3/8" ply vs. 1/2" ply

[bdann]

Hi all,

My buddy and I are knee deep in our weekender project and we've run into a bit of a snag.  Our local supplier of marine plywood doesn't stock 3/8" ply, and they told us their supplier wouldn't carry a "weird" size like that.  So my question is, would it be a major blunder to substitute 1/2" ply for the 3/8"?

Thanks,

Brian

[Barry Pyeatt]

Where are you located?  I would look for another source.  3/8" isn't a particularly "Wierd Size".   If you go to 1/2" ply you will find that you will have an issue trying to get it to form the cabin top.  It is a fairly difficult bend with 3/8" and would be more than a little difficult with 1/2".   The deck in 1/2" wouldn't be a problem other than trying to pull it down into the sweeping curve from bow to transom to attach it to the stem, forward bulkhead and main bulkhead where there is very little material to attach it to.  The rocker to the bottom is a difficult curve to achieve with 1/2" in the forward sweep for some builders.   Getting the bottom attached to the keel will give you an idea of just what you are facing in trying to bend 1/2" ply. 

  By putting in a heavier deck and cabin top you are building in more resistance to these parts taking the "fair" curve that you are looking for and applying more resistance to each joint and attachment point.  The stringers are going to need to be able to take the additional stress.  The adhesives are going to be under more stress, and you would likely need to use more screws spaced closer together to get things to work.  The sweep of the deck from forward bulkhead to the main and to the lazerette will be flatter than designed due to the thickness of the 1/2".  This will have a bit of an effect on how your cabin, hull sides and deck sides are shaped. The deck will want to resist bending to a curve to attach it to the stem, bulkeads, and to bend to fit the curve of the lazerette and transom.

If you are unable to find 3/8" marine ply locally, you can order it from other sources out of your area.  Or, instead of 1/2" ply for the cabin top you could use two layers of 1/4" to laminate together.  Much stronger and easier to lay up and work with.

The deck "with more muscle" can be formed into the sweeping curve you need fore to aft.  But the biggest challenge will be to get the curve from side to side over the lazerette and transom. You may need to beef up the stringers a little on the main bulkhead as well as on the forward bulkhead, lazerette and transom.  Here is where quarter sawn stringer stock will benefit a great deal in strength.  Especially if you cut and attach them so that the deck fasteners will go thru the grain rather than with it on the stringers.  Epoxy would be a better choice than other adhesives as well.

There is another factor involved and that has to do with the overall balance of material use vs. application and resulting strength.  The structural balance is pretty darned good with the thicknesses recommended.  When you go to heavier, (thicker) material, you make that portion much stronger than the rest of the project.  You are also making more internal stress within the project.  Ideally, if you were to have a major damage issue, the entire boat would come apart more or less equally as designed.

[bdann]

We are in Dallas.  I have a suspicion our supplier just doesn't want to special order anything in small quantities, and rather than telling us that, has told us he can't get it. 

Thanks for the advice, we'll be sourcing 3/8" elsewhere (not sure where yet...).

[lewisboats]

Look for someone supplying MDO in 3/8 or someone with marine 9mm (which is near to a whisker 3/8")

Steve

(edited to add: I did a google on "marine plywood in dallas tx" and this is the first of many that came up: http://www.dixieply.com/dallas.shtml )

[Roger Peterson]

Try "Plywood of Fort Worth" on I-35N (West) at loop 820. I bought 3/8" marine fir for a selway-fisher Waterman Canoe a few years ago. They also have one and two sided MDO in 3/8" as well. I will be using the MDO on my next build.

[bdann]

pardon my ignorance, but what is "MDO"?  That place in Ft Worth is likely where we'll end up going.

[Roger Peterson]

MDO = medium density overlay. It is a fir or sometimes pine plywood made with waterproof glue and covered on one or both sides with a resin coated heavy paper-like material that is very smooth. It is used mostly for signs that are intended for outdoor use. It takes paint very well and won't "check" from weather like marine fir plywood. I think 3/8" is the thinnest available. If I remember correctly, it costs more than the marine fir but much less than occume or other import hardwood marine ply.

[bdann]

After another phone call to our supplier (Wood World in Dallas), as it turns out they can after all order 3/8" marine ply.  I guess it just matters who you talk too!  I plan on posting photos of our build progress (once we actually make some!)...stay tuned.