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Fir marine ply


Scott Dunsworth
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I found some good looking marine fir plywood with no visible voids on the edges 3/4 inch 7 ply ($65), 1/2 inch 5 ply ($55). What are the thoughts about using this for interior bulkheads and structures only, on my 28? This is locally so no freight charges make it about half price of meranti and a third of what Okoume is.

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Thanks Par. I have no idea if that is a good price for fir marine, but to be able to pick up a sheet in twenty minutes is a big plus. As I understand it checking cracks is the biggest problem with fir. Which brings me to my next question should I , or must I epoxy all that stuff that is going to be painted inside?

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Scott:

 

Several years ago, I purchased a sheet of 3/4" marine fir that I used to build a laminated rudder. Two sections glued together to make it 1 1/2 inches thick. I seem to recall it was listed as A/B, and the B side may have had as many as 10 or more "footballs" in it and both sides did eventually develop a lot of surface checks. I also found a lot of internal voids as I started cutting it up. But having said all that, it was structurally sound and everything I used it for worked out fine. But even though I did epoxy coat the surface prior to painting, any areas that had the "footballs" did eventually show print through.......again, a cosmetic blemish, but not a structural one. I found the same thing with a sheet of 1/4" marine fir ply. Same cosmetic issues, but no structural ones.

 

If this was going to be used for internal bulkheads and internal decking........in places you would not likely see it or if it's going to be painted, it should work OK. I would make sure any edges are sealed up, but anything glassed into place with tape along the edges would do that. You might also consider the moisture content when new..........it may need to dry down some to become fully stable. I have found that to be the case with some exterior plywood. When new, the surface seems to be bit wetter than a month or so later, once it has been exposed to air movement. That might be the case with the marine fir too. It didn't come with the surface checks.........those developed over time.

 

My current glass boat has cockpit locker divider (painted) made of 1/2" marine fir and it has been in place over 30 years with no apparent issues. As near as I can tell, it was simply tabbed along the edges and the surface just painted.

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There used to be a product that would stabilize the grain on fir plywood called Firzite. It has not been available for many years and wouldn't be used under epoxy anyway. I found this post about another product that apparently is similar to Firzite. 

 

"Check out a product called "Benite", it's made by an outfit in Seattle called Daleys. Try a search for them on the internet. not sure what their web address is. You see this product in the marine stores around the PNW. I was in Flouder Bay Boat Lumber in Anacortes, WA. yesterday talking with Erica (the proprietress) and saw some on the shelf. Nice folks at Flounder Bay, definitly worth a visit for anyone that finds themselves in that neighborhood. Anyway, it reminded me of your post here J. Dillon. I've used it in the past with success. Good luck."

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Douglas fir will check and the only way to prevent this from being a problem is a sheathing heavy enough to prevent movement and moisture ingress. On a 3/4" sheet, this is a pretty heavy sheath, say two layers of 10 ounce as a minimum. This is about a 1/16" thick sheathing (a little shy of this actually). For other thicknesses of plywood, try to get 1/12th to 1/10th of the total thickness (fabric and plywood) in sheathing thickness, if you want to stop checking. Simply put, if you skin a 3/4" plywood board with two 10 ounces layers, you'll have a board that's .85" thick, which amounts to about a 13% sheathing to board ratio. At least I've found this rule works pretty good on Douglas fir, possible a little heavier than necessary, but the point is to have no issues, not a potential for no issues, so I've errored on the too heavy side with this formula. I've seen 3/4" plywood bulkheads check, under 6 and 8 ounce sheathings. I've seen 3/4" decks check under two 6 ounce sheathings, hence the heavy schedule recommendation.

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  • 1 month later...

I found some good looking marine fir plywood with no visible voids on the edges 3/4 inch 7 ply ($65), 1/2 inch 5 ply ($55). What are the thoughts about using this for interior bulkheads and structures only, on my 28? This is locally so no freight charges make it about half price of meranti and a third of what Okoume is.

If it was mine I would use exterior grade  okoume plywood and coat it with resin after you fit and make all of the necessary cuts and holes. If you completely seal the wood it should never rot. Glue the carpeting on top and enjoy.
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Okoume is fine, but if you look at the cost differences, things change. For example 3/8" (9 mm) Douglas fir is about $40 a sheet and 3/4" (18 mm) about $65, yet 3/8" Okoume is $102 and 3/4" is $182 per sheet.

 

Additionally, carpet that can't be removed or rolled up will kill plywood pretty quickly. I've replaced countless hundreds of feet of soles, because carpet was employed. Small boats shouldn't have carpet, unless you just like mold, mildew and eventually problems. Carpet needs to remain relatively dry, so larger craft can enjoy it, but small boats (30' or less) just will soak any carpet aboard on nearly every outing. Walking on carpet wears away the coatings, below, even light fabric sheathing, which eventually lets moisture in to do its deed. It doesn't matter the type of carpet or how it's attached.

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