Gerr Could be wrong this one time.

I am aware Dave Gerr no longer contributes to this site, a great pity. The Nature of Boats is my favorite.In boat strength there seems to be a contradiction, which goes like this:
There is nothing quite so good as plywood for decking, and it seems ok to plank up a transome with ply. However plywood is not recommended for planking a hull. There are many ply designs on the net. I think the ones i've seen are meant to be wood framed and ply covered. One manufacturer even make their keel and frames by laminating from their own wood stock. George Buehler recomends plywood Panels in 3 or more layers over frames on single or double chine hulls which he claims are easier to build.
I would use 9" planks at 45, 135, 225 degrees. I agree ply wouldn't do for a round bilge boat, but a Dory is a different matter.
Let me be clear I'm not having a go at D.Gerr I just disagree about plywood. If I was in the market for a boat he would be my designer of choice.
He states a plank of Fir has less than half the breaking strain of a similar plank in Fir plywood, I agree so what. Plywood is still better. If both planks were stressed along the grain the ply losses if the Fir plank were stressed across the grain it would lose. I don't know but would be inclined to bet that the Fir can take much less than the ply across the grain. There is no across the grain with ply.
The Romans made very effective shields from plywood. Vikings lost that technology and made their shields with flat straight circular grained boards with leather shield binding. they could be pierced with a spear or smashed with an axe relatively easily. Not so the Roman shield. Do we really need to relearn what the Romans knew 2000 years ago. A heafty wallop from a club could split a Viking shield down the grain. Do the same to a Scutum and you just make the Roman angry

 

Geer hasn't any real problems with plywood planking, though did and justly point out that it's weaker longitudinally, than solid wood of the same thickness. To further his point is an issue, not much discussed called rolling shear failure, within the sheet on highly loaded and/or flexed plywood. This is very common with appendages, where continuous movement in use, will cause a plywood rudder blade (as an example) to fail long before a solid wood strip planked version of similar dimensions. Yes, there is an "across the grain with plywood" and in this vein, plywood is a minimum of 30% weaker than longitudinally. This in concert with he being as aware as the rest of us, in regard to the quality of APA grades in recent decades, makes plywood less desirable than it once was. Lastly, plywood is heavier than solid wood of the same dimensions, so weight critical design choices need to be evaluated, to justify this choice. Don't get me wrong, I have lots of plywood designs, it like everything else has limitations, given certain criteria.

 

if the loads are all in one plane or direction, than plywood is not as strong as solid sawn lumber of the same thickness.

In places where the loads are in many planes or directions, than plywood is an excellent choice. A hull in a pounding surf can have loads in many directions, but in calm weather or rough, a mast will generally have only bending loads, so a solid sawn (or laminated with the grain all in one direction) would be stronger than the same thickness of plywood. that is why do not see plwyood used in masts.

 

I know that's not the point of this thread but I would like to comment on the following:

So compressive stresses on a mast are not important ?.
Anyway, I think the plywood does not work well under compressive stresses. In my opinion, the appropriate application for plywood is as panel, never as stringer nor pillar. But here I would like to know other opinions.

 

Again, plywood needs to be evaluated for application. A mast wouldn't be a good place for it, as 1/3 of the grain is in the wrong direction, just adding to bulk and weight. The reason you see it employed so much in small craft is simple; cost, area per unit, ease of machining and strength/stiffness to cost ratio.

There are lots of materials that are far superior than plywood, but instead of costing $50 a sheet it might cost $350 per sheet. Then there's the ease of machining issue. Sure a honeycomb or foam core panel is much lighter, but you'll have to skin them with fabrics, which require more fabrication, often careful fabrication, while a plywood sheet can be cut with about any tool, by any person with rudimentary skills. I run into this all the time and currently have had this conversation with a client this morning, for a transom replacement material. He wants inert (of course) to eliminate this need again, but when he gets the cost of Cossa, pour in poly and the other usual choices, he quickly understands why plywood is the typical choice.

It's not the perfect material, but is a very cost effective one, that can provide much of the physical attributes desired and this is hard to beat.

 

I agree compression on the ends of a plywood sheet would be its weakest point.
Parr I just discovered you have designed two new sail boats congratulations. I have never seen a prettier sailboat than the Rocky ketch. Rocky as you know is plywood covered so must also suffer sheer weakness which I did not know about. With a 13mm ply bottom she could be planked in 5mm fir because ply is 30% weaker.
The cold molding process is plywood created by the builder so that must also be 30% weaker in longditudinal stress and have the same sheer weakness.
Your answer has created a new question. How did you arrive at 9-13mm for hull bottom panels, and if Rocky's bottom were made up of 3 layers of plywood set at 45, 125, 215 degrees would it be stronger.
mik

 

These threads remind me of the three blind men describing an elephant. There is no mystery about relative strengths of plywood and timber planking. It has been long accepted that the strongest wood boat hull on a strength/weight ratio basis is a cold (or hot) molded veneer one. Plywood is not far behind and when dimensional stability is considered both are way ahead of the timber boat.

Its all just engineering to suit the occasion.

As for masts, there is (usually) little compression on a free standing mast and (usually) little bending on a stayed mast. Lots of room for variation though. Depending on the application, plywood can be very useful on spars, although mostly as connective webbing and not as primary members. Many wing masts have been successfully built in plywood.

 

Agreed, you have to work with what you've got. As a rule the thickness is replacing say solid lapstrake planking with plywood doesn't change. The replacement plywood isn't as strong longitudinally, but in many applications it would be a glued seam, which is inherently stronger. Even if it wasn't, it's stability is desirable, plus the wider widths that are possible too.

I've seen box section mast where the sides where plywood, with solid wood fore and aft staves. This is a heavy, but relatively stable spar, but not the best choice. As to the lightest wooden build method, I'd suggest the Lord method, which uses a fair percentage of plywood as the lightest, though more flexible than typical of wooden structures. It's really a core composite type of build, but can be amazingly light, though likely not a fair comparison to a true cold (or hot) molded veneer build.

 

not in the small craft unstated masts I build. Even so, swan lumber is still good for compression vs. plywood for the same reason; half the grain goes in the wrong direction in plywood.

End grain compression strength might be 2000 psi, but cross grain compression would be only 400 psi (sometimes less, depending on species).

so a mast in both bending and compression, would be better off with all the grain going all the same way, as in a solid sawn piece of lumber, or laminated so all the grain goes the same way.

 

Ok, in that case I agree. Thanks

 

Bending and Compressive Stress

A long beam (such as an unstayed mast) in bending stress will have compression stress on one side of the beam, and tensile stress in the other side. These stresses act along the axis of the beam. Shear stress at right angle to these stresses is generally minor.

With a stayed mast, the tension in the stay wires shows up as additional compressive stress in the mast, again acting along the axis of the mast (beam). The additional compressive load in the mast reduces the tensile stress on one side of the mast, while increasing the compressive stress in the other side of the mast.

So in summary, bending stresses in a mast (or any beam) are always evaluated as tensile and compressive stresses. Considering bending forces in any beam structure inherently involves compression and tension stresses, that is fundamental to stress analysis.

 

I think we're talking about compression along the axis of the mast, a load which compresses the mast in that direction. Of course, pure bending, resulting in tensile and compression, but that's not what we talked about, imo

 

Stresses

I think not, but I will run some stress numbers to find out.

In my previous post I was trying to point out that only the stresses along the axis of the mast are important, and that even with just pure bending there are significant compressive stresses existing along this axis. The other compressive stresses caused by stays in tension are in addition to the compression stress formed by the bending moments.

In these types of problems, a combination of forces are in action, and are evaluated by the engineering process of "superpositions", which is reasonably accurate for ridgid structures. Forces on the mast are caused by the force of the sail attachments to the mast, the boom attachment to the mast, and the forces from stays, plus boom weight, which is typically minor with respect to the mast stress condition. This is a simple model, many sail plans become much more complex, but the general method still prevails. All of these forces are balanced to the forces and moments at the base of the mast.

Every stress analysis has these factors at work, and depending on the particulars, some of them will dominate the result, others will be typically insignificant. Analysis of numerous boom angles must also be examined to assure review of the entire spectrum of load conditions.

 

I was always amazed that F1 tunnel boats survived so long as in that size it was hard to beat plywood I guess?
The requirement to have carbon kevlar safety cells pretty much said why bother with plywood

 

I think what Gerr was referring to was replacing planking in a carvel planked situation with plywood planks which to my knowledge has not been attempted except for small lapstrake craft .As for cold molding many John Wright yachts here up to 40ft were cold moulded with no stringers with 2 6mm layers of planking at 90 degrees .Now quality timber is not available they are defaulting to 2 layers of 10 -12mm plywood planking. Over 50 ft 3 layers Some boat designs call for 4 layers of 6mm for boats of 50-60ft but this is starting to get into composite territory as the resin and glass
is getting towards 50% of the hull So clearly the strength of plywood is taken into account