plywood thickness to size craft ratio

Hi. I'm new to this forum,
I just finished building a tortured ply out rigger canoe of my own design, 20ft long 16 in wide. What I want to know is if I decided to build the same design for a catamaran of 40ft, what thickness ply should I use, I used 5mm ( double 2.5 mm sheets), for the 20ft and it is stiff and strong,
Is there some rule that helps decide size and strength of wood for differing sizes of boat.

 

There's no rule to determine an ideal thickness for a given size, only what comes out of an analysis of the design in terms of intended use. Different plywood species have somewhat different strength characteristics too.
Shape also comes into play. A conical shape has an inherent stiffness compared to a flat panel. Spans between supporting frame members matter.
However, rule of thumb and experience play a large role so it pays to see what has worked in the past, what other similarly constructed boats built for similar use have been built with.
One could say that 5/8" fir plywood would work well with many 40 ft designs. but 1/2" okoume might be okay on other 40 ft designs. Multihulls are built much like monohull racing boats--- light and stiff and highly engineered. The designer therefore considers a host of aspects such as bulkhead spacing, skin thickness and material, and seam taping schedules as interrelated.
Multihull designers come closer to aircraft designers than any other type of boat.
A 20 ft boat is a fun place to start designing since it won't be called on to cross oceans or to cruise for long periods of time. But a 40 ft boat needs a designer who is also a talented engineer.

 

thank you Alan, for nice reply and info, yes I am thinking more along the lines of monoque design, here in the philippines the local boat builders make outrigger boats of all sizes, watching construction of one 40 footer, I was not impressed, they use a lattice work of frames and stringers and cover with 1/4 inch ply.
Surely 5/8 or 3/4 inch ply with a lot less frames would be stronger and lighter.

 

Not necessarily lighter, but 1/4" seems way too fragile for a 40 ft boat. A minimum of 1/2" plywood would be my thinking. You can make a boat stiff enough with light scantlings and lots of framing, but you need toughness too. You can only have toughness with a thicker skin. I recommend that no matter what you do, you pay a good multi designer to assist you when the time comes to build the bigger boat. At a minimum, see how successful designs have been built.

 

plywood thickness

Hi Alan. Yes I agree in some ways, but although I am truly an amateur, I have built and modified a lot of boats, but my interest is not really in building, but in designing and finding new ideas, and I do not assume any boat I build will be capable of long distance cruising, or weathering heavy sea conditions, no I seldom go farther than 15 miles offshore and at 63 yrs old, now if it's rough I stay at home (different when I was young) because for me sailing is for pleasure now not adventure, if you make me consult a marine architect, you will be destroying my hobby and my interest, I don't make boats for other people to use, only myself, and try then out very cautiously and carefully over a long term, until I am satisfied they are safe and workmanlike.

 

I understand, and I'm that way myself. My recommendation was a standard one. It might not apply but it was offered in case it might save someone from some misery.
Your way of designing has merit. The holistic or Gestalt approach, you might say.

 

You are describing tortured plywood or stressed skin construction here.

It does not scale up linearly afraid. The Geougeon brothers of WEST SYSTEM RESINS did some experimenting with it .It`s in their book " GEOUGEON BROTHERS ON BOAT BUILDING.

You can approach this another way :

http://www.google.com.au/url?sa=t&s...lv26BA&usg=AFQjCNGKlNdVIB0uLm2vGsO3eR6VL1KB_A

http://www.google.com.au/url?sa=t&s...lv26BA&usg=AFQjCNG4yULhLE8WCzqCjcVebs4lf528Pg

Hope this helps.Good luck.

 

May I suggest that if you wish to torture ply, then do it in thinner layers, such as diagonal /tripple skin laminate. The ply works so much better, and the end build is not under tension anyhow as the layers lock each other up. Thick single tortured ply is not such a good idea. It is also very hard to do as a sheet of ply only really bends on ONE plane...hard to believe I know, but it is true, the torture method tries to break this simple rule, but it can only succeed in limited areas.

 

Cold moulded ply is the name we use here in Aussie land.

 

I think it does scale up linearly, I don't see why not, but there are other factors at work here than just length. The stresses of sailing are quite different from the stresses of paddling. however, I routinely use 3 mm for my canoes, around 12 ft, and they are stiff and tough enough for their purpose, flat water recreational paddling. based on that, 5 mm for your canoe sounds about right, and it worked. There are a lot of kit boats in the 16 ft range out there that use 4 mm, so there does seem to be a linear progression at work here.

Let me put forward my own (monohull) sailboat, which is only 10 ft (car topper). The designer suggested 1/4" ply sheers and 3/8" bottom. After consultation I reduced those to 4 mm (5/32") sheers and 6 mm (1/4") bottom, because my method of construction imparts additional stiffness from scantlings and I also use closed box construction.

A monohull sailing boat has to withstand forces from the mast, rigging and centerboard, these forces necessarily pass through the hull. For a catamaran these forces are primarily distributed through the aka or central frame. The hulls (vaka) can probably be more lightly constructed than a monohull. However, applying the more or less standard canoe practice of 1 mm/4 ft to a 40 ft craft suggests a skin thickness of 10 mm. That would be the bare minimum

So I agree with Alan that you would be taking a chance using 1/4" skin thickness. You might get a wickedly light and fast boat ideal for racing, but you will have to do as serious racers do and pull it out of the water after each race and thoroughly inspect and maintain it. Whateveer you do, a few water tight buikheads would be advisable!

As far as home designing is concerned, I have to admit all my canoes are my own designs, and the first 2 were pretty useless. But I learned from them and I think I understand the business of canoe design well enough for my own purposes now; at any rate my canoes look good and are fast, stable and light. But when it came to a sailboat I did not have a feel for sailboat construction and scantlings or the forces involved; I got a lot of insight (and help) by buying a plan directly from a designer. However in your case, working on a new design the charges are likely to be a lot more!

A 2:1 size increase is a lot to undertake in one step without theoretical knowledge and training, and an intermediate boat size would be a sensible precaution for a do-it-yourself designer. As well as size, also consider the application. For example, if your smaller boat is for day use on a lake and your bigger boat is for serious ocean cruising, for example, far more is involved than a simple size increment.

 

I was actually referring to tortured plywood. And I certainly do see " why not ".

You will find that the method quickly reaches its limits .Go beyond that limit you will no longer be able to compound curve the plywood without severely limiting hull shape.Try compounding 10 mm plywood !!!!!!!

If you don`t believe me , search out results by Geougeon Brothers.The brothers proved that and published data on it.

Bingo Landlubber !

They used (thinner) plywood and cold moulded additional layers inside the hull where more compounding was required.
You could do that with biax glass or plywood , or both .By their`e own admission they themselves stated that Tortured Plywood is " hit and miss " seat of the pants" boat building.

Furthermore , they found that you can actually get two distinctly different hull shapes with plywood of identical thickness , but different species .
Some species bend better ( more ) than others.The " bendier " ply gives more displacement due to more semi - circular section.The stiffer plywood gives a more " v" shaped hull , less displacement. Test panels will give a reasonable estimate.



Doubling the size is bound to result in an inferior or even dangerous design , for the reasons you pointed out.

 

Good point about torturing ply; I was thinking ITO just a simple bend.

While I am only a wannabee boat designer I am an experienced engineer and I think I understand the mechanics involved. With linear scaling the bending forces involved should increase as the square of the linear dimension, since torque increases as the 4th power and increasing the bend radius and the length each reduce the force required by 1 power each. The material stresses remain the same.

So IMHO scaling up a design using the same factor for dimensions and skin thickness should result in a buildable design. Whether it is still a good design is doubtful as you say. Obvious by increasing thickness alone, one will rapidly encounter the material's stress limit.

It is hard to understand why ply with a stiffer wood species would give a different shape except for a particular class of compound bend when the ratio of stiffness in the 2 planar axes is different. For an isotropic material the combined twist and bend of a typical boat plank most often combine to form a rolling bend with a smoothly varying predictable axis and zero stretch or compression except across the thickness. I am only addressing hard chine hulls here, by the way; obviously anything remotely spheroidal is another case entirely. Sometimes I use that predictability when I am cutting a rolling bevel along a chine to get a better fit; the data can be teased out of Free!Ship - play with the resolution in the perspective view in Shade mode and it will emerge as a moire pattern.

Sometimes I encounter cases that require the sheet material to stretch and compress along its length, producing a visibly convex or concave shape. I am not speaking here of the optical illusion that makes a plank look concave just because of the viewing direction - the familiar "hollow entry" - of course. The material adopts a saddle shape.

I have found Okoume and Luan ply bend well, Baltic Birch not so well but it is not a good boat material anyway.

 

Yes , to be fair , I should have added that the Geougeons also mixed plywoods of different "construction " that is to say that some was 3ply
and some the preferred 5ply.I would never attempt using 3 ply for compounded plywood.

Plywood quality would also be of optimum importance. any slight void on inside or centre plies could cause failure of course....Lots of variables....

 

Found that out the hard way: use Luan door skin for the first boat, lovely stuff, 2 good face veneers no sign of voids. Sheer luck. I kept trying to find more decent stuff, gave up after 2 years and ante'd up for the marine ply.

No comparison, but also x5 price. I shopped around and got lucky again; enough for a couple of years production at half price - slightly water stained but perfect otherwise and 90% has no stain at all. Now if I could just do that with epoxy and clean lumber ...

 

Yep....