Subject: Re: [baidarka] knot question for the experts
From: Peter Chopelas (pac@premier1.net)
Date: Tue Mar 04 2003 - 22:19:01 EST
Wayne wrote:
> The one imperfection is a small knot that runs top to bottom of
> one board, never showing on the wide face of the board. The knot is 1/4
> inch dia on top and 3/8 on bottom, is 3 feet in from one end. the gunnels
> are 3/4 inch thick, so it just barely attains half the thickness.
I always use knotted wood and have never had any problems, I would rather do
that than throw away an otherwise perfectly good piece. I just position the
pieces with the knots in places where their effects are not important, or
minimized. This way there will be no loss in strength because the
critically loaded parts will fail before the ones with the knots will. I
have used even stringers with knots, I position them in the outer third of
their length, always keeping clear wood in the center third. I have never
failed one.
If the knot is centered in the board, it probably does not affect the
strength that much anyway, unless it ends up right in the middle of the span
of the final member (where the bending stress would be highest).
I would not put it in the splice since likely the knot wood is harder and
less flexible than the regular wood. The wood on either side of the scarf
would not flex the same, introducing an internal stress riser right in the
middle of the scarf. Also try and avoid any fasteners or mortises through
or near it since it could split out for the same reason, it is very hard and
brittle.
I would position the knot furthest towards the ends of the finish piece
(away from the center), and away from any scarf or mortise. The side with
the smaller end of the knot showing (1/4 inch) should be in the tension
zone, and the larger in the compression zone (if applicable). The
compression zone is the upper or inside when you bend a piece of wood ends
up (in a "smile"), the tension is the lower side, or "outside" of the bend.
In a normal gunwale installation the upper edges are the compression edge,
and lower edge is the tension edge.
You also may get a bit of distortion around the know as the wood dries and
ages, it may not matter much if the piece is held in place securely with
lots of lashings and bracing. But just to be safe, try and use it in a
place where a bit of warpage will not effect you much (near the ends and
above the water line).
If you are concerned about strength right around it you can simply splint on
the outside a small piece of clear strait grained scrap (make the splint 5
to 7 times the width on either side of the knot if possible). This would be
especially important is there is lots of grain runout around the knot.. That
is where the grain has to pass round the knot, it gets cut when the board is
made. The splint will add strength but will affect the way it flexes. You
again have to use some judgment on where you used. I have done this over
knots on the keel, with the splint on the top side.
Doing these kinds of things I have been able to make use of almost all of
the wood I rip down from a plank, with little scrap. Though it does take a
bit of judgment to understand where the critical loads are on each member,
and position the knot, or other defect, accordingly.
Peter
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