Combining Foam and Plywood construction in the Same Hull

[CET]

I am considering designing and building a 20’ boat and have a question for the experts here. It will be a planing hull with moderate deadrise, will be used mainly for fishing and will be powered by an outboard in the 120 hp range. Originally I planned to use plywood and assemble the hull using the stitch and glue technique. However, I would like to include some flare toward the bow which would entail compound curvature and would thus not be developable, precluding the use of plywood panels for the topside. So, what I would like to do is use plywood for the bottom panels using the typical stitch and glue process, but then build the topside panels using foam sandwich construction. This would, of course, require a more costly male mold, but I don‘t mind.

My question is this: could there be any problems with using the two very different materials in the same hull? For example, plywood and foam may react differently to the stresses of a rough sea. Would those differences create undue stresses at the chine area where the two materials meet? Are there any other considerations I should be aware of?

I would appreciate any advice or insight. Thanks in advance.

Charlie

PS - both the plywood bottom panels and the foam top panels would be covered in mutiple layers of biaxial glass and epoxy.

[FranklinRatliff]

Plywood is itself a form of sandwich composite.

And many of the resins and adhesives that work with wood also work well with foam/fiberglass.

If you're just planning to build a one off, making a mold could be more trouble than it's worth.

[Chris Ostlind]

Hi Charlie,

Skip the foam and use cedar strips instead. They can follow the contours just as easily, have very close structural characteristics compared to the plywood and keep the whole structure in the wooden zone for an easier build process. The difference in weight for the amount of surface you mentioned is negligible.

You can still build to a set of forms for the strip section and a simple male structure for the S&G hull panels. There's a lot of info on the web regarding the building of a hybrid kayak in which the hull panels are S&G and the decks are stripped.

Chris Ostlind

[CET]

Hey Chris, great to hear from you. Thanks for the suggestion about cedar strips. I’m not really familiar with that construction method, although I’m aware it exists. A few questions. How thick and wide are the strips? Would I need to laminate (cold mold) veneer or plywood over the cedar strips for transverse structural strength, or would biaxial cloth be adequate for that? I assume the strips are applied longitudinally only and thus it seems they would not offer much transverse structural strength. I’m interested…where could I find more info about cedar strip construction, calculating core thickness and lamination schedule requirements, etc.?

Thanks,

Charlie

PS - did you get the last e-mail I sent in our exchange several weeks ago? Attached to it was a Rhino file of an 18’ Tolman skiff with internal structure, center console, deck, etc. It was a large file and since I didn’t hear back from you I thought the file size may have prevented it from going through.

Thanks again.

[FranklinRatliff]

You could take a look at balsa as the core for the top deck instead of foam.

A note on sandwich panels. With no change whatsoever in skin thickness, simply doubling the core thickness makes a panel eight times as rigid.

[CET]

Originally Posted by FranklinRatliff

You could take a look at balsa as the core for the top deck instead of foam,

Thanks for your replies, Franklin. How would the use of balsa be a benefit as compared to foam in this particular application?

Thanks,

Charlie

[FranklinRatliff]

If people are going to be stepping on or walking over the deck, balsa core can be more durable than foam.

[Chris Ostlind]

Charlie,

I can lend you a set of books with an astounding amount of scantling info for your "spare time" calcs. You'll hate me later for the places it will take you.

The strip dimensions are calculated from scantling requirements and the widths are mostly determined by the radius of the arc you wish to build. They need to follow the arc while not requiring too much material removal during fairing which changes the overall thickness of the build.

I think Charlie is talking about the flared forward sections of the bow that he'd like to incoprporate that differ substantially from the normal bow surfaces capable from developable plywood. Is that correct Charlie? You'd like to flare the upper sections of the bow out and away from the plywood surface? You intend for the boat to be essentially open up front for utility and fishing?

I did not get the Tolman skiff file mailing you mentioned, though I can accept really large file sizes (couple of hundred megs) Try it again and in the meantime, I'll send my phone number off-list so we can get connected for the book transfer.

Chris

[CET]

Franklin - Chris is correct about my intentions to use the foam (or cedar strips or balsa) for the side panels rather than the deck. I don't think I was very clear in my originalpost. My bad.

Chris - thank you for the offer about the books. I just replied to your e-mail.

Charlie

[Buildboats]

How about orientating the grain of the plywood in the bow, so he can create the flare he is looking for... and laminating thinner panels together if the flare is deep (cold molded)? yes "don't bother building a mold" you'll be building 2 boats then you might as well build a cat

[CET]

Originally Posted by Buildboats

How about orientating the grain of the plywood in the bow, so he can create the flare he is looking for... and laminating thinner panels together if the flare is deep (cold molded)? yes "don't bother building a mold" you'll be building 2 boats then you might as well build a cat

Thanks for your reply, Buildboats. As I understand the process (I must admit my understanding is pretty limited) cold molding requires a male (or female?) mold to which to affix at least the first layer of panels. Is there another method to cold mold without a mold that I am not aware of?

Thanks,

Charlie

[Buildboats]

Sure its done over a frame instead of a mold then part of the frame can be removed... do a search online for cold molding and you'll see lot of info on it. There is a site I found that cover what you want very well I'll have alook for it

[Buildboats]

got it this might help you out a lot http://www.kbeddoeboats.com/pages/2/index.htm

[CET]

Originally Posted by Buildboats

got it this might help you out a lot http://www.kbeddoeboats.com/pages/2/index.htm

Ah yes, I'm familiar with Kevin's Tolman boats...the Standard he built first and the Jumbo detailed in the link you provided. However, the hull panels on all Tolmans are fully developed, meaning there are no counpound curvatures like what I would like to incorcorate into the boat I am designing. I would post a picture but I'm not smart enough to do anything that high tech. Here's a link to the Parker Boats Web site. All of Parker's designs have the bow flair I'm looking for.

http://www.parkerboats.net/

Unfortunately that's not possible with the stich-and-glue plywood construction method used to construct Tolman's designs. I do really like the Tolman designs, though, and the simplicity of their construction. Thanks for replying and posting the link.

[Buildboats]

Yes I didn't meant that this link was for cold molding. Sorry it was sent to me a couple days ago so and thought it was a good look at how he built the bottom and stuff. Heres a good shot of cold mold curve http://www.sculleyboatbuilders.com/const_hull.html is the site I got this from