Strong and stable: PBO’s Secret 20 is strengthened before being turned in the next step of her construction. David Pugh reports
The time has come at last – nothing now stands between us and turning the boat upside down, prior to panelling the bottom to complete the hull.
It’s a landmark moment in the project, but a heart-stopping one nonetheless.
See the full set of articles: Building the Secret 20 kit boat with PBO.
Ben Meakins and I have clear memories that, not so long ago, we were almost tiptoeing around the boat to avoid destroying our painstaking alignment of the frames with the keel; now we expect the epoxy fillet joints which locked that alignment in place to hold the keel as gravity tries to wrench it from the frames during the rotation.
At the same time, the edge of the hull will be subject to significant force while the boat briefly rests on her side.
With that in mind then, this month a lot of our work has been concentrated on ensuring that the boat’s structure is as strong as possible, given its half-built nature.
An obvious point of maximum stress is the joints between the frames and keel, so we started our stiffening operations there.
Frame to keel joints
We initially filleted the frame to keel joints with plain fillets of WEST SYSTEM epoxy, thickened with filleting blend from the same brand.
Although fillets are strong, in this case they are very short so we opted to back them up with some glassfibre tape.
Working along the keel, we primed each fillet and the surrounding area with unthickened epoxy.
No preparation was required as we’d used peel ply on the fillets, which when removed leaves a textured, blush-free surface ready to accept more epoxy or another finish.
Into the wet epoxy we pressed a layer of 50mm glassfibre cloth tape, using a brush to stipple the epoxy into the weave, adding more epoxy as required.
Finally, we finished the joint with another layer of peel ply.
Apart from leaving a prepared surface for future work, when using cloth the peel ply also helps by wicking excess epoxy away from the joint.
This is important for strength: if too much epoxy is used the glassfibre cloth can ‘float’ to the top of the epoxy, leaving a weak layer between the cloth and the substrate.
The tape will help spread the load from the joints across a significantly larger area, making the joint much stronger.
Sheer and decks
After fitting the hull sides last month, we needed to set the sheerline to its final profile.
Using a long, sharp plane to ensure the line remained smooth, we trimmed the plywood edge and stringers together to ensure a smooth sheer along its length.
As the decks are finally fitted once the boat has been turned right-way-up again, we’d assumed that we could move straight on to fitting the gunwales at this stage.
However, building a kit has its advantages – careful reading of the instructions suggested that it would be wise to temporarily fit the decks first.
There are two reasons: firstly, having the decks in place makes it easy to ensure the gunwales are set high enough to cover the deck edge, and secondly that it’s much easier to trim the decks to the shape of the hull without the gunwales in the way.
In the kit, the fore and aft deck panels are roughly cut to shape, while the builder is left to work out the side decks.
We decided it would be easiest to work from the bow, so put the foredeck in position first. This comes as two panels, joined at the centreline.
They fitted reasonably well, but to minimise the amount of filling required we first chiselled the cutouts around the stem to give a snug fit.
Then, using a long straight edge, we marked a centreline down the foredeck beam to help us align the two panels.
The edges butted together relatively neatly, but we found it worthwhile to lightly plane them to make the edges absolutely straight.
With the panels butted together and clamped in position, we were able to see that the aft edge was not completely straight – the joined panels were slightly longer at the centreline than at the edges.
To resolve this, we used a straight edge to scribe a line across both panels, then removed them from the boat before clamping them together on the work bench.
A long plane made short work of truing the edges, and once the panels were refitted left us with a neat, straight line to butt the side decks against.
To hold them down, we used screws into the foredeck beam and stringers.
The kit left us with most of an 8ft x 4ft sheet of plywood to make the side decks.
We marked them in the simplest way possible – ensuring one edge was straight, we butted the sheet against the foredeck and drew around the stringers with a pencil.
Once removed, this gave a clear shape. We cut them out using a jigsaw, staying about 10mm outside the line to give us some room to manoeuvre as the panels were screwed into place.
We repeated this process four times in total, covering the side deck framing in two sections per side. The aft sections we trimmed off square at frame 10, which marks the forward edge of the aft deck.
The aft deck provided with the kit unfortunately proved too small.
Whether that was due to the part being cut incorrectly or perhaps we have made the stern wider than planned is unclear, although we were careful to take the stringers which define the deck edge to the corners of the pre-cut transom.
However, the upshot was that we needed to make a new aft deck.
Our local supplier was having temporary difficulties sourcing 6mm marine plywood, so we bought a sheet of cheap WBP ply.
We’ll replace it with marine ply later, but the WBP will give us an accurate template from which to cut the new panel.
Before we could shape and fit the temporary aft deck, we needed to plane the top of the transom to follow the shape of the aftmost deck beam.
Using a long straight edge laid longitudinally along the deck supports, we marked a series of points to show where the deck would meet the transom, before joining these points with a flexible batten to scribe a curved line.
We them planed the transom to the line, keeping the plane flat to the deck.
A long plane helped here – with the back of the bed supported by the deck beam, we were able to accurately set the angle at the top of the plywood.
The deck was simple to make, again by placing the sheet in position, drawing around it and cutting it slightly oversize.
At this stage, although the panels were fitted and the joints between them were neat, the inner and outer edges were a bit of a mess.
We could have spent the better part of a day planing the edges to shape, but an easier option presented itself – the router.
With a ball-bearing copy follower bit set to the depth of the deck plywood, an exceedingly dusty quarter of an hour trimmed the plywood edges neatly to the stringers.
The aft edge of the aft deck we left overlong – we’ll trim that later – and the tiny areas inaccessible to the router we quickly fettled with a pull saw, block plane and sharp chisel.
Fitting the gunwales
Finally, we could move on to fitting the gunwales.
These are in a beautifully figured timber called Queensland Silky Oak and are aesthetic as well as functional, so we had initially wondered whether we should fit them later in the project to avoid their being damaged when we roll the boat.
However, they provide significant extra thickness and strength to the sheer, adding nearly 20mm of extra timber and increasing the laminated sandwich made by the gunwale, hull skin and stringers to four layers.
The gunwale timber came in four lengths, which we had scarfed into two towards the end of a previous work session.
In this project, the scarfs will later be protected by the chains (timber attached at the gunwale to spread the shrouds and support the jib turning blocks).
When choosing the lengths for scarfing we were careful to make sure that any natural curves or twists in the timber worked with the shape of the hull, and this paid off as we offered up the gunwales as they required little force to make them match the sheer.
To fit them, we started at the bow. Although it’s tempting to bevel the gunwale to butt neatly to the stem, it’s better to round them off as they will otherwise foul the straps of the stemhead fitting later on.
This makes the job simple – we transferred the angle of the stem to the gunwales with a sliding bevel, trimmed them and used a block plane to round them off.
To ensure the rounding was identical, we used a mortise gauge to mark the point where the curve should start.
With the ends trimmed, we dry-fitted the gunwales using screws every 200mm or so, setting them about 1mm higher than the deck to allow for final shaping, and drawing a line underneath to give us a datum point for the epoxy.
The screws are counterbored into the gunwales to allow for plugs; we found the counterboring easiest to achieve before fitting, using a drill press.
The sticky finale
The final task was to glue the gunwales in place.
To do this, we unscrewed the gunwales and all the decks, setting the decks to one side for later fitting – we didn’t want to inadvertently glue them to the gunwales!
We then wiped the oak with acetone to remove the natural oils before priming the gunwales and the hull side with unthickened epoxy.
To glue them in place, we used epoxy thickened with a mix of microfibres and colloidal silica, mixed to a paste just thick enough not to run.
We held them in place using stainless steel screws into the holes made previously, and were careful to remove the bead of epoxy that squeezed out – otherwise the decks won’t fit later on.
To finish, we cut plugs from an offcut of the same timber and epoxied them in place over the screw heads, allowing the epoxy to set before trimming them flush with a block plane.
As published in the July 2017 issue of Practical Boat Owner magazine.