Building the Didi 120 "Passion XI" - Keel Support Grid

This is the 3rd 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 building the grid structure that supports the ballast keel and distributes the large loads that result from sailing in wild conditions and, if navigation is not as good as it should be, hitting the bottom.

Photo numbering continues sequentially from post to post.

Photo 12. The ballast keel loads are carried by laminated timber floors, which are beams that cross the bottom of the boat and pass through the centreline girder. The floor that aligns with the chainplate semi-bulkhead is being laminated in place against the backbone and stringers, from which it receives its shape. A host of clamps is needed to coax the layers of laminations into the required shape and squeeze excess glue out of multiple joints. The laminating is best done in stages because gluing all layers at once requires a lot of clamping pressure, which distorts the stringers outward, slightly flattening the shape of the floor from the desired curve. The post at left in the photo is a temporary support to hold the backbone at the correct height while the floors are being laminated.

Photo 13. The floors are in various stages of lamination and shaping. For the shorter floors, David chose to laminate a few layers on the hull longitudinals to lock in the basic shape, then to add layers as needed to build to the correct dimensions and to fine-tune the shape to an accurate fit against the longitudinals. The floors all taper in thickness towards the ends, so laminating is done with progressively shorter lengths of timber, then a belt sander is used to smooth them to the final shape. The hardwood shoe that will form the base on the outside of the hull for the ballast keel can be seen at the top of the photo.

Photo 14. The floors closest to the camera have still to be cleaned and trimmed to length. The others have been fine-tuned to shape and trimmed to length. The longest floor extends around the turn of the bilge at both ends. It aligns with the chainplate semi-bulkheads, to stiffen up this most heavily-loaded part of the hull that carries keel and rigging loads.

Photo 15. The floors have been glued in place to the longitudinals and the keel bolt holes have been drilled. Doublers are being glued to the forward faces of the floors to compensate for the timber removed in drilling the keel bolt holes. This doubler has been completed on the floor on the right and is being done to the other floors, using blocks and wedges to hold them in place while the glue cures. The floor on the left is the long one that connects with the chainplate bulkhead and will later be glassed to it. That junction is at the level of the settee top, which strengthens the junction as well. The settees are also structural, serving as girders to support the ends of the floors. The cleats that will bond the settee tops and fronts to the bulkheads can be seen on the two full bulkheads at the sides of the photo and the small semi-bulkhead in the centre. The tangent stringer can be seen on the other side of the hull, with the plywood doubler over which the junction between flat and radiused plywood panels will be formed.

Photo 16. This view shows the keel support structure clearly. This boat has a deep keel with very low CG, so the large loads applied to the hull require a very solid grid to distribute them away from the immediate area of the keel root. The floors and backbone structure do that work, with the floors carrying the transvers loads and the backbone carrying the longitudinal loads from groundings. He is starting to fill in the spaces between the longitudinals with timber glued to the floors. Limber holes will be formed through this timber filling to allow bilge water to get to the pumps.

Photo 17. These are the engine beds, which are formed from two layers of 12mm plywood. The aftmost laminated floor is located at the end of the girder and passes through the beds to link them, extending this strength member as a double girder through to the companionway bulkhead. Also passing through the beds, at left in the photo, is a sawn floor in two pieces, linking a pair of semi-bulkheads that are part of the aft cabin and heads compartment. At the midpoint of the floor is a laminated plywood connector bracket through which the stern tube and exhaust will pass and into which the front end of the stern tube will be bonded.

Photo 18. The centreline girder for this design is very solidly constructed to cope with the very large loads that potentially result from striking a rock with the toe of the keel. In this photo the spaces between the floors have been filled solid with timber, with the grain running fore/aft. The limber holes that can be seen running through the timber have been lined with ¼ sections of plastic pipe, set in epoxy, to seal the end grain of the timber. A boat with shallower and more lightly-loaded keel may have an H-section girder but the ISO calculations require this one to be solid timber.

Photo 19. The keelson is being dry-fitted as the final piece of the girder. It extends through the bulkheads at the ends of the saloon but is too long to be fed through them. It has a scarph to allow it to be fitted in two pieces with the scarph glued during fitting. The girder extends to the end of the walking area in the forecabin, picking up compression loads from the mast that are transmitted via the bulkhead in the right of the photo and the post that is glued to it.

Photo 20. The ends of the laminated floors extend through the settee fronts, which are structural, serving as deep girders and part of the keel support structure. The junctions between the floors and settee fronts are bonded with epoxy fillets.

Photo 21. The spaces under the settees are integral water tanks, so the junctions have to be waterproof as well as durable so that there is no future cracking to cause leaks. This is achieved with epoxy fillets reinforced with glass tape. This view is looking into one of the tanks through an access opening.

Diagram. Master section, showing details of hull and deck structure. Read along with the construction drawing in the previous post.

The next post about this build will be preparation for and fitting the hull skin.