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.