[joyinPNW]

I'm a little late in suggesting this but I have the same boat, circa 1991, and Ward from Beneteau in S.C. is a great resource for specific design questions or locations of thru-hulls, etc. I believe if you sent him the numbers off the Beneteau plate he can help you with knowing what the original plans were. He's a busy man and email is usually best but I've had great luck. Google Ward from Beneteau and I think you'll find him quite easily.

We love our 35s5!

 

[eherlihy]

Why am I not seeing a backing plate?

It should look something like this:

- or this (Photo credit to Maine Sail / Compass Marine)

 

[gbgreen59]

Why am I not seeing a backing plate?

It should look something like this:...

The backing plate is there. I used 1/2" G-10. I decided not to go with a round shape. I went with the shape of the base plate as seen in the picture below.

....sorry I posted without the image. See the baseplate image in the next post.

 

[gbgreen59]

Thru-hull baseplate...1/2" G-10. I decided to go with this after seeing it in an installation.

 

[Stumble]

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]

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.

 

[Stumble]

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.

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.

 

[mitiempo]

I would use either G10 or fiberglass and epoxy it to the hull.

 

[eherlihy]

Stumble / Greg's post is spot on. In the case of backing plates, bigger (in terms of area) is always better.
If I had the $$$ I'd like to see my entire hull backed by G10.