Pooduck Frame - how do I fix this

[mwybo]

Several of the Mahogany strips cracked at a bend while I was laminating my Pooduck frame.

http://media5.hypernet.com/cgi-bin/UBB/ultimatebb.cgi?ubb=newtopic;f=1

The area of separation is about 2 inches long and goes three layers down. My thoughts are to use a router to clean out the area and fill it with epoxy/wood flour. Is this the correct approach?

There are many firsts for me in this my first boat. First scarfs, first lamination, first time the garage has really been cleaned out. (and first time posting an image here so I hope that works.) The lamination overall went relatively well although I found I just could not bend all the strips at once so I am doing half at a time. I did this with the inner and outer stem and it worked fine and gave me a more stable platform to bend on. I am using the technique in the John Brooks book with the moveable blocks. I am not sure I would do that again.

Also, it is really true that you cannot have enough clamps!

Mike http://www.imagestation.com/album/pictures.html?id=2131748286&idx=1

[mwybo]

Oops, ignore that link for the image and try this one.

http://www.imagestation.com/album/pictures.html?id=2131748286&idx=1

Mike

[Bob Smalser]

http://www.imagestation.com/picture/sraid155/pbdb13d17d6c6e974ba1776d27195ad8e/f55dd473.jpg


I sure wouldn't...epoxy alone doesn't have near the longitudinal strength of those lams.

Assuming you used epoxy, I'd simply remove the broken lams on the bandsaw followed by cleanup on the jointer and relam those layers....going a tad thinner on the lams and adding one more. If you used something other than epoxy, you have to machine to completely clean wood. With epoxy you don't have to be as fussy.

One of the real bennies of epoxy is that after you scuff that old epoxied surface up, the reglue is just about as strong as with clean surfaces.

No other glue I know of can say that.

You can clean up curves on the jointer by setting it to a 32nd for a fine shaving or your can do it on the beltsander.

[ 01-23-2005, 12:42 PM: Message edited by: Bob Smalser ]

[JimD]

I wonder if you could do like Bob says only just scarf in shorter sections of the laminations, or even shape in a solid piece with a good long taper on the ends to provide plenty of purchase?

[mwybo]

Bob, it is glued up with epoxy. Of all the tools you mentioned I have a jig saw. I like Jim's idea about scarfing some smaller pieces into a patch. There will be another 8 or 9 layers on top of this sandwiching it in. Does that make sense to you from a structural integrity perspective ?.

[John Meachen]

Is the grain a little on the steep side in the broken laminates?It would pay to inspect the replacement strips closely as the bend does not look too severe.It might help to use an extra strip without glue as a clamping batten.Good luck with the repair.

[Bob Smalser]

Sure you can scarf if you can get 6 to 1 or better. The issue is can you?

Remember that merely filling the cracks as they are now gives you 100% worthless end grain as a gluing surface, and a scarf works only because it has the higher proportion of face or edge grain to end grain.

And a saber saw followed by a block plane and/or sanding block with 60 grit will work well enough.

[ 01-23-2005, 04:48 PM: Message edited by: Bob Smalser ]

[WFK]

When doing a glue-up such as this, you should be able to do it all at once. I'd first do a dry run making sure the laminations will bend, and clamps and blocking is right where you want it. Another thing that can be done to help stop cracking is to apply a steel strap over your last ply. Starting in the middle and working out both ways with lots of clamps with slow even pressure should give you a succesful job.

Bill

[mwybo]

Thanks Bob for your comments, I am going to take it off the modl tonight and see just what I can do.

Bill, I did a dry run and left the strips on the form for a week, that is why I was surprised when it cracked. I am working with limited space and experience and a very cold garage (-15 in Montreal the last few weeks). I used the technique that screws wooden blocks onto a flat surface and then bends around these blocks. This allows me to bring the flat surface (3/4 inch plywood) into the house to cure. However, bending the wood around the blocks causes thay ply base to move around and since I am pushing it by curing this in the living room I thought it best not to propose screwing the ply into the dining room table. I would use the method in the Shellback book, bending around a vertical plywood form, if I were to do it again.

Anyway, I am going to take a look tonight and see what I can do. I will post my proposed solution for comments. I have found that the discussion and opinions on this board to be invaluable to a new builder like myself.

Mike

[Frank Wentzel]

Mike

Are you sure you don't have an epoxy-starved lamination? I'm afraid that, with working in the living room and trying to avoid the mess of excess epoxy, you may have not used enough. If the wood wasn't sealed with a coat of partially cured epoxy before gluing the layup you may have had most of the epoxy absorbed into the wood and little remaining at the glue-line. The picture appears to show failure in the epoxy bond area between the strips. A good epoxy bond is much stronger than the wood.

/// Frank ///

[Billy Bones]

Sorry to say this but I'd redo the whole thing with thinner laminations. I'm pretty sure, though, that the frame is way overengineered so if you try to salvage it, you probably won't be sorry.

When stack laminating like this, you should be able to EASILY bend all the strips around the frame by hand. In fact, you'll know you have the right thickness and number of lams if you can bend ALL the pieces around the jig with only slightly more effort than bending a single stave.

And, do it inside where temps are warm. Let all the bits warm up before gluing. Send the significant other out on the town and do it on the kitchen floor with a sheet of builders plastic sheet if you have to. Gluck.

[JimD]

Doesn't look glue starved to me. Looks like the wood broke before the epoxy had a chance to cure.

[mwybo]

Jim, you're right, the wood cracked just as I finished clamping. I have taken the piece off the mold. I have about 1/2 inch of springback at the ends which I should have taken into account when placing the blocks.

I do not think I could easily get a 6:1 or better scarf as Bob Smalser suggest so I think I am going to start over with thinner lams and chalk this up to experience. My son suggests I try breaking it to see just how strong it really is. Any ideas on how to do that and measure how much pressure this will withstand? I have read a lot of comments about the pooduck stem and frames being overengineered, might be interesting to know just how strong they are.

Mike

[JimD]

Mike, I've made similar laminated frames with doug fir and they are really strong. Is the wood you're using quite dry? You might try the poor man's steam box meaning soak the lams in water for a couple days, and/or pour a kettle of boiling water over them, and quickly clamp them into place, let dry. Remove the clamps and they will likely have retained most of the bend. Then epoxy them together. As for the spring back I over compensated once and ended up with too tight a bend in the frame and couldn't use it.

[Steve McMahon]

I used dry mahogany so I cut the lams a bit thinner.

http://www.imagestation.com/picture/sraid111/pe2565c828816d7162685093256f86157/f910388c.jpg

http://www.imagestation.com/picture/sraid112/pb7c19369d79385e5430509d419b47b6f/f90238ba.jpg

If I were you I'd scrap the first attempt and chaulk it up to practice. Keep at it - the results will be worth it. If you use the search button and type in "pooduck skiff" you will find a wealth of info under this section of the forum.
Keep posting too!

[mwybo]

Jim, it is pretty strong and it has only half the lams on it. I thought about pre-bending but wasn't shure how to do 7 foot pieces. Might give that a try on the next attempt

Steve, I have read all your posts, and Billy Bones' as well before starting this project. You both have made some nice modifications. I do not have the equipment or the experience that many on this forum seem to have. This looked like a fun project which it has been so far. I also am using mahogany and did not realize that I should be making the strips thinner than called for in the plans. I will try that with the next attempt.

Mike

[Bob Smalser]

It's no big deal...you are out 3 strips and 30 minutes of work. The cost of doing business.

They crack occasionally mostly because of grain runout in individual strips, which is difficult to read in mahogany and even more difficult in thin mahogany. Push a felt tip pen into the edge grain of your board stock and see if the ink seeps in on the diagonal instead of dead straight.

If you can see it, more grain runout in your stock or very dry, kilned stock mean you need thinner strips...you adapt the plan to your materials where you have to.

You can overcome grain runout by using a thin steel backing strap on the outside of the bend. That'll save some runout stock by holding it in place until the epoxy cures, but can be detrimental to the overall strength of the lam, as stock with enough grain runout to crack on such a shallow bend doesn't add all the strength the designer intended in that scantling because the wood between the glue is weak. These designs are generally overbuilt sufficiently to accommodate that, but it's something to think about in a real light boat or when reducing scantlings beneath the design.

Ideally, you correct your bending stock for grain runout by ripping and resawing as required...and that can mean a lot of waste with run-of-the-mill stock. One of the many reasons I have my own trees and my own sawmill.

http://pic3.picturetrail.com/VOL12/1104763/6035832/81088143.jpg

Here's a shallower bend but with considerably thicker DF strips being lammed right on the boat to save form materials with as many lams as will lock in the set (to be covered by an electric blanket overnight for a good cure out in the cold)...

http://pic3.picturetrail.com/VOL12/1104763/6035832/82385088.jpg

...and the rest of the lam done in the shop using the first strips as the form. Tomorrow, the mahogany tiller yoke will be lammed right over top of it to match, but with thinner strips because the store-bought Mahog stock isn't the perfectly-cut and perfectly-airdried that DF is.

[Paul Scheuer]

mwybo: It looks like you would have been ok with a few more clamps. If you're really short of clamps, think of blocking the pressure points so that you have one clamp pushing in two places.

I wouldn't have expected that assembly to break. What is the bend radius ? .

[Don Bailey]

mwybo,
I built a Pooduck 3 years ago and it is a great boat. That laminated frame is an important part of the boat. I'd chalk it up to experience and try again. I made mine from 1/8th strips of mahogany and made it in one fast glue up using slow cure west epoxy. This was also my first build. If I can be of any help with questions just email me. See profile for email address.
Don B

[Don Bailey]

mwybo,
When you are doing that glue up you got to start tightening the clamps in the middle first and work toward the ends so the wood can slide a little as the bend increases. This reduces the kind of pressure points that caused the wood to crack for you. In my glue up for the Pooduck frame I used at least 30 clamps. Each piece slides about 1/8th to 3/16 at each end during the clamping process.
Hope this helps,
DonB

[mwybo]

Thanks all for your comments and help. I will start over on the frame. I ripped the (thinner) strips today and acquired a few more clamps so that should help. I will glue it up sometime this week. The plywood for the strakes is scarfed and epoxied so assuming I get the frame laminated this week I can start cutting pieces next week, setup the building frame over the weekend and then I am building a boat!

[mwybo]

Just to close this off, I have redone the frame using thinner strips and doubling the number of clamps. The result is noticeably stronger. Thanks for your help.

Mike