Although "knee" can refer to any bent piece of wood that takes stress,
the term usually refers to a particular type of bent piece of wood.
Knees are most commonly used in wooden
boat construction.
Recall that all wood has growth rings which happen to be places of
weakness in the wood. These rings are represented in a piece of lumber
as its grain. If you simply cut, say, the stem of a boat out
of a straight grained board:
___
|///|
|///|
|///|
|///|
|///|
|///|
___________/////|
/////////////////
|////////////////
you're placing the wood's weakest area at the point of its greatest
stress. Your boat will sink in the first storm.
So, ideally, you want the grain to follow the curve of the wood:
___
|||||
|||||
|||||
|||||
|||||
|||||
___________/////|
/-----------'/////
|----------------/
Ok, so wood grain is hard to represent in ASCII.
How do you get a piece of wood like that?
-
Steam bending. Place a length of green wood in a steam box
for a period of time, until it gets soft, then bend it into the shape you
want over a frame. This can be done for smaller knees, such as the
seat supports in a small rowboat, but green wood can't really take a lot
of stress.
-
Lamination. In other words, take strips of wood and epoxy them together,
creating in effect a bent chunk of plywood. Of course, epoxy hasn't always
been around, and really treats a plane harshly.
-
Grow it that way, or find a piece of wood that grew that way naturally.
These are the most satisfying knees to use. Smaller knees can be
made out of the crooks of apple or cherry trees. Although you can
get larger knees out of the places where hardwood (usually oak) trees branch, the hackmatack
(aka tamarack or larch) tree grows in a most fortuitous way. Of course
the trunk is upright, but a larch has a horizontal tap root, making the
whole assembly shaped like, you guessed it, a knee.