: I have just finished my Guillemot and for a fall project I have decided to
: install a skeg. It is a simple matter to cut the hole in the deck,

Why do you want a hole in the deck ? The "traditional" form of drop skeg
retracts into a narrow box entirely inside the boat, with a control running
through a tube to some mechanism near the cockpit.

: but how
: do I cut a hole in the hull so that it lines up with the hole in the deck?
: I don't think a jigsaw will fit in the boat, it's too tall.

Logically, it is going to be easier to cut your slot from the outside. The
technique needed is just the same as if you were building a skeg in a boat
before the hull and deck were mated. Cut the slot narrow and then widen it
out carefully to the size you need, keeping it nicely centred. You can
start the cut the way Nick's book suggests for cutting hatches, carefully
lowering a running saw onto the line of the cut. But a skeg slot is going
to be at a point where the hull has a considerable deadrise, so the blade
might get pushed off to one side. I started my cut by drilling a hole on
the centre line (unlike a hatch, you don't need the bit of wood you will
remove) and keeping the saw vertical.

If the hole ends up a bit ragged after cutting and sanding, you will still
fill any gaps with your epoxy/wood-dust mix when filleting the box into
position. Being on the bottom of the boat, even a really tatty job will
not affect the looks of the boat too badly, except perhaps when you are
rolling :-) But getting a good slot is easier than it sounds - I'm sure
you'll produce a neat result.

Andy