Question for Jacques (plywood composite strength)

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Dallphin
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Question for Jacques (plywood composite strength)

Post by Dallphin » Tue Mar 12, 2019 1:24 pm

Jacques:
I found your articles on Plywood Composited Testing and Testing Resin in the "how-to" tutorials section of this site to be excellent! I an working on a boat design and was wondering if you had more composite information available from your tests. The type data presented in the article is exactly what I would be looking for! I am trying to build light and would like to get a feel for what the composite equivalents (#layers and bidirectional glass weight(s) lamented to 4mm marine ply core) are for non-laminated "native" 6,9, 12mm plywood.



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Re: Question for Jacques (plywood composite strength)

Post by jacquesmm » Tue Mar 12, 2019 2:36 pm

Many designers have asked for my scantling rules but I don't give them away, sorry. They are based on comparative testing that was done almost 30 years ago at a local university lab and paid for by a fiberglass supplier. Perfected over time but even then, there is no simple rule.
Mc Naughton publishes wood-glass-resin scantlings, I find them to heavy and they are not based on epoxy anyway.
Dave Gerr covers calculations in detail in his book "The Elements of Boat Strength". That is the best source.
I recommend to start there but do not expect to find a rule that say all boats of 21' need 1/2" ply with one layer of this and one layer of that.
Another good book is by Arthur Edmonds: Building A Fiberglass Boat, He was my predecessor at S2 Yachts and his book explains calculations for many materials that you can turn in a spreadsheet.
At bateau2.com, you can see pictures of the testing bench that I used to compare different resins, glass and ply composites compared to single skin glass. Not all resins are equal and that is why I can't and don't want to list a simplistic rule.
One good shortcut would be to take my data from one of my plans of a similar size and type but beware of things like panel span, designed speed and displacement etc. Gerr explains why.
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Dallphin
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Re: Question for Jacques (plywood composite strength)

Post by Dallphin » Tue Mar 12, 2019 6:57 pm

Jacques:

Thank you for your response! after your explanation I totally understand your position. I also appreciate the references to the other literature sources. Secretly I have been wanting to do this testing for myself for a long time so now I have an excuse. I have bought several of your plans over the years and the one I built was wonderful and exceeded exceptions. I built a PK78 over 20 years ago and it is still in use today. Built it from AC plywood so now it is over weight with all the amateur epoxy and glass repairs different owners implemented thought the years to address checking. Lesson learned; always use Marine Ply regardless of the cost.

I exclusively use your MarinEpoxy and think it is wonderful stuff. Am testing the UV resistant formulation on the latest boat I built. I am hopeful that I can just coat the interior of subsequent boats and do no varnishing or painting. I hate fairing to the point of good topcoat finishes !!!!!!!!!

Do you offer a panel routing service? If so what considerations do you need in the associated cad file?

Once again thank you for your response(s).

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Re: Question for Jacques (plywood composite strength)

Post by jacquesmm » Wed Mar 13, 2019 9:26 am

For the routing, you should ask Reid at BoatBuilderCentral. I am semi-retired: I still design boats but do not own BBC anymore.

For the scantlings, I was thinking mostly of planing hulls. Displacement boats are much easier to calculate. Start with the two books I listed. Drawing the lines is easy compared to the engineering work.

Did you check the Westlawn basic course? It is a good introduction to yacht design.
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Re: Question for Jacques (plywood composite strength)

Post by Dallphin » Wed Mar 13, 2019 11:26 am

Jacques:
I have looked into Westlawn and plan to pursue it when I retire in about 3 years. As far as the referenced books; I will be ordering them this week. I have been using ProSurf3 for hull development and analytics. Once happy with the design I export the lines and panels to a 2D Cad program to develop the bulkheads and other details. I sure have a lot to learn but it has been a great experience so far.

When the time is right I will get with Reid about the CNC routing. I am working up a big order to "stock the shop" in preparation to build a tender I am designing and building for my brother. I am eyeballing the structure based on some of your plans and experiences to date. Ultimately I will verify the structures integrity through testing, composite sample strength testing, operation, and gross exposures like drop testing. Once I am satisfied that it will not fall apart on my brother I will give it to him to see how it holds up with use and abuse.

I believe I may understand your retirement status. I too want to semi-retire and design/build boats for enjoyment, to cover the costs of materials, and excursions to the different wooden boat shows. I know too many people who worked their whole life and died within months of their retirement. They usually had no plan or purposed which is certainly is not a good thing. I plan to retire as early as possible and keep engaged to ward off the grim reaper. :lol:

Once again thanks for the mentor ship.

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Re: Question for Jacques (plywood composite strength)

Post by jacquesmm » Wed Mar 13, 2019 12:23 pm

A displacement speed dinghy is easy to design. It's not really a composite hull but a stitch and glue boat made from 1/4" ply.
No need to study yacht design for that.
Once you get in planing hulls or larger boats it becomes a very different story.

Prosurf is good, I used it to design many of the boats at bateau.com but I prefer Rhino and Orca. Once you factor in all the modules, you will spend around $ 3,500.00 either way.
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Re: Question for Jacques (plywood composite strength)

Post by remedy32 » Wed Mar 13, 2019 3:19 pm

Thanks Jacques for sharing some very useful information.

Those references will be a great help as my Pelicano 23 project continues. Having built 3 of your designs I've become very comfortable and confident with the strength and longevity of your true composite designs. It was a great surprise to see just how much wood can be used in a non or semi composite stitch and glue build. In this case roughly double that of the similar sized NV23. Essentially a plywood boat that happens to be stitched rather than screwed together. If I were not so taken with the Pelicano's aesthetics I would have gone in a different direction. I'm working hard to convert to a true composite and those references should really help me put together safe, workable scantlings for my build.

Bill
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Re: Question for Jacques (plywood composite strength)

Post by jacquesmm » Wed Mar 13, 2019 4:32 pm

Yes, that Pelicano 23 is a nice design. The hull shape is very close to my LB26 scaled down 10% and many builders have asked for a smaller outboard version. I may put that kind of boat on my list.
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Re: Question for Jacques (plywood composite strength)

Post by OneWayTraffic » Mon Mar 25, 2019 3:06 am

Vectorlam from Vectorply is another online software package that calculates stiffness tensile stress etc of most laminates, cored or not. It also calculates panel properties given span and length and expected pressure.

There is also Eric Greene’s pdf book on marine composite design where there are some formulas on sandwich panel stiffness: enter values for Young’s modulus dimensions etc into a spreadsheet and calculate away

I also have Geer's book.

The problem with all of this is that it does not replace the years of skill and experience of a qualified NA. Boats in use undergo tremendously variable conditions for long periods of time. I have for the last few years been waiting for finances to allow building the bigger boat I want. After building a little D5 dinghy I would not want to be without a good set of plans. I am not a boat designer.

I do the Other stuff comparing Jacques specifications to Geer etc just because I enjoy the maths and physics of it.

Edited for Grammar that I couldn't do on the iPhone.

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