Tabbing question...

[tpenfield]

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?
.

 

[TANSL]

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.

 

[fallguy]

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.

 

[tpenfield]

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.

 

[powerabout]

How about the question of small to large laminate or large to small in the joint when tabbing?

 

[wet feet]

Start small and increase successive layers.

 

[fallguy]

How about the question of small to large laminate or large to small in the joint when tabbing?

This highly debated question comes down to quality.

Noone has shown, that I am aware of the small to large or large to small are structurally better.

But from a workflow perspective, largest first offers a couple of things: 1. The likelihood of sanding through the widest layer of tabbing is practically eliminated vs the other way. 2. Air intrusion at the overlap of larger successive layers is eliminated. Now, if you are tabbing say 150/100/100, you will still have some air intrusion in the 100/100 overlap, but 100/100/150 has two locations versus one in the other method.

I will add, it is sometimes counterintuitive to go largest first, because it feels like you get a shingle effect, but this is all just a hope and nothing real has been shown to come from it.

 

[gonzo]

Large to small has better fiber alignment. Starting with the small pieces makes the fibers bend in an S and there will be a gap, hopefully without an air bubble.

 

[TANSL]

No, that´s not correct. Each layer wider than previous one.

 

[fallguy]

No, that´s not correct. Each layer wider than previous one.

Or for sure the opposite of Gonzo?

 

[TANSL]

I felt that Gonzo's statment wasn't right and I said what I thought.. I may be wrong or have misunderstood, but I think you have to put down a layer that overlaps by about 25 mm, then another layer that overlaps the previous one by 15 or 20 mm, and so on. That is, the narrowest layer first, and successive layers getting wider and wider. Something similar, not just the same, to what the drawing shows.
If you could explain where I've gone wrong, I'd be very grateful and would have no problem correcting it.

 

[wet feet]

TANSL is stating what every designer and composite engineer I have known puts on their specifications.I have requested links or references to specifications that differ from that in the past and none have been provided,but "feelings" get mentioned.

 

[latestarter]

I don't have a view on the subject from a structural point of view however I prefer the look of starting small so the outer surface is continuous rather than stepped.

I am puzzled by the diagram TANSL has posted as it does not represent either view.

If I am reading it correctly the first layer is long on the left side short on the right then second layer short on the left long on the right, last layer half way between the 2 on each side?

 

[fallguy]

That picture defines staggered tabbing and doesn’t even agree or reply to the question.

In his picture; here are the tape lengths:

1. 55+25+k or 80+k
2. Same as 1
3. Same as 1&2

You’ll note, these 3 staggered tapes are all the same and net to a combined 240+3k.

There is an alternative method where:

1. 110+k
2. 80+k
3. 50 +k

And you’ll note the total tape amount is the same, but the numbers of times the tapes change in their plane on the bottom is zero. In the drawing, tape 2 changes in plane on the right side as it goes over tape 1. And then tape 3 changes in plane on the left side as it goes over tape 2. This method allows for air entrainment at those two locations.

If you apply the tapes smaller to bigger; you can entrain air in two locations both sides or 4 places.

And when you sand, you can sand through those plane changes easily and effectively eliminate the longest tape; in the longer over shorter method.

So, it isn’t feelings. If the designer specifies say 150/100/100, then stagger is required, but typically here in the US, we’d lay 150/100/100, not 100/100/150. Again, to reduce the air entrainment likelihood and the potential for sandthrough. And yes, it is specified this way. Just because we are not providing you with citations doesn’t mean we are using our feelings. Good heavens; such a proof of theorem.

What is really going on is we are getting into surlyville here, where someone thinks their way is the only way, and so they refute the objective. On the other hand, the air entrainment is not that much and sandthru not super likely except light tabbing,

But if we are honest, when you fin roll a 150/100/100 tape set; you realize air does end up in the laminate at the overlap of the 100 over 100. For say a 1708 tape; those are about 1mm thick or so and they end up with a triangle of air approximately 1mm high by 1mm long which is inherent in the overlaps. Sometimes, an excess of resin fills the margin, but usually there is a bit of air in this stagger, or even after perfect fin rolling; the physics of a one mm step takes over and the resin lays down leaving air in the laminate.

And so, staggered tapes result in some air entrainment; so why do we do it?

In the case of 150/100/100, say over a glued seam, let’s assume the stagger is 25mm. This means the seam is overlapped 75/25 and 25/75. In a more perfect world, all seams would be 50 min over the seam, so we could tape 100/125/150 or 100/150/200 or the other way 150/125/100 or 200/150/100, but doing so adds weight and it happens fast, and so we allow a seam with some inherent air entrainment to reduce the total weight.

But, at least here, we avoid entraining air in tabbing, when possible.

 

[TANSL]

If I am reading it correctly the first layer is long on the left side short on the right then second layer short on the left long on the right, last layer half way between the 2 on each side?

The figure comes from ISO 12215-6. It is probably a very scientific arrangement, extremely correct from a theoretical point of view but difficult (impossible, I would say) to put into practice. That is why I have specified that my solution is similar, not the same, as the one in the figure.