Timber varieties and moisture content

Couple of quick questions for you US folk....

What would be the preferred species for constructiong a wood-epoxy monocoque hull?

Before sealing with resin, what should the moisture content of timber be?

 

You didn't say, but I suspect you mean plywood over sawn frames.

Depends on where you are and what it costs...on the West Coast, Douglas Fir is probably the most common species for both uses and you want it at the M/C where it is stable in your working environment and not so dry that steambending is difficult. For boats built outside under a tent, that's 18-20 pct M/C.

 

Thanks Bob - nope not talking about ply - I don't think! They are a couple of questions from my design course. The first, I think is referring to epoxied diagonal double-planked hulls. The only mention I can find is for port or white cedar....
As far as the moisture goes, the only info I can find in my texts suggests that for general boat building it should be between 12 & 16%. No info as to whether it should be different (lower?) if sealed with resin.....
I've hunted through all my other books by Gerr etc, but have come up with nix....when all else fails, read the manual (or ask some people who know!! )

 

Bob, I find that I have to disagree with you on one small point. Building with wood encapsulated in epoxy, one should use wood that has a very low m/c so that there is insufficient moisture in the wood to allow rot to begin within the matrix. This is usually accomplished with a m/c of between 9 -12%. Curves that are too small in radius for the stock to bend around at this m/c are usually dealt with by building up thinner plies rather than by steaming. High m/c as found in steamed timbers may inhibit the epoxy from bonding fully with the wood fibres.

I agree that Doug. Fir is the preferred material for cold-moulding & strip-built wood-epoxy hulls, both for its workability and strength-to-weight ratio. Kiln-dried material, properly stored indoors is the preferred working material. Building outdoors under a tent in moist climates is not the best facility for wood-epoxy construction, although many successful boats have been built this way.

 

" Building with wood encapsulated in epoxy, one should use wood that has a very low m/c so that there is insufficient moisture in the wood to allow rot to begin within the matrix. This is usually accomplished with a m/c of between 9 -12%. "

No argument...I just assumed the typical frame-plywood-fabric/epoxy hull.

But you are quite correct. Build that boat outside under cover or even in an unheated shed during the wet season, and it won't take too many weeks for that 9pct M/C solid stock or plywood to approach 18pct. So think thru the whole plan, not just a part of it.

"The only mention I can find is for Port Orford or white cedar...."

One of the reasons is that cedar moves with the seasons about a third less than solid DF does...your USDA Wood Encyclopedia has all the data...and I'd consider that when locking all that hull planking together with epoxy, although cold-molding in thin veneers rather tan thick double planking will behave similar to plywood.


Encapsulating solid planking...or cold molding for that matter...is outside my area of expertise...so I'll stop talking and start listening.

 

Yup, a little bit of climate control (read: shelter, temp control, humidity control) does wonders for the quality control of the product.

 

Will,

The Gougeon Brothers on Boat Construction, Wood and West System Materials, this book is almost 25 years old, but I think it is still the best reference you will find for wood and epoxy used in boats. They also have great news letters filling you in on their research.

They recommend 12% or less in wood composite. It is interesting that one of their concerns is that boats can be built with up to 18% moisture level but if that level drops after built it could affect the structural integrity because of internal stressing.

Gary

 

Will, For you information, Douglas Fir is sold here as oregon and is imported.

Some other timbers in use here are Western Red Cedar, and Kauri for hulls. For some of the harder wearing components spotted gum, and tasmanian oak

Plenty of options and you'd probably be better talking to an old shipwright for workability.

Choice also depends a lot on availability Get it seasoned (dryed out) but it will take up moisture from the atmosphere.

Brett

 

Thanks guys,
I thought that Gougen Bros would have some info in it - I'll have to send off for a copy. Any mention of a preferred species for wood-epoxy Gary? And no concern about the age of the info - this part of my text was 1st published in 1978.....one of my criticisms of the Westlawn course - much of the material is simply too old....
Incidentally to correct my earlier post - one of the species I mentioned was Port Cedar - that should have been Port Orford Cedar.

 

Will,
There are several ways to build a wood-epoxy monocoque hull. If it is strip type construction, the wood type is selected by weight, grain, and availability. It changes from area to area. For cold molding the most often used is quality marine grade plywood, usually made from tropical woods like Okoume, Lauan, and Meranti, because of low expansion and contraction rates.

Gary

 

Will;

The Wood Encyclopedia is one of the books you need if you are going to work with cold-moulded designs. The Gougeon book is also excellent. I spoke with Meade about updating it years ago, but nothing yet. There is also an out of print book by John Guzwell that is excellent on the subject. Ian Nicolson's Cold-Moulded and Strip-Planked Wood Boatbuilding is okay, somewhat useful but dated.

Wood is wonderful stuff, great for boatbuilding but specific species are appropriate for different uses. There are a number of methods of building cold-moulded or wood-cored hulls. Certain woods will work much better than others for the job. Questions such as; how much structural dependency are you placing on the fiberglass skin, is that skin even fiberglass, what are your weight/strength criteria, cost/availability, and framing method? All these feed your preferred species selection.

Twenty years ago the cold-moulded boats I was involved with were almost entirely framed with laminated Honduras Mahogany. Hull planking was four layers, inner & outer of H Mahog. running fore & aft, and two diagonal layers (core) of Atlantic White Cedar. Mahog. was substituted for the cedar in high load areas such as chainplates and rudder/skeg. These boats were of the "frameless" or what I call "thick-skinned" cold-moulded construction. They have no framing except bulkheads, floors, and deckbeams/clamps/carlins. Deckbeams were laminated Ash.

The Mahogany was used for a number of reasons; it bonds really well with epoxy, at the time it was easy to come by in long clear lengths, it's easy to work, tight grain structure means good strength/weight, and the wood is very strong in rolling shear (two pieces of flat grain sliding against each other). We oriented at least half the planking fore & aft to deal with the tremendous bending loads from the rig. The decks of these boats was started with a thick t&g layer of yellow cedar (port orford) running fore & aft, covered by two diagonal layers, then glass and teak.

The above was for sailing vessels, powerboats were constructed completely differently. As they were high-speed boats the loads are much higher on an individual panel. Thick-skinned frameless is not appropriate in a lightweight structure. So we used longitudinal framing and all diagonal planking. We built bottom stringer "I" beams of Mahog plywood with caps top & bottom of Douglas Fir. The planking was also Douglass Fir, usually all Fir in the four bottom layers and Fir with Western Red Cedar core layers in the topsides. Keel & clamp were laminated mahogany or fir. Again this is somewhat dependent on your glass sheathing, in the latter boats the glass is heavier to try to minimize print-through from the plank lines.

Lately I've decided that we should be using as much local wood as possible, so I substitute Douglas Fir for the Mahogany. The Fir bonds very well with epoxy, it's not impossible to come by, and it's almost as good as Mahog. in rolling shear. The latest strip planked and glass sheathed cruising sailboats are planked with Doug. Fir.

All the best, Tad

 

Gary;

Ha, you nipped in ahead while I was writing.

To avoid confusion I should say something about cold-moulded plywood construction. Why would you do this?

Cold-moulded plywood construction seems to be very popular with the custom sportfishermen builders in the Carolinas and Florida. I believe they use it for a number of reasons; it's available, reasonably priced (if anything is!), and it does work. These are high-speed boats with lots of longitudinal framing. The hull form is not developable, so they saw the plywood into strips about 12" wide and put them on diagonally, in opposing directions. Often they have a glass or Kevlar layer in the matrix, and a heavy glass layer on the outside.

The main problem I have with this method is that some amount of wood in the plywood is cut into little short pieces, and it's running in the wrong direction. This is in comparison to solid stock with all the grain interlocking and running in the same direction. But the plywood is available, and it's stable, so they don't have as many print-through problems. I still believe they could build lighter and stronger boats using solid stock, but perhaps they could not find the material?

Clients often come to me asking to use cold-moulded plywood rather than strip planking in a sailboat. This makes no sense for the above reasons, plus the usual longitudinal loads on a sailing yacht. But few really get it, I've even seen someone who cut his plywood into strips!!!

Tad

 

As usual, Will asks the questions and while searching for the answer or reading the responses I learn something.

Gary

 

Tad, I agree completely with your observation about cold-molding with plywood. The sportfishermen could probably be designed with a developed form bottom and planked with multiple layers of plywood left full size. This would probably be easier, faster and stronger. The topsides though, I would build with strips or thin layers of solid wood due to all the shape, broad flair up forward transitioning to tumblehome aft.

One builder does still use solid wood for their sportfisherman though, Rybovich. If I am not mistaken, they triple plank diagonally with mahogany and then glass over the outside with a fairly heavy laminate.

 

As Gary suggests, I ask a question and I wind up learning way more from you guys than ever expect to. Your responses and efforts are all greatly appreciated. I mustn't tell too many others about this site - so I can keep you all for myself!!

I finally found the answers I was after (or rather that my lesson quiestion was chasing). They suggest Western Red Cedar as the preferred species and a moisture content of around 11%.
What's interesting is that the text was written twenty + years ago, and with the exception of Tad using it as a core, nobody else suggested W. Red.........