much (usually 1.5 to 2X the total skin thickness) to cause stress risers . Hand lay up or vac infused would always use anti compression bushings and fender washers to distribute load more evenly. Sometimes a backing plate is substituted.

 

So, I did not do anything too fancy drilling and the holes may be imperfect, so I hear you saying good holes would offer about 300# each only for 1200# total. Ouch.

 

RW did substitute the core. We went from Gurit M80 to Gurit M200 behind the fixture.

I did not use compression bushings. If I just change the holes to include compression bushings; these could be made larger, but it mght be tricky to make the holes now and get the thickness of such a bushing to match the core. Then does the bushing get skinned back in on the inside or just bedded into epoxy putty? Hard to use a backing plate if the depth is too much. The m200 core area might not be large enough for the compression bushing to be real wide either...would need to measure..

If I redo anything core wise; the local core would be M80 because I don't want to work up in the narrow area of the hull where the bracket is too much. Too hard to do good work there. If I decore a section of the hull from the inside, I could make it solid glass.

Ad Hoc suggested the fastenings be weaker than the hull. The hull at 1200 pounds now is weaker than one of the fixture bolts. Using the Wing Guy's 20,000 pounds and a 3/4" compression bushing and removing an area of core and making is solid to say 0.625" would result in 0.75 (seems area is greater than diameter) times 0.625 is 0.46875 times 20000 is 9375# per hole. But the M80 isn't that strong in nearly any fashion, nor is the epoxy bondline.

Too tired to post more..

 

I've got to get busy, but here are a couple ideas:
Core can be removed from outside with a hole saw, and if you are very careful, you can leave the inside skin intact. You need to wire brush (with a cup in a drill) to get the remaining skin thoroughly clean, then start laying up discs of glass.

>> compression bushing and removing an area of core and making is solid to say 0.625" <<

The required area is that area that won't rip the skin to which it is attached. The actual potential failure can be complicated, but if you provide enough area in shear, counting the two areas shearing on each of the top and bottom skins, you should be good to go. So for example, if your core replacement is a 2" square, then there are 8 inches in shear x .040 x 20000, so 6400. 20,000 psi is my guess for triaxial/epoxy in shear. There is probably a better figure than mine. Either careful (very) tapering (like a scarf joint) or a lap joint can provided continuity between the core replacement and the skins.

 

How did you arrive at those values?