A Visual History
The Building of Aslan
Cabin Hatch / Companion Way 
Door Combination
Updated 08/26/00
I decided to incorporate a slide-out, swing down companion way door into the hatch itself.  In order to do this, the hatch sides have to be 4" high to allow the door to clear the curve of the cabin top, which makes it much too high to use molding on grab rails as the track as shown in the plans.  So a track has to be added to the top of the cabin.  In this picture, you can see the track, the front of the hatch, the port side of the hatch with a 1" wide strip of UHMW plastic on the bottom, and the half lap cuts in the front and side piece.  A butt joint would work as well here, as the hatch "floats" on top of the cabin and is not stressed by the vibration of the boat as other joints are.  But I like to make compound joints (sick, I know.)
This picture shows the track on the top of the cabin on the left.  It is 1 1/4" wide x 3/4" high and 40" long, with a 3/8" x 3/8" rabbett cut in the outboard side (it is barely visible in the end of the track.)  One track runs along each side of the cut out in the cabin top.  I made the track out of mahogany, which will be stained and oiled later.  The forward end, shown in the picture, is rounded, and the aft end is cut on an angle to match the cabin bulkhead angle. 

The sides are 4/4 red oak, 4" w x 27" long.  The bottom edge is cut on a 14 degree angle to allow the sides of the hatch to narrow slightly at the top, while compensating for the angle of the cabin top.  A rabbett was then cut 

in the bottom of each side, on the inboard side, for the runners.  The rabbett is 1/2" wide and just 3/16" deep, allowing the runners to stand proud of the bottom by 1/16".  Ultra High Molecular Weight (UHMW) was used for the runners -- UHMW is a very slick plastic, almost like teflon, that is long wearing.  It is available from most plastic suppliers, and in woodworking supply catalogs, in various thicknesses and widths.  It cuts easily with a table saw.  The runners are 1" wide, 1/4" thick so they project out 1/2" to lock in under the rabbett in the track.  The runners are shorter than the track -- just 21 3/4" long --  to allow the hatch to lift off the cabin top before it reaches the mast (they are positioned toward the bow.)   At the aft end of each side, the oak was trimmed to match the angle of the cabin bulkhead.
This picture shows the front, port side corner of the hatch and track.  You can see the half lap joint I probably spent too much time on, the track with the rabbett on the outboard side, and the UHMW plastic runner under the side piece.
Here's a wider view of the hatch frame.  Because of the curve of the cabin top, I put the tracks on first, then the sides with the UHMW plastic runners under the tracks, and measured for the front and the rear brace.  The curve for the front was taken from the curve of the cabin top, trimming it to allow 1/4" clearance while the assembly slides back and forth.  A 3/16" groove runs the length of the sides, 1" down from the top on the inboard side, to accommodate the brass pins in the slide-out, swing down companion way door.
I used the piece cut out of the cabin bulkhead to make the slide out, swing down door.  To make it overlap the opening, I used 1 1/2" x 3/4" fir, with a rabbett 1/2" deep and 7/8" wide cut in it (a piece of it is in the photo.)  Lengths were glued along the bottom and sides of the cut out piece (the sides were longer than 21 1/2" at first, and then trimmed after the top cross member was put on.)  The curve of the cut out piece was trimmed off flush at the top, 20" from the bottom of the cut out piece itself.  Then the top cross member was made, a 27 1/4" long piece of fir 1 1/2" wide and 1/2" thick.  The rabbett in the top of the side pieces was extended out to the sides so the top cross member could sit just proud of the cut out piece.  You can see small triangular pieces filled in at the top corners.
This shows the door in the hatch frame for one of many "test fits."  At the bottom, you can see the rear support that has the same arch as the front of the hatch, but is recessed just 1/2" into the top of the sides.  On the door assembly, 1/4" brass pins are glued into the ends of the top cross member to ride in the grooves on the hatch sides (you can see one of the pins in the top left of the photo above.)  Using test fits, and making sure there is plenty of play available to avoid binding, my pins protrude from the top cross member 9/16" of an inch.  I used brass "sheer pins," available at marine stores, which are normally used in outboard motors.  Less than $2 for both pins.  Brass "mending straps" installed across the groove toward the cockpit provide a stop for the pins; removing these straps will allow the door to be removed for maintenance.  A removeable cross member, simulated by the strip of oak under the door in the photo, will support the door and provide a stop mechanism so the hatch cannot be pushed out of the tracks inadvertently.
Here's the hatch on the cabin top, with the plan's original port style mocked up with paper cutouts.  The higher hatch does change the profile of the boat, so we decided to experiment with different port designs.

The hatch has oak flooring installed on top.  It was sanded and stained, then sanded with wet sandpaper and generous amounts of boiled linseed oil.  Sanding with lindseed oil makes a slurry that further seals the pores of the wood, but won't interfere with marine varnish later on.

We decided on 5" round port holes, and went to three of them instead of two.  It just seemed to fit the new profile of the boat better.  Once I get my new band saw, I'll make mahogany port trim instead of the 1/4" fir plywood shown here.
A side view of the hatch, as it will be positioned when closed.  The 3" overhang will match up with a 3" wide hand hold to assist when climbing into the cabin (this will visually connect the hatch to the cabin bulkhead angle.)  The overhang is necessary to provide rain cover when the slide out, swing down door is closed.
I should stress that this hatch design is not waterproof, although I think it probably will work as well or better than the original method in the plans.  I think the UHMW plastic runners, captured in the tracks, will provide a smoother opening hatch that is less apt to bind.  The slide out, swing down cabin door saves some valuable space, but it relies on overlapping the cabin door opening to keep water out rather than hermetically sealing the opening.  I'll incorporate some kind of latch later on, but I don't think I'll go to the trouble of making a positive seal.
UPDATED 8/26/00
Here's the nearly finished hatch.  At first I thought I would have to incorporate a "stop" for the hatch near the front of the cabin top to prevent it from sliding too far forward.  But I designed a way to make the hatch stop when moving it forward, support the companionway door while its sliding, and make it easy to remove the entire hatch all at the same time.

I had already designed a "stop" when sliding the hatch back to cover the companionway by gluing laminated strips to the cabin top.  They were located so that when the hatch front hits the laminated strips, it stops and the hatch extends 3" over the door.

This is a length of 1/2" PVC pipe with caps on the end.  1/4" brass "sheer pins" have been epoxied into the caps so the pipe will rest in the groove inside the hatch.  Then, grooves were added angling down from the companionway door groove toward the cockpit, and the pipe is allowed to float in this groove.  Brass straps across the intersection of the two grooves trap the pipe in its groove, and prevent the sliding door from hanging up.  This pipe supports the companionway door as its being slid back.  By locating the pipe so that it "stops" on the laminated strips as the hatch is being opened, I provided an internal stop to prevent the hatch from being pushed too far.  The trick was angling the grooves for this pipe down and toward the cockpit, so that the pipe is trapped against the end of the groove, and cannot migrate up as it could if the grooves were simply vertical.
Here, the door swings up and is ready to be pushed back into the hatch.  Notice how the pipe supports the door.
Here, the door is pushed all the way back, and the hatch has been pushed toward the bow of the boat, opening the companionway completely.  Note that the pipe is stopped against the laminated strip (you can see it on the left, just below the left pipe cap.)  This provides a positive stop, unless you reach in to the pipe, and flex it up.  1/2" schedule 40 pipe is just flexible enough to flex up to clear the laminated strips, allowing the hatch and door assembly to be pushed forward and free of the side rails for removal.