Alaska Yellow Cedar as structural material

[Venchka]

I need engineering help or pointing in the direction of referrence material regarding the strength of Alaska Yellow Cedar for the interior structural framing of a glued lap boat.

The boat is Iain Oughtred's Caledonia Yawl. The backbone consists of laminated stems (2" x 1-1/2") and keelson (1" x 4"). Floors are attached to the keelson. Laminated outer stems, keel and skeg cover the plywood planking and complete the backbone.

Several professional builders suggested Douglas-fir or sasafrass for the interior framing. I don't trust the Douglas-fir here on the Gulf Coast and sasafrass isn't easy to come by. By all accounts, yellow cedar is about as durable as wood gets, insect resistant and relative light.

Here is my proposal: Use Alaska Yellow Cedar for the inside structure (backbone, floors, centerboard case) and either Black Locust or White Oak for the exterior half of the backbone. It seems to me that the inner half of the backbone is well supported by plywood (12mm & 9mm) and the outer hardwood keel structure.

Is there a book or formula somewhere that would tell me:
A. Is the yellow cedar strong enough to replace Douglas-fir?
B. Or, how much should I add to the cross section of the yellow cedar to equal Douglas-fir?

I have a lot of information on the strength of both woods, but I don't know how to evaluate the information properly. Yellow cedar seems strong, but is it strong enough?

I hope there's a wood engineer out there who can shed some light on this problem/solution.

Thanks to all.

Wayne

[RGM]

I'm certainly no "wood engineer" but I have played around with the stuff a little bit. Here's some info, if it helps. At 15% moisture content a cubic foot of White Oak weighs about 47 lbs., Doug Fir (coastal) at the same moisture content is 34 lbs. per cubic foot and Yellow Cedar is 32 lbs. per cubic foot at 15% m.c. As you surmised, Yellow Cedar is considered slightly weaker than Doug Fir. Big deal, strength has alot to do with quality and lack of defects. Yellow Cedar is more decay/bug resistant and nicer to work with. Personally, I love working with the stuff. If you are concerned about obtaining suitable, tight grained "old growth" Doug Fir then perhaps you should go ahead and use Yellow Cedar. I think that it will probably work out better than the fir for you. Price and availability is probably an issue for both. Where are you getting it from? Are you getting it shipped direct to you from the PNW? Do you need some sources? Good luck.

[imported_Conrad]

Great wood, may not hold fastenings as well as fir, but given the lighter weight you could up the thickness by half to compensate and use longer/thicker fastenings.

[RGM]

I've been rooting around thru my old copy of WOOD: A Manual for it's Use As a Shipbuilding Material, prepared by the US Navy. Great reference by the way, hard to find now, printed by Teaparty Books. Anyway, here's some interesting test results for Doug Fir (coastal close grain) and Yellow Cedar. The tests were conducted on small pieces of clear stock, air dried to 12% m.c. How small I don't know. If you drop a 50 lb weight on a sample of this Yellow Cedar the wood will fail when you reach a drop height of 29 inches. Under the same size and conditions a sample of Doug Fir will fracture at a drop height of 31 inches. And now for the ever so important ball embeddment test. The amount of load required to imbed a 0.444 inch ball to half of it's diameter in the side and end grain of the Fir and Yellow Cedar samples is? Fir- 900 lbs for end grain, 710 lbs on the side, Yellow Cedar-790 lbs for the end grain and 580 lbs for the side. There you have it folks. Hey, it's lunch time here and I had a little time to kill.

[ 08-28-2002, 02:29 PM: Message edited by: RGM ]

[Venchka]

RGM:
I've talked to Timberwright on Vancouver Island so far. Waiting for a frieght quote.

Intuitively, I feel like the yellow cedar would be adequate as the top layer in an oak/plywood/cedar sandwich. I just wish I knew which of the strength figures to use to relate yellow cedar to Doug-fir.

So, if I don't go sailing among bowling ball size hailstones and I avoid being shot at with 00 buckshot, the yellow cedar might work? Sounds good to me.

The bottom line seems to be: Good quality yellow cedar which won't rot would be stronger down the road than starting out with suspect Douglas-fir which may last as long.

Wayne

[TR]

If you are getting cedar from B.C. then why not Douglas Fir? I think part of the question involves which glue you are using to do these laminations.

The stack of laminations in the boat's backbone is loaded in bending. This bending creates a sheer load in each piece of your lamination; this load is called a rolling sheer. Documentation by the Gougeon Bros. has shown Douglas Fir bonded with epoxy is very strong in rolling sheer, which is why the designer specified it. Epoxy bonds in yellow cedar can be questionable, because of the oil in the wood. The yellow cedar bonds could be as good as those in fir, but they might not be. Think about hauling your boat over a beach roller when she’s half full of water.

Bonding Oak with epoxy in structural members is also questionable, and Black Locust is difficult to work. Why not use wood that is easy to work and bonds beautifully.

All the best, Tad

[Venchka]

Thanks everyone for all of the good information. Someday I will find a use for some good Alaska Yellow Cedar. By the way, there is evidence that if the yellow cedar is standing dead for several years before felling, the strength of the wood approaches Douglas-fir. Interesting.

As it turns out, I think I have located enough black locust close enough to the building site to make trucking it to the boat feasible.

The boat is going to be heavy but STOUT!

Thanks again everyone.

Wayne

[TomRobb]

I konw, I know, everyone wants to second-guess the designer's specs. It seems to be part of our disease syndrome.

"heavy but stout"?? Why not just go with what Oughtred spec'd? Ya paid good money for his talent and expertise. If you can get proper Doug Fir, use it :rolleyes:

[Venchka]

TomRobb:

Iain Oughtred didn't specify the wood species for the backbone. Several builders I spoke to wanted to build the backbone out of Douglas-fir. No doubt it works well, assuming good stock, in cold climates. The builders who wanted to use it were all WAY north of the boat's eventual home here in the Swamp. I don't think Douglas-fir will hold up as well as other more durable woods-white oak or black locust. Yes, there is a weight penalty for durablity. That was why I raised the question of using yellow cedar in place of the next two lightest suggested woods: Douglas-fir or sasafrass.
Truth be known, "sinker" cypress from south Louisiana for the inner half of the backbone and oak or black locust for the outer half of the backbone is probably the best compromise in the weight vs. strength vs. durability vs. availability problem.