Is bulkhead tabbing now redundant?

Ok tunnels, thats a nice description of what youve built in the past... it doesnt help me calculate / engineer a bulkhead-hull joint/bond for another application - the purpose of the thread.

The idea im working towards, is that provided we can calculate the stress in the bondline/joint area for a given problem, we can then try to find a new, high strength adhesive compound, with enough mechanical properties to exceed the calculated stresses in its own right, and thus not require additional tabbing fibres. For example, the HPR5 adhesive compound i linked to earlier has;

28Mpa shear strength as tested bondline lap shear when bonded to 2024 T3 aluminimum alloy,
And solid castings of it tested as, 56Mpa tensile strength, 90Mpa flexural strength, 2895 Mpa tensile modulus. They also claim a 30x increase in peel strength, but dont publish any numbers. It was specifically design for bonding to metals, but i beleive the mechanical properties could well be upto the task, i just need to be able to calculate the stresses in the fillet bonding area, but how?

 

If I understand correctly, what you are asking is common in aircraft. Unless it is a cheap homebuilt bulkhead joints are not taped. The problem is the bulkheads have integral molded in flanges which requires unique tooling for each part. If saving money is the plan that is not what you want. If you just want to use big fillets as the joint then you will need to do a bunch of testing to qualify that process and determine where it can be applied.

Another option is prefab angle that is bonded on. Obviously, the straight runs would use prefabricated composite angle that is bonded in place. Curved sections could use prefab angle but it will have to be used in numerous short sections and thick bondlines, depends how much curve. You could probably eliminate 90% of taped joints.

 

Yes, as adhoc eloquently tells the story of needing to demonstrate the properties of a non-standard way of doing something new, with new materials, but fails in describing how to go about it. This is the second part of the problem.

First i want to be able to calculate the stresses, second i would like to build a sample and test it.

So for the testing;
I mentioned in the first post, the gougeon bros method of testing an epoxy fillet with plywood frame, by applying bending force to the frame at right angles to the joint at a point 8 times the thickness of the frame away from the joint, until failure. If the fillet failed, then it was deemed no good. But if the frame broke and the fillet remained intact, then it was considered good enough. A simple cantilever test. But is a simple cantilever test testing the correct load case? I dont think so... so what is a more appropriate test?

And im looking for the maths in order to calculate the stresses the bondline needs to handle if i know the loads applied to the hull panel. A simple formula, or series thereof, would suffice.

 

Errr nope. It is described in the paper I referenced, which you said you read. I’ll quote direct from the paper:

“ This paper outlines and experimental procedure devised to assess the strength of…”

If you read the paper and understand it and what, why and how they did it, then you have your answer.

As I noted previous, the procedure is explained here:
http://www.boatdesign.net/forums/fi...head-joining-question-45735-2.html#post603791

If your knowledge of structural design is lacking that is something for you address. It is not the fault of those who can.

From the previously reference paper, if you understand how to conduct experiments to establish the strengths, which answers your second problem, the first can then be answered by way of learning how to design structures; this is not a case of just reading a text book. Once you are familiar how to do this, a second supplementary paper will also assist: “Finite Element Analysis of Top-Hat-Stiffened panels of Fibre-Reinforced-Platsic Boat Structures” By Eksisk O., Shenoi RA., Moy SSJ., Jeong HK., Marine Technology No.44 January 2007.

It is not a case of here is the formulae XX and plug in the numbers and out pops YY, and.. hey presto. If it was that easy, you would not be asking such questions.

Structural design and of boats is not a 5min learning process. I read so many times this same request for a “simple” equation type of request. It is rather like asking a heart surgeon, which knife shall I use to try my own open heart surgery! It takes many many years of reading and application of theory to practice to fully understand and appreciate all the implications of a structural design that may or may not be boiled down into one “simple” formula.

If you do not have the time, patience or otherwise to go through this process, then I suggest you pay a NA to do this for you. As they have.

 

My thinking, exactly.
Attached my imagination of a only bonded flange as one option.

The critical spots I would see here are
A: possible concentration of forces
B: The same, but even more
C: Possibly loaded on shear force

If the orange panel is standing relatively free and the blue panel would flex, bend or twist (torsion of a hull?) I would expect an issue here that requires further stiffening members or an alteration of 'C'
If the orange panel is further attached to other structural components, I see a chance that it can work.

However, I personally find Ad Hoc's version from the earlier thread much more attractive.

The entry value for calculation will be pressure I reckon. This can be derived i.e. from building guidelines (ISO, GL, etc.: here you find the formulas you seek) or individually calculated by own 'experiments'/tests. In case of a changing dynamic load there will be tension on one side, pressure on the other. And next moment the other way round.
When the adhesion strength of the bonding material is known you can calculate the required surface. A flexing will result in tension within the laminate (.ca the area at 'C' in the sketch) and with the glass properties results in the laminate schedule here. At the same location I also see a force that tries to shear the core from the laminate.
If I wanted to go for a limit, I now would test the arrangement. Another way would be to multiply with a safety factor - which is to be paid with extra weight and/or costs.
The 'Gougeon Brothers Test' is not bad in my eyes: It applies force with a reasonable lever arm. If time and conditions allow, I would extend the test by applying a specified force in increasing steps, and include observations of what happens before it breaks. Also a closer examination of the sample after it broke will give conclusions on the loads flow.

 

Typically for a bulkhead installation, the fillet would be a secondary bond.

Secondary bonds are only as good as the surface prep.

Tabbing greatly increases the surface area of the bond.

Id be careful when eliminating tabbing

 

Just whack plenty of bouyancy foam in your boat groper, that way any structural failures will be more of an embarrassment than an emergency.

 

Ive seen very light raceboats with bulkheads only filleted and stitch tabbed . That race boat, a 30 footer, was built for the double handed transatlantic race. I believe Howlett was the designer.

 

"My boat" is not being built this way, besides 90% of the structural stuff is already complete. This questioning relates to the next boat i will build, so i often consider the design of it, when i have nothing better to do...

There are more efficient ways of building things, building methods and the design to accomplish it are always evolving, look how far weve come in the last 50 years alone... imagine what the next 50 holds...

Spoke with another marine engineer today, she basically echoed Adhoc`s comments in that - "with composites, almost anything is possible - thats the beauty of composites. However when you looking to do something outside the norm, your on your own, engineering can only take you so far especially with safety factors of 3.5, beyond that you have to take a risk and put your own money and neck on the line to prove its satisfactory for its purpose"

which is exactly what ill have to do, such as all the pioneers have done in the past with new and innovative ideas to prove they work... build a boat and then try to break it, preferably within swimming distance of the shore

 

Seems to me that theory follows practice as much as practice follows theory in boat design and construction, trial and error is the name of the game, but you need a back-up plan if the unproven fails at sea......a good lifeboat or copious volumes of bouyancy foam !

 

if you make some test panels and test to break you'll know more than anyone else.
Trouble with areas bonded like that cars and planes have this these days, quality control is very important, easy to do on production line hard to do in boat yard

 

I have heard of one or two builders of GRP boats for the leisure market sticking their bulkheads in with Sikaflex and no bonding.Not sure I would be happy going to sea in them myself,but if the owners of the companies in question would like to make a rough weather trip to prove my concerns groundless, I might be persuaded.As post #26 says there is no substitute for doing your own testing-expecting customers to be your guinea pigs is irresponsible.

 

You appear to have a romantic yet myopic view of “pioneers” in the same vein as GaryB, not sure why, it is called design engineering, nothing new. If you’re still wondering about for a way forward and if your engineer friend (why ask the forum if you can ask her for advice?) is unable to provide further guidance, then you can at least comply with class rule procedures:

 

Uh oh, is this more "embittered, sedentary, dull, negativism"?


Hehehe...

 

Its not that hard to do ,really !!

At last some useful information .
Most of the boats I've been involved with in recent years have been built to DNV survey standards !!
But there is a point to take into consideration , this is a minimum standard that is required , so yes you are allowed to exceed there standards as long as its done with there approval and is better than they have specified !!
BUT if you only work to a minimum standard you learn very little !!,but if you exceed there standards and make better and ask and get there advice what can be done to be able to exceed there specs and be consistently consistent you will have a surveyor for a friend for life !!,
Building the 147 foot boat was all done to DNV AND INSPECTORS HOVERED IN AND AROUND TAKING PARTICULAR NOTE OF WHAT WAS BEING DONE , BY WHOM , AND HOW WAS IT BEING DONE .
My department we welcomed the inspector with open arms and later as the project wore on we hardly ever saw him .
The guys in the actual assembly were scared stiff of his visits but on the other side of the coin the surveyor knew full well our place of work and our workmanship was way better than there minimum standard and test results of all samples taken from our department always exceeded there minimum expected !!
All testing was carried out by the Auckland university engineering department ,composites division !! a place Mr Richard Honey of Gurit lectures frequently !!
Was only when the big man from head office DNV Australia and his followers came to visit that we saw the inspector again .
The big man even made a special point of walking away from the group and came to see our department , what we were doing , how we were doing it and to shake hands with not only myself but the whole of my team and a pat on the back all round for work well done throughout the entire 4 year project was a proud day for us all that's for sure !!