steam bending stems

[tptbum]

Hi guys,

The trouble with building your first boat is that you keep wondering if you screwed up beyond repair, or at least I do.

This one is about stems. I steamed the stems, bent the stems, clamped the stems, let the stems dry on the forms. Then when I unclamped them they sprang open, maybe 4 inches at the bottom.

My neighbors, who are all pretty good hobbyist woodworkers, assured me that this is normal and the only way to compensate is to build a seperate stem form, with a 20% overbend. I thought that when I epoxied them they would be unable to spring open, but surprise surprise, they did it again.

Is this normal? I keep thinking maybe I didn't steam them long enough or something.

any advice??

John

[Glen Smith]

Laminated stem springback should be minimal if any at all after gluing with thickened epoxy and allowing the epoxy to cure for at least 24 hours before removing the stems from the mold. Making a separate form with 20% overbend is not a good idea IMHO because IF you get springback, it will be unpredictable.

I have had some stems springback maybe 1/2" after gluing but they were still fairly easy to install and this did not compromise anything.

After removing the glued-up stems from the mold, tie some string across the open end (like a bow) to keep the shape required until ready to be used.

[JimND]

Some springback is normal, how much is hard to predict for even experienced woodworkers unless they are really into steambending and have done it for hundreds of hours. Even then, different woods are different. The best way to know if you have applied even steam and gotten the wood hot enough is that the wood will bend to the desired shape and feel fairly relaxed in that position. If it wants to springback much while in that position and takes a lot of force to hold it in that position, you probably have not gotten it hot enough or the choice of wood was not appropriate. Even done correctly there will be 1-5% springback. 4" seems a little excessive but that also depends on how long your stem is. On a short stem that is less than a foot, that would be excessive, but on a long stem that is 18-24" you can probably still work it. Obviously the less you have to force it to the required shape, the easier it makes the task, but you will have to be the judge of that. How much force does it take to get it back tight to the form?

[KARKAUAI]

I found the whole steam bending thing difficult and unsatisfactory, maybe just not patient enough, but I gave up on it. You can accomplish the same thing with a heat gun, or cut 1/8" laminates and don't worry about steaming or heating (what I'll do next time).

[Dean in Eureka, CA]

I built my stems three times and all three sets were acceptable.
I did change my mind after the first set and switched from basswood inners to ash inners.
I only let mine dry on the form for one day, but then tied them in position with a rope and snugged them a bit tighter than the bend on the form and let them dry haning up for about a week. The second set was fine, I was worried that they were too narrow after I planed the laminations, if only I had known... All the extra little bit of width I had in the third set (Inners) got turned into dust with my sanding sticks during shaping.
I made good use of my two "practice" sets.
I took the outers and ripped them into layout guides for my centerline and 1/8" leading edge.
I clamped my final outers on top of my "practice" inners to keep the perfect bend during constuction of the hull. (Oh yaa, I accidentally made my stem forms too wide with double layers of plywood, so I had an extra stem form that was wide enough to mount both outer stems and "practice' inners on while I was doing the stripping)

Just a little extra work there....

[John Michne]

I have used hot soaking (boiling), a steam box, and a heat gun. The heat gun works best, and is the quickest with no drying time required - glue the stem up as soon as it cools. My web site has the illustrated details. Go to http://michneboat.com , click on Builder's Corner, then scroll down to Making Stems.

- John

[Shuswap Pat]

I have tried steam bending stems and beck beams for kayaks with mixed sucess. When it works, it is great, but when it doesn't - ARHHHHHHHH! If you are going to steam bend, make sure your wood is NOT kiln dried. Air dried or green is best. I have also tried Johns method with the heat gun, and that works too. It is real handy for straightening long skinny pieces like the shear clamps on a S&G kayak. The best sure-fire method I have found is to cut thin strips, 1/8" or less, and a good adheasive. They will bend easily, and willl not spring back. If you cut them out of one piece, mark them, and glue thme back in the same order, it will be hard to tell they are not one solid piece.

Pat

[tptbum]

I have concluded that maybe I did not steam them long enough. I think the ash was kiln dried, I steamed it for about an hour, but I think my steam tube was too big. I used a 4" PVC pipe (I had some handy scrap) I think if it were 2" or 3" the density of the steam would have been better. Anyway it never bent easily, it always fought me pretty hard to get on the jig. After I had put the epoxy on it was very hard to get it back on the jig.

I will go back out to look for more ash (air dried), perhaps I will try the heat gun next time. It gives me hope that I am not the first one to be frustrated by this step in the process.

My strips are milled and I am anxious for something to glue them to.

John

[davidb54321]

John, I used a piece of 2 inch pvc and it worked fine. I know how you feel, finally getting strips milled and forms set up and NOW THIS! Patience. Before long you'll be making good progress every day and enjoying yourself.

[JimND]

To add to my comments above. You didn't mention if you are building a laminated or solid stem. I usually don't mess with solid outer stems because I think laminated are stronger and more stable. I normally laminate my stems from 1/8" strips. I also normally just soak them down with water about 30 minutes before forming and then use a heat gun. I seldom steam bend anything for boatbuilding, although I use it for furniture.

If you want to use solid stems, then you should try to find air-dried wood, which may or may not be possible in your area. Kiln-dried will generally not bend as easily as air-dried, but it will bend. I soak my wood for at least one hour before forming, longer if I know a particular wood is very dry, especially this time of year as our humidity is very low up here in the winter. It takes a certain temp for the lignins to become elastic, and the general rule of thumb is 1 hour per 1 inch lumber thickness, assuming you are generating enough steam. With lumber under 1/2", you can coax it with the heat gun if it isn't hot enough from the steam box, but with a full 3/4" lumber this becomes less effective, but you could try it. Your wood should be coming out of the steambox too hot to handle with bare hands. If you can handle it, it is not hot enough.

[mtpocket]

My steaminer tube is 4" PVC 5 feet long, capped on one end and has a 1-1/2'Y for the inlet. My gunnels are 3/4"x3/4" kiln dried cherry. The first attempt failed at bending. I steamed for one hour, removed the gunnel and tried to clamp it to the hull to the exact curve of the redbird. It was like bending steel. I had waited too long. I made a template, transfered it to a junk piece of plywood, attached cleats and tried again. This time I steamed for thirty minutes, removed the gunnel from the steamer and immediately laid it on the jig and made the bend. Everything went great. It was like bending rubber. I left it clamped on the jig for thirty minutes to let it cool. When I removed it from the jig, it never changed at all. The trick is to make the bend quickly, within the first 15-20 seconds after removing it from the steamer.

I tried the heat gun several times on another gunnel while one was in the steamer and never really got the hang of it. In order for it to get hot enough to bend, I kept on charring the surface. I spent more time with one curve and the heat gun than it took to steam one so I gave up. Many have mastered this but I have not. In addition, I had to spend several minutes sanding away the burn marks. Those from the steamer had no signs of raised grain or burn. Also, I was free to work on other things while a gunnel steamed. With the heat gun, my hands are full. I have seen where some write about having to wait a couple days for the steamed wood to dry. This is true unless you use the heat gun to speed this along. I didn't do this because I had other things to work on. My way of thinking is that if you bend the gunnels before you are ready for them, they will have plenty of time to dry. The stems can be bent and left to dry while you are building the strongback and cutting out the other stations. My vote go to the steamer.

[Patricks Dad]

I'll cast my vote for steam as well. Last night I attempted to bend some cherry stock 1/4" X 3/4" as tightly as I could (not for stems but for an inside curve on decks we are building). Information I've seen so far says 6" radius is as tight as you can go. This wasn't tight enought to suit me. I built a form with about 4.5" radius to experiment with.

I tried the following:

A. microwave oven: A flat glass pan with 1/4" of water in the bottom. Layed the cherry piece in the water and nuked it (it's only 16" long). After about 25 minutes (flipping it periodically to keep both sides wet), I was able to bend it fairly tightly but it did break before I could get it completely around the form. With an extra pair of hands (Patrick's out of town), I might have been able to do it quick enough to complete the bend. But I gave up and went to plan B.

B: Heat gun: I had worse luck with the heat gun. The wood scorched before it would bend. I hadn't soaked the wood in water prior to the attempt. That probably would have made a big difference. I wasn't able to get much of a bend before the (scorched) cherry broke. I quickly moved on to plan C (I probably didn't give the approach a fair shake)..

C: steaming: I soaked 8 pieces of cherry in the bathtub over night. This morning, I re-habed my steamer to use a shorter narrower PVC pipe and fired up the grill. I was able to bend 4 pieces of cherry simultaneously aound the 4.5" radius form without much difficulty. As my form was 1.5" thick, I was able to stack 2 sets of 4 pieces of wood (one group above the other) on the same form. They are drying on the form now.

My gunwales are gonna get steamed...