Rob,

 

The weight does sound discouraging, but if you don't mind a couple more questions:

 

I am pretty confused on the pricing issue. I go online to one of the big box home improvement stores and I find 1x6x8' tongue and groove cedar (3/4" x 5.5" x 8' actual dimensions) for $13.00 a piece. By my calculations, that is 2.75 board feet, or $4.75/board foot.

 

When I go to a boat building store (noahsmarine.com, which is usually pretty cheap), I can get 8 cedar strips (3/4" x 1/4" x 8') for $27.00. Since it is only 1/4" thick, that comes out to about 1 board foot.

 

So, is the $27/BF for the marine version versus $4.75/BF just the markup of cutting thinner slices for marine use, or are these not really the same wood? How would I know if the lumber is usable (for places where I actually want the 20mm thickness) ?

 

Comparing to 19mm foam (also from noahsmarine), a 4x8' sheet costs $211.00, which is 24 board feet, for a cost of $8.81/BF.

 

- Gardner

 

On Mon, Jul 8, 2013 at 5:25 AM, Rob Denney <harryproa@gmail.com> wrote:

 

Because panel stiffness increases as the thickness cubed, but only linearly with the component materials' stiffness the panels will all be similar thickness.   

Cedar does have lengthwise stiffness which is approx the same as 150 gsm glass each side of foam.   foam absorbs about 200 gsm of resin, cedar virtually none when hand laid, various amounts under vacuum depending on how much grain runout there is and how well the planks are held together.    Vacuumed laminate contains about 50% the weight of the laminate in resin.  Select cedar weighs approx 300 kgs per cu m (very variable), H80 foam about 80 kgs (also variable).

Assuming both are made on a table so that one side is finish quality and bagged/infused the weights would be

10mm panel of cedar with 200 uni each side will weigh about 3 kgs (cedar) plus 3 x 200 (glass and resin) plus 50 gsm each side for soakage = 3.7 kgs per sq m

10mm panel of foam with 400 gm each side will weigh about 0.8 kgs (foam) + 3 x 400 (glass and resin) + 200 each side for soakage = 2.4 kgs per sq m

20mm panel of cedar with 400 db each side will weigh about 6 kgs (cedar) plus 3 x 400 (glass and resin) plus 50 gsm each side for soakage = 7.3 kgs per sq m

20mm panel of foam with 600 gm each side will weigh about 1.6 kgs (foam) + 3 x 600 (glass and resin) + 200 each side for soakage = 3.8 kgs per sq m 

You will have to find out your own local prices for materials to make a cost comparison, but in Aus, the foam is cheaper, particularly if Ve is used.  The cedar should use epoxy, although I suspect that ve under vacuum would be more than adequate (never tested it).  

Quality lightweight ply is about the same price (in Aus)  and weight in the thinner sizes, but needs a layer of glass on the outside and 3 coats of epoxy on the inside.  It is more wasteful as offcuts cannot easily be used, unlike foam and cedar.  

Another way to get a timber finish is to use light (200 gsm) glass and veneer on the table side of the laminate.  Would need to perforate or split the veneer, but the glass would give a clear finish.  This would allow lighter glass for the rest of the laminate.   A cockpit floor will need some non slip on it which will mostly cover the timber anyway.  Fancy veneer on the underside of the deck would look very cool.  

Dennis,

Those panels would still need some glass on the outside to stop them splitting when bent.  You might want to use two layers of cedar with a layer of glass between them to prevent this and make the external finsihing easier than adding glass after it is bent.

rob

rob

On Sun, Jul 7, 2013 at 12:20 PM, Gardner Pomper <gardner@networknow.org> wrote:

 

I have noticed that since I have been lurking, the frequency of dumb questions is way down, so I decided to do something about that.

I once came very close to buying a 50' Roger Simpson strip planked catamaran. One of the things I loved about it was the way the interior showed the cedar through the fiberglass. 

I did a quick google and I have been unable to find a comparison of cedar strip vs foam on a cost and weight basis.

Note: I am not talking about traditional cedar strip boatbuilding; I am still interested in making flat panels, but out of cedar instead of foam.

I was thinking that perhaps this would be cost effective in areas which might otherwise require thick foam, such as the deck over a bunk, or the cockpit floor. The price of foam seems to go up linearly with thickness, so a 4'x8'x1" sheet of corecell runs over $300 US. Could I accomplish the same thing cheaper and more attractively with cedar and fiberglass for a 4'x8' cockpit floor?

- Gardner