: I need to cut a slot in my hull 1/2" wide and about 14" long using
: an awkward router. My thinking was to cut two 1/2" thick plywood
: strips and glue 1/2" spacers betreen them at each end making a
: 1/2" slot. Then fasten this slot pattern over the area I want to cut
: out and plunge a 1/4" router bit through the boat and let part of the
: smooth bit shank guide against the pattern slot sides. It seems like this
: would work.

: John

Another approach is to use a guide made out of relatively thin plywood - usually 1/8" or thereabouts - with a thicker (usually 3/4") piece attached to the top. Once you have the two pieces screwed/glued together, take your router, using whatever bit you will be using to make your cut, and hold the router base against the 3/4" piece. Plunge the bit down far enough to cut through the plywood, and run the router the length of the piece, trimming the excess plywood off. That will give you a straight guide for that router and bit combination that shows you exactly where the bit will cut. You could attach it to your hull with double-sided carpet tape when you make your cut.
Just make sure you use plenty of tape so that the guide doesn't creep while
you are cutting.

______________________________________
//////////// 3/4" piece///////////////|
|-------------------------------------|
| 1/8" bottom piece trimmed to width|
---------------------------------------
bit cuts along this edge

The one thing you have to remember is that the same spot on the router base has to be against the guide (the 3/4" thick piece) every time. If your router base has a flat side, that's easy to remember. If it has a round base, you would probably want to mark the base so that you know which spot needs to be against the guide.

If you need to cut a slot where the guide won't lie flat, such as on the keel of a v hull (like on the S&G boats), you can make a guide that will trap the router. You'll build a rectangular frame into which your router base will fit snugly. You can attach spacers to the underside of the frame to raise it up and level it. You might want to consider cutting just a little bit into the
ends of the frame with your bit so that you know where the edges of your cut
will be. You could also consider adding a plywood base and cutting a slot in it
with the bit you will be using. That would keep the router level, and would let
you know exactly where the edges of your cut will be.

XXXXXXXXXXXXXXXXXXXXX 3/4" thick frame XXXXXXXXXXXXXXXX
X X
X center space sized to fit router base X
X plywood base for jig is optional X
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX