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Marine plywood thickness

Posted: June 28th, 2020, 7:39 pm
by stockton
I am trying to replace the floorboards of a 30 year old Avon inflatable. The boards appear to be made of plywood which is about 11mm thick, which does not seem to be a standard imperial or metric dimension. Could anyone explain what thickness plywood I should be trying to purchase ?

Re: Marine plywood thickness

Posted: June 28th, 2020, 7:49 pm
by Breelander
stockton wrote:...plywood which is about 11mm thick, which does not seem to be a standard imperial or metric dimension....


12mm seems to be a standard thickness, for example here....

http://www.chilterntimber.co.uk/product ... thickness/

Re: Marine plywood thickness

Posted: June 28th, 2020, 8:47 pm
by 88V8
Maybe it was 5/8" and it's swollen in the damp.

Is it crucial to match the thickness?

V8

Re: Marine plywood thickness

Posted: June 28th, 2020, 9:16 pm
by dspp
stockton wrote:I am trying to replace the floorboards of a 30 year old Avon inflatable. The boards appear to be made of plywood which is about 11mm thick, which does not seem to be a standard imperial or metric dimension. Could anyone explain what thickness plywood I should be trying to purchase ?


Recently I had to do some marine GZ calcs and I used these as a data source for some of the hull build-up, they may help.

https://www.boulterplywood.com/MarinePlywood_4.htm

Frankly, for a little Avon, I don't think it matters.

regards, dspp

Re: Marine plywood thickness

Posted: June 29th, 2020, 9:36 am
by stockton
It is a moderate sized Avon (340) and more or less essential to match the thickness. Hence my puzzlement.

Re: Marine plywood thickness

Posted: June 29th, 2020, 10:08 am
by stockton
It would appear that plywood standards changed around 2014 and prior to that were manufactured to a 1972 standard but I cannot find the 1972 standards.

Re: Marine plywood thickness

Posted: June 29th, 2020, 3:19 pm
by todthedog
All marine ply used to be made to imperial thickness and sizes and then rounded up or down to the nearest metric equivalent . That was when the UK made most of the plywood manufacturing kit. This could be the case if the original is old like me. :D

Re: Marine plywood thickness

Posted: June 29th, 2020, 3:55 pm
by jfgw
todthedog wrote:All marine ply used to be made to imperial thickness and sizes and then rounded up or down to the nearest metric equivalent . That was when the UK made most of the plywood manufacturing kit. This could be the case if the original is old like me. :D

7/16" then?

Julian F. G. W.

Re: Marine plywood thickness

Posted: June 30th, 2020, 9:55 pm
by bionichamster
You say that the old plywood is “about 11mm” which isn’t very precise given that what is available is 12mm thus only 1mm difference.

Is the old stuff slightly more or slightly less than 11mm? If it’s the former them maybe it is just a worn down 12mm?
Is 1mm really going to make a difference? If you think it is then Could you Sand down The New 12mm stuff slightly on one side prior to application of an appropriate sealer?

Bh

Re: Marine plywood thickness

Posted: July 1st, 2020, 9:08 am
by redsturgeon
Do you mind being more specific as to why it is essential to match the thickness down to the last mm?

John

Re: Marine plywood thickness

Posted: July 1st, 2020, 6:19 pm
by stockton
Two reasons which make a thicker plywood undesirable; one, the weight which is significant, and two, the boards fit into a space formed by the fabric at the back of the boat and the extra thickness will strain the fabric.
I am in some doubt as to whether a 9mm ply would be strong enough.

The reason for my original question was that if I can discover exactly what was originally used I might well be able to source an exact replacement but at the moment I am having difficulty understanding the original spec - 7/16 sounds most probable but then what surprises me is that 7/16 or 10 mm is such a useful thickness that it is not available on its own merits.

Re: Marine plywood thickness

Posted: July 1st, 2020, 7:34 pm
by bionichamster
stockton wrote:Two reasons which make a thicker plywood undesirable; one, the weight which is significant, and two, the boards fit into a space formed by the fabric at the back of the boat and the extra thickness will strain the fabric.
I am in some doubt as to whether a 9mm ply


Could you sand a slight 1mm+ taper along the edges Of a 12mm board that the board is the correct width where it sits in The fabric but slightly thicker in the center?

Bh

Re: Marine plywood thickness

Posted: July 1st, 2020, 7:53 pm
by stockton
bionichamster wrote:Could you sand a slight 1mm+ taper along the edges Of a 12mm board that the board is the correct width where it sits in The fabric but slightly thicker in the center?
Bh

That is what we have ended up doing. The weight difference remains inconvenient.

Re: Marine plywood thickness

Posted: July 1st, 2020, 11:03 pm
by dspp
stockton wrote:
bionichamster wrote:Could you sand a slight 1mm+ taper along the edges Of a 12mm board that the board is the correct width where it sits in The fabric but slightly thicker in the center?
Bh

That is what we have ended up doing. The weight difference remains inconvenient.


That sounds like the newer ply is denser, i.e. better quality.

regards, dspp