Mid-Deck: Puttin' on the Lid
Well, mid-winter progress has been slow. Not steady. Slow.
Our original plan was to button up the exterior of the boat by the end of autumn, then spend winter puttering on the (heated) interior while coursing through sail and upholstery projects at the cabin. But no. Our boat remains unenclosed and therefore unheated, and the cabin has been teeming with construction personnel. In other words, both our work environments have been only available in fits and spurts.
Still, we've managed to creep the mission a bit.
I favor two kinds of deck in a Triloboat; flat, sloped decks at the ends (more of these in future posts), and a curved, constant section mid-deck.
Sheet materials, when curved, assume one of two sections; conic and section-of-cylinder. When longitudinal edges are parallel sections-of-cylinder result.
|In the mid-ground (2nd and 3rd from front) are two bulkhead arcs |
defining the shape of the mid-deck.
The darker hued one is the template, from thin ply.
A box barge/scow from sheet materials lends itself to parallel sheer lines, both in plan and profile. Consequently, section-of-cylinder decks are the easy choice. If a curved sheer is desired, fauxworks are possible, but multiply complexities through adjacent structures.
All longitudinal lines across such a deck run flat, straight and true, with layout and construction ultra-KISS. The sections of such a deck (as if sliced like a loaf of bread) are constant... a single template can be used for the deck arc and the hull which it covers. Transverse members follow identical curves at any point along the deck's length. In this case, the outboard foot on each side was allowed to run straight, resulting in sloped planes. This eases bending sheets, and simplified the one foot wide side decks along the Pilothouse.
For this boat, we're using ply-foam-ply (aka SIP) construction. Since heat rises, insulated overheads are especially effective, and - if nowhere else - make an attractive return on investment in warmth and condensation avoidance.
Generally speaking, in any arced, composite deck, the lower panel will be the thicker to equal thickness, relative to the upper. Both are working in compression, and the lower (following a smaller radius, tighter curve) bears most of the weight. The top layer mostly has to be thick enough to stand up to point loading (dropping an anchor, say) and spread loads without deforming the filler material (foam) between layers.
In our case, we've been happy with 1/2in lower and 1/4in upper, separated by 1 1/2in foam. This saves 25% of deck ply weight for about 125lbs, all of which is high in the hull. That's the equivalent of a small adult on deck.
We sized the first layer, using the mid-line as reference, and trimming to land on bulkheads. Full-width edges land on the sheer logs, with about an inch of overlap (1in/ft of crown). Bulkhead upper faces are insufficient landing for single layer construction, but by the time all layers are added in ply-foam-ply, there's plenty of total surface area. This layer is glued to the hull structures with 3M5200. We glued, and fastened along the mid-line, then bent the ply down and fastened, working our way outboard.
Glue was to have been water-based TiteBond III, but it has been defeated by winter cold/wet. So Gorilla Glue (Liquid PolyUrethane) is doing the job. We try to work at temps above 35... despite warming the glue, if the ply is too cold, GG goes to heavy honey consistency, and it sucks up considerably more. Bad news both in terms of supply and $$.
|We're using the left hand option. |
Note the But over the wide 2x framing overhanging the hull side.
Longitudinal framing is 2x4, with 2x5 along the outboard edges. Framing provides nailers for the ply sheets, helping to keep close contact between ply and foam. Down the mid-line, the port and starboard sheets butt along one of these. Outboard, they overlap the 2x5 which is extends outboard to form eaves.. later, strips will be added outboard to complete the ply run to eave edges. Caps will be added to the 2x5's outboard edges to provide a stable basis for the eventual fabric/resin coating.
Transverse framing is built up from two layers of 3/4in (= 2x). We glued these up, using the salon bulkhead as a jig, then cut them to fit between longitudinals.
|Slightly over-bent to spring back about true.|
We put a nail at one end to maintain position while allowing sliding as ply strips bend to spec.
Note the plastic to protect the blkhd from glue drip.
We cut the foam with a Japanese saw (very thin blade; cuts on pull stroke), checking with a Quick-Square to ensure a right angle cut. Most framing ended up plus or minus 1/16in. We went for tight, but not too tight fits. They seem to work best when considerable pressure is required to force them down into place, but not so much that they arc up and away from the lower ply. Foam edges compress slightly, and the glue acts as a lubricant. No kerfing was required to accommodate the bend (may need some for the tighter radius Pilothouse).
Note: Be sure not to push the dry foam all the way down... it's WAY hard to get back out!
We dry fit each piece, averaging the differences at each end, and used a rasp to fine-tune any over-size. Any small gaps are filled by the expansive Gorilla Glue. We had one biggish gap (about 3/16in), which we filled with a glued slice of foam.
At assembly, we smear glue onto the upper face of the lower ply and framing edge faces. Insert foam. Smear upper face of foam and framing. Position upper layer of ply (prefit and marked for nails), and fasten from the mid-line, working outboard. Clock starts ticking with the first glue, and we have about 20 minutes per 'sheet' (including foam).
If there's any gaposis (squishy release sounds when you depress ply between frames), extra measures are needed. Clamps can't work... an extreme possibility is a long screw driven through ply-foam-ply and into a 2x block on the low face. Work fast, though, as the glue will be setting up, making any gaps permanent. To date, we've managed to use live weight; simply squatting in place until the glue has set up. Good time to make up for hasty, glue-job irritability.
So... as of this writing, we're half-way on the mid-deck. Two weeks (and maybe more) private time is on our horizon. All we need is a little more, unseasonable warmth.
Who knows? Maybe we'll get a lick of work in!
|Completed to s'brd, foam fitted port and aft, empty bays awaiting foam port and fwd.|