Stem Forms

[ticonderoga]

This year for our winter project, my son and I will be building a 16 foot canoe off of a form we built from plans from Stewart Boat Works. I will be building the form for the stems shortly and would like to know how much "springback" can I expect and how do I alter the stem profile to take this into consideration. We assume that the stem must be dead on the plan profile when we take it off the form to end up with the desired curve. Thanks for your input, Joe

 

[Worth Gretter]

Joe, I made stems recently and had a lot of trouble. The first ones split when I steam bent them. I'm sure part of the problem was the oak, which had a little bit of grain run-out to the side. I had tapered and beveled them before steaming, which was also a mistake since all the work was lost when they split.
So my second attempt, which worked fine, was to laminate them in three layers. Actually, it was all one piece of wood for each stem, because I just cut two slits most of the length, stopping before the last foot or so that is straight and sits on the bottom ribs. Then I steam bent them and after that epoxied them. One advantage of this method is that any springback after steaming can probably be overcome by the strength of the epoxy. Finally, I beveled and tapered them.
Hope this helps - assuming you want to laminate them. It's not completely authentic and traditional, but it doesn't show due to the bevel.
-Worth

 

[MGC]

WRT form, the stems will indeed spring back. Consequently, overbending them is necessary if you want to try and match to plan. I generally make my form so that the part that is inside the hull is pulled at least a few inches or more past where it should be. I make the form so that the tip and the curve of the stem match the plan. Then, I soak, steam and bend. It is important to pay attention to the grain of the wood, to make a few extras, to use a backer band and to work fairly quickly. I make sure that when the piece comes out of the steam that I get it bent and fixtured immediately. And then I let it sit...the longer the better. A week or more is ideal.
So what if the stems spring a bit? Matters not. When you assemble the hull, you will center and square the rails to the stem and tack them together. You can move them around to fit your form. It's not until the decks are in and the planking is on that you lose the ability to adjust the shape. And, symmetry is nice, but I can guarantee you that there are not two canoes that have ever been built off of one form that are perfectly identical. Not to suggest that perfection should not be a goal, it's just not all that apparent in the finished product.
Personally I would not laminate stems. I've never had a problem making either inside or outside stems.... you may wreck a few as you learn, but it's par for the course.

 

[Michael Grace]

Agreed with springback and assembly comments. Boatbuilding is not an exact science, and deviation from exact standards is tolerated in the construction process. As Mike says above, you can move things around to get them where you want them. On a recent restoration I compared the stem curves from the two ends of this factory-produced canoe (a symmetrical design), and they were strikingly different.

To add to the steam bending discussion, you certainly can bend solid wood with care and consideration. Here is some straight-grain, air-dried white oak that I recently bent to a strong curve. The bending stock was 1"+ (the face that you can see) x 2". I bent the single piece, then sliced and shaped two stems from it.

As for springback, I don't have a formula; just guesstimate based upon experience. Somehow it usually seems to work. The hard curve doesn't seem to change a great deal but the top and bottom of the stems are likely to move considerably with relaxation. Thus, when making a form I draw the curve accurately, then draw and cut the top and bottom of the form a bit tighter, leaving the hard curve alone.

Installed (ignore the paint, please; it's not finished yet):

 

[Todd F]

Michael, what is the black material you have between the bent stem and the clamps?

 

[Andre Cloutier]

steel band to prevent tension failure during bending. most necessary on extreme bends. remember too, after its split into single stems they are somewhat moveable and you can compensate for some springback when mounting them. like Michael says, guesstimates and experience rule the day. if its all too much, laminate and shape.

 

[ticonderoga]

Thanks for all the advice. I will bend both stems as one piece to be cut later, and I’ll add some bend to both ends of the stems and leave the hard curve as on the plan. Joe

 

[Worth Gretter]

The steel band is very helpful. I had trouble finding a suitable strip of metal but settled for something from a big box store that halfway worked. I think I should have looked at Metal Supermarkets.

 

[Todd F]

Looked like a flexible plastic, which may work. Had not seen a band that looked like that. Thanks all

 

[Dan Miller]

Michael, what is the black material you have between the bent stem and the clamps?

Looks like the Lee Valley Strap Clamp (https://www.leevalley.com/en-us/sho.../42173-veritas-strap-clamps-for-steam-bending). Notice in addition to the strap, Michael's photo shows the ends of the stems being constrained in compression. That's another key for success.

 

[patrick corry]

I learned a significant amount watching Engels Coach Shop on YouTube. The host is a traditional coach and wagon builder in the western US who employs lots of steam bent parts in his work. He has several great videos on bending methods. Impressive work.

 

[Todd F]

Thanks Dan, exactly what I was looking for.

 

[Dan Miller]

I should have mentioned that I've made straps from lumber strapping that resulted from dumpster diving at Home Despot.

 

[Michael Grace]

Good eyes all. Dan has it - my strap in the photos is from Lee Valley. Last I looked they offered a larger version and a smaller version. Great product. Like Dan I've also made compression straps myself from found metal strapping. However you get it, a good compression system makes all the difference in the world. Then too, so does quality lumber, and soaking and steaming properly. The compression strap is more than just supporting the outside face of the wood. As you're trying to finesse a piece of lumber into its new shape, it experiences very different forces on the inside vs. outside of the bend. A good approach to bending is about avoiding cracks and breaks from stretching and bending on the outside face, while at the same time avoiding compression failure on the inside face. Obviously it can be done, so read, discuss, and try. Success is very satisfying.

 

[ticonderoga]

For the compression band, I was going to use an extra band from building my canoe form. Will this work?

 

[MGC]

It should work perfectly as long as it is not too "stiff". You want it to be able to "follow" the wood as you bend it while backing and supporting. If it's too "stiff" it might not stay as tight to your stem as you need it to.
These Lee Valley straps are intriguing. I wonder if that is what Rollin used in the figure 8 bending demo.

 

[Rollin Thurlow]

I think half the fun of canoe building is completing a successful steam bending operation. Of course half the frustration of canoe building is a failed steam bending operation. I do have one of Lee Valley's bending straps and it works great. Its very effective but it really is almost a two or three man operation to hold the handles and another person to loosen the pressure on the wood. I have used it when I'm doing steam bending classes to see how tight of a curve I can make with it and have gotten down to a 4" diameter circle with 3/4" square stem stock. The one in the picture was a 6" diameter, but over the years it has opened up a bit. There was so much pressure on the strap it took two people on each handle to do the bending and another person operating the ratchet to loosen the pressure.
In the shop, for almost all the canoe bending needs, all I use is a 1/8" steel strap that has a hook bent at one end. The hook acts as a stop for one end of the rail or stem. At the other end of the stem or about 6 ft down the rail, I clamp the steel band to the stem or rail. using a pad of wood to protect the rail or stem. This system acts like a poor mans Lee Valley strap. The bent hook prevents one end of the wood from slipping. If the wood can not move in the strap, as it is bent, it puts the wood in a great deal of compression and the wood will buckle on the inside of the curve. Fortunately almost all the time the clamp will slip just a bit even when its really really tight. This slipping will release a bit of the compression but if it slips too much then the outside surface of the wood will be in too much tension and the face of the wood will split which is the most common failure that occurs in steam bending.
The nature of all woods is that it can be compressed much more than it can be stretched or put in tension.
When bending a tight curve such as on the Molitor there is as much as a inch of difference between the length of the inside surfaceof the stem and the length of the outside surface of the stem. The inside of the stem will try to compress 1/2" and the outside will try to stretch 1/2". The outside can not stretch that much so the face of the stem will split. By securing the stem to a steel strap so that the wood can not move, when the unit is bent, the wood can not stretch because of the stops on the steel band. The more the unit is bent the more the wood is compressed, until the inside surface of the wood fails and the inside buckles. Before that happens, if a bit of pressure is released from one of the stops, the wood can stretch a bit , the inside compression is relived just a bit and the outside surface is put into a bit more tension.
For strap material, thin sheet metal will work just as good as the heavy steel, its just harder to install stops, clamps slide off and the bent hook system doesn't work. When I need a wide strap such as for a 5" wide coaming which is fairly thin, I cut out what I need from a roll of aluminum flashing stock. For more serious bends such as for a 3" wide Morris stem, I'll use a 1/8 wide steel strap that is 4" wide.