Framing with Cedar?

[Charlie43]

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[MarkR]

Hi Charlie,

Welcome to the club!  Good topic.  Brings up some interesting thoughts.

WRC is very soft, lightweight and easily worked - great for strip building and planking.  My opinion is that in a strip planked boat, the WRC is a core material that does not significantly contribute to the strength of the structure which is primarily provided by the epoxy & fiberglass sheathing it.  The cedar could be replaced with foam. The key to that structure is the space between the fiberglass structural components.  So, if you were to frame with cedar, it would be good to wrap it with glass for strength.  But at that point, why not use CVG fir and avoid the hassle of doing the wrap?  Probably a similar weight, too.

That being said, on my cedar strip canoe, I used Alaskan yellow cedar for outwales, then applied a 3/32" hickory veneer as a more durable rub rail surface.  It's fine on a 38#, 14 ft canoe.

Here's a good resource:  http://www.wood-database.com/?s=western+red+cedar

Essentially an engineering spec for your wood components.  

[Charlie43]

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[MarkR]

Hi Charlie,

I imagine that WRC will make an appropriate framing material. but I'd like to make a case for Douglas Fir.  It's nearly double the hardness of WRC, so resists bruising better even though it is relatively soft as far as absolute hardness goes.  It's incredibly strong.  It excels in both Modulus of Rupture (MOR) and Modulus of Elasticity (MOE) in combination with relatively low weight.  

Doug Fir is stronger than the classic boatbuilding framing of Eastern White Oak for a given weight.  The result is that you can use a smaller member to perform a similar task.  Fir heartwood is very rot resistant.  There is a reason that the West Coast builders of the past were stuck on CVG Fir even when WRC was plentiful and cheap.  The classic combination was fir framing and cedar planking.  The softer cedar planking was easy to fit and absorbed impacts rather than breaking.

Of course, builder's preferences come in there, too.  You can properly structure a small boat with cedar assuming that the builder takes into consideration how long they expect that boat to last and how much of a beating they expect it to take.  

Cheers!

Mark

[Charlie43]

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[MarkR]

Charlie,

You know I once worked with an architect who changed the way I thought about mixing woods.  Ken's thing was "Wood is wood."  He would mix species with wild abandon, allowing each species to shine through clear finishes.  And I always liked it.  It made me more willing to try different combinations of woods - usually with good results.  Of course, there is always stain...

As far as structuring with different woods, I am familiar with the more traditional boatbuilding timbers, but don't know anything about using walnut in boats.  Is it rot resistant?  That's a big consideration unless you plan to completely encapsulate it in epoxy and make sure it stays that way.  And how is it for strength?  I always assume clear woods for framing as that's the way structural properties are derived.  A bit of wild grain or knot is a failure point and not nearly as strong as a clear, tight grained timber.  Acceptable as a rub rail, perhaps.

I'm not sure if you know this, but in case you don't: The older WRC is (as in older tree) the darker and browner the finished material.  So you might want to hand select clear WRC at the lumberyard for color & grain.  What you will find is that the darker and browner material will usually be more clear as well.  It is the heartwood.  You can buy 2X material green and rip it.

As far as proportion - I agree - it has to look good while doing its job.  What I found on my canoe (14' cedar strip with two decks and floatation chambers; 38#) is that sometimes you can use one material to structure with and a second material as an abrasion resistant layer.  An example of that are the outwales which are Alaskan Yellow Cedar sheathed with a thin veneer of hickory as a rub rail.  I didn't alter the vertical dimension of the outwale, but did make it thinner in cross section.  Then, with a spaced inwale it made a very strong and light gunnel assembly.  On that project I also learned that I didn't need to stick with the lumberyard thicknesses.  If it needed to be 5/8 rather than the 11/16 from Crosscut - set up the bandsaw or tablesaw.  Make it so!

A technique that I learned from my latest build is taking a very light member (CVG Fir ribs in this case) and wrapping with four ounce fiberglass cloth and epoxy.  It increased the strength of that rib immensely.  I'm assuming that the little 3/4" thick rib would probably need to be 2 - 3 times the cross section for equivalent strength.  But that means a lot of rather fussy work.  

OK - so I've blathered enough!  Have a great week!

- Mark