Black Water Thru-Hull Location

It may be too late, but there is a reason backing plates are round not triangular. It helps distribute the load more evenly, and prevents point loading. What you have really isn't any different then just mounting the plate directly to the hull.

If you can change it I would. If it's already installed I probably wouldn't tear it off. But keep in mind for next time.

gbgreen59 said:

I've seen it the triangular base on another installation on a cruising yacht.

I don't think the bronze base should be mounted directly to the hull. The G-10 bonds well with the adhesive. I don't believe the bronze would adhere well to this.

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The shear strength of 5200 is effected somewhat by substrate, but in the case it really doesn't matter much.

5200 sheer by substrate
Fiberglass - 362psi (G-10)
Gell coat - 519psi
Bronze - 252psi

The reality is that if the thru hull is mounted directly to the hull the limiting factor is the Bronze-gel coat bond of 252psi. If the thru hull is mounted to the backing plate first the limiting factor is the Bronze-fiberglass bond of 252psi. Either way the failure point should be the bronze joint.

The reason for the backing plate is to spread the load. Which is why you need a very stiff, incompressible material (G-10 is great for this). But if the backing plate is the same size as the fitting there is no surface area to distribute load over.