Correct way to Tab bulkheads

[UserError]

The surveyor who advised on my keel stubs rebuild stated a preference for smallest first, that in the event of any laminate separation, each new layer edge represented a new barrier to the separating forces, whereas once the 'big layer on first' starts delaminating, there's nothing to stop it. I buy that reasoning behind smallest first. Vaitses shows small layer first and Staton-Bevan shows largest first - you pays yer money. As has been said, if it's done properly, its probably a moot point.

[s/v Jedi]

Originally Posted by UserError

The surveyor who advised on my keel stubs rebuild stated a preference for smallest first, that in the event of any laminate separation, each new layer edge represented a new barrier to the separating forces, whereas once the 'big layer on first' starts delaminating, there's nothing to stop it. I buy that reasoning behind smallest first. Vaitses shows small layer first and Staton-Bevan shows largest first - you pays yer money. As has been said, if it's done properly, its probably a moot point.

An argument as convincing as the opposite side but during tests it was demonstrated that it doesn’t matter which way it is done. Biggest mistakes one can make: using resin other than epoxy, not mixing epoxy properly, not using the aluminium thin roller for removing air bubbles and excess resin, leading to voids and resin rich layup.

Looking at my own boat, what I wish they did differently: add a top layer of peel ply for a smooth result, then after full cure I wish they rolled on barrier coat to keep it clean easier. I hate raw fiberglass.

[coopec43]

"Crystic Crestomer®structural marine adhesives are
used by the leading boat builders around the world.
Customers using Crestomer 1152PA to bond bulkheads
to hulls with fillet joints gain the following advantages
compared to conventional laminated joints:

• Superior external cosmetics with zero print through of the joint
• Potential labour savings of more than 60%
• Better internal aesthetics, cleaner looking joints
• Considerable weight savings
• Much cleaner and easier to use – improved working conditions
• Improved productivity
• Significantly reduced styrene emissions
• No grinding required

https://www.scottbader.com/wp-conten...uide-uk-lr.pdf

[Dave_S]

No expert, but that's never been a barrier for me 🤣. I think it would depend on the bonding strength between layers. i.e. In failure, does it fail by the layers separating or crack through. I imagine a properly applied joining would only crack not separate. If this is the case, I guestimate it would make no difference which order you place them. I do think you should go in either descending or ascending order to allow a progressive flex across all of the joining material.

If it cracks near the joint, then there should have been more material near the joint and/or comparitively less material away from the joint. The intention is to share the flex or loads evenly so that one section isn't closer to failing than another section. This will provide the strongest joint for a given quantity of material. If it cracks next to the joint your intention of dispersing the loads hasn't worked and you had a weaker joint from the start.

[thinwater]

I know what the manuals say. I know what I have done and what I have seen in production and custom boats.


We're talking about placing rigid (you aren't using upholstery foam!) foam under the bulkheads, then covering that with 2-3 layers of biax on both sides. The foam is nearly rigid. But more to the point, the 6 layers of biax will be as rigid as steel over such a short distance. Finally, if the tabbing flexes more than 1/100th of an inch while sailing, I make a bet that it will fatigue and crack in short order. The joint is effectivly rigid, as far as the foam is concerned. And that is a good thing, or the foam will degrade quickly.



I can understand that fitting a bulkhead tightly will push the hull outwards and make a hard spot. But if the bulkhead is fitted with some gap, even 1/10th of an inch, The hull will remain fair and the bulkhead attachment will be completely rigid compared to the hull 1-inch away whether fitted with foam or not. The foam will not degrade due to movement anymore than foam in the rest of the hull. The foam does NOT change how the hull moves or looks under normal and even storm conditions.


The only time it is going to matter is if the boat is in a collision and strikes the bulkhead. Then there is some potential softening of the impact. But not much and this only applies to minor strikes. My gut (and I could support this with calculations) is that a bead of polyurethane will not support enough to make any difference. It is effectively a void in the joint, which is not all bad.



I offer that the foam makes no difference unless it is several inches wide, an inch thick and shaped into a smooth transition, and then the tabbing is actually supporting the loads and needs to be multi-layer. You need to think of the tabbing as either being the sole support of the bulkhead, supporting 100% of the load, or skip the foam and use the tabbing just to keep the bulkhead in place. The middle roads make no structural sense.


Of course, in most cases it just does not matter much. Either works.

[s/v Jedi]

If you use 3 layers of 1708 tape for the tabbing, then you get a laminate that is just a touch over 1/8” thick. I would suggest to just take some 2’ lengths of 4” wide tape and laminate three layers on some plastic wrap on a bench, let it cure, then see how unmovable stiff it is.

You will find it is very flexible, as is all fiberglass laminates used for hulls. When you go up in thickness, it will be for larger pieces and flex is there over the longer distance.

I read somewhere that carbon fiber hulls will be as stiff as plywood hulls. And plywood still flexes, even when bent into compound curves.

It’s good to have that flex because without it stress cracks would be a common thing.

Edit: about the foam: even the highest density foam will prevent print through.

[wyb2]

Originally Posted by s/v Jedi

If you use 3 layers of 1708 tape for the tabbing, then you get a laminate that is just a touch over 1/8” thick. I would suggest to just take some 2’ lengths of 4” wide tape and laminate three layers on some plastic wrap on a bench, let it cure, then see how unmovable stiff it is.

You will find it is very flexible, as is all fiberglass laminates used for hulls…..

But make that bench test piece in the form of an L like it would be when used as tabbing, and try to bend it in plane with one of the legs.

But that would actually only represent if the tabbing was just a structural rib bonded to the hull with no bulkhead. So now make one bonded to a chunk of plywood, with the appropriate air or foam gap, and try to flex it in plane with the plywood. You’ll likely need a hydraulic press and hear some nasty cracking sounds by the time you observe noticeable flex.

[Chotu]

Originally Posted by thinwater

I know what the manuals say. I know what I have done and what I have seen in production and custom boats.


We're talking about placing rigid (you aren't using upholstery foam!) foam under the bulkheads, then covering that with 2-3 layers of biax on both sides. The foam is nearly rigid. But more to the point, the 6 layers of biax will be as rigid as steel over such a short distance. Finally, if the tabbing flexes more than 1/100th of an inch while sailing, I make a bet that it will fatigue and crack in short order. The joint is effectivly rigid, as far as the foam is concerned. And that is a good thing, or the foam will degrade quickly.



I can understand that fitting a bulkhead tightly will push the hull outwards and make a hard spot. But if the bulkhead is fitted with some gap, even 1/10th of an inch, The hull will remain fair and the bulkhead attachment will be completely rigid compared to the hull 1-inch away whether fitted with foam or not. The foam will not degrade due to movement anymore than foam in the rest of the hull. The foam does NOT change how the hull moves or looks under normal and even storm conditions.


The only time it is going to matter is if the boat is in a collision and strikes the bulkhead. Then there is some potential softening of the impact. But not much and this only applies to minor strikes. My gut (and I could support this with calculations) is that a bead of polyurethane will not support enough to make any difference. It is effectively a void in the joint, which is not all bad.



I offer that the foam makes no difference unless it is several inches wide, an inch thick and shaped into a smooth transition, and then the tabbing is actually supporting the loads and needs to be multi-layer. You need to think of the tabbing as either being the sole support of the bulkhead, supporting 100% of the load, or skip the foam and use the tabbing just to keep the bulkhead in place. The middle roads make no structural sense.


Of course, in most cases it just does not matter much. Either works.

Precisely!!!

This is exactly how it works.

[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!

[admiral 31]

Originally Posted by UserError

The surveyor who advised on my keel stubs rebuild stated a preference for smallest first, that in the event of any laminate separation, each new layer edge represented a new barrier to the separating forces, whereas once the 'big layer on first' starts delaminating, there's nothing to stop it. I buy that reasoning behind smallest first. Vaitses shows small layer first and Staton-Bevan shows largest first - you pays yer money. As has been said, if it's done properly, its probably a moot point.

That is the way I was taught (smallest first) & for exactly the same reason.

[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.

[Mike Burch]

Originally Posted by UserError

The surveyor who advised on my keel stubs rebuild stated a preference for smallest first, that in the event of any laminate separation, each new layer edge represented a new barrier to the separating forces, whereas once the 'big layer on first' starts delaminating, there's nothing to stop it. I buy that reasoning behind smallest first.

+1! Smallest first. Then if one layer has only a dodgy grip on the surface it is trying to bond to, it will matter less. Besides, on a practical note, it's a lot easier to get the bulkhead held steadily in the right place by using a few small bits of glass first rather than trying to manoeuvre a large strip of wet glass into place accurately on a surface that isn't held rigidly.

[smj]

Originally Posted by Mike Burch

+1! Smallest first. Then if one layer has only a dodgy grip on the surface it is trying to bond to, it will matter less. Besides, on a practical note, it's a lot easier to get the bulkhead held steadily in the right place by using a few small bits of glass first rather than trying to manoeuvre a large strip of wet glass into place accurately on a surface that isn't held rigidly.

I’ve found the best way to hold a bulkhead in place is to “spot weld” it with a thickened fast setting epoxy. Place a few dabs where needed to hold the bulkhead true, and make the welds where they will not be sitting proud of your future filet. So once cured run your filet and your tabbing in one go without worrying about the bulkhead moving or the welds getting in the way.