Improving Cabin Top Stiffeners

The project that has been keeping me busy lately has been designing and making the cabin top stiffeners for Windsong.  This project has been one of the most difficult ones for me, mostly out of indecision on which option I wanted to take but also with plenty of setbacks. The original stiffeners are molded fiberglass and were attached to the cabin top with screws and a weak chopped fiberglass and resin mixture.  They did an adequate job of keeping the cabin top from flexing, but never appeared to be that strong.

I didn't realize just how much they were holding the cabin top shape until I began to remove them  and saw the cabin top begin to sag.  Before I removed the bulkheads I am replacing, I had to prop the top up with beams.  As described in this post, I raised the top when installing the beams.  Not very much, but enough to make it a stronger, more convex shape (it was sagging/concave in areas otherwise).

I was originally planning on reusing the original stiffeners and improving their purpose by heavily glassing them onto the cabintop.  Unfortunately, when I started to consider many of the modifications I plan on making to the interior I began to see limitations with using the old stiffeners.

The first problem was the molded light bases.  I didn't want to be forced to place lights where those are, or have the bases bare and ugly.   If I really wanted to reuse the original stiffeners, I considered cutting the light bases out and glassing them in flush.

Furthermore, I am changing the original vinyl headliner to hard panels screwed into furring strips.  The end points of the original stiffeners extended about 1.5 inches, where I only want 1/4" furring strips.  I began to modify the stiffeners to have 1/4" ends, but soon realized I would have to lengthen each stiffener to compensate for the ends I cut off:

I figured out a decent way of adding length to the stiffeners, it would just require some crafty epoxy and fiberglass work...something I am feeling confident in.

But then I brought a couple of the stiffeners back to the boat to get some measurements on length, and the final straw broke on using the original stiffeners.  I tried to fit one in position and the curve of the cabin top was now changed due to the support beams and the original stiffeners didn't have the correct shape.  They are hard fiberglass and not very flexible, so fitting them now seemed more trouble than it was worth.

Therefore, I concluded making new stiffeners would be necessary.  I mused for many weeks over the decisions leading up to this, and continued for many more trying to figure out the best way to make them.  Originally I decided on using some pre-formed foam to glass in, such as these.  Unfortunately these would cost nearly $40 per stiffener (6 total, 7 maybe) and that seemed ridiculous to me.

After much contemplation, I decided to use strips of plywood laminated together and heavily glassed in.  Once glassed, they would be faired and painted; with the original teak trim installed.  At first I figured that I would rip 1/4" ply into strips and laminate them together.  I wanted to make them approximately 2"H x 1.5W" x whatever the length is of each stiffener.  This would require about 8 - 1/4" ply strips each.  I had a sheet of 1/2" ply that I wasn't using yet, and decided to experiment with it first.  If the 1/2" strips were flexible enough to conform to the cabin top, it would take only 1 sheet of ply for all of them and would make this project a lot faster.

I took the ply over to a friends table saw and we ripped it into 2" strips.  I then took the strips to the boat to make some test bends, and it all worked out well.  I had my materials, my design, and I finally got on with the construction.

The process started by grinding the cabin top where the old stiffeners were in preparation for epoxy/glass.  Of course, it was a dusty mess and it was awful having all of the at fiberglass flying around inside.  But good glassing technique requires it.  I found the spots on the cabin sides where the stiffeners would end, and glued on a plate of 1/4" ply that will be part of the furring strips:

I then measured and cut the first of three layers for each stiffener.  The first layer is then screwed into the cabin top, forming the proper curve of the stiffener.  Before screwed in, the beam and cabin top is wetted out with epoxy, then the beam is slathered with cabosil thickened epoxy.  When screwed in, the glue squeezes out and I create a filet:

I repeat this process for each stiffener two more times, waiting a day in between each layer for the glue to dry (so screws can be removed).  The final beam is rounded on the edge for easier fiberglass layout (same with the filet).  I made sure to form a thick and solid filet on the end pieces.

After two layers:

All three layers glued in:

As of now I have all of the stiffeners glued into place, and they appear to be quite strong as is.  But with a couple layers of bi-axial cloth layered over them will really make them tough and the cabin top should be much improved.   After laying down the glass, I will fair and prep for paint.  Once these are finished I will finalize the installation of the bulkheads, and be ready to focus on the deck paint prep.  I have decided to hold off paint prep on the interior until I finish the decks.  Too many leaks make keeping the interior clean a difficult task.  For a preview, here is the new main bulkhead being fitted: