Structural panel bulkhead joining question.

In this example... the 3 ply ,off set , Laminate stack is built with the widest tape on the inside next to the fillet. .

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In the other example posted PDF, the 3 ply laminate stack layup is reversed with the narrowest laminate tape against the fillet and the widest laminate on top of the stack.

Which layup technique is the correct engineering solution ?

 

Emm: Both.
For Epoxy building you normally beginn with the widest layer.
The other way is preferred for Polyester laminates.

 

Hmmm.....................................

Sounds like the answer I got when I was a kid.

Dont ask so many dam questions and get back to work !!!!!!!!!!!!!!

 

Well that’s the “beauty” of composites. You can make the material to suit the application.

Trouble is, is when you come across people who do just one method of application over and over again..refine it and it suits their build method. Its perfect…for them and their application and type of boat. BUT, it does not automatically mean it is applicable for every boat.

Composites are anisotropic, not isotropic. Thus you must, as any designer, first, establish the loads, the load paths you wish to encourage and then what material properties you want or need for THAT application. With an isotropic material, that’s easy. However with an anisotropic material, the final composite is to satisfy that application. The coupon test one does is to ensure that the design intent is satisfied.

So what does all this mean??

Well,. Take a simple beam and place it under a load, what’s the stress??...well depends upon the structural stiffness, the EI. The E being the Young’s modulus and the I being the second moment of inertia of the structural joint.

So if I state that an I beam of X and X and Z dimensions would be suitable…er…suitable for what? If the material, the E, is steel, it is then easy, it is related to just the I, its dimensions. Same for any isotropic material, just a different value.

But with a composite the EI is always different; therefore the resulting stress is not so straight forward to calculate.

Oh, ok, well, let’s use the same composite layup as boat A for boat B, then there is a consistent E. Sure no problem. Ok, you bend the beam and get XX the stress, perfect….er…nope.

As soon as you apply that E to another application, the geometry is different, in all 3 directions…ergo the EI is also different. Bummer!! A boat is 3-demsinonal…not to mention you won’t be using the same guys to make the layup, nor same equipment nor the same temperature or humidity etc etc.

So whilst composites are great, as you can add whatever, wherever you want…the down side is that it is not always applicable from one boat to another. Oh the WTB is a bit tighter on boat B because the bunk is bigger, oh ok lets add a bit more overlaminate at the fillet, opps…you’ve now changed the geometry and the material properties. And on top of that it is highly HIGHLY quality control dependent, unlike isotropic materials, in their raw state.

So if there is just one minute difference between boat A and boat B in the geometry of the structure and the material quality and layup and not to mention the size/displacement speed etc of boat A to boat B…then nothing transfers. It is now different, thus the response of the structure to a given load shall also be different.

That is why builders tend to stick to their tried and tested preferred method. But show them that boat B is different then they ***** and moan it is not the way to do it….well, how do they know if they haven’t done the coupon test to confirm their quality control and also performed their structural calculations to establish the load paths and stresses and of course the deflections, their labour efficiency, overheads etc etc….they can’t.

The only transferring is the methodology of how you arrived at a previous successful structure. Is it applicable…well, using the same methodology will tell you..yes or no. A designer wont, or shouldn’t assume, but confirm facts before saying so. A builder will often just say yes or no without knowing any of the aforementioned. Gives a good impression of knowledge, experience and confidence…but if you probe and ask pointed questions about XX or YY and get little in reply other than just do it trust me, believe me etc…that speaks volumes. Just means they know a lot about their way..but little else.

Designing a structure in composite is more tricky that it appears. The basics are straight forward, but as with everything..its all in the testing to confirm the material properties and the details that are applicable to that part of the structure; which is of course driven by all of the above

 

NO "technocratic polypusher" Now get it right !

 

ok so in just a few simple words what are you actually trying to say???? That its going to rain next week ??

 

Your way is not the only way, and nor the best in most cases.. (but works!)

 

its better to get to the point than chase around the bushs !!
You need to have a cup of tea and cakes when reading those posts and when you do get to the end you forgotten what the hell that subject was about and are non the wiser !!

 

When i tab things in,eg,a bulkhead, i actually use both methods in the same aplication, ie ,i use the same width double bias tape for the whole thing and offset the appropriate amount with each layer to establish the taper so it ends up the same except on, say,the hull side of the fillet the longest overlap is on the top and on the bulkhead side of the filet its on the bottom of the stack. I really dont think it makes any difference and it means i dont need to have 3 or 4 different width rolls on hand.

Steve.

 

Well im sorry to edit most of your post Ad but although you are correct I hasten to add that most manufacturers do'nt know or don't care if the bulk head join fails or buckless in any way.

They simply say you lifted it wrong!

What is really serious is the deck to hull join allowing water in the the gap between bulk head and hull. Its all to easy to mis this join and allow water to run down the gap between hull and bulk head causing a right olde mess. Just running round the inside with a 4 inch band of of glass on the deck to hull jopin does nor help.

 

Ad Hoc, just a couple of questions, what does WTB mean? watertight bulkhead perhaps? also, are you saying that a 12mm radius is optimal? and finally, how good is the bond between the vinylester (or epoxy) to the crestomer adhesive filet? i ask this because i have not yet used any of these adhesives having always fileted with a blend using the resin i will be tabbing with, i did a quick read and i think they mentioned the crestomer being urethane based and having good adhesion to other substrates but thats not the same as other common resin systems having good adhesion to it, im assuming they do. It looks like you use it the same as any other system, lay down a bead, position the bulkhead and filet with the squeezout. I am interested in trying some of the other adhesive systems such as plexus and this is the first time ive heard of crestomer although i have heard of Scott Bader, i dont know if we have their products here.

Steve.

 

Sure...Ive often used offset tapes , of the same dimension, to tab in bulkheads when a two ply layup was required .

Ive also been supplied drawings from well respected NAs that use the method described in AdHocs post with for both two and three ply layups.

Ive never learned whether its a practical solution or the correct engineering solution.

 

MP, provided each method can be executed equally well, it makes no difference.

However, in practice, its better to do it in the way ADHOC`s drawing shows because if you do it the other way and overlapping the layers above the previous, you tend to get a void forming where the lap is, due to the next layer bridging the thickness difference of the previous layer. Im sure you would have seen this during the course of your work from time to time...

 

Correct.

Not at all. A 12mm fillet radius was allocated for this application using this method with said materials coupled with the experience of the laminators. Hence you need to know what your laminators experience is, what methods they like, and then decide how best to get the response of the structure to loads that the laminators can do that is within their ability. It is not the same as…’oh just add a 6mm bracket in the corner’….as with aluminium, for example. It is all about balancing the need for flexibility and strength coupled to what the laminators can actually do, and with what materials and, consistently!!

To the best of my knowledge it was extremely good. However, this product application was recommended to us, via the laminators. It was their preferred product. Once this was proven to the satisfaction of Class, it became a production and hence a QA issue. There is a poster here Herman, he would be able to give you a far better insight and a reply with considerable experience and depth to your question than I could simply because my knowledge of this product is also limited.

I don’t design with composites that much, thus I am reliant more upon the laminators, since as I have noted above, each yard/(laminator) has their own prefer way of laying up a boat. It is highly quality control related which I have no control over, unlike with isotropic materials. Thus I design a structure to meet their ability rather than insist upon my preferred methods from my own previous shipyard's tried and tested methods; which are always different!

Agreed. But....that's the hard part...can they do it and do it consistently??

 

Frosty, you and I think of the same faces right now, I guess.
But to be fair to serious builders (and outside of Thailand we are a majority ), I would like to point out that especially structural details are very much taken for serious. No builder wants his product breaking on the first sea trial...