Structural panel bulkhead joining question.

Need to look at broken boats to see what stays and what goes !!

The corner cove has a lot to add in the way of durbility and during stress from the movement of the hull skins or pressure exerted in the case of a collission water tight bulk head that could have enormous in one side only and the glass one side wanting to be stripped off the hulls surface and the otherside in compression not forgetting the bend the bulkhead will get This bending is where a larger cove comes into play and the glass layers will and do hold together and in nearly all cases in repairs the glass had peeled off the bulhead because of insefficent bonding width onto the wood in the case of plywood the top veneer simply stripped off at the glue line . where small of no coves were used the glass ripped off the pressured side both the hull bond and also the bulkhead bond !! the compression side the glass ripped and broke as the bulkhead was bent and pulled away and simply slipped over the top of the broken glass still attached to the hull skins .
What do you use to cove with ?? flexable no !! a limited flex yes but has to bond really well to all glass suraces the hull !the bulkhead !! and the glass being lad over the top !!
Mei try to use the same resins all the time so what do you make coving mix from ?? a little talc and little more aerosill !!,Never Q cells add a little more micro fibres untill you get a consistancy that wont slump on a vertical surface !! The mix need to be wet enough to stick to all surfaces when pressed in and coved . The cove size that best i found is use a 30mm dia plastic pipe with a smooth end , easy to roll glass round the corners , a big enough radius where glass wont break and the cove filler is sufficently strong enough to not break or simply peel off the surface its stuck to !!
And you thought its just another corner !!!
Broken boats are a gold mine of information if you take the time and make the effort to take a camera with you !!
What type of glass should be used ??
Glass strands are whats needed in the right place across the corner !!
so double bias is one choice and triaxle is another choice plus if they have a sprinkling of csm on one side thats even better . each of these glasses the glass strands need to run across the corner !!dont forget the bonding surface on the hull and on the bulkhead needs to be as big as is practical .
Its all just common sense !! there no magic formulars ! and it all works !!

 

Anyone with a weeks laminating experience can execute either method equally well, 1708 for example is just .055" dry thickness and it is easy to work out any void, that said i have seen just about every poor practice over the years, usually (but not always) on production boats where the boats are built by people who do the same work day to day and are only there for the paycheck. We are discussing optimum methods of doing things and slight variations make no measurable difference but with many/most of the older production boats from the 70s for example the bulkhead would be cdx plywood,roughly hacked to fit with a jigsaw (with a drunk in control) touching the hull in just a few places, no foam spacer, no filet, gaps from 0"-3/4" gap and then tabbed with polyester with mat and rovings (there was no biax back then) so the tabbing went through and met the tabbing from the other side in the places where the gaps were wide, the transition where we would have a filet, they would have a huge void and to top it off they didnt bother to stagger the widths much of the time. Ive seen this time and again from many builders and amazingly, for the most part the only failure would be the polyester to plywood interface or else deck leaks had caused the plywood to rot out between the tabbing.
As to the original question, i have in front of me the b/h to hull drawing from a well known NA for a 44ft cat, this is a typ drawing for all such joints in the boat and it shows an additional 6" wide strip of DB1708 down the hull before the b/h is installed, then it shows triangular foam fillets each side of the b/h, (this is a balsa cored hull) then 2 layers of 1708 with taper. He doesnt show the layers drawn individually so no preference of how you stagger the layers but due to the tabbing extending out past the 6" strip down the hull you do end up with same .055" step down onto the hull skin. He doesnt show a dimension for the foam fillet because obviouslt it will vary based on the location in the hull but he shows the longest tabbing layer extending 2" onto the b/h from the edge of the edge of the fillet and 4" onto the hull from the edge of the fillet. I think that adding that 6" strip will take care of your issue.

Steve.

 

For me its all about quality control. I spend very much time supervising subcontractors performing a variety of tasks. ... Woodwork, glass work, metal work, paint work, fitters

Im always on their *** to do it the correct way.

In the perfect world , for structural projects, I have a set of drawings from a naval architect and tolerate no variation from what is drawn. Unfortunately the world is not perfect...and many times I have to make up my own...quality control..standards as I go.

Difficult...particularly when I see contractors going quick and dirty, or doing it their way. I inevitably have to troll thru old drawings to locate a particular detail , then rub it in their nose. THIS IS HOW YOU MAKE A DIESEL TANK INSPECTION HATCH FLANGE

Composites work and its detailing are a mystery to me. Ive only ever been involved with one high class composites project .

 

Technically, widest tab first is best as it leaves the secondary bond between the tabbing and hull and bulkhead skins with one continuous interface with no resin rich lines at each step of laminate transition. It also transmits loads more evenly as there is no interruption of tabbing laminate 'flow/flatness/plane-ality ' or whatever it's called, where the laminates have to transition .055" or so from one plane to the next. It's like built in wrinkles in the laminate.

I think though that such detail only comes into play in the highest end type of construction, for example aircraft or ultra light boats, etc. Whenever I tabbed things, I always went widest tape last as it ended up a cleaner bit of work, leaving only one 'loose' edge of fiberglass nastys that would have to be cleaned up.

But, as per usual, the topic has drifted from the OP. And, not uncommonly, almost immediately. I think that yes, you need to have a continuous inner hull skin and so you should tape the panel butt joints before the bulkhead is tabbed in place. I'm guessing the bulkheads are what gives shape to the hull and you don't want to lock the hull shape in with the butt joint tapes as then the hull fairness might be compromised. Is there any way to do them both at once, such as wet out the tape and then install the bulkhead and tabbing before the tape has set? Or set the tape dry, then the bulkhead, and then wet out the tape and tabbing all at once?

 

That is what Ive been told.

Widest tab first, the primary laminate and primary bond.... then secondary tabs and bonds on top.

The primary tab fibers are also shielded from workmans grinders and have the best bond.

 

strength of materials and not very complex

Strange, not one suggests scientifically proven solutions. Everyone forgets that the foam inside has significantly different properties, but the stress flow doesn't discriminate. As it goes through the shown detail it would would rip apart the foam or interface between the foam and the fiberglass (or whatever). There is need in reinforcing not only on the outside skin, but also inside the foam. There is a need to transfer the stress from one part to another by means of more rigid stress passages thus avoiding the weak inner layer of foam.

That is why there are foam "cut-outs" in areas of high stress concentration. These areas are filled with epoxy with microfiber and glass micro-balloons. There are numerous ways to reinforce a T-connection. Another way to reduce stress concentration is to use stiffeners and spread the load to the point when foam could take it. Stiffeners are used in large scale production, mostly with steel, as it is less labor intensive.

For a foam sandwich I would do a detail in the attached picture. I showed the detail with the inner skin removed. You could see the reinforcing lattice. After this is done, the bulkhead is attached and covered as you showed it.

I hope it helped.

 

I do agree in part with what you saying but thats not the way to do it its a little agricultual . Tying the outer and inner skins together and making a place for the mounting of bulkheads is really important but theres more to it than just a hard something under the bulkhead to push to the outer skins and then glassing to the inner skin .. adding lots of glass and making a non flexing location is creating more problems than its solving in a loaded stress situation . The joint and everything else needs to be compatable with its surrounding area being the inner and out and core in between and that what you discribed is not the answer!!!
The true proceedure is written in the installation instructions , i looked and read and had to take time to really think about it before i understood .

 

I believe the introduction of stiff connection will work only in case when this stiffness would distribute stresses. Spreading the stress reduces its intensity. When this intensity is less or equal of the carrying capacity of foam it will work. This is exactly what stiffener plates, also extra layers of glass, foam cut-outs filled with epoxy, etc.etc. do.

I have done plenty of "agricultural" tests and compared them to the theory in my life to be certain about this topic.

 

If you feel its necessary Ad Hoc showed 2 different ways to do what you are talking about, however, as is usual in these types of threads folks tend to get overly academic and try to solve problems that really dont exist. Folks like Derek Kellsal have been building foam cored boats much much longer than any of us (longer than almost anyone actually)with great sucess and im pretty damn sure he doesnt do anything more than the usual detail. There is rarely any need to do more than what has worked for decades in a similar situation as what you are doing. The loads on bulkhead tabbing is spread nicely over a lot of surface area, in all my time building and repairing boats i have never seen any type of issue at a bulkhead/hull interface other than a plywood b/h rotting out between the tabbing or just delaming from the b/h where polyester resin was used or a poorly prepared polyester secondary bond to the hull, i have never seen any kind of issue with the hull core, foam or balsa, in a bulkhead area. The only hull issue ive seen is bulkhead print through on solid glass hulls.

Steve.

 

I agree. On carbon race boats that see abuse many times more severe on around the world races, flying off waves at 30 knots, rig and keel loads much larger than the typical cruiser, sometimes have bulkheads installed with NO tabbing! To save weight in some areas they are put in with rubber toughened epoxy...period. Most of the time they do fine. In my experience in trying to save weight I have noted many times that tabbing material is way overkill on many boat claiming to be "light".

 

My understanding of Gropers project is the hull panels are infused both sides at once on a flat table, the panels are cut to shape and then joined into each other to form the hull, vaguely like using sheets of plywood to make a stitch and glue hull. Is that correct?

The issue never was the method to be used for tabbing. The only issue in the OP is whether the inner skin of the hull composite laminate should or should not be continuous when hull panel butt joints occur on the edge of a bulkhead. The question is can the bulkhead tabbing take the place of, and perform the same functions as, the inner skin of the hull laminate.

I say no, I think in this situation the inner skin performs one function and the tabbing performs another and you need both.

Taping the joint first might be a little time consuming or difficult, but if the joint is not taped first and then it turns out it should have been, a little difficult and time consuming task will be replaced by a very difficult and very costly task.

I'd like to hear what Mr Groper thinks about all this discussion and what he decides to do.

 

(Are the inner hull panel joints)

To be, or not to be

(taped before the bulkhead is installed),

that is the question.

 

I understood it as a combination question. Can the bulkhead tabing also be used to join the inner skin of the hull panel.

 

As i said in post #47, a naval architects drawing on a plan sheet i have shows a 6" wide strip of 1708 down the hull before the bulkhead is installed, the tabbing extends further onto the hull than 3" each side of the bulkhead, this is for all the bulkheads, not quite Gropers situation but not a bad idea and deals with his problem, it also adds to the inner skin thickness and helps isolate the hard spot (b/h)from the core. I would not do it like Gropers drwg.

Steve.

 

Yes, I agree...

What do you think?

In post #12 you say

which is a bit different, as that objection is to the panel joint itself being over the bulkhead, no matter how the panels are joined, much less with no inner skin.

So, I'm under the assumption that a correctly made scarf in ply, or foam sheets joined with a "structural bog", create a homogenous material and the location of bulkheads shouldn't matter. Maybe that's not actually correct. ?