View Full Version : Steam Bending Iroko failures

11-20-2016, 01:07 AM
I haven't had much luck in posting photos, so I'll give it a try. I"m following Thorne's method of posting photos. If they don't show up, I'll keep trying till they do.

Anyway. . .

I'm making new toerails for Amie. The old ones were oak and very rotten (and about 30 years old). I'm making new toerails out of Iroko. The forward toerails worked well, the aft ones, not so well

11-20-2016, 01:12 AM

The forward toerails

11-20-2016, 01:38 AM
Photo worked

11-20-2016, 02:21 AM
Here's where we started running into trouble



The wood is iroko, 1" thick x 1.5" high. It's kiln dried (I don't know where to find any other kind), and was soaked in water for about a week before steaming. It was steamed for 2-2.5 hours.

I'm not sure where we'll go from here. We have some stock and can try bending more, but I'd like to know what errors we've made before we try it again.

As I said, the other pieces had a very gentle bend and we had no trouble.

Thoughts? Opinions?



11-20-2016, 02:27 AM
maybe it helps if you round the post where the bend is the stongest, it lokks to me that this has caused the breaking

ETA I always bend wood INTO aform instead of over it, that and as many others say laminating will help.

11-20-2016, 02:47 AM
Three things:

1. Kiln dried timber is not suitable for steam bending. Steaming works by softening the lignin bonds between the cellular structure of the timber. During kilning the lignin gets permanently set and hardened and no amount of soaking will soften it again sufficiently for successful steaming.

2. Even with green timber your set-up is likely to cause a breakage as your former is producing hard spots. You need more formers – or a solid shaped section – where the break is at the point of maximum curvature.

3. As an aid to prevent break-out you can use some sort of flexible backer strip which can be bent around the outside of the timber being bent as you bend it around your former.

When wet (that is not kiln dried or excessively air dried) Iroko is an excellent timber for steam bending.

Probably lamination is your best solution if you have to use kiln dried timber.


11-20-2016, 02:52 AM
We tried that on the second bend. It still broke where we rounded it (we rounded 2 of the posts, and the wood fractured at both rounded posts).

That does bring up a question-would it have worked better with solid form (like a sheet of plywood bent around those vertical posts and clamped to that) rather than the individual radiused posts? I mean, I believe it would have helped, but would it have helped enough?

11-20-2016, 03:02 AM
Thanks George. Where can one find bending iroko on the west coast? I can check with Edensaw, but they've even had trouble coming up with bending white oak in the past.

I had read too that iroko was excellent for bending. Hence the disappointment. But, the more I think about it, the more I agree that some type of smooth form and smooth backing board should help.

We thought about laminating. It presents a couple problems (that I see). One is that the laminates will have to be bent somehow, so we may have a similar (although less of a) problem. The other that comes to mind is that the laminations will be attached to the deck with screws parallel to the glue line, and we could get separation there.

Maybe I'm overthinking it too.



11-20-2016, 03:09 AM
Laminating is the answer, I wouldn't worry about fastening parallel to the glue line, especially if you use numerous thin laminates.

For bending, the only way is local woods. Everything else is kiln dried to be legal for import (kills the bugs.) You may be able to find some locust that is air dried, but it won't take that severe bend you show I don't think. Some yew may be a nice choice, gum wood maybe. Local tree guys with a band mill will probably have the exotic air dried stock you need.

Save yourself the headache and laminate. Or go with oak again.

11-20-2016, 03:46 AM
What George said. We used aliminium strips as backers on moulding projects to avoid point loads. The kiln dired stuff will always be brittle. Ripping it down and laminating would be the way to go. Stuff fresh enough to steam will look almost 'green' inside when cut, we used to get teak and iroko in large baulks that would be green on the inside, you are not likely to find that with a local supplier.

11-20-2016, 04:08 AM
I doubt you can get green Iroko – as J.Madison says.

Actually you have been surprisingly successful considering it's kiln dried. You do have a bad pinch point where the break occurred – making all that section of the former solid, so you can clamp at more regular intervals might work – not just ply – you need something solid you can clamp to. It needs to run from the straight section around the worst of the curve.

And if you can get a length of thin (say ⅛") spring steel or similar to use as a backer (on the outside of the bend) that would also help. Or possibly a length of say ╝" or ⅜" wet oak or other local timber would do as a backer – anything that doesn't break going around the bend but which will support the outer surface of the timber.

Steaming timber allows the cells to compress a lot but not stretch much (less than 1%). So breakages mostly occur on the outside of the bend, although occasionally the inside of the bend will crumple. If you can prevent the outside from stretching this will help – but that is kind of difficult. If your backer is spring steel then you can possibly clamp it to the timber being bent so that the clamps are intermediate to those clamping the timber to the former, which will help stop the outside stretching.

When we were building classic carvel boats, we had quite a selection of spring steel lengths just for this purpose in tight reverse bends. The heel of the timber (the steam-bent frame) was tucked into its housing in the wood keel (so that end couldn't move to stretch) and then the steel backer was held over the reverse tuck part of the timber as it was "walked" up the temporary stringers.

You are steaming too much as well – about an hour per inch of thickness is the usual recommended amount – and the steam wants to be saturated, not dry – so only at about 212║ and under no pressure. We used to reckon that steaming for too long "dried" the timber and made it less easy to bend and more susceptible to breaking – I don't have any scientific data to back that up however apart from years of boatyard practice!

But it's a lot of work and trouble and expense for something that might not work!

Cheers -- George

Max F
11-20-2016, 05:34 AM
Like the others said, a thin backing of steel or aluminium makes a big difference.
It has to be fastened solidly so that it won┤t slip while bending.
Instead of stretching the wood fibres there will be only compression.
Wood fibres tolerate much more compression and only little stretching.
With such a compression strap we bend kiln dried oak brutally. No trouble.
I keep the timbers for 4-6 weeks submerged though the be sure they are soaked completely.
Good luck

11-20-2016, 05:36 AM
I have successfully 'bent' iroko for the beam shelf. The point about kiln dried is absolutely valid but it was still ok for the beam shelf because the curve is not extreme. I am collecting the iroko for the bilge runners (see the thread about removing the bilge keels) and I am expecting the curve to be moderate to be followed by the iroko without steaming. A point that hasn't been made as far as I have noticed is that iroko can (probably frequent enough to say 'will') have an irregular or 'wild' grain. My view after doing bits with iroko for 25 years is that iroko would be very unsuitable for that extent of bend simply because its natural grain.

11-20-2016, 05:49 AM
I reckon it was steamed for too long, I have found if you over cook it it breaks much easier.
Years ago i had a job putting a heavy rubbing band on a workboat, it was Iroko, 3" deep & 1 1/2" thick, we fastened it in at the bow & eased it round with big g cramps bolting it as we went. It wasnt steamed but we did wrap sacks round it & pour boiling water on it to help it.
Got one side done & started on the other side then Ray the shipwright told us we were farting around taking too long & started winding the clamps fast. There was a loud bang, the rubbing band snapped & threw a 12" g cramp into his jaw knocking him out.
Blood everywhere & he had to go to hospital. Happy days!

11-20-2016, 05:52 AM
Could you split that section ( it being close to the end ) on a bandsaw, then glue the kerf later. 2 x 1/2'' layers should steam and bend OK.

That's quite a tight bend for 1'', especially KD timber.


11-20-2016, 08:15 AM
Well, capn', I'm surprised no one has mentioned the orientation of the grain here. Tough, though, to find straight grain on sawn lumber. Maybe some of your other stock is better.
I've seen KD wood, even Iroko, bent in. Of course I don't know the specifics, but the grain was straighter.
Steaming is done until the wood is as hot as possible, but can't get over 212 deg F because that is as hot as steam at atmospheric pressure can get. Also, it cools fast when out of the box and you are 50 feet from the forms.
Just saying, lots of variables involved. Did it take 2 hours to get the timbers hot? If so, maybe you need to improve your rig. :) Maybe more heat to have more steam produced then the amount leaking from the box.
Offering my 2 cents anyway.

Here's where we started running into trouble.


So, maybe the safer solution, as has been mentioned, is to split the thickness and scarph-in that aft section.

11-20-2016, 08:31 AM
Looks like a good candidate for the Plastic Bag Method.

wizbang 13
11-20-2016, 08:38 AM
rip em, lam em.
you won't even need to throw away what already broke.
Or...push the board into a mold, not pull it around.

Dave Hadfield
11-20-2016, 08:51 AM
Make them from black locust?

David G
11-20-2016, 10:19 AM
Three things:

1. Kiln dried timber is not suitable for steam bending. Steaming works by softening the lignin bonds between the cellular structure of the timber. During kilning the lignin gets permanently set and hardened and no amount of soaking will soften it again sufficiently for successful steaming.

2. Even with green timber your set-up is likely to cause a breakage as your former is producing hard spots. You need more formers – or a solid shaped section – where the break is at the point of maximum curvature.

3. As an aid to prevent break-out you can use some sort of flexible backer strip which can be bent around the outside of the timber being bent as you bend it around your former.

When wet (that is not kiln dried or excessively air dried) Iroko is an excellent timber for steam bending.

Probably lamination is your best solution if you have to use kiln dried timber.


George covers most of it here. I've bent Iroko - kd, of course - successfully. But I didn't find it 'excellent' for bending. Just looked in Lincoln, and they call it 'moderately' well suited for bending. That's my experience. Also the straight grain - aka, NO runout at the tightest bend - would be important.

1. It's not quite THAT straightforward. It depends partly on what mc the timber was brought to in kiln... and, to a lesser degree... how gently/slowly that mc was arrived at.

2. Indeed

3. This one is the key. From your fotos, that's the failure. Timber will stretch FAR less than it will compress. One needs a bending strap to forstall the stretching bit, while forcing the bend into compression (they are sometimes called 'compression straps' for that reason). Lee Valley sells the key bits to allow you to create your own, and their kit comes with excellent instructions. Or back issues of Fine Woodworking for articles on creating one. There are also several good books on the topic of bending that provide strap how-to.

AND... unless you foresee yourself don't a bit of bending in your future... you might be better-served by going the bent lamination route. Messier, but far simpler. Very little failure rate. For Iroko... I'd use g-flex.

11-20-2016, 03:27 PM
A better bending fixture and a tension strap should work there. Sometimes with bending there are no "short-cuts", you have to do everything right including selecting the material. (The easy way out would be to kerf it before steaming)

A quick google search on "steam bending, tension strap" found some good info here...


John Meachen
11-20-2016, 06:06 PM
I have to agree with the assessment of the bending jig.It looks like it requires the wood to take a sharp bend precisely at the point where the breakage occurred.I would have located at least two additional uprights in that area,or even gone for the recommended solid former as it would have allowed the iroko to follow a less abrupt change of direction.

11-20-2016, 07:21 PM
Plastic bag method. The beauty of the bag is that you can occasionally try the bend and see if it will go, without removing the heat. You can bend and clamp, bag and all. You may still need a backing strap, but I am guessing once you try the bag, you will never go back to the box. (I still use the box for ribs, but they are easy). I am thinking you steamed too long too.

11-20-2016, 09:20 PM
Thanks for all the help. I think we'll laminate the pieces to the shape we need.

I'll put up some pics when we get done. It will be a couple weeks.

Jay Greer
11-20-2016, 10:10 PM
I am really sorry to see you experiencing the problems you have been going through with bending! You are working with a wood that I have never attempted to bend and so, I have not ability to comment on it.
By tradition most of the woods both I and my contemporaries have worked with are woods that are well understood for their bending abilities. Most of them that I work with are still green and unseasoned and so lend themselves well to bending after having been in the steam box. Of the woods that I know will bend easily, I would recommend White oak, rock elm, maple and ash as well as locust black or honey. However, all the woods mentioned should be obtained fresh cut and green if steam bending is to be expected to work. The use of a bending strap on the outer surface of the wood is always recommended if a high degree of bend is needed.

11-21-2016, 01:57 AM
Thanks Jay. I chose iroko mainly because some of the trim on Amie is already iroko (rubrails, hatch trim), and partly because of the staining problems I've seen with oak. Yes, if I kept up with the varnish I wouldn't have that problem, but I haven't been very good at it-and a lot of the old oak trim was already varnish free and stained. I had read, somewhere, that iroko was used on a lot of English pleasure craft (for trim, framing and planking). But I've forgotten where I saw that (maybe I made it up). I think that laminating 1/4" pieces of iroko should get me what I'm looking for.

And speaking of bending failures-about 6-7 years ago we made a new cabin top for Amie. The old one was White Oak, so I decided to go with it again. I found a 15ft piece x 15" x1" to bend (the cabin and cockpit sides are oval in plan view), not being aware of the problems of bending KD oak. After steaming it for an hour, we tried to bend it around the form-and broke the form. We put it back in the steambox and rebuilt the form. We got the piece back out, and with lots of muscle, got it to the shape we wanted-for a couple days. Then, the piece started bending and straightening in weird ways. I thought some how water may help-wrong. Snapped like a toothpick. It's disheartening to see your $200.00 piece of wood splinter. Anyway, we found a piece of green oak from Edensaw that worked just fine. Lesson learned. be cautious bending KD oak (and iroko now, too). I'll try to find some pictures of the broken piece.



Jay Greer
11-21-2016, 01:16 PM
Sounds like you chose the wrong grain pattern for bending that oak!

Jay Greer
11-21-2016, 01:24 PM
A member of my family once salvaged a load of Iroko that had been used as paneling in a restaurant. I allowed my surface planer to be used to clean it up when I was asked and did not pay much attention to how the project was progressing as I was working elsewhere. The wood was so damned hard that they burned up my planer which had to be rebuilt! No wonder that wood won't bend for you!
The grain looks good but, in my opinion, that African teak is from the devil's forest!

11-21-2016, 09:21 PM
I've heard it can be rough on tools.

Here's that coaming we bent (the green oak one)




David G
11-21-2016, 09:29 PM
Iroko, like teak and other species with some mineral content, can be a bit hard on tools. You'll only notice it, though with hss tooling, not with carbide tooling. Unless you're running hundreds and hundreds of board feet of something.

So what have you decided to do?

11-22-2016, 12:38 AM
I need to talk to my partner in crime (since we do the steam bending at his house). I think I'll rip some iroko into 1/4" thick strips and see if they will dry bend around the curve. If not, I'll try steaming them a little, and clamp them in a couple days to dry. When dry I'll glue them up. If that fails, probably buy some more iroko and cut to shape (the oak ones were cut that way).

Suggestions (other than use something besides iroko)?



11-22-2016, 12:41 AM
I'd be wanting to try again with one piece, steamed, for the aesthetic value of a clear-finished toerail. Try more clamping stations, and square up your blocking to the radius. Bring it in evenly, and good luck; this should be doable. / Jim

11-22-2016, 02:52 AM
Thanks Jim. I thought about the visible glue lines on the varnished toerail, and I think I can live with them. With enough clamping, the glue lines should disappear and only the change in grain should be visible.

The thought of steaming it again had crossed my mind. Iroko has really wild grain structure, and even all these precautions can't guarantee some breakout of the grain.
Still, food for thought. I would look nice if we could pull it off.


David G
11-22-2016, 10:02 AM
A bending strap would go a very long way toward eliminating the breaking also... if you decide to try re-bending. Without it... I predict sorrow. But bent laminations, as you mentioned, should work just fine.

wizbang 13
11-22-2016, 10:16 AM
" With enough clamping, the glue lines should disappear "
Whops, be careful there or your laminated rail will fail as well!

Eric Hvalsoe
11-22-2016, 12:41 PM
As wiz suggests, you don't particularly want epoxy glue lines to disappear.

11-22-2016, 05:41 PM
Man, you olde salts need to get out and try new things.

Bag Method.



11-22-2016, 08:05 PM
I apologize. There has to be a glue line. No glue line=starved glue joint. I meant that the glue line won't be the most prominent visible feature of the finished piece.

Fitz, it's not that I don't like the plastic bag method. On really long pieces I don't see any way around it (as you show on the Coronet video). Our pieces are about 6 ft long, and the 50 gal hot water heater and 20ft steambox are nearby. So we're pretty well set up.

And now you got me thinking again-laminate them or steam one piece. . . That said, it's nice to have options.



Eric Hvalsoe
11-23-2016, 01:08 AM
Harvey, did you look for good bending oak in the first place? Admittedly not always easy to come by.

Your iroko stock not looking good for steambending for reasons allready pointed out.

I often coat bending stock with linseed oil before going in the box. Especially if I feel it is on the dry side. Might even pickle it in boat sauce for a few days. Or drop it in the lake for awhile. None of that reconstitutes the cellular structure of dry stock, but it can give you a little moisture to work with and coating with linseed oil or boat sauce will keep the piece from drying out as badly in the box. Or the tube. Or the bag. There is also the idea of boiling rather than steaming. Steam time is particular tricky when the stuff is relatively dry - really narrow time window.

11-23-2016, 06:37 PM

No, I didn't look for bending oak. I wanted the iroko to match the rubrails you put on 16-17(??) years ago-which still look good. If bending the stern quarter toerails doesn't work, I can always cut them out of larger stock. I had heard about coating the parts in linseed oil before steaming, but had forgotten (also, it was in the box before I got there).

One nice thing about bag boiling is that you can gradually add pressure, and, with the straps inside and out, should reduce cracking. Some things that worry me about that method are that it seems messier to work with, and the scalding hazard goes up. Maybe I'm overthinking it, or maybe we would need to do more planning to pull it off. I do have visions of hot water all over the floor (and us).

We'll see what happens. I've got a lot of prep work before the boat is ready for the toerails.



Eric Hvalsoe
11-24-2016, 12:30 PM
Ahh. I have no recollection of doing the fore toerails. You're right, you can also saw the pieces out.


01-07-2017, 11:53 PM
It's been a few weeks. Between holidays, out of town and having the flu (for 2 weeks now), it's been slow. But we're showing progress again.

I bought some more iroko this week, and Mark R milled it down to the proper dimensions, then started soaking it. The grain on this particular piece was much better-a lot less built in stress after it was cut.

We reinforced the jig with 3 strips of 1/4" ply to provide a smooth bending surface


Note that there still is a hard point at the 4th upright. We moved the upright in a little to ease the bend there.

We steamed the iroko for a little over an hour before trying the first piece.



This piece turned out better than any previous effort, but we still had some compression failure on the inside radius and a little break out of grain on outside radius (covered by a clamp in the photo)


We could hear some cracking as we bent it around, but it looks like it might be servicable.

The second one came out better.


with minor compression failure on the inside radius


After the pieces were clamped in place, we wrapped them in wet towels and plan to keep the tightest bend wet for a week or so. The hope is that it will reduce the stresses in the bend. We'll see. If that sounds like a poor approach, please tell me.

I'm hoping/planning on these pieces working on the stern quarter of Amie's deck. I'll post some photos when we get them installed (hopefully before the end of the year . . .)

Thanks again for all the comments and suggestions.