Tabbing question...

Greetings,

I have a question about tabbing, as I build my extension bracket. Generally you see 2-3 layers of tabbing, but I wonder what are the deciding factors of using 3 layers instead of just 2?

Certainly, the strength needed is a factor, but I wonder about other factors? Length & width of the tabbing, etc.

I'm currently tabbing in the 'bottom plate' of my extension bracket to the stringers. . . .I'm planning on using 3 layers all 20" long. 8" 1708 , 6" 1808 , 4" 1708 widths. (3/4" radius fillet) This will give me 0.135 thickness at the joint and 160 sq in of bonding area. (VE resin)

The joint will have 20" x 0.135" = 2.7 sq. in. of cross-section. with a conservative approximation of 40,000 lbs of overall strength. 2 layers would get me in the 27,000 lb range. . . . either would be plenty for my application.

So, I guess I'm wondering how much is too much? I assume both bonding area and overall thickness of the fiberglass laminates are concerns . . . just wonder what matters more, etc?
.

 

Check the direct, tangential, and intralaminar stresses in that area, and you'll see if you need stronger or wider material for the overlap.
I suppose my answer might sound pedantic, but I don't know any other way to give the right answer.
If you want to avoid technical jargon, look at your neighbors' boats at the dock.

 

When you look at your work; you’ve done enough glasswork to know where major concerns exist.

The transom to pod sides, for example, is automatically going to need more; probably more than 3. The load from the engines is transferred to the hull through what components? Those need to be very strong. Short of doing an accurate loads analysis; this is the gutcheck method.

 

Yes, I think that I calculated needing 5 layers for the transom. The bottom plate, which I just tabbed in, was questionable as to how much load there would be and how many layers. 3 should do the trick.