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jalmberg
02-12-2010, 10:14 AM
In the interest of time and money, I've decided to use marine plywood to plank my lapstrake dingy.

I've just cut out a pattern for the garboard, and when trying to bend it around the station closest to the bow (the point of maximum bend), the outside layer of the cheap 1/4" plywood I was using for the pattern cracked.

So, now I am thinking I may need to steam bend the marine plywood plank, so it doesn't crack...

I'm pretty sure the plank just needs to bend at this one forward station... the run is pretty straight right to the transom from there.

Question: can I use the 'wrap board in rags and pour on boiling water' method of spot bending this bit of plywood? Or will boiling water cause internal glue failure, or some other dramatic failure that will ruin my day?

Thanks!

knottyBuoyz
02-12-2010, 10:28 AM
I've often read about the "Boil" test for marine ply's so I don't think a brief exposure to your boiling water will affect most marine ply's. On smaller radii you might have to kerf the backside to get it to conform to your curve.

Mike Vogdes
02-12-2010, 10:56 AM
I've often read about the "Boil" test for marine ply's so I don't think a brief exposure to your boiling water will affect most marine ply's. On smaller radii you might have to kerf the backside to get it to conform to your curve.

Using hot rags and boiling water should help, definitely stay away from kerfing the panel, that will dramatically weaken or ruin the plywood.

jalmberg
02-12-2010, 11:23 AM
It's 3/8" marine plywood

wizbang 13
02-12-2010, 11:29 AM
The cheap temp broke, don't worry yet about the marine. it has no voids. The voids in the other ply were probly what caused the break. Wetting ply should not be nessessary. Another potential problem is you may be attempting a compound curve (torture)?Sure the station is correct? No kerfing.

Cuyahoga Chuck
02-12-2010, 11:50 AM
It's 3/8" marine plywood

Marine what? Marine fir? Marine okoume? Marine meranti? None of the above? The devil is in the details.
Also, how big is this dinghy? The shorter the span where the girations have to be accomplished the harder it will be. Especially with 3/8" ply.
Is this glued lap or metal fastened? With glued lap you could use " for the garboard and overlay it with 6 oz. glass and nobody would be the wiser. And the laminated garboard would be as strong or stronger.

JimD
02-12-2010, 11:53 AM
It's 3/8" marine plywood

Marine what? Occume will take the biggest bend, much easier than meranti or fir. Keep in mind if you use hot water to soften the wood it will leave water stains. Going to paint it?

Thorne
02-12-2010, 12:06 PM
No one expects....


...the Wooden Boat Inquisition!!!
http://people.csail.mit.edu/paulfitz/spanish/tt3.jpg


This can be done the easy way, where you tell us what design, exact materials and their dimensions, etc ---- or the hard way, where we attempt to pry the information outa you bit by painful bit.

You decide....

Just don't make us bring out...THE COMFY CHAIR !!!
http://people.csail.mit.edu/paulfitz/spanish/tt11.jpg

MiddleAgesMan
02-12-2010, 12:11 PM
A small glued lapstrake dinghy shouldn't need plywood planking thicker the 1/4 inch. Starting with 3/8 material of any kind may be the source of all your problems.

holzbt
02-12-2010, 01:20 PM
The boat in question is Atkin's CABIN BOY.

http://www.boat-links.com/Atkinco/Dinks/images/CabinBoy-3.gif

I thought you were going to use 1/4" for the sides and 3/8" for the bottom.

TimH
02-12-2010, 01:35 PM
best to bend it before it becomes plywood - before the glue between the veneers dries :)

Boatsmith
02-12-2010, 01:50 PM
I agree with
the fellow above who said you won't have the same problem with good (I like okoume) plywood as with cheap stuff. Also what the guy said about 1/4" for the sides sounds
righton.And the guy who implied that you are being stingy with details nailed it. If you can't make the required bend with the ply you need ,do it with 2 layers of thinner ply. David

jalmberg
02-12-2010, 02:07 PM
The boat in question is Atkin's CABIN BOY.

http://www.boat-links.com/Atkinco/Dinks/images/CabinBoy-3.gif

I thought you were going to use 1/4" for the sides and 3/8" for the bottom.

And I thought this was a simple question ;-)

Roger, you are 100% correct. I mis-remembered what I bought. It is 1/4" okoume (made by Joubert, if that matters).

-- John

jalmberg
02-12-2010, 02:11 PM
The cheap temp broke, don't worry yet about the marine. it has no voids. The voids in the other ply were probly what caused the break. Wetting ply should not be nessessary. Another potential problem is you may be attempting a compound curve (torture)?Sure the station is correct? No kerfing.

Actually, the bend isn't all that much. I was pretty surprised when the pattern cracked. I was bending it around the chine logs, which are made of white oak. The white oak bent around that curve with no problem at all.

Yes, the station is correct.

jalmberg
02-12-2010, 02:22 PM
Sorry if these are dumb questions, but I have zero experience with plywood... I'm also trying to figure out the best way to cut the curving planks out of the sheet, without wasting too much of this valuable stuff. My best guess is a jig saw. Any other type of saw I should consider?

jalmberg
02-12-2010, 02:23 PM
best to bend it before it becomes plywood - before the glue between the veneers dries :)

I think you're on to something there, Tim...

willmarsh3
02-12-2010, 02:30 PM
I built an 11' Shellback dinghy with 1/4" doug fir marine grade for the lapstrake sides. I made the bottom with 1/2" doug fir marine grade. I don't remember anywhere that it was especially difficult to bend.

When I later built the Elver I had a hard time bending the shear clamp. After looking things over for a while I discovered that I had built the stern post 1" too high. I trimmed it down and it was a much more natural fit.

jgmarine
02-12-2010, 02:39 PM
Hi,
Try using a 1/4" okume, and then a1/8 veneer ply over that.

Jack.

jalmberg
02-12-2010, 03:08 PM
Oh well, I guess I'll just try it and see what happens.

Candyfloss
02-12-2010, 03:24 PM
John, this should not be a problem. 1/4" okoume should fly round that.

Jigsaw is best.

Cuyahoga Chuck
02-12-2010, 03:30 PM
Sorry if these are dumb questions, but I have zero experience with plywood... I'm also trying to figure out the best way to cut the curving planks out of the sheet, without wasting too much of this valuable stuff. My best guess is a jig saw. Any other type of saw I should consider?

The big thing with curbing watage id nesting the pieces to get the most out of what you have. And with planks that tend to be simitar shaped that can be troublesome.
In Tom Hill's book. "Ultralight "Boatbuilding" he demonstrates a technique that will cut waste about as far as is possible given the difficulties.

jalmberg
02-12-2010, 04:51 PM
John, this should not be a problem. 1/4" okoume should fly round that.

Jigsaw is best.

Will soon find out. I'm not going to bed tonight until that garboard is cut out, bent on, and fastened. :cool:

holzbt
02-12-2010, 05:42 PM
Your Japanese pull saw will work fine for this if you'd rather not use power tools. If you must use something with a trigger a circular saw should be your first choice if you are comfortable using one. It's difficult to get a nice fair cut with a sabre saw. Not a big deal on an 8' plank, just a little more planing on the edges to get things fair.

You are planning to cut the gains (lap width rabbets at each end of plank to make the planks flush at the stem and transom) before fastening the planks permanently.

jalmberg
02-12-2010, 06:15 PM
You are planing to cut the gains (lap width rabbets at each end of plank to make the planks flush at the stem and transom) before fastening the planks permanently.

Yes... but I'm going to try your bevel-less trick for the laps. I think that will save a lot of time. My main goal at this point is to finish up so I can get back to FL.

Lewisboats
02-12-2010, 09:52 PM
One way to avoid tear out is to use a razor knife and cut the surface veneer on or just outside the line then cut with your saw. This leaves a nice clean edge to either use or plane from.

mackaye
02-13-2010, 12:33 AM
Laying very wet towels on the plywood overnight has always worked great for me. Sometimes, with a considerable curve, I might take a few days to slowly bend in the full curve. If you try to do all of a considerable curve all at once and too fast, you risk breaking the wood. If you sense that you're trying to take the wood too far at one time, just make sure to keep it wet and to take your time bending it.

Cedric Rhyn
02-13-2010, 02:19 AM
Perhaps not relevant here, but Designer John Welsford in one of t'other of his writings suggests that for areas of serious bend in a plywood plank it pays to cut a sheet of ply corner to corner then take the plank out of the angled edge, it then has the grain in all of the veneers angled and will bend both more easily and further before it lets go.

Cedric Rhyn


In the interest of time and money, I've decided to use marine plywood to plank my lapstrake dingy.

I've just cut out a pattern for the garboard, and when trying to bend it around the station closest to the bow (the point of maximum bend), the outside layer of the cheap 1/4" plywood I was using for the pattern cracked.

So, now I am thinking I may need to steam bend the marine plywood plank, so it doesn't crack...

I'm pretty sure the plank just needs to bend at this one forward station... the run is pretty straight right to the transom from there.

Question: can I use the 'wrap board in rags and pour on boiling water' method of spot bending this bit of plywood? Or will boiling water cause internal glue failure, or some other dramatic failure that will ruin my day?

Thanks!

Mike Vogdes
02-13-2010, 08:10 AM
And if you don't already have enough on your mind .. sheesh.
You want to make sure your boat framing is well braced. When bending and torturing plywood into shape you inadvertently apply alot of pressure on the framing, making it want to rack, or worse blow apart.

cookie
02-13-2010, 08:25 AM
Perhaps not relevant here, but Designer John Welsford in one of t'other of his writings suggests that for areas of serious bend in a plywood plank it pays to cut a sheet of ply corner to corner then take the plank out of the angled edge, it then has the grain in all of the veneers angled and will bend both more easily and further before it lets go.

Cedric Rhyn

Xlnt point. Still, if you must bend either parallel or perpendicular to the face grain, ply theoretically bends easier in the direction parallel to the face grain. Don't know why, but that's what the books say.....

Lewisboats
02-13-2010, 09:18 AM
Xlnt point. Still, if you must bend either parallel or perpendicular to the face grain, ply theoretically bends easier in the direction parallel to the face grain. Don't know why, but that's what the books say.....

Plywood is made (mostly) of uneven #s of layers...with the outer sheets grain direction being the more numerous. If you bend parallel with the grain you bend along the axis that is naturally weaker and, with plywood, against the fewer veneers that compose the inner perpendicular layers.

jalmberg
02-13-2010, 12:36 PM
Perhaps not relevant here, but Designer John Welsford in one of t'other of his writings suggests that for areas of serious bend in a plywood plank it pays to cut a sheet of ply corner to corner then take the plank out of the angled edge, it then has the grain in all of the veneers angled and will bend both more easily and further before it lets go.

Cedric Rhyn

That would be one expensive plank.

jalmberg
02-13-2010, 12:41 PM
And if you don't already have enough on your mind .. sheesh.
You want to make sure your boat framing is well braced. When bending and torturing plywood into shape you inadvertently apply alot of pressure on the framing, making it want to rack, or worse blow apart.

That's not really a problem in my case. The framing is strongly built and reinforced by the keelson and chine logs.

No, the problem at the moment is that the first pattern I developed by spiling did not fit at all. I suspect that my spile batten was edge set. I hate to do it, but I'm going to start over from scratch and use a three part spile batten.

The good thing is, I've learned a few things from doing it wrong once. Hopefully, the next attempt will be a bit more successful.

Back into the shop!

mcdenny
02-13-2010, 08:14 PM
John,

1) Hard to beat 1/4" luaun floor underlayment ($10/sheet) for pattern stock.

2) Getting done fast when you are learning and getting done good are mutually exclusive.

jalmberg
02-14-2010, 08:46 AM
John,

1) Hard to beat 1/4" luaun floor underlayment ($10/sheet) for pattern stock.

2) Getting done fast when you are learning and getting done good are mutually exclusive.

Well, I have definitively discovered the problem with bending my pattern stock -- it is the plywood I was using. It cracks waaaaay too easy to be used for this purpose. In fact, it cracks if you look at it too hard.

I've heard of luaun, and I'm going to go out tomorrow and buy a sheet to try. I need something decent to work with.

To your second point, I am learning this truism the hard way, I guess. In fact, it is not possible to rush this stuff, anyway. I would *like* to go faster, but its not happening.

My first attempt at making a pattern for my garboard was a total, dismal failure, but I know a heck of a lot more as I start on the second try.

Someone on another thread was talking about how a photo record of your build is a good thing to have. I'm actually discovering that lots of photos/videos are invaluable for figuring out what you did wrong. I am just putting together a video of my recent failure, and can see the root cause of the failure in the very first clip. Everything I did after that was doomed to failure, but I didn't know it.

Well, live and learn, as they say.

cookie
03-07-2010, 10:06 AM
Xlnt point. Still, if you must bend either parallel or perpendicular to the face grain, ply theoretically bends easier in the direction parallel to the face grain. Don't know why, but that's what the books say.....

Whoops, did I say parallel :o
Checked the books and should have said that plywood bends easier in the direction perpendicular to the face grain. Sorry...

Mrleft8
03-07-2010, 10:20 AM
Whoops, did I say parallel :o
Checked the books and should have said that plywood bends easier in the direction perpendicular to the face grain. Sorry...

The books can say what they want. In my (considerable) experience you were correct the first time.