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.

Building the Didi 120 “Passion XI”- Bulkheads & Framing

The previous post, "Didi 120, Bringing the Didi 38 into 2022", described the development of the Didi 120 design from its predecessors in this design series, the Didi 38 and Didi 40cr. “Passion XI” is the working name of the prototype of the new design and is being built by amateur builder David Edmiston alongside his house in the suburbs of Sydney. We are going to follow his project, from the start through to completion, In a series of articles or photo essays. Thanks to David for providing the hi-res photos.

Although very modern in concept, this design uses a fairly traditional longitudinal girder backbone structure on centreline. This provides longitudinal stiffness to carry rig loads and resistance to grounding damage. It is combined with laminated transverse floors to carry keel loads and spread them into the rest of the hull. During construction, these structures, as well as a system of stringers and sheer clamp, are installed over bulkheads that are pre-cut to measurements and diagrams provided in the drawings and/or a table of offsets. This forms the skeleton to which the hull skin is fitted to complete the hull.

There are aspects of this project that are particular to the radius chine plywood method used for this design but overall the basic construction sequence and procedures are similar for most plywood boat projects. This series of photo-essays may help potential amateur boatbuilders to figure out whether or not a big boat project might be in their future, whether or not their skills and resources of time, money and endurance will see them through.

It can help with understanding the photos to refer to the drawings of this design in the previous post. The construction drawing shown here will clarify the basic construction layout. The plan view shows hull construction below centreline and deck construction above centreline. 

Photo 1.  Making bulkheads. The shapes are drawn onto the plywood from measurements on the table of offsets and diagrams on the drawings. The slots for stringers and backbone have been cut with a router and template for accurate placement of the longitudinals. The cordless drill gives scale to this large bulkhead, which is under the cockpit. It is cut from two sheets of plywood and scarphed at the joints. When built from a CNC kit the panels have jigsaw joints instead of scarphs. Other bulkheads are stacked against the wall in the background. The nearest bulkhead in the stack will be positioned mid-way along the forward berth. It has a laminated trim around the opening to soften the edge for any crew sleeping on that berth.

Photo 2. These bulkheads are ready for setting up. The cleats for joinery components are already glued on, ready to receive plywood fronts, tops and shelves. They are all located by measurements from centreline or waterline drawn onto the bulkheads. This reduces the time needed to build the interior joinery further into the project, while increasing the accuracy of setting out that joinery. The cleats are triangular in section, saving 50% of the weight compared with square cleats of equal size. Laminated roof beams have been fitted, holding the tops of the bulkheads securely at the correct widths. The surface has been primed with white epoxy primer to preserve it against weather during construction in an outdoor building site. Areas that will be glued or will be cut away later have been left unprimed. Also without primer are a narrow vertical stripe on centreline and a horizontal stripe low down on the bulkhead, where the centreline and design waterline (DWL) have been drawn and are needed for future reference. At the upper corners of each bulkhead, at the deck edge, large cleats have been glued on diagonally across the corner for gluing and screwing the sheer clamp. On these designs the sheer clamp sits diagonally across the corner to give a clean interior structure in that area and allowing a rounded deck edge on the outside.

Photo 3. This is the transom, with stiffeners and doublers glued on. It is about to be epoxy-coated and primed, so the blue painters’ tape is masking off areas where glue or epoxy fillets need adhesion onto raw timber. The stringers pass through the transom and will be epoxied in place. Short lengths of beige masking tape spaced around the perimeter show the locations of the hull and deck stringers, where the tape protects the gluing areas. The tape that protects the centreline marking is peeled back and the drawn centreline can be seen.

Photo 4. Setting up the bulkheads on the building stocks. Each bulkhead is bolted to two legs, which are themselves bolted to the rails of the building stocks. They must be set up vertical, at the correct fore/aft positions, centred and at the correct height, all within 1mm accuracy. This can be achieved by means of a plumb-bob hanging from a taut centreline string above the boat for vertical alignment and a laser level for height. This allows any bulkhead to be checked at any stage during setup, irrespective of other bulkheads. Alternatively, it can be done with a laser level that has both horizontal and vertical lines to check level and centring at the same time. For this method the bulkheads must be set up in sequence from one end of the boat to the other. The bulkheads must also be stabilized with bracing to hold them firmly until permanent longitudinal structure secures them. The fastenings must be bolts or large screws because they will eventually be carrying the full weight of the completed hull structure plus any people who may be working on top of the hull.

Photo 5. Stringers are set into slots in the edges of the bulkheads. Here they are being dry-fitted to the flat areas of the side and bottom panels to check for fairness of the stringer runs, also showing overall fairness of the hull shape. The area without stringers is the radiused portion of the hull. The backbone is also visible, being test-fitted on centreline.

Photo 6. The radius stringers have been added. The junction between flat and radiused hull skin is made with a plywood doubler fitted to the tangent stringer. What looks like a broad stringer is the plywood doubler over which the joint is made, with the tangent stringer under it.

Photo 7. The backbone is being dry-fitted to check for fit and curvature to where it must fit against the stem bulkhead. The backbone is too stiff to take the required curvature in the bow, so a horizontal saw-cut is made through the timber to allow it to be laminated in place. The large piece of plywood that looks like a longitudinal bulkhead is a temporary support for the stem bulkhead to hold it accurately in position. This boat has a plumb bow with the stem formed from a narrow bulkhead of multiple layers of plywood, with a solid timber nosecone bonded on later in the build process.

Photo 8. The backbone, stringers and tangent doublers have been glued to the bulkheads. The stringers will be trimmed off flush with the front face of the bulkhead before the nosecone is glued on. The backbone will be planed to a V-shape to match the dihedral angle of the bottom and that work has already started while fairing in the stringers against the sides of the backbone. The sheer clamps have also been fitted.

Photo 9. Side view of the same stage seen in Photo 8. The junction of the backbone with the stem bulkhead is made rigid and reinforced with a knee laminated from multiple layers of plywood. The knee is housed into grooves in the backbone and bulkhead. The stringer just above the tangent doubler has not yet been glued in place, it has been pulled into its correct position and secured with a rope to establish the alignment for planing the backbone dihedral angle in the forefoot area.

Photo 10. The chainplates of this design are inboard, i.e. they are bolted to a semi-bulkhead on each side, not to the hull skin. The loads are best transferred into the structure if those bulkheads align with the load direction. To do this, the bulkheads point toward the mast. In this photo David has the bulkhead clamped to temporary timbers that terminate at the mast support post on the major bulkhead. This boat has a deck-stepped mast standing on top of a reinforced bulkhead, which I have found from personal experience with my personal boats to be the best arrangement for a wooden boat. This places all of the structure under the mast in compression rather than having the compression loads on a keel-mounted mast foot attempting to force the backbone structure off the bulkhead, requiring stainless steel tie-bars to contain the loads.

Photo 11. Here the chainplate semi-bulkhead has been bonded to the stringers. The inner edges will be trimmed to final shape during fitting out of the interior. In this photo the sheer clamp can be seen, diagonal across the corner between hull and deck. The hull side skin has been fitted almost to the semi-bulkhead. The inner face of the tangent stringer can also be seen, with the narrow stringer projecting inward from the broader plywood doubler.

This build series will be continued in future posts, the frequency dependent on build progress.

Didi 120 Build Project in Sydney, Australia

The Australian magazine Australasian Amateur Boatbuilder & Kitboats (AABB) was running a series of articles about the construction of the Didi 120 prototype, by amateur builder David Edmiston. The magazine published its final issue a few months ago, with David's boat half-built. This is the first of a series of posts that will duplicate the articles that were published by AABB and then will follow the rest of the build through to launch. This one describes the development of the Didi 120 from the Didi 38, on which all of our radius chine plywood designs are based.

These articles show the boat as drawn for David Edmiston, with deep draft keel and rudder, along with a large racing rig. Alternative options will be available for shallower keel and rudder, as well as a smaller cruising rig.

Didi 120, Bringing the Didi 38 into 2022

I drew the Didi 38 in 1994, a fast light displacement boat for my own use for trans-ocean racing, with an emphasis on downwind sailing. It did that successfully and the resulting “Black Cat” has stood up well to 26 years of ocean sailing. One of the many builders who liked the building method that I developed for amateur builders was David Edmiston in Sydney. He wanted to build a variation of the Didi 38 that I had previously drawn, the Didi 40cr. That was basically the Didi 38 with longer stern overhang and extended accommodation for more cruising comfort. But David wanted a bit more cockpit space, so he commissioned a modification to the stern for more deck beam aft and a T-shape cockpit, with more flared aft topsides to make this work. The result was his boat “Passion X”, which he and his crew have raced with good success in the region.

Midway through 2021 he started discussing potential modifications with me to upgrade the performance a bit. He was weighing up the pros and cons for modifying the keel and rig against building a new boat. After throwing some thoughts back and forth for possible modifications, he was leaning toward modifying rather than building. Scheduling to draw a new design for David was far from my thoughts. In the midst of this, Sydney went into full pandemic lockdown mode that looked like lasting a long time, which decided David to take the lockdown opportunity to make good headway with a new boat project, starting asap. For me that presented a problem to squeeze it in with other projects on the go already. I started on it while winding down the last of the Cape Deseada 36 design for another Sydney client.

The Didi 38 is a light and fairly slim boat, in the interests of an easily-driven hull and easy motion at sea. Fast on all headings in light to moderate breeze and strong reaching/running courses, it falls off a bit in strong winds to windward. David wants more sail-carrying power for around-the-cans club racing, so called for more beam as well as a deeper keel, with lead ballast concentrated in the fabricated steel bulb rather than split between fin and bulb.

He thought that the new boat would be near enough to the shape of “Passion X” that I could cut that design down the middle, spread the two halves apart 400mm and fill in the missing bit in the middle, at the same time changing it to a plumb-bow and very short stern overhang, to maximise waterline length. It sounds doable and that process can work with physically modifying an existing boat but that is not the way that boat design works to create a good overall design. It is much less problem to just start from the beginning and draw a boat that targets desired criteria.

David ordered material right away, based on his previous build. He also started preparing the build site, with the new boat to be assembled on the same building stocks as “Passion X”, alongside his home. That really turned up the pressure on me, not yet having started to draw the boat and the builder aching to start. There are design cycles that have to be worked through and short-circuiting that order of development can generate problems. David may have found it a bit frustrating as drawings went back and forth, progressing another step each time toward what he needed before putting saw to wood for the first time. Scantlings, basic construction detailing and bulkhead diagrams were needed for work to start but they couldn’t be done prior to many iterations of hull shape, running hydrostatic numbers and doing structural calculations. I fed those drawings to him as soon as they had progressed enough to be of use to him but it was only after many weeks of drawing that I could see a bit of daylight between my progress in the computer and the progress on the boat that was taking shape literally on the other side of the world.

David was very clear about what he wanted in the new boat. He enjoyed the build process of “Passion X”, so wanted the construction method and detailing to be the same. The layout works well for him and his crew, so that basic interior was also to stay, with small changes to make better use of the wider hull. The deck configuration suits him and his mature crew, so that was a given as well. From those parameters plus the long waterline, everything else in the concept came together. The longer waterline gave more usable interior length, so some rearrangement was done to nicely match the interior to the rig and ballast keel support structure requirements, as well as the rig to the underbody to ensure good sailing characteristics.

In order to get Cat A racing certification in Australia, the new boat must meet ISO structural requirements. I drew the Didi 38 to the requirements of American Bureau of Shipping Guide for building and Classing Offshore Racing Yachts, as applied at the time, so it was going to be interesting to see how the structures compared when designed to different scantling rules.

The ISO scantling rules proved to be considerably more complicated but, overall, the hull and deck structure came out quite similar. The exception is in the grid structure that carries the ballast keel loads and distributes them into the rest of the hull, as well as the keel bolts that transfer the keel loads into the grid structure. Much of this is due to the considerably lower CG of the keel, applying larger forces on the hull structure.  But some of it may be due to tightening of requirements in response to a few boats losing their keels around the world.

The keel of the Didi 38/40 is relatively slim, with low frontal area. That is great for downwind racing and other situations that don’t require lots of lift. The bulb has a delta form, with hard bottom edge that works well on flat water but struggles a bit when pushing hard to windward in lumpy water. The new boat is more of a round-the cans racer, with more emphasis on windward performance. Her keel is thicker, with a foil section that has better lift/drag characteristics. The extra thickness and redesigned construction makes a stiffer keel that is better able to carry the increased ballast loadings. The bulb, also delta-form, is larger to accommodate the increased ballast volume, with modified toe shape to improve flow in lumpy water, also softening the shock-loading that accompanies an accidental grounding. Both keels are flared at the root to reduce bolt loadings.

The rudder is a deeper and slightly slimmer spade, with a more forgiving NACA 000 foil section and options of either tiller or wheel steering on the same basic configuration. My preference on a boat of this size and type is a tiller for fast and intuitive response but I think that David will make his decision further into the build. The shaft is exposed inside the boat to allow fitting of a quadrant for wheel steering as well as a short tiller for electronic autopilot.

The rig is configured to make it easy to sail and versatile for around the cans and distance racing. With less emphasis on overlapping headsails, it has higher-aspect headsails and proportionally larger mainsail. The new boat has more displacement and wetted surface, so it has more sail area to compensate for those changes. It has an on-deck retractable sprit for asymmetricals as well as a conventional pole for symmetrical spinnakers. The mast is supported by double swept-spreaders, with the V1 wires close to the rail and the D1 wire inboard. The broad staying base gives better support to the mast, to better handle the loads from the much greater righting moment, also giving more scope for headsails with moderate overlap to sheet through the gap and easing crew movement past the shrouds. It has a babystay to improve stability of the deck-stepped mast, as well as an inner forestay for a staysail or heavy-weather jib. As with the Didi 38 and all of its derivatives, the mast stands on top of a major bulkhead that is stiffened by a T-intersection and a timber post, bearing down on the centreline hull girder. This arrangement places all of the structure below the mast in compression, the mode that is best for composite wooden structure.

David likes a clear cockpit, so on “Passion X” he replaced the bridgedeck-mounted mainsheet traveller of the Didi 38/40 design with a 4:1 tackle on a bridle on the cabin roof, with the tail led forward along the boom to the base of the mast. I have duplicated that system on the Didi 120 deck layout but an alternative with traveller mounted on the cabin sole will also be on the final drawings. From the mast base, all halliards, reefing lines etc. and the mainsheet go back to banks of jammers and a pair of winches.

Along with the cleaner cockpit, the bridgedeck companionway has been replaced by a full-height companionway with washboards to control water in rough conditions.

My work continues ahead of David’s progress, with detailing of ancillary items to finalize for completion of the design.

Characteristics

	Didi 38	Didi 120
I	14.35m	16.40m
J	4.10m	4.50m
P	14.30m	16.00m
E	5.35m	5.90m
LOA	11.50m	12.00m
LWL	10.33m	11.60m
Beam	3.40m	3.80m
Depth to DWL	2.25m	2.60m
Lightship Displacement	4000kg	4850kg
Ballast	2000kg	2440kg

Here are the words of David Edmiston, my client for the design and builder of the prototype.

I have long been an admirer of the designs of Dudley Dix and I enjoyed my experience building my first Dix design the Didi 40 CR2.  Passion X as I named her has been everything we expected both from a cruising and club racing perspective.

We have worked hard to extract the best racing performance from Passion X but I had a sense that the concept could be improved particularly with windward speed in heavy airs.

After extensive Velocity Prediction Program evaluation I concluded that to get the performance I wanted we needed a heavier, deeper keel, a longer waterline and a taller rig all within the same overall length. From that point building a new yacht seemed the logical option.

High on my list of wants was for the design to be to the ISO standards so that it would be accepted for the highest category ocean racing and for the design to be available to other prospective builders at a reasonable cost.

On the specifics I wanted the longest waterline reasonable on a 12 metre yacht, high form stability from a 400 mm width increase and good windward heavy air sailing characteristics from a deeper heavier bulb. By choosing a taller rig we could eliminate overlapping genoas for the same foresail area and gain some area in the taller mainsail.

As for the construction details and the general arrangement I was very satisfied with my current Didi 40 Cr2 and wanted only to replicate what we had but wider side decks and a wider bow. After Dudley accepted the design commission, he found ways to use the extra 400 mm width for some very pleasing incremental improvements in the accommodation and Dudley kept introducing new improvements such as the innovative keel design.

I am particularly pleased to be building the prototype of a thoroughly modern club racer cruiser. I know it will be an awesome vessel and I hope that it inspires others to build their own yacht.