Plywood vs laminated strips for rudders or center boards?

[Greg Luckett]

I saw where Charlie and Laura had trouble with the plywood dagger board (I think that was it?) breaking during the Texas 200, and then Charlie spoke of the merits of the laminated, and then during a phone chat with Graham he mentioned that the laminated type is superior to plywood too.  My question, now that I have thought about it some more:  Why??

I have built both rudders and dagger boards with plywood in the past without any problems, and see today that Joe Nelson did the same for his CS20.  Since plywood has layers at cross grain to each layer, that makes it stronger than a simple laminated strip construction, in my way of thinking.  I do fiberglass the ones that I have built.

[John Burritt]

Plywood centerboards MAY warp, or (as in your case) MAY NOT.  Properly constructed laminated boards won't.  We've been doing this in Graham's boat class "forever."  Take a piece of plain sawn 2x fir and cut it in strips whose width is just a bit more than you want the thickness of the centerboard.  Turn the strips on edge, reverse every other one end for end to reverse the grain, and epoxy them together, making sure that you keep it flat.  Now your finished board is made up of reversed quarter sawn strips.  Now shape the board and you'll have one that not only stays flat, but looks great.  I've never seen Graham do this with a rudder, but there's no reason you couldn't.  Rudders aren't as apt to jam.

I seem to recall hearing that Charlie got pushed sideways into something.  Any board will break if you put enough side force on it.  Building one the way I described above will be as strong, and sure won't jam in the trunk.

[Greg Luckett]

John,

I do understand the laminated board construction and still appreciate hearing more about it from you, so thank you.  What I am puzzled about is why the plywood ones warp and the laminated ones do not?  It could be a size thing, both thicknesses and length X width.  It may be the way that I build with plywood that makes the difference?  The last one I did was a rudder for a Catalina 22.  I do not simply use a factory panel and then cut it down, but take two factory sheet thicknesses and laminate them together.  Examples:  2 6mm thickness to make 12mm, or two 3/8 to make 3/4 thick, etc., then I shape it, then I fiberglass it.  I have never had a warping problem with the several that I have made over the years.  I always use the best quality plywood that I can find too, which today are the marine grades.  This is only good for painting though and do not look as pretty as the laminated strip ones do for finishing bright.  I was at a local marine supplier and saw plywood ones which were finished bright, but they were single thickness....not the way I like to build....but they looked nice.

I plan to use the laminated strip type on the Spindrift I am building....it looks way too pretty not to use.

Incidently, is it just me, or is plywood warping a lot more than it use too?  It seems like it to me.

[Greg Luckett]

John, I am envious that you can attend Graham's classes. 

Much of what I do takes a lot more time for me to figure out what should be done and in what way.  If I can see something that is already made and/or being made, I can usually understand faster and duplicate it better.  That being said, I use the next best things, such as books, pictures, emailing/forums, and phone calls.  I appreciate that you are helping me to understand this topic.  Thank you.

Greg.

[Tom Lathrop]

Greg,

You still need the answer to one of your original questions on relative strength of plywood and solid lumber.  The cross laminations of plywood give it nearly balanced strength in the direction of both laminates while solid lumber is optimum in the long grain direction and minimum in the cross grain direction.  Plywood strength is between the two extremes of solid wood. This makes solid lumber superior for centerboards and rudders while plywood is superior for hull planking.  It just depends on what kind of strength is needed as to which is better.  When you are standing on the end of a centerboard trying to right a capsized boat, it is comforting to know that it is made of solid wood and not plywood. 

The tendency for solid wood to warp is minimized by the vertical lamination and alternate flipping of each laminate orientation.  The natural tendency of an individual piece to warp is minimized by the canceling effect of flipping adjacent pieces.  Plywood can still warp, especially if it is subjected to any unbalanced force or moisture for an extended period of time.  Inferior plywood can warp with no apparent cause.  We are experiencing a lot of this in Chinese plywood right now.  Awful stuff.

I recently built 12 daggerboards and 12 rudders for Optimist prams from plywood.  Some of these show some warping and some don't.  All are usable and the force on them from sailing and the weight of small sailors when capsized will not be a problem.

Incidentally, John does not "attend" Graham's boatbuilding class.  He is an unpaid teacher (except when substituting).  I did this for many years until John came on board.  Both of us have benefited greatly from this association.

[Greg Luckett]

Thanks, Tom, for the explaination, which makes better sense to me now, thanks both to you and John.  Incidently, I was thinking of "attend" as not being just as a student, but as anyone at the class.  It is good to know John's function in the class, as well as yours.  I am still envious.  It would be great to have a DVD similar to what the Stevenson's put out from you folks.

Anyway, thank you.

Greg.

[John Burritt]

Greg

I agree with Tom that plywood is getting more and more "interesting."  However, I don't think the Chinese have a corner on this.  For example, five equally thick laminations suddenly aren't.  Maybe the skins are thin, maybe thick.

Going back to the glued up centerboards, the only failure I've seen with the ones we have done in class was with one that spent too much time in a hot car while only partially shaped - split on one of the glue lines.  Added a small insert & reglued it - good to go.

Working with Graham in class has been a great experience.  Gives you the opportunity to gain experience on lots of different designs.  Different boats go together lots differently, even though they're all "stitch & glue."  

You're absolutely right, seeing it done first hand is hard to beat.  Even after all this time, though, I'm still finding new and innovative ways to "create corrective opportunities."

John

[Tom Lathrop]

John,

I should have been more clear which plywood I was talking about.  The really bad stuff is some 3/4" material labeled as birch that we used for the furniture that we built for the county Heartworks recreation center.  It had 11 laminations and still warped badly.  Some other similar plywood that George Duffy bought for about $70/sheet and claimed to be high quality had similar problems.  Also 11 laminates.  I am not really sure where either of these came from so maybe it was not China but Southeast Asia anyway.  Buyer beware.

[Charlie Jones]

Actually we didn't get pushed sideways- we DRIFTED sideways and grounded so softly we didn't even feel the board break- we SAW it floating out to the side and Laura realized it was our board. This on a boat that MAYBE weighs 400 pounds with nothing aboard. I would guesstimate we were making 1/2 knot sideways. Snapped that sucker flush with the bottom of the boat hull.

The only reason I had a plywood dagger board is that I built this boat from a kit, in a short period of time and the board came with the kit. We were rushing to get 4 boats ready to show in Strictly Sail, Chicago. Had I had the time originally I'd have redone it in solid strips.

The new one is FOR SURE done that way

[Greg Luckett]

I am probably not as shocked as you and Laura were when it happened, but it shocks me that it broke so easily.  What did the laminations look like along the break? 

Darn it, I wish I had made it to the mess about.

[Ken_Potts]

  Plywood warps differently than it used to partly because of the way they peel the veneers.  They essentially chuck a log up in a lathe and then shave a contiuous veneer (kind of like a pencil sharpener).  In the past they would start with a big log and peel it down to a little log and process the remainder into something else.  Because of the skyrocketing cost of lumber cut from big (old growth) logs they are now cutting the big logs into dimensional lumber and cutting the smaller second growth logs into veneers for plywood.  Taking the veneer from a smaller diameter log yields a material that really wants to bend back to its natural shape.

  Not completely sure if I'm right about that...

[Tom Lathrop]

Ken,

We used to live near High Point, NC, the heart of the old furniture industry which is now more likely in Indonesia.  Almost all plywood cores are rotary cut since it makes for a much more efficient use of the log.  Only face veneers of quality wood is flitch cut with a slicer blade.  The rotary cut lathes could not cut smaller than about 6" diameter so there was a lot of "heart of poplar" available for free.  Some people built lake cabins with these cores.  I don't think this has much to do with plywood warping though they may be able to cut to smaller diameter now. 

I suspect that the Asian mills are not drying the plies adequately before gluing them up and the result is unbalanced moisture content which results in warping as the plywood dries out.

By the way, the green heart of poplar is the most foul smelling wood imaginable.  Think of a full baby diaper.

[Ken_Potts]

  I don't know if I want to smell the Heart of Poplar cabins, then ;D

[Tom Lathrop]

  I don't know if I want to smell the Heart of Poplar cabins, then ;D

After it dries out, it is ok.

[Maria CS20 104]

Having played some with force vectors and materials being "distructively" tested for sheer strength, plastic deformation etc., on both on metals and wood, including V-notch testing on metals at different temperatures, I think (not really sure), if board is covered in epoxy ONLY, it's elasticity is reduced to side forces if a fault in introduces.  The minimal wear that can occur at the base of the centerbard well or dagger well as it moves sideways in the well and a little forward and backwards with different speeds can score the surface epoxy, much like glass cutting scoring along a very specific and fairly fine line.  I would think this scoring can be increased in muddy or sandy water.  With continued use wear can score it more deeply than suspected and with little applied side stress, the momentum and weight of the boat along with the board pressed against the square bottom side of the well like a fulcrum,  any sustained side pressure on the tip of the dagger or centerboard blade would make it easier to snap.  I have played with glass and it does not take a fully scored plate to cause it to snap on a straight line under the right circumstances, think straight pivot point at the bottom of the well.  I believe, could be absolutely wrong, but I think it is basic physics along with materials flexibility/ rigidity of the composition (natural or induced).  I also think a really good case for fiber glassing to increase wear resistance and cross sectional strength on a wide flat piece that is clearly subject to potential wear at a specific point.

Just an engineers thoughts ??? ???

Jim A

[Maria CS20 104]

Just looking at the pic provided it really looks similar to a brittle fracture in highly stressed metal, especially with straight edge and short fiber tearing in the board?

Ideas?

Jim

[Greg Luckett]

This discussion just gave me an idea.  What would the effect be if the strip construction used a V groove, male on one side of a strip, and female on the mating side of the next strip.  It would be a sort of tongue and groove and should be strong.  Biscuits or dowels would not be needed then either.

[Apteryx]

I have thought of mating surfaces, too, but I don't know if the grain orientation would be a factor.  The stress on the overlapping portions of a vee would be on the grain's weak axis.  Also, half the glue would be in tension, half in compression - I have no clue what that would mean for the strength of the bond.

Worse, I just bought a biscuit joiner.  I dread the thought of not being able to play with the new toy!  ;D

Here are a couple of other centerboard questions.  I bought Douglas Fir for my centerboard.  Is that suitable?  All I could find were 4x4s, so I've ripped them into 2x2s (about 1 3/4 per side).  I could stretch my clear-grain wood if I used rectangular, not square, strips for the laminations.  Understanding that the grain needs to flip-flop as depicted in Graham's plans, is there any problem with fewer strips in the lamination?

Also, I don't have clear-grain in enough full-length pieces.  Is there any problem with interspersing partial length strips between full length?

Any advice appreciated.  My boat building experience... lemme see, today's Friday...  I've been boat building for about 72 hours.  'Course a lot of that was just arranging stuff in the shop. 

[Greg Luckett]

You know, now that you mention it, I bet my boat building experience in the last 10 years is about 10 days, the rest moving stuff around too. ;D

[Maria CS20 104]

Greg, biscuits are a better choice because the diagonal stress is at 90 degree angle and although you may increase the surface area by 50% or so with V slot, you reduce the stress capability because it probably mates at close to 45%.  As others have noted, maximize the strength by  increasing the angle against the grain is maximized around 90%.  It is also dependent upon the relationship of the grain orientation.  Alternating the grain increases the strength and minimizes the potential to warp, does make the wood susceptable to rough surface when planning as I think I noted before.  Biscuits are fun and easy and provide additional strength points over a large join at 90 degrees and it does not take many spaced evenly apart.  I used a 3 by 2 alternating join with the biscuits.  Between first board 3 biscuits and next two, next three and on and on.  With the epoxy you should get a really strong join.  Concern I would have with a "V" cut is the slippage between the pieces when you try to press and clamp.  Even with biscuits, the boards will slip and it is really important to have FLAT surface to press pieces against.  I push and HOLD DOWN, even weight down sometimes against clamp bars, and then I am more able to apply higher consistant and constant pressure with the jaws of the clamps.  Again physics, minimize ability to misalign as pressure is applied and the work you will have to do to push hold down or pull in the strips to keep them flat against clamp bars.  (Trust me, it ain't never going to even or level, just do the best you can and it will work out).  It is not uncommon in my experiance when joining strips that they will "buck" as the uneven edges come together and are pressed together, usually because  I join edges in the rough (ruff), not necessarily true to square and they move, but taking time to square is very time consumming (eats up good wood too) and I do not think necessary when you are joining with thicken epoxy, and adding an additional 1/2" or more thickness to the board you are making.  Making the boards thicker (as far as I am concerned) makes it more more easily be planned to square and true, as I am sure you know.  Of course making strips as square as possible is best to prevent slippage when clamping, but you will still have to press the wood down with hammer, blocks and back, into the pipe clamps for flating and squaring as best as possible that you can, when max pressure is applied.  I use my son's weights to help keep pressed across the joins.  Does it always work, NO, but it will come close enough to make it good to plane in if you make sure you add enough thickness to allow thorough planning of any piece as you know.  I would always go with the biscuit joiner, read many acticles and data that indicate it works best for butt joins.  Choice you have to make is how close should the biscuits be, can't tell you a thing about that, I did mine at around 10 to 12 inches apart over a 5 foot work piece, like I said on a 3 -2 alternating setup.  And again, it is critical to get really good and I think thickened epoxy coverage with really good epoxy.  8)  Hope this helps, just more info from a dumb engineer.

By the way I am learning from stupid stuff I already did and had to re-do, NOT and EXPERT

an EXPERT is a "has been drip under pressure"  Usually with no real life experiance!

Jim Atkinson  CS 20  #104