Allowable Bearing Strength - Bolted Connections

[Renoirpierre]

So I've been thinking about bolted connections to fiberglass hulls. I see this topic as pertinent to understand chain plate connections, drogue connection plate connections, cleats, winches, etc.

I would appreciate it if someone can review my understanding and description below and confirm if I'm correct or incorrect.

I recognize that there are a lot of different loading scenarios but let's talk about a bolted plate in pure shear (e.g., chainplate that is perfectly in line with shroud forces)

To determine the bearing stress I would take the force placed on the fastener and divide it by the diameter of the bolt x thickness of the material.

or

Fp=P/(db x T)

so, looking at the fiberglass bearing stress of a chainplate restraining 1,000 lbs with a single 1/2" fastener (bear with me (lol)) going through a 1/2" thick hull it would be:

Fp = 1000 lbs/(0.5in x 0.5in) = 4,000 lb/in2 (PSI)

The compressive stress of fiberglass is ~30,000 PSI in the strong direction per page 2 below (and 15,000 PSI in the weak direction).

https://www.fibergrate.com/fileshare...signishape.pdf

So ignoring factors of safety and dynamic loading factors, 4,000 psi is less than 30,000 psi so the joint would be good for this (unrealistic and simplified loading scenario)

If we can agree on a common understanding of bolt bearing stresses I think there are additional topics around series drogue plates that would prove interesting.

[Bowdrie]

I generally leave the math to engineers.
But consider this; let's say that your 1/2" bolt was placed thru a 1" hole in the fiberglass.
You can see that in that instance their would-be what amounts to a "point loading" in which all the load would be placed on a tiny sectional area.
Looking at the bolt end-on one can see that the circumference of the hole between the 3:00 around to 9:00, (the bottom hemisphere,) carry's no load, whilst at 12:00 the load is greatest, the load tapering off between 12>3:00 and 12>9:00 at a rate following the sine of the angle?
Of course, I'm just talking out loud and what I've said could be totally wrong on all counts.
I would be pleased if an ME explained that, then I could learn too.

[markxengineerin]

It would be nice if it were so easy, but there are many more factors to consider even in a simplified scenario. For one, the "compressive stress" is not the material property you would want to compare your bearing stress calc to, since that's not the same thing. It is difficult to find any reliable material properties for fiberglass, especially a specific fiberglass, with a specific fiber, resin type, resin amount, layup strategy, etc. I have never seen "bearing stress limit" as a published material property for anything except actual bearings and bearing coatings. Neither compressive stress nor "bearing stress" are for sure going to define the failure mode when overloaded. The result would depend on the bolt material, how tight the bolt is, how close the bolt holes are to the edge of the materials involved, and many more factors.

Being practical, it would be smart to significantly over-design the joint if it's safety related (the practice of most boat builders, and the simplified rules of thumb found in boatbuilding textbooks) or to do some testing that's applicable to your individual scenario, if you're really trying to save weight or have another special goal. There are specialists who can calculate/simulate this type of scenario, but it's far from simple/easy, and definitely not worth getting into for a general application.

I went through a similar question/answer process trying to decide on a bolt torque for my chain plates- notes attached- and concluded that it's easy to torque the bolts highly enough that the hole ID's will not see any shear at all, making it a moot point. The weak point in the rig should not be the chain plate connection to hull, ever. On most boats (assuming all parts are brand new) I suspect the weak point is the wimpy stainless tangs to the mast, or the turnbuckles. If you're talking about a drogue, I might start elsewhere in the system and see if there is a different weak point that's an easier place to start.

[Bowdrie]

^Thanks for clarifying the unclarifiable.

[Bowdrie]

Originally Posted by markxengineerin

On most boats (assuming all parts are brand new) I suspect the weak point is the wimpy stainless tangs to the mast, or the turnbuckles.

This reinforces what a NA told me years ago, to wit. That if the turnbuckle was the weak link, you could carry a spare, but if a wire or something else let go you were in real trouble.