Correct way to Tab bulkheads

[Chotu]

Yeah, as much as I love your wealth of information, Jedi. And that you know all the materials really well typically. You don’t have this one right. You should look at these posts. They are correct.

Regarding the need to use a hydraulic press to break or bend this type of bulkhead installation, that’s accurate. Not onlythat, the plywood would tear apart before you even begin to get a movement in that joint.

I’m not sure how many bulkheads you have installed before, Jedi, I don’t know your background. And I don’t know if you are just picturing crappy tabbing like a lot of production boats use. You know. A couple little squares of fiberglass here and there? Maybe you are picturing that? If you do it correctly, it doesn’t move. At all. Ever.

[Celestialsailor]

Originally Posted by wyb2

The logic that it gives the largest adhesion area doesn’t really make sense. Every layer is put on with resin, the final surface area that sees resin is the same regardless of order.

I agree the surface area is the same no matter which way you do it. I have seen both from manufacturers and repairs from pros.

[double u]

small first, largest last will result in "steps" in the layers. I wonder if this will not result in a slight weakening of the fibers under tension

[Benz]

Originally Posted by double u

small first, largest last will result in "steps" in the layers. I wonder if this will not result in a slight weakening of the fibers under tension

There are no detectable "steps" in that method, perhaps since the ends of the glass splay out as the next layer goes on; whatever the reason, I always get a smooth taper with that method.

[coopec43]

I wouldn't necessarily hold up production yachts as "the best practice" as they must always keep costs in mind. I have to keep cost in mind so I used polyester to "tab" the bulkheads and I bet I'll never have a problem.(My hull is Airex cored)


Beneteau just glue there bulkheads in "a two-component polyurethane adhesive manufactured by LORD® Corporation".
The comment #24 (above) also states

"Crystic Crestomer®structural marine adhesives areused by the leading boat builders around the world


As most production yachts these days have cored hulls (which don't "flex") they are able to use an adhesive rather than "tabbing"

[team karst]

In my experience, tabbing failures using 3 layers of biax on each side are rare and are usually adhesion failures.
Thinking about flexibility a bit. Consider a theoretical tabbing job using a hinge on each side of the bulkhead that is securely adhered. So a totally flexible/hinged on each side system. Would that not be stiff as an assembly ? Point i am trying to make is a stiff assembly can be made with flexible components. Another example is an I beam. Two thin flexible members attached by another flexible member, the web. But together, quite stiff.

[Bowdrie]

Empty fiberglass hulls are quite flexible, even on thick "crab crusher" boats.
The tension of the headstay/backstay works to "bulge out" the sides in way of the mast, and the lower shrouds are trying to squeeze the hull and lift the deck, so there is a certain component that at least "self-cancels".
Deck vs keel stepped obviously have different parameters, deck stepped needs a "compression" post. keel stepped needs a "tension" post/rod.
At the same time the deck is in compression whilst the hull bottom is in tension, neutral axis generally somewhere between WL and cabin sole.
Typically, the heavier loads on bulkheads are withstanding the wracking/twisting of the hull, and how the interior joinery is installed can have a large influence on the hulls ability to prevent twisting.
Boats that are heavily compartmentalized, with everything bonded in can accept fewer and less structural strength on the bulkheads than boats that are more "open" and have interiors that are more or less "tacked in place".

[Bowdrie]

Jeepers Batman, after reading my own post I realized I used too many words to make things un-clear.
A thousand pardons.
Regarding OP of tabbing.
We assume that the bulkheads are capable of withstanding the compression loading.
My comments regarding the wracking/twisting was supposed to be geared to showing how the wracking/twisting loads are absorbed by the tabbing.
Those loads are trying to twist the bulkhead out-of-plane, and the tabbing transfers that loading to the fairly rigid, (we hope,) bulkhead, (and any joinery that's attached to it).

[Seabeau]

Originally Posted by coopec43

I don't think it matters which way you lay up..

I used an Airex "Buffer" to spread the load. I primed the timber with a thinned down acetone/polyester resin (10% acetone) before laying the glass.

I cut 2in. holes about every 8in. around the perimeter of the bulkhead. I then fiber-glassed from each side so that the tabbing on both sides joined through the holes. Probably an over-kill but that is what Bruce Roberts recommended in his designer jottings. You can see the "holes" in the bulkhead in the following photo

I also use the holes drilled around the bulkhead's perimeter, but I first saw the technique, used in a boat building book by builder Ian Nicholson. In my opinion, there is no stronger system.

[bruceb]

I have to add my thoughts!!

Some years ago, Beneteau built a run of 30 foot cruiser/racers at their then new SC USA factory. The layup directions from the engineering dept were not followed - Maybe a language translation issue? - The factory crew put the small tabbing on first followed by the progressively larger strips. After the boats were put into service in a racing series and loaded up, the chain plate bulkheads started failing. The company had to go back, grind out all of the problem bulkheads and re-install them correctly- with the big piece on first!!! They didn't have any more issues!
I had a French built boat of the same model with properly installed bulkheads, used it for hard racing, and I can assure you it does make a difference. There are two of the same model at my sailing club, now about 30 years old, and still racing and still solid and tight as ever.

Be careful of information from "experts" on the internet and PUT THE BIG SHEET ON FIRST!!

Some boats are so well built (heavy) that probably any method will work but when you get into lighter "engineered" layups, I think I would chose my experts carefully.

B

[thinwater]

I think I need to explain again.


If the foam actually flexed any measurable amount in use, in such a small constrained space, it will delaminate, fatigue, and turn into dust. This is true of hull sections that flex a significant amount, and the angle we are considering, in such a small space is significant. Therefore, the function of the foam is NOT to provide flex or to soften the joint. Obvious if you think about it clearly.



What is does do, like every use of core, is increase stiffness by increasing the leverage of the skins, and in this case spread the load, by making the "foot" of the bulkhead wider. It does not soften the joint, but rather reduces the chance of localized fatigue from a hard spot by spreading the load over a larger area, which reduces sharp flexing of the hull near the bulkhead. The tabbing is flexible, as Jedi mentioned, near the edges as the hull panel flexes, but the support between the hull and bulkhead is rigid (the foam does not compress).


Basically, it rounds the joint radius, makes the foot wider, and allows the bulkhead to support the hull more, not less. It is not intended to reduce hull flexing as much as to reduce acute, fatiguing flexing at the bulkhead locations. Whether it is needed depends on the stiffness of the hull panel.

[double u]

Originally Posted by Benz

There are no detectable "steps" in that method, perhaps since the ends of the glass splay out as the next layer goes on; whatever the reason, I always get a smooth taper with that method.

the "steps" will be about the same as in woven cloth, which is inferior to biax fabric just because of the slight directional change the fibers make as they cross each other.

[JAFO]

Originally Posted by s/v Jedi

using an aluminium thin roller to work the air pockets out.

I prefer bristle rollers for this. No more 'chasing' air bubbles all the way to the edge! Instead, the air just escapes from the cloth, right where it is.

[bruceb]

Load differences.
Bulkheads, like many parts of a boat are not all equal, and proper design has to allow for different loads, and therefore, their proper attachment. With many bulkheads, most of the loading is in compression with some shear from hull flexing. However, a chain plate puts a vertical load that is trying to pull the bulkhead and/or the hull pan structure up and away, an entirely different force. Tabbing is not going to be the same either.

Adding a foam cored hull into the design often requires a more complex solution to distributing the compression and rigging loads.

B

[JAFO]

Originally Posted by s/v Jedi

during tests it was demonstrated that it doesn’t matter which way it is done.

Not doubting you at all... but if you have it, I'd love a link that I can rub other people's noses in!