Results 1 to 34 of 34

Thread: Rule of thumb for plywood thickness

  1. #1
    Join Date
    Sep 2017
    Location
    Vilnius, Lithuania
    Posts
    53

    Default Rule of thumb for plywood thickness

    Hi,
    I have noticed that for similar length of boat designers specify different thickness of plywood. Boat of 14-15 ft could be built with 6 mm or 9 mm plywood, what should result in substantial difference in boat weight. What is the reason?
    Is there a "rule of thumb" for the boat size (or other parameters), relation to plywood thickness?

  2. #2
    Join Date
    Jun 2000
    Location
    Pleasant Valley NS Canada
    Posts
    25,064

    Default Re: Rule of thumb for plywood thickness

    Not rule of thumb, scantlings derived from either first principles or decades of recorded experience. The thickness of the hull skin - be it plywood, solid planks, fiberglass or metal - can vary due to the speed of the boat, whether it is intended for inshore of offshore use, and what the spacing of the support structures (frames, bulkheads, and longitudinal stringers) beneath it is. Essentially, the faster, farther offshore, or greater the structure spacing, the thicker the hull.
    Hope for the best, but plan for the worst.

  3. #3
    Join Date
    Sep 2002
    Location
    On the river, Auckland, New Zealand
    Posts
    7,183

    Default Re: Rule of thumb for plywood thickness

    Quote Originally Posted by mmd View Post
    Not rule of thumb, scantlings derived from either first principles or decades of recorded experience. The thickness of the hull skin - be it plywood, solid planks, fiberglass or metal - can vary due to the speed of the boat, whether it is intended for inshore of offshore use, and what the spacing of the support structures (frames, bulkheads, and longitudinal stringers) beneath it is. Essentially, the faster, farther offshore, or greater the structure spacing, the thicker the hull.
    All of that. I have a set of tables published in "Professional Boatbuilder" magazine, the sister publication to our hosts here, that give me a guide to allowable deflection of the hulls skin under a stated (proportional to the boats weight) for a range of anticipated uses. I've added to that over time, and use it in any design that is a bit outside my usual area of design.

    John Welsford
    An expert is but a beginner with experience.

  4. #4
    Join Date
    Jun 2015
    Location
    Graceville, Florida
    Posts
    157

    Default Re: Rule of thumb for plywood thickness

    Dave Gerr explains it pretty well for those of us who are inexperienced, in his book Elements of Boat Strength.

    regards,
    Joe
    These days, everything I do is just "puttering around"

  5. #5
    Join Date
    Feb 2009
    Location
    Australia
    Posts
    358

    Default Re: Rule of thumb for plywood thickness

    We use commercial scantling's such as ABS. Some parts we do use First Principles

  6. #6
    Join Date
    Feb 2010
    Location
    Albuquerque, NM
    Posts
    30,237

    Default Re: Rule of thumb for plywood thickness

    When I built my epoxy lapstrake 15" Lincolnville Salmon Wherry, Walt Simmons ( designer ) said one could get away with 6mm if you were rowing only, but to use 9mm with some sawn frames if it was to be sail-n-oar, so that's what I used.
    Gerard>
    Albuquerque, NM

    Don't believe Republican lies.

  7. #7
    Join Date
    Sep 2017
    Location
    Vilnius, Lithuania
    Posts
    53

    Default Re: Rule of thumb for plywood thickness

    Thank you for comments.
    John, it would be interesting to see your table.
    I have available sheets of 6 mm plywood and want to build at least 4,5 x 1,8 m daysailer (bigger than I have now) to be sailed in lake or lagoon. Is fair to assume that if I take design for 9mm plywood without longitudinal stringers and add stringers, than the boat would be strong enough? Or add glassing?
    Best regards,
    Jonas
    Last edited by JonasK; 10-20-2017 at 01:58 AM.

  8. #8
    Join Date
    Jan 2010
    Location
    Northern Europe
    Posts
    12,927

    Default Re: Rule of thumb for plywood thickness

    6mm should be ok for planking, but depending on weight and whose rules you look at, you do not want more than 30cm square area unsupported by either a frame or stringer, according to Firth-Jones.

  9. #9
    Join Date
    Feb 2009
    Location
    Australia
    Posts
    358

    Default Re: Rule of thumb for plywood thickness

    you do not want more than 30cm square area unsupported by either a frame or stringer, according to Firth-Jones.
    Hmmm, bit small I think, especially for a small rowing boat or sailor. I'm just going by First Principles and class rules

  10. #10
    Join Date
    Dec 1999
    Location
    Oriental, NC USA
    Posts
    5,336

    Default Re: Rule of thumb for plywood thickness

    Quote Originally Posted by JonasK View Post
    Thank you for comments.
    John, it would be interesting to see your table.
    I have available sheets of 6 mm plywood and want to build at least 4,5 x 1,8 m daysailer (bigger than I have now) to be sailed in lake or lagoon. Is fair to assume that if I take design for 9mm plywood without longitudinal stringers and add stringers, than the boat would be strong enough? Or add glassing?
    Best regards,
    Jonas
    I would say that the majority of plywood sailboats of those dimensions are built with 6mm skins. I have Firth Jones's books but would doubt that he has actually verified the 30cm claim by experimentation. I think that is not a good rule and overly cautious. I know of many such sailboats with relatively huge unsupported 6mm panel areas that have survived a long life of competitive racing and are still active. One way to stiffen a 6mm ply hull without adding lots of weight is to build in lapstrake.

    This 16' Lapwing has only one bulkhead well forward and no frames at all, built in 6mm occoume and is plenty stiff and strong for the purpose. Side tanks/seats do add lots of stiffness.
    Attached Images Attached Images
    Last edited by Tom Lathrop; 10-20-2017 at 07:46 AM.
    Tom L

  11. #11
    Join Date
    Jan 2010
    Location
    Northern Europe
    Posts
    12,927

    Default Re: Rule of thumb for plywood thickness

    I believe Jones was building a Bolger Catboat, if i recall, the gap between chines and frames was 2ft- 60cm, which was too much for 6mm ply plank, and if i recall correctly, he put a stringer in between the chines bring the supported area down to 1ft- 30cm.
    My point being, you can skin with a thinner ply, but you have to make up the strength in framing......not that it will help with puncture resistance, quite a difference between 9mm and 6mm ply in that regard.

  12. #12
    Join Date
    Sep 2017
    Location
    Vilnius, Lithuania
    Posts
    53

    Default Re: Rule of thumb for plywood thickness

    Hello All,
    Thank you for interesting and valuable comments.
    I had one boat with 6 mm 3 ply skin. Wide bottom panels did not feel like to stand on them. My present Challenger13 has 9 mm bottom panels and it is OK to stand on them. Side panels are 6 mm , the structure quite robust. So I agree, that 6 mm skin if supported every 30x30 cm areas should give stiff structure, but for punch resistance 9 mm skin is much better.
    Best regards,
    Jonas

  13. #13
    Join Date
    May 2010
    Location
    Port Townsend, Wa.
    Posts
    861

    Default Re: Rule of thumb for plywood thickness

    Re: scantlings. it's the wild west: so unless you take some one person's design as gospel, prepare to be confuddled. I purchased access to the McNaughton data, bought the Gerr book and the Gougeon Book,, talked to the dudes at West Systems (before they passed on), emailed other builders and designers, plied the staff at the WoodenBoat school with lots of beer, etc, etc. and the recommendations for building the Chapelle "Western Lakes Mackinaw" in strip composite, ranged from planks of 5/8 to 1.25" and glass from 6 oz exterior only, to 20 oz inside and out, which is basically a fiberglass boat with some wood as a core. At some point, I think that one needs to give up thinking that there is some established science or engineering in this. If I had tried to design first I would never have gotten off the dime.... I have maybe gotten to the nickel or quarter radius ...... still working..... :-(
    Tools / Materials / Techniques / Products


  14. #14
    Join Date
    Feb 2006
    Location
    Pennsylvania
    Posts
    9,458

    Default Re: Rule of thumb for plywood thickness

    Bolger wrote somewhere that he figured when designing what ply thickness he wanted, and would then specify the next size under -- knowing full well that all his amateur builders would upsize no matter what was specified. That way, he argued, the boat would be built correctly.

    I'm sure this was all said tongue in cheek, but his point was that what's adequate with plywood just doesn't seem like enough to the beginner builder.

    And there is yet another variable -- what kind of ply is being used? They're not all the same. My 30-year-old Wisp is all 6mm (top quality hardwood) ply, hardly any framing, and still as good as new.
    -Dave

  15. #15
    Join Date
    May 2010
    Location
    Port Townsend, Wa.
    Posts
    861

    Default Re: Rule of thumb for plywood thickness

    Quote Originally Posted by Woxbox View Post
    Bolger wrote somewhere that he figured when designing what ply thickness he wanted, and would then specify the next size under -- knowing full well that all his amateur builders would upsize no matter what was specified. That way, he argued, the boat would be built correctly.

    I'm sure this was all said tongue in cheek, but his point was that what's adequate with plywood just doesn't seem like enough to the beginner builder.

    And there is yet another variable -- what kind of ply is being used? They're not all the same. My 30-year-old Wisp is all 6mm (top quality hardwood) ply, hardly any framing, and still as good as new.
    What was I thinking? I built the Bolger Glouchester Gull as specified and while it was easy to carry it was too freaking tiddily. If I was to build another I would go for 3/4" floor, 1/2" sides and maybe some room for ballast..... K

  16. #16
    Join Date
    Sep 2017
    Location
    Vilnius, Lithuania
    Posts
    53

    Default Re: Rule of thumb for plywood thickness

    Quote Originally Posted by Woxbox View Post

    And there is yet another variable -- what kind of ply is being used? They're not all the same. My 30-year-old Wisp is all 6mm (top quality hardwood) ply, hardly any framing, and still as good as new.
    Once I tried 6 mm birch plywood -marine grade. It was very stiff, probably as stiff as other types of thicker - 9 mm plywood. That could stand for "positive" side.
    On negative: (1) plywood twists on it's own, (2) heavy, (3) probably taking in very little epoxy, (4) at exposure to moisture (even from air) very quickly getting gray - developing some sort of mold, I guess.
    Can anybody say more positive features of birch plywood to be used bor boat building? Or it is totally useless?
    Regards,
    Jonas

  17. #17
    Join Date
    Jan 2010
    Location
    Northern Europe
    Posts
    12,927

    Default Re: Rule of thumb for plywood thickness

    With regards to birch, i will just say that Francois Vivier built his 25ft cruiser from it, but it is imperative on epoxy encapsulation. As epoxy is supposed to be vapour permeable, it will be interesting to see how she is in 20 years.

  18. #18
    Join Date
    Jun 2013
    Posts
    3,412

    Default Re: Rule of thumb for plywood thickness

    The Australian Lightweight Sharpie was built in 4mm ply and had up to 1 metre between frames. They are 20ft long, between a 505 and FD in speed, and race very happily in the open ocean in very windy areas.

    Scow Moths were made from 1/32" (o.7mm) birch aircraft ply. It made repairs easy in some ways as you could cut it so easily with scissors.

    A Class cats are 3mm; I think Taipan 4.9s cats are probably the same thickness and they last for decades sailing at high speed in the open ocean, driven by a 28' high wingmast.

    While these are obviously at the extremes, they do show that some rules of thumb are..... well let's just say dubious.
    Last edited by Chris249; 10-23-2017 at 04:04 PM.

  19. #19
    Join Date
    Sep 2017
    Location
    Vilnius, Lithuania
    Posts
    53

    Default Re: Rule of thumb for plywood thickness

    Hi Chris,
    Very interesting.
    Regards,
    Jonas

  20. #20
    Join Date
    Apr 2017
    Location
    Nelson, New Zealand
    Posts
    54

    Default Re: Rule of thumb for plywood thickness

    +1 for Elements of Boat strength.

    The trouble with using Lloyds rules or others is that they need a lot of experience to use them and they are orientated for racing yachts (in my opinion) and not for more hard use boats. Others like Nevins or Herreshoff require workmanship that would make a Chippendale furniture maker look ham handed.

  21. #21
    Join Date
    Feb 2009
    Location
    Australia
    Posts
    358

    Default Re: Rule of thumb for plywood thickness

    The trouble with using Lloyds rules or others is that they need a lot of experience to use them and they are orientated for racing yachts (in my opinion) and not for more hard use boats.
    I don't agree with that at all

  22. #22
    Join Date
    Jun 2000
    Location
    Pleasant Valley NS Canada
    Posts
    25,064

    Default Re: Rule of thumb for plywood thickness

    The blanket dismissal of classification society's scantlings rules as being "orientated for racing yachts... and not for more hard use boats" doesn't sit well with me, either. I regularly use ABS and DNV Rules in my design work, and have used Lloyd's on occasion, and found that, if anything, they tend to be a bit on the heavy side (ABS especially). I find that the afore-mentioned Herreshoff method to be on the light side for offshore work. One of the 'black arts' that us designers who do this sort of stuff for a living learn to do is to determine what set of scantlings rules is best applied to a given project to result in the best hull for the application, and still be able to prove to insurers that the scantlings are based on valid calculations. Scantlings aren't a subject that can be distilled into a 'one size fits all' series of calculations; experience and rationalization of the varying parameters must be applied to achieve an optimum result.
    Hope for the best, but plan for the worst.

  23. #23
    Join Date
    Jan 2010
    Location
    Northern Europe
    Posts
    12,927

    Default Re: Rule of thumb for plywood thickness

    Most of the large yacht builds i was involved in used Lloyds rules, and some built under Lloyds supervision. My opinion was it was on the heavy side for what might be termed "pleasure boats", but do generally have a long life under working conditions.
    So although 3mm birch ply has been used, it does not also mean it might be suitable for a family dinghy that gets beached with kids jumping in and out of it, i think thats where the rule of thumb comes in.

  24. #24
    Join Date
    Sep 2017
    Location
    Vilnius, Lithuania
    Posts
    53

    Default Re: Rule of thumb for plywood thickness

    How the birch ply was encapsulated?
    Quote Originally Posted by skaraborgcraft View Post
    Most of the large yacht builds i was involved in used Lloyds rules, and some built under Lloyds supervision. My opinion was it was on the heavy side for what might be termed "pleasure boats", but do generally have a long life under working conditions.
    So although 3mm birch ply has been used, it does not also mean it might be suitable for a family dinghy that gets beached with kids jumping in and out of it, i think thats where the rule of thumb comes in.

  25. #25
    Join Date
    Sep 2002
    Location
    On the river, Auckland, New Zealand
    Posts
    7,183

    Default Re: Rule of thumb for plywood thickness

    Quote Originally Posted by Chris249 View Post
    The Australian Lightweight Sharpie was built in 4mm ply and had up to 1 metre between frames. They are 20ft long, between a 505 and FD in speed, and race very happily in the open ocean in very windy areas.

    Scow Moths were made from 1/32" (o.7mm) birch aircraft ply. It made repairs easy in some ways as you could cut it so easily with scissors.

    A Class cats are 3mm; I think Taipan 4.9s cats are probably the same thickness and they last for decades sailing at high speed in the open ocean, driven by a 28' high wingmast.

    While these are obviously at the extremes, they do show that some rules of thumb are..... well let's just say dubious.
    For lightweight plywood construction, I size the stringers according to the distance between frames, and the plywood according to the shortest distance across the largest unsupported panel area. But watch out if you are using three ply, some three ply sheets are much stiffer in bend along the grain of the outside lamells than they are across, and we tend to use the ply with the grain orientated along the boat when the stiffness is needed acrosswise between the stringers.
    I've helped build Tornadoes in the past, 3.2mm gaboon plywood tortured to the max, amazingly strong. Where are the real limits? I dont really know, I tend to stress my smaller boats to cope with being hauled at speed along rough roads on a trailer and that puts higher local loadings on than any sailing.

    John Welsford
    An expert is but a beginner with experience.

  26. #26
    Join Date
    Jan 2010
    Location
    Northern Europe
    Posts
    12,927

    Default Re: Rule of thumb for plywood thickness

    Quote Originally Posted by JonasK View Post
    How the birch ply was encapsulated?
    I believe external glass sheathing and a "minimum" of 3 coats of epoxy on the inside of the hull.

  27. #27
    Join Date
    Nov 2012
    Posts
    1

    Default Re: Rule of thumb for plywood thickness

    Nice thread, I've been going back and forth on this. Planning on building an Argie15: no sails, outboard only. And yes, I've looked at (and purchased) quite a few skiff plans but this one speaks to me. Scantlings call for 1/4" ply for basic hull, but I'm considering using 3/8" due to outboard use only and the water here in coastal Georgia can kick up a bit. No centerboard, etc. Will add doublers on transom for support. And maybe a little weight forward to keep the nose down. Weight will increase, but that's expected. Still in thinkin' stage. But the Selway-Fisher Cape Cod Dory skiff keeps tugging at me, too. Too many boats, not enough time. Age old story.

  28. #28
    Join Date
    Dec 2011
    Location
    North East England
    Posts
    1,387

    Default Re: Rule of thumb for plywood thickness

    Quote Originally Posted by skaraborgcraft View Post
    I believe external glass sheathing and a "minimum" of 3 coats of epoxy on the inside of the hull.
    I have lurked and enjoyed this thread from the start.
    The answer to the OP is very application dependent and I would say go as thin as you dare and stiffen as required. Obviously I refer to inland day sailing.

    My personal opinion is that reinforcement should be on the inside. Water pressure will put a tensile stress on the inner skin and the reinforcement is best in tension. As a simple analogy think of SOF craft. External sheathing is more for abrasion protection.

    Panel deflection is a function of the square of the span length and so it is clear that if you can reduce the span, using stiffening or more chines you can reduce the plywood thickness. However as stiffness is a fiction on the cube of the thickness as you go thinner stiffness will soon disappear.

  29. #29
    Join Date
    Jun 2000
    Location
    Pleasant Valley NS Canada
    Posts
    25,064

    Default Re: Rule of thumb for plywood thickness

    Bob, you may be a bit over-cautious by increasing spec'd ply thickness by 50% You will probably gain significant strength without the weight (and expense) penalty by going up to 5/16 (8mm) ply, if you can get it in your area.
    Hope for the best, but plan for the worst.

  30. #30
    Join Date
    Jan 2010
    Location
    Northern Europe
    Posts
    12,927

    Default Re: Rule of thumb for plywood thickness

    I can give an example of going the other way. The bilge panels on my 15ft family skiff were supposed to be from 9mm. I knew i would be giving the outside a light sheathing of glass for abrasion resistance, so used 6mm. Not been an issue in over 5 years, though admittedly it is not overloaded or used in extreme conditions. I do reccommend sticking to the designers spec....

  31. #31
    Join Date
    Dec 2011
    Location
    North East England
    Posts
    1,387

    Default Re: Rule of thumb for plywood thickness

    Quote Originally Posted by Bob_Hinely View Post
    Nice thread, I've been going back and forth on this. Planning on building an Argie15: no sails, outboard only. And yes, I've looked at (and purchased) quite a few skiff plans but this one speaks to me. Scantlings call for 1/4" ply for basic hull, but I'm considering using 3/8" due to outboard use only and the water here in coastal Georgia can kick up a bit. No centerboard, etc. Will add doublers on transom for support. And maybe a little weight forward to keep the nose down. Weight will increase, but that's expected. Still in thinkin' stage. But the Selway-Fisher Cape Cod Dory skiff keeps tugging at me, too. Too many boats, not enough time. Age old story.
    1/4” to 3/8” will double the panel stiffness with a 40% increase in weight, but only a 16% cost increase. All very interesting stuff.

  32. #32
    Join Date
    Sep 2017
    Location
    Vilnius, Lithuania
    Posts
    53

    Default Re: Rule of thumb for plywood thickness

    Quote Originally Posted by Bob_Hinely View Post
    Planning on building an Argie15: no sails, outboard only. Scantlings call for 1/4" ply for basic hull, but I'm considering using 3/8" due to outboard use only and the water here in coastal Georgia can kick up a bit. No centerboard, etc. Will add doublers on transom for support.
    Hi Bob,
    My first built boat was Argie 10, similar shape to Argie 15, so some experience may be relevant.
    Bottom panels were to be strongly twisted in forward section, whish means, that thicker plywood may be much more difficult to shape properly, especially if you are going to stich and tape. If you would build on temporal frames, you can incorporate permanent longitudinal chine stringers - for stiffness and for ease of forming thicker panels.
    For side panels I would advise to stick to original specified thickness and then cover outside of the boat with glass fiber.
    You can also look at threads for Cape Cutter 19 building - the issues of twisting thick forward ply panels were discussed there.
    Regards,
    Jonas

  33. #33
    Join Date
    Dec 2011
    Location
    North East England
    Posts
    1,387

    Default Re: Rule of thumb for plywood thickness

    Just done a rough calculation
    if you had a 6mm thick panel 1meter wide and reduced the thickness to 4mm the panel width would need to be reduced to to 0.73meters to keep the deflection the same.

  34. #34
    Join Date
    Feb 2009
    Location
    Australia
    Posts
    358

    Default Re: Rule of thumb for plywood thickness

    The blanket dismissal of classification society's scantlings rules as being "orientated for racing yachts... and not for more hard use boats" doesn't sit well with me, either. I regularly use ABS and DNV Rules in my design work, and have used Lloyd's on occasion, and found that, if anything, they tend to be a bit on the heavy side (ABS especially). I find that the afore-mentioned Herreshoff method to be on the light side for offshore work. One of the 'black arts' that us designers who do this sort of stuff for a living learn to do is to determine what set of scantlings rules is best applied to a given project to result in the best hull for the application, and still be able to prove to insurers that the scantlings are based on valid calculations. Scantlings aren't a subject that can be distilled into a 'one size fits all' series of calculations; experience and rationalization of the varying parameters must be applied to achieve an optimum result.
    I totally agree with you on this MMD, especially when it comes to insurers. Better to engineer to set of rules than simply guess... and hope for the best. Insurers love finding ways to avoid paying out. Its their job (LOL)

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •