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From a Bare Hull:  Cabin Trunk (Page 7)

Cabin Trunk Deck Beams

The next step in the cabin trunk construction process was to laminate and install a series of transverse deck beams across the opening between the two sides.  The width and length of the opening called for 5 or 6 laminated beams to support the overhead, depending on the size and spacing of the beams.  Because of limited overhead clearance to start with, I intended to keep the beams as low profile as possible, so it looked like I would shorten the spacing and use 6 smaller beams to span the area.

Much earlier, I had transferred the shape of the overhead curvature, as defined by the shape of the midships bulkhead (at the aft end of the cabin), to a scrap of pine for future reference.  Now, I used a jigsaw to cut out the shape as a template, for marking purposes only.


With 6 beams to fabricate, I hoped to build several duplicate laminating molds so that I could speed up the process and laminate several per day.  To build the simple molds, I began with a sheet of plywood, which I covered with plastic for protection.  Then, I marked the shape of the beam template directly on the plastic in four locations.  Since the curvature required was not extreme, I built the molds of small blocks of scrap 2x4 that I screwed to the plywood in 7 locations for each mold.  This became tiresome, and after building two molds I decided that finishing the beams in three laminating days would be just fine, so I called it quits and moved on to other related tasks.

Next, I had to mill the various strips that would form the curved beams.  From some mahogany stock, I milled a total of 18 strips, each a little over 1" wide and about 3/8" in thickness, and close to 8' in length.  I only needed a maximum of 7 feet for the widest beam, but for the moment kept all the strips uniform. The 18 strips, at three strips per beam, was enough for the 6 total beams required.  The milling process took the remainder of the afternoon.

Over a period of three days, I laminated the six total beams required to span the coachroof opening,  two per day.  I glued each beam from three strips of mahogany set in thickened epoxy resin adhesive, and clamped them tightly to my forms for 24 hours till they cured.  At the end of the period, I had six messy beams ready for final milling.

Since the beams were laminated with one side pressed against smooth plastic, that side was relatively flat and smooth, and was suitable for running against the planer table.  Therefore, I could use the planer to remove the bulbous epoxy spillout on the top edge, and acheive a uniform thickness.  Over a number of passes, I cleaned up all six beams till both sides were smooth, and the beams were consistently about 1-1/4" in width.  Now they were ready for final fitting and installation.


First, however, I needed to work on some layout.  Since I had planned a keel-stepped mast, my first order of business was to locate a beam on each side of the mast step, to begin the reinforced area around the future mast partners.  To locate the first (after) of these beams, I laid a strip of plywood (one that I use in the shop as a straightedge) across the cabin trunk and aligned it so that it was parallel with the midships bullhead.  With a plumb bob, I located the after edge of my built-up mast step in the bilge, and aligned the plywood so that the bob was just a bit aft of the step. 

After double-checking the measurements from the bulkhead, I marked the cabin trunk on each side, representing the forward edge of the beam.  Then, I made a second set of marks to locate a beam on the forward side of the step area.  The distance between these beams turned out to be just over 9", and was close enough to the forward edge of the cabin trunk that no more beams were required there.

With the first two beams located, I laid out the remaining four beams.  By pure dumb luck, the space remaining between the midships bulkhead at the aft end of the cabin and the beam aft of the mast partners turned out to be such that the spacing between the remaining beams, keeping them evenly spaced, ended up within a sixteenth of an inch of the preexisting spacing already determined by the two mast partner beams, so all six beams ended up evenly spaced.  This was a pleasant surprise.

With the remaining four beams located, I moved on to trimming and installation.  Each beam required angled cuts at the ends to allow them to fit into similarly angled notches that I would cut in the cabin trunk walls.  With each beam (one at a time) set on top of the cabin trunk in the proper position, and using marking and cutting techniques more or less identical to those used when I installed the short sidedeck beams earlier in the building process, I first marked the end of the beam at a point about halfway through the thickness of the cabin trunk, or about 1/2".  I had previously marked the fore and aft edges of the beams on the trunk, during initial layout.  Then, I marked the bottom edge of the deck beam at its intersection with the cabin trunk, which would equate with the bottom of the angled notch once the beam was pushed downward into position.


    


Other marks required on each beam included the top edge, which was in line with the mark on the cabin trunk for the depth of the notch, and plumb marks on the inside of the cabin trunk itself, as well as a final horizontal line at the depth of the beam, which signified the final edge of the notches that would be cut into the trunk.

(Note:  this is a challenging process to describe.  It would be easy to show you, but it's hard to put clearly into words.  Please forgive me.)

With the various marks made, I cut the ends with a small backsaw, and then cut out the angled notches in the cabin trunk with the same saw, holding cutting carefully along each edge of the marked opening and angling back to the mark halfway through the thickness of the trunk.  Then, I made several additional cuts in the center to make chiseling out the waste easier, which was the next and final step.


    



The first beam required several minor adjustments (shortening) before it fit snugly into place, since it was entering into a highly curved section of the cabin trunk.  The remaining beams' fitting went quite a bit quicker, but the entire process was still rather drawn out and labor intensive, and took up the bulk of the day.  Note that the final beam, not seen in these photos, would end up directly against the bulkhead.


    


In the end, though, the beams clearly and pleasingly defined the shape of things to come for the coachroof.  It was clear that some beam fairing would be required, particularly at the forward end, to ensure that the coachroof maintained the proper curvature had angle forward, but this was to be expected.

 


With the fitting complete, I was ready to prepare for final installation.  Before removing the beams, I marked 2" in from each intersection with the cabin trunk, on each side of the beam; this was to locate the stopping point for a chamfer detail that I planned to rout on the lower edges of the beams, in keeping with the chamfers on other deck beams throughout the boat.


Down on the bench, I routed the detail on all 6 beams, and sanded each with 120 and 220 sandpaper until they were smooth and ready for finishing.  Then, I installed the beams permanently with thickened epoxy resin (using mahogany wood dust for color), and a single bronze screw in each end, driven in from the top of the beam where it wouldn't be seen.

Once the beams were installed, I pretty much left the boat alone for the rest of the day, as I didn't want to accidentally strike or run into one of the beams while the epoxy was still curing; the beams were so low that it was very easy, and common, to accidentally run into them.

When the epoxy cured, I sanded the joints and began the varnish process on the beams, which I wanted to complete before installing the overhead permanently.  Over several days, I built up a number of base coats of gloss varnish on the beams and inside of the cabin trunk, which really started to bring out the character of the wood.

Next:  Installing the coachroof.  Please click here to continue.>

 
 
 
 
 

All photos and text on this site 2002-2009 by Timothy C. Lackey and Lackey Sailing, LLC
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