How far will the plywood bend?

[GRussell]

Plank is 6mm okume, 3 1/2 wide, including 5/8 lap. Transom is cherry (end grain to the planking fasteners). The gap is about 1/2 at its deepest. The concavity starts about 15 inches forward of the transom. I have a feeling that there is more to this than just squeezing the heck out of it and gluing and screwing it into place. I could use some sage advice.
Thanks in advance
Guy

[Gib Etheridge]

Have you tried it with a scrap? I think it will take the shape, but only if you place a shaped pad between the plank and the clamp to avoid point loading. What boat is it?

[Bob Cleek]

This is one of the problems inherent in using plywood as a replacement for natural wood. Plywood will not take a compound curve, which is what it appears you have there, albeit perhaps not a severe one. You can only bend plywood in one direction at a time. Here, you have your plank bending to follow the curve of the hull in one direction and now you want to bend it in the other direction about ninety degrees to the first bend. You are trying to get the plank to bend across its width, rather significantly, actually, for a length of about fifteen inches, while the plank is already bent in the other direction along its length. Fortunately, the run is fairly flat aft, so you might make it. Moreover, you haven't notched the edge of your transom to accomodate the lap, resulting in an unavoidable hole created by the lap of one plank over the other. (Maybe somebody figured you could just fill the resulting hole with epoxy?) A natural wood plank could be steamed and bent into shape, but not plywood, in which the grain in each layer runs at 90 degrees to the layers on either side of it.

You can try to force it into place and hold it there with clamps (and a shaped forming pattern block as suggested above) and then add screws until the googe hardens. This is a very questionable approach, since you are trying to fasten the plank into the end grain. As it is cherry, it is possible that it will hold a screw under that sort of pressure, but screwing into end grain is NOT done because end grain does not hold fasteners. (Then again, neither does epoxy hold as well on end grain.) (There should be a "fashion piece" around the inboard edge of the transom with its grain running 90 degrees to the grain of the transom planks into which the plank fastenings are driven.) This is called "torturing" the plywood. It MAY work. It MAY not. If it doesn't, blame it on whoever decided to use plywood plank a boat with a shape like that.

Another possible solution is to run three, four or more saw kerfs along the length of the outside of the plank for as far as the bend you want is supposed to run, with a depth of about three quarters the thickness of the plywood plank. This will "corrugate" the plywood, allowing it to bend more easily. Once the plank is in place and fastened at the transom, you can fill the kerfs with epoxy and sand it fair. This is a half-assed way of doing it, but perhaps it promises as much a chance of success as anything else.

I'm interested to know if the boat was designed to be built this way or if you are just going it alone. If this was the way some plans or kit said it was supposed to be done, something's very wrong.

[Lewisboater]

I think it would be better to kerf the inside rather than the outside. This will place the outside skin in compression rather than tension...which has never worked for me very well. Better you take a piece of scrap the same size and try it each way to see which works better for you.

[Gib Etheridge]

If it fails it will show longitudinal cracks on the inner face. A trick for dealing with that will be to apply 2 oz. FG cloth in epoxy to the inner face for the last 15" or so. This will make it somewhat harder to bend, but should protect the inner face from fracturing under tension. You'll know once you try it with a scrap.

[boatkid]

I don't know much about this, but.... If it's a lapstrake, shouldn't the stem/stern be rabbited for the strakes? It appears that if you 'let-in' the forward portion of the strakes, then isn't the issue is pretty much solved? Again, I'm no pro, but if it was mine, I would saw the glue joint at the stem/stern, rabbit it for the strake, ' let it in', then fit the next strake by repeating the process. In my mind, when viewing the stem/stern from the front, it should almost take on a herring bone pattern like you see on those fancy teak decks. Am I missing something?

[andrewpatrol]

What about glueing some extra wood to the transom to take some of the curve out, provided you 're not finishing clear that is? Also if you kerf the surface of ply you could put hot water on opposite face to help it bend, then clamp to let it dry then glue

[TerryLL]

This will never go as is. If you try to crank the plank home the screw heads will likely pull through the plank. The problem you're facing is exactly why the plank lands on a properly designed ply-lap hull are flat. The ply stock you're trying to bend on is not appropriate for the design. Another problem I see here is the short end grain at the heel of the transom, which can be kept from splitting with a knee on the inside.

At this point it's reasonable to consider either a different design or a different planking material.

[Oldad]

"You cannot ben plywood in two directions..." yet I have done it many times. Tortured plywood. Saw kerfs would help. Okume bends easier than meranti so that is a plus. Do you plan to finish the transom bright?
It will take more than epoxy to hold that in place. If you kerf the plank you will have to use fairing compound to achieve a pleasing line.

Oldad wondering what design and what info came with the plans.

[Mrleft8]

I think the shortgrain Cherry transom is going to snap before that plywood bends to that shape.
That said... Keep the outside face very dry, like hair dryer dry, and the inside face very wet, like swabbing it with a wet sponge wet, while you're clamping it into place. You probably ought to do both sides simultaneously to keep the transom straight..... If it doesn't snap anyway.....

[TerryLL]

I've had some success steaming Okoume, but with that amount of gaposis I do believe steaming will not get you there. Worth a try perhaps.

[Sayla]

I've had some success steaming Okoume, but with that amount of gaposis I do believe steaming will not get you there. Worth a try perhaps.

Totally, definitely steam - and on scrap first at that - too much work to go and break a good plank - and cut a male bearing block to shape, for under the clamp - if you don't have steam then boil the end in a big pot in the kitchen for 15 minutes (you'll get to find out if the glue is truly marine grade too!).
As mentioned, it is a compound curve, but not very much curve fore & aft - lucky, as the new scoop shape will also try to resist the plank bend in the fore & aft direction.

sayla

[Wacoflyr]

If you look at some of Joel White's designs, he does this very thing at the transom. Check out the garboard of the Pooduck, for example. However, the bend in that design is nowhere near as extreme as this, and it was difficult to make it look good. I do not believe that this can succeed, except maybe by the saw cuts mentioned above. Obviously, we all want to see an after photo!

[upchurchmr]

Bob,

I sure wish people would not continue to say plywood cannot be compounded. I had a Tornado that was completely compound plywood, but it was 4mm not 6 like this example. I don't think you can bend 6mm in that tight of a curve whether or not it is compound curved. However if you got 2 - 3mm pieces of plywood and offered up the unit with epoxy between the two plys it certainly would for and cure to the same strength as the 6mm piece.

Gougeons must be pretty sheepish thinking they compounded ply on multiple boats. BTW their book is on the internet free digitally, perhaps you could review how to compound curve plywood.

This is one of the problems inherent in using plywood as a replacement for natural wood. Plywood will not take a compound curve, which is what it appears you have there, albeit perhaps not a severe one. You can only bend plywood in one direction at a time. Here, you have your plank bending to follow the curve of the hull in one direction and now you want to bend it in the other direction about ninety degrees to the first bend. You are trying to get the plank to bend across its width, rather significantly, actually, for a length of about fifteen inches, while the plank is already bent in the other direction along its length. Fortunately, the run is fairly flat aft, so you might make it. Moreover, you haven't notched the edge of your transom to accomodate the lap, resulting in an unavoidable hole created by the lap of one plank over the other. (Maybe somebody figured you could just fill the resulting hole with epoxy?) A natural wood plank could be steamed and bent into shape, but not plywood, in which the grain in each layer runs at 90 degrees to the layers on either side of it.

You can try to force it into place and hold it there with clamps (and a shaped forming pattern block as suggested above) and then add screws until the googe hardens. This is a very questionable approach, since you are trying to fasten the plank into the end grain. As it is cherry, it is possible that it will hold a screw under that sort of pressure, but screwing into end grain is NOT done because end grain does not hold fasteners. (Then again, neither does epoxy hold as well on end grain.) (There should be a "fashion piece" around the inboard edge of the transom with its grain running 90 degrees to the grain of the transom planks into which the plank fastenings are driven.) This is called "torturing" the plywood. It MAY work. It MAY not. If it doesn't, blame it on whoever decided to use plywood plank a boat with a shape like that.

Another possible solution is to run three, four or more saw kerfs along the length of the outside of the plank for as far as the bend you want is supposed to run, with a depth of about three quarters the thickness of the plywood plank. This will "corrugate" the plywood, allowing it to bend more easily. Once the plank is in place and fastened at the transom, you can fill the kerfs with epoxy and sand it fair. This is a half-assed way of doing it, but perhaps it promises as much a chance of success as anything else.

I'm interested to know if the boat was designed to be built this way or if you are just going it alone. If this was the way some plans or kit said it was supposed to be done, something's very wrong.

[JimConlin]

Plywood can be compounded, to a considerable degree. See the Gougeon chapter 25.

This is the transom of my 'Liz' pulling boat. It was planked with 4mm. okoume.

I doubt that you could pull 6mm. okoume down into that curve.

The only way I could think of to get that plank done on the OP's boat would be either to use two plies of 3mm okoume scarfed somehow to the 6mm plank farther forward or kerfing which would take some fairing and maybe a little glass. I think it'd be easier to do on the outside.

As lefty mentioned, I'd worry about the short grain across the 'wineglass stem' of the transom. It needs supplementary reinforcement cross-grain. Maybe drilling up through at least 8" of it for a 'drift' of maybe 1/4" all-thread set in epoxy. Or maybe a routed dado on the inside hidden behind a knee. All that cross-grain joinery will need careful encapsulation to survive.

[mick allen]

If you gain or rabbet the garboard (or the pair) to nothing at the transom , the 13mm gap (1/2") is reduced by the 6mm thickness to 5mm instead of 13mm - big difference to the issue. A little heat or steam may do the rest plus there's no gap at the lap anymore. If heat doesn't work alone, the inside slicing may make up the rest of the gap.
Instead of saw cuts, use a depthcontroled razor knife to reduce the apparent cut width appearance inside the boat.

an alternative, anyway.

[mcdenny]

The transom really needs flat lands to take the plywood. I'd epoxy on little crescent shaped pieces to create flat lands. If you care careful with grain direction and use some of your cherry scraps you could still have a good looking bright transom.

[larsmann]

What about 2 layers of 3mm stock? The thinner ply will probably make these bends, and you'd end up with a "double-planked" hull, so to speak.

[Yeadon]

What about 2 layers of 3mm stock? The thinner ply will probably make these bends, and you'd end up with a "double-planked" hull, so to speak.

I like this idea (though honestly I really haven't read the entire thread).

I'd consider cutting the plank off a couple feet back, then scarfing on the thinner pieces for the final run at the transom/skeg. You'd put on one, get it to fit, then laminate the second piece on there. After a bit of fairing, no one would ever know.

Yeah, it'd be easier to to flat lands ... but the transom will look super sexy as is.

[Mrleft8]

Well...... I just took a 3" wide by 3' long piece of 5 ply D-fir plywood and cupped it to a 52" radius form at one end, and around a 52" radius form along it's length.... Not much trouble doing it either. I still have doubts about the shortgrain transom holding up, but it sure looks like the bend is doable...... I'll take a picture in a few minutes.....

[kc8pql]

Have a bandsaw? Split the end of the plank, in effect giving you two 3mm thick pieces. Cut back maybe 2/3 of the way to the last frame and try the bend. If it works, take it loose, apply epoxy into the kerf and fasten it in place. That short grain on the transom bothers me too though.

[Gerarddm]

I think the end grain is a deal killer. I'd redo the transom entirely so that you were screwing into something that will actually hold.

[Mrleft8]

I got curious, and decided to really give it a serious test..... I cupped the ply to a 10" radius, and a 52" radius lengthwise curve.....

[Mrleft8]

Huh...... But it seems that all of a sudden, I can't post a picture?......

[Mrleft8]

[Mrleft8]

[Yeadon]

Interesting idea ... first dry-fit the plank, then pre-bend it before installation.

[JimConlin]

Have a bandsaw? Split the end of the plank, in effect giving you two 3mm thick pieces. Cut back maybe 2/3 of the way to the last frame and try the bend. If it works, take it loose, apply epoxy into the kerf and fasten it in place. That short grain on the transom bothers me too though.

That's a really good idea, elimination some hopeless complication. Dunno if you could do it, but a patch of glass cloth dragged into the kerf might stiffen it back up.
I'd drill through one of the 3mm plies near the head of the slice and inject epoxy. Only way to be confident that he joint is wet.



Lefty- Don't tell Cleek. A guy needs his illusions.

[upchurchmr]

Remarkable demonstration of compound curvature. There still may be practical problems which everyone is talking about. It certainly would take cauls on each side to allow clamping similar to what you used. I wonder about cutting the transom back on the sides with the endgrain and laminating board(s) on the sides to spread the fastening/ gluing loads and reinforcing the grain. It actually might visually enhance the shape of the transom, assuming you will clear coat it.

I may have to try this myself just to really learn the lesson.

[AJZimm]

I faced this exact same problem in the same location with Hornpipe, except that I was trying to cup 9 mm okoume. I couldn't do it, even with steaming it. I ended up doing what Yeadon suggests, using, in my case, 3 layers of 3 mm ply for the last 3 ft of plank. I used the boat itself to clamp the whole rig together while gluing and it was quite a lashup, but it worked. Here it is after laminating and trimming and cutting the gains

[pumpkin]

I can't believe how clean your workshop is! I envy how clean it is.

[chuckt]

I find it inconceivable, with a title such as this thread has, that Bruce, aka Wizbang, has not chimed in.

[Mrleft8]

I can't believe how clean your workshop is! I envy how clean it is.

I've been working like a slave to neaten it up enough so that Jim Ledger doesn't have a conniption fit in 2 weeks, when he comes here for the EBS....

[GRussell]

Thanks to all for the ideas. I tried just squeezing it in dry and that did not work. The test piece split a couple of ways. I cut a series of kerfs as deep as three plys, filled them with goo and clamped it in place. It shows promise. Next, I'll try some heat/steam and see how that goes. I'd really like the plank to take a set so the screws don't have to work so hard.
The design is Ken Basset's LIZ, designed for conventional planking, so I guess you could say I brought this on myself. The transom is supported with a generous knee so I think it will be OK. Thanks again to everyone. I'll keep you posted.
Guy

[Mrleft8]

Even with a knee. Each fastener will also act as wedges, and contribute to the problem. A knee as shown, with a cross ways grain will crack the shortgrain even faster with expansion/contraction.

[GRussell]

We have a winner! Steamed two test pieces for more that 1/2 hour (got to 203 degrees, which is as good as it gets at this altitude). Clamping was a nightmare, but after 24 hours, they seem to be holding their shape.

Also tried cutting kerfs with a back saw and filling them with goo. Worked ok but I got a split on the back side (one kerf was a little too deep) and the facets on the front side may have been difficult to smooth out.

Anyway, thanks again for all the suggestions. I'm never disappointed with the advice you folks provide.

Guy

[Mrleft8]

Pictures please........ If I can do it, so can you....... Come on........

[GRussell]

Here ya go.



I wish someone had noticed that the gains are wrong. In my zeal to hang this plank before my vacation (New Your for the grandkids and CT for the Woodenboat Show. yippeee!) I cut the gains, mounted the plank, only to discover that while the outside looked fine, the inside edges of the planks do not meet. The garboard gain needs to be beveled (an easy fix), and the next plank should be beveled on the inside from the full thickness at the start to half the thickness at the transom. So it looks like I'll be lopping off the last three feet or so and scarfing on a new piece to have another go. I'm a dope.

[Mrleft8]

Looks promising, nonetheless. That little lump may be problematic, but who knows.... I'm still leery of the shortgrain transom, but stranger things have worked out just fine, despite what common sense would dictate....
Hope you can make it to the EBS while you're here for the show.

[GRussell]

The photo is of the test pieces. I eased the edge on the mold and the lumps went away and the planks were fair. I didn't think about the short grain until you mentioned it, and I probably would have done it differently if I had. I'll just have to keep an eye on it. Leaving tomorrow. What's an EBS?

[Mrleft8]

"Elbow Bending Symposium"...... (AKA party ) If you look in "people and Places" you'll see a thread titled: "Margo's WBS EBS" or something like that.....

[gordocutter]

I bought the plans for the Cape Cutter from Dudley Dix, and it has the same problem in the stem. The solution in the plans is to cut some scarfs in the plywood in the critical points so it can be more flexible and once in place you fill the gaps of the scarfs with epoxi.

[gordocutter]

I forgot to mention, you have to cut just one or two layers of the plywood (depend on the thickness) no all of them.