Correct way to Tab bulkheads

[coopec43]

Originally Posted by HeywoodJ

All the more reason to do it very neatly. Exposed stuff is easy to work around, but there is nothing like reaching into a hidden hole because you have to work on something way out of the way, and stabbing yourself on some strand of fiberglass sticking out from the tabbing (or anything else)

I'd prefer a perfectly smooth finish in all the place I can't see than all the places I can.

I always use welding gloves to avoid my hands being torn to shreds. All I can say is your fiber-glassing must be pretty damned rough!

(You can see how dangerous my fiber-glassing is)

[Tsalta]

Thanks.

Logically it seemed to me that smallest first may be stronger and have more redundancy, but as noted there are very different opinions. I would love to hear from a structural engineer for the actual math on the options.

I think in the meantime I’ll be doing smallest first.

Cheers
Jeff

[Adelie]

As an engineer I can tell you the question is beyond first principles analysis. Someone needs to do the testing. My sense is that the advantage one order may have over the other is small enough to not make a significant difference unless you were trying to build an ultralight.

[smj]

If the area that your laying glass is going to be faired I was taught large piece first, that way if you break through that first layer of glass while grinding/sanding it’s the smallest piece.

[Apollo19.5]

Originally Posted by smj

If the area that your laying glass is going to be faired I was taught large piece first, that way if you break through that first layer of glass while grinding/sanding it’s the smallest piece.

That is what I was taught as well and always have done since. Never failed me yet in too many years

[Ballsnall]

Originally Posted by smj

If the area that your laying glass is going to be faired I was taught large piece first, that way if you break through that first layer of glass while grinding/sanding it’s the smallest piece.

So you have sanded through one layer of glass, the strength reduction is the same regardless of whether it's the smaller or larger layer.

Both ways are fine but largest last is easier and neatest, this is especially true when patching holes as the layup can be done on a table prior to application on the hull.

[Chotu]

Originally Posted by s/v Jedi

Indeed, the foam doesn’t provide any additional strength and an air gap would be okay, but to lay properly radius corders with the tabbing, having something to work against is required or you get voids in the laminate.

The bulkheads don’t move, but they flex as does the tabbing. The whole hull flexes. Plywood is very stiff and will make noise when the edge touches the hull. Not just from flexing, but also changes in temperature and moisture.

Having a boat that doesn’t flex at all, I didn’t really think about that situation.
My boat is stupidly stiff. I got a good deal on 1” inch core when the plans called for 3/4” inch core.

However, the glassed fillet shouldn’t be flexing owing to the geometry. It should be transferring the loads of a flexing hull to a flexing bulkhead in a floppy laminate situation. It’s essentially the same thing as L channel.

The idea is to float that bulkhead away from the hull so that it cannot touch it when it flexes or has temperature changes and keeps hard spots away from exterior fairing. That’s the whole idea of what we are talking about. I’m sure you know that. So those aren’t really a factor. It won’t make any noise at all because it doesn’t touch. That’s why even an air gap is fine.

And that wouldn’t count as a void. Putting the foam in there isn’t going to change anything vs an air gap. Neither air nor foam nor yesterday’s lunch is doing anything at all in this case if used as a buffer between the hull and bulkhead.

[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.

[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.

[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.

[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.