Marine Plywood vs Baltic Birch Plywood

[bateau]

This is my first post to these forums, hello everybody and nice to meet you eh! :)

So here's yet another marine plywood question. From what I understand, MP uses mostly the same glue as regular exterior ply, and the material it's made of is of little relevance because it's coated in epoxy. If I'm correct, the main outstanding feature of marine plywood is it's consistency. Consistency in lack of voids, and of ply thickness so you don't get weird stuff happening when bending a compound curve. Is this correct?

So, would that mean that Baltic Birch plywood - which from what I know is made to the same standards in quality - can be used as marine plywood?

I don't think I'll be saving any money by going with BB, but it's easier for me to get than MP. I can go down to the city for the MP, but if I only need a little piece to get me through the weekend I can get BB locally, and hence the question.

I don't know if BB is even available in Aus, however so you guys might not know what I mean.

I'm starting to build my first boat very soon - plans are in the mail, so likely I'll have more questions in the future. Thanks for looking!

[Boatmik]

Birch ply can be very nice ... a lot of aircraft grade ply is birch for example.

Exterior, marine and aircraft ply all use the same glue.

The difference is the quality of the veneers. This is not just a longevity issue ... it is a strength issue too - particularly for 3-ply (3 layer) plywood. Voids also mean that you are asking the epoxy to do a really hard job of making sure there is not a single leak into that void.

The other area that might be relevant is in terms of the weight of Birch.

I couldn't find Baltic but the figure for British indicates a specific gravity of 0.67
http://www.simetric.co.uk/si_wood.htm

Most plywood weights for small boats are around 0.45 to a bit over 0.5. Some like Gaboon/Okoume are around 0.35

So a boat built of Birch might be a bit less than 14% heavier than one built out of more normal plywood or about double that built of Gaboon.

Of course it depends how much extra structure goes in ... because the whole boat is not plywood.

Best wishes
Michael

[Theodor]

Welcome to the forum Bateau .. what are you planning on building?

As for the Birch thing, I saw this survival Pommie guy build a traditional birch bark canoe on Discovery channel. I think his name was something like Ray Mears. It seemed to me that birch is an ideal material as far as its wear and tear in a wet environment - I have no scientific basis for such a comment. It is purely my own observation: since birch trees seem to cover a large percentage of high alpine (i.e. snowy) areas, it must be pretty good in dealing with rot.

My 2c worth.

I think you'll also find people on this forum constantly ask what locally grown tree varieties are good for boat building - you've asked a good (and common) question.

Mark

[Boatmik]

With a completely traditional canoe like a birchbark the material itself is responsible for keeping the water out and the wear and tear.

With modern building we can rely on coatings and surface finishes ranging from paints through to epoxy and glass. The result is the timber choice becomes less important.

MIK

[PAR]

In BC your typical choices would be Okoume, (BS 1088 or 6566), Meranti (often sold as BS 1088 compliant Hydrocore), Meranti BS 6566 compliant (also sold as Aquacore), Sapele (very nice African mahogany, but getting quite rare) and Douglas fur or western larch which will come with a USA marking of APA 1-95 (WBP marine). The Douglas fur is difficult to finish, as it checks and splits like my ex-wife's face. Most often you have to 'glass the surface then fair this smooth.

[Slavo]

Most often you have to 'glass the surface then fair this smooth.

Are you talking about the boat or your ex-wifes beauty routine :D

[Boatmik]

haha Slavo ... one of the greatest yacht designers of all time, Francis Herreshoff (who PAR has run into - luckily or unluckily I am not sure) used to make the same joke.

He compared a beautifully designed and built boat with a plain finish to a second grade boat with beautiful finishing ... like putting make-up on a beautiful woman!

MIK

[bateau]

Thanks for the replies! Much appreciated.

I hope to mostly use MP for the boat, but this topic came up as an interesting question that I thought I'd post, and use as an excuse to introduce myself to the forum :)

Theodore - I'm going to build a Pocketship. I have my garage all cleaned out and organized, some epoxy waiting, and I'm going to Vancouver to pick up some plywood this weekend. All I need is the plans to arrive and to find some lead ballast, then I'm off to the races!

Thanks fellas.

As far as birchbark canoes go, from what I understand they used pitch to seal the knot holes to make it waterproof. Essentially a birch bark canoe is just a skin on frame, with the skin being made of birch bark...... I don't know anything, really, about the water resistance of Birch. I do know however that if you get it wet thewater will stain it before you put varnish on so....

Speaking of using 'local' materials - I hope to scavenge my jobsite for most of the odds and ends I'm going to need for this boat. Cedar or the cabin deck, maybe some poplar or fir for the spars. I have a whack of white oak, although I don't know if I need it......... Ok, I"m rambling now :D