Steam bending questions

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Veteran Member
Sep 22, 2008
Anyone have any advice. I'm replacing the rub rail on my 40' Nova and I'm building a steam box to bend the wood. I'm using 17' long pieces of Purpleheart that are 2" wide by 3/4" thick. Rule of thumb is 60 min per Inch so I'll cook the wood for ~ 45 to 50 min.

For my steamer I'm planning on a closed system with a condensation return line.

I'm using 4" schedule 40 plastic pipe and planning on using a 5 gal propane tank as the boiler. I'll put it on a propane crab cooker for the heat source. I'm wondering if it'll end up generating enough steam to fill the ~ 20' pipe. I have a pressure cooker relief valve for any over pressure, which I'll tap into the pipe. Another concern is the return tube. I'm using 1/4" copper tube which I'm planning on running from the low spot of the 4" pipe back to the propane tank where I'll tap some threads for a connection point just below the top of the tank. Will that work for the condensation return line?

All previous bending was with small pieces which were set in place immediately. When setting this piece I planned to screw it into place. Any idea how long a piece can be out of the steamer before it becomes unworkable?
Instead of steaming it why not rip it!* Take a circular saw and make ½ cuts across so the board will bend easily, bend to shape and then fill the cuts with wood filler/glue* to make ridged again.* The number of cuts is dependant on how much the bend is.* *
I have steamed Teak for cap rails , and since the cap rail is curved with the thick side getting the bend, loads of extra time is needed.

If you can build a jig and overbend at least 10% as even steamed the wood will want to spring back.

When I planked my boat in 95 we used a 10" dia steel pipe and built a fire under the bottom end (which was caped) and the pipe filled with water and the addition of a little linseed oil.
These planks were 4 x 1 3/4" and up to 25" long.
Once they had boiled / steamed for an hour or 2 we took them out using hessian sacks and then clamped the planks in place and nailed and clinched them.
This has to be done whilst the planks are still relatively hot hence the potato sacks.

The timber was spotted gum an Australian eucalypt hardwood.
The one thing yet to be mentioned is the fact that very soon after the steam enters the plastic pipe, the pipe will sag and collapse !! Please don't ask me how I know Better to get some aluminum or steel heating / vent pipe that you can sheet metal screw the sections together with.
I used regular 5 inch domestic vent pipe as it was the least expensive and supported it with a 2X4 on edge between 2 saw horses.
Other than the plastic pipe problem I don't see any problem with what you are doing but please let us know how you make out as I will be attempting the same thing within the next few months - steam bending purple heart of about the same dimensions that is.
Good luck
John Tones MV Penta
Sidney, BC
I've heard that PVC has the problem with bending with heat and that ABS does not.* The proof will be in the doing.* I have a strongback that I use for strip building kayaks that's 16' long.* That will be the bed for the pipe.* I'll start the process on Thanksgiving weekend (weekend after next) if all goes well and will post my results.
Something that may make your project a bit easier is if you were to screw temporary cleats to the boat that would allow you to clamp the steamed purpleheart into place while it is still hot. When I was bending oak pieces for the stern of our boat that is what I did for the first full length, clamping the steamed wood into place and then screwing it down. This gives you a good clamping place for the remaining pieces. It does not take very long in my experience for the wood to cool and start to stiffen up so you have to work rather fast, helps if you can engage another pair of hands as well, if you are handling long pieces.
Actually on my boat the deck has a short section of wall above the hull deck join which will act as my form - plus it's about the only spot where I can get the clamps to work. The wall runs about 2" above the actual deck and has a ever so slightly tighter radius. The plan is to take the wood out of the steamer and immediately clamp one end to this section of the deck and work it around from there. I have 12, 12" wooden screw clamps which should do the job plus I'll bring along some pipe clamps. Once the beginning end is clamped, I'll do the full bend and have my helper throw on the minimum number of clamps so as to keep it in place. Then after it's bent end to end, I'll go back and adjust any hard sections with additional clamps.

I'll leave it in place until the next day and then attempt to remove the clamps to see how much spring back I have - if acceptable, I'll then remove it and then re-attach it at the hull deck join. My hope is that the bend will set and I'll be able to manually put it back into position with some clamping and screw tension.

I'm doing it one piece at a time just incase this does not work. If it doesn't then I have a backup piece and will go to plan "B" which will require some extra pieces of wood under the outside legs of the clamps so as to "extend" their reach "over" the hull deck join area. Then I'll bend to the exact location where it will be attached. (I'd like to not have to do this as setting the clamps with this extra piece of wood means more fumbeling around which always takes longer.)
The plastic pipe will get soft. You can use steel or a box made of 2x6's. The crab cooker will make plenty of heat. The purple heart will steam well. Teak does not steam well at all. The oils in teak prevent the hot water from softening the cell walls. On a scale of 1 to 100 teaks steamability is around 3. David
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