Building the Didi 120 “Passion XI”- Hull Skin

This is the 4th of a series of posts that follow David Edmiston's build of the prototype of the Didi 120. To follow the series, start with "Didi 120, Bringing the Didi 38 into 2022", then follow through chronologically. This post shows preparation for and fitting the hull skin. This changes the skeletal framework into what looks like a boat.

Photo numbering continues sequentially from post to post.

Photo 22. This view of the bottom structure of the hull shows the keel support grid, comprising backbone and keel floors. All have to be shaped by means of planes and belt sanders to form a fair surface for the plywood sheets to lie on, with contact with all surfaces to which they will be glued. This includes forming the dihedral (V-shape) onto the keel timber, which can be seen as having a peak on centreline. A guide for planing is created by using a long hand saw to cut into the backbone at each bulkhead, cutting down until the saw teeth just touch the edge of the bulkhead. The saw-cuts from the two sides will meet on centreline of the keel timber and planing down to the bottoms of those saw-cuts creates the required V-shape. Between bulkheads, the keel floors, stringers and tangent stringer can be used in the same way with the saw to make guide cuts. Limber holes run through all solid timber alongside stringers etc. to lead bilge water to the low points, where the bilge pump strum boxes will be. These openings must all be sealed either with multiple coats of epoxy or ¼ sections of PVC or GRP pipe bedded in epoxy. If you are wondering what the blue patches are in the limber holes, as I did when I saw them, they are reflections of the blue sky above in the gloss surfaces of the epoxy coatings.

Photo 23. The cockpit sole and seat fronts have been installed at a convenient stage of the assembly of the skeleton and is bonded to the bulkheads. The cockpit increases the rigidity of the structure to help it to hold shape during construction, helping to resist the forces applied by the multiple longitudinal timbers that are all pulling in different directions. It is also much easier to finish these surfaces and radius the corners with a router when working down-hand with the boat upside down, rather than working above you with dust and shards flying in tight areas when the boat is upright.

Photo 24. This is the opposite view to Photo 23, looking at the inside of the upside-down cockpit. The sole is supported by the triangular fillets at the edges. Triangular foot braces will be fitted to the top of the sole to assist with secure footing when well-heeled, serving also as stringers to stiffen the sole. This keeps the underside of the sole clean and flat, without head-bumping framing over the aft berth.

Photo 25. Much of the interior joinery can be built before the skin is fitted. The settee front and top can be seen at the top of the photo. Below that are the shelves that will be behind the backrest of the settee. Fitting them before the skin is fitted is quicker and easier than doing it later in the upright hull and the dust and shavings fall onto the ground instead of having to be vacuumed out of the nooks and crannies inside the boat. If fitted into a skinned hull, the edges of shelves and divisions must be carefully cut to fit against the skin. If fitted first they can be cut a few mm over-size, then easily planed back to the correct line for a good fit. It is a bit awkward to fit the shelves, fastening them with temporary panel pins to the triangular fillets, working upside-down. But it is well-worth the effort to do it this way.

Photo 26. This is how the interior looks when ready for the skin to be fitted. This is looking aft through the galley and nav areas toward the aft cabins and companionway from the front of the saloon.

Photo 27. The first of the bottom skin panels fitted in the bow. There are various ways to join the plywood panels together, including butt joints with butt blocks, scarf joints, jigsaw joints or butt joints with glass tape both sides. The builder has used butt joints glassed both sides for the bottom panels, so the ends of the panels are left square. For scarf joints the ends of the panels would be tapered over the last approx. 100mm before fitting, with the opposite taper planed onto the next panel. The outer longitudinal edges land on the tangent stringers, which have plywood doublers over which the junction between flat and radiused hull skin is formed. The centreline joint will be glassed over, after first being planed down a small amount to compensate for the added thickness of the glass layers.

Photo 28. The side and bottom panels have been completed, leaving the radius still to do. This is looking into the water tanks under the settees, for which all joints must be totally watertight. It is easiest to seal as many of those joints from outside as possible before doing the radius skin rather than trying to do them through the hatch openings after the completion of the skin. The edges of the side and bottom panels have been rebated half the thickness of the plywood. The first layer of the radius fits between the sheet edges and the second layer fits into the rebate.

Photo 29. The first layer of the radius is being fitted. It is done in vertical strips approx. 300mm wide (1/8 of the sheet length), which are spiled (shaped) at their meeting edges to form a close fit. The radius is tight in the bow and gradually opens up throughout the length of the hull, with this large radius aft for a powerful stern with a clean wake. The strips are cut with the surface veneer running fore/aft because the radius becomes too tight further forward for strips with the grain running the other way to conform to the curve.

Photo 30. The second layer of the radius is being fitted, set at an angle to the first layer. The builder is using screws through temporary wood strip doublers at the ends of the strips to pull them in, flattening out the slight bulges that can occur from fastening with screws alone. 

Photo 31. Both layers of the radius skin have been completed and fairing has started. The bow is finished with a hardwood stem that is built up in layers onto the front of the small bow bulkhead, then shaped to a rounded bow. This is fine at waterline for wave penetration and broad at deck level to increase flare in the topsides for reserve buoyancy.

Photo 32. The Ballast keel bears onto a dense hardwood shoe that forms a base on the bottom of the hull, to transfer the loads into the hull structure without risk of crushing the softer plywood skin. Here the shoe is being test fitted onto the flat surface that has been planed onto the hull to receive it and to which it will be glued after a layer of glass is laminated onto the hull. Both hull and shoe have been drilled with pilot holes for the keel bolts.

Photo 33. Fairing of the radius has been completed. The hull skin and stringers have been trimmed back to the final slope at the stern and the boarding step has been built, which creates a sealed void aft of the transom. The exposed areas of the inner face of the hull aft of the transom will be closed with a layer of thin plywood fitted over the stringers, with a timber capping closing in the aft end, creating more sealed voids. All surfaces of the hull and deck must be sealed with multiple coats of epoxy to prevent moisture penetration over time. Sealed voids and areas that will be difficult to access later during the build must not be forgotten because those are areas susceptible to problems in the future.

Photo 34. With the hull now closed in and forming a roof over itself, there is plenty that can be done inside the hull to keep the progress going when the weather is not playing nice. This photo shows the forepeak, with the epoxy coatings and painting done. The same has been done in the forecabin, from the sheer clamp to the upper tangent stringer and the structure of the forward berth has been built, as well as the fixed portion of the berth top. The open section of the berth top will be loose for access to the stowage below. The anchor locker has also been built in the forepeak, with the locker bottom and its two transverse stiffeners visible through the bulkhead opening. The bulkheads have unpainted stripes on centreline because the centreline that was drawn onto the plywood, when the bulkheads were made, is needed as a reference point throughout construction.

The next post in the series about this build will cover exterior coatings on the hull and installing the stern tube for the propeller shaft and the port for the rudder shaft.