Modern Performance

~ Long distance racing version of Didi 120

~ Proven construction methods

~ Detailed drawings for amateur builders

~ Long waterline for maximum performance

~ Robust construction

~ Set up for single-handed racing

~ Flickr Album

Our Didi range of radius chine plywood designs started with the Didi 38/40, my own "Black Cat", as the prototype. Designed in 1994/5, that became one of our most successful designs and spawned a long series using the same basic construction methods. That radius chine plywood design was an experiment that has grown into an expanding range of monohull and multihull performance concepts, primarily for amateur builders.

The Didi 120gsc design was commissioned by a client who intends to sail in the 2031 edition of the Global Solo Challenge non-top single-handed race around the world. He liked the Didi 120 that was built in Sydney, Australia, but wanted a boat that would be set up for single-handed racing rather than the crewed around-the-cans format of the Didi 120. He also wanted it to be beefed up a bit in the areas that historically are most likely to give issues. Those are rig strength, steering failure and hull resistance to impact with flotsam. We also had to meet the additonal requirements of the race rules where they differ from normal cruising standards.

Aside from those changes, I introduced some of my own recommendations to make this boat more confortable for the crew sailing through some of the nastiest waters in the world. They are in deck configuration and interior features. Of course, a custom design commission like this must also fulfill the wants of the client. In this case the client is a sailor in Romania, who will build it in his factory in the construction industry.

The most obvious visual difference is the pilothouse deck configuration. This is to allow clear all-round visibility from the nav station, helped by a pair of ventilation hatches over the seat for viewing the rig. The pilothouse structure extends over the front of the cockpit for protection of the companionway and to give a protected cuddy area to sit outside but out of the weather.

Hull construction and detailing is generally the same as the ISO-standard structure off the Didi 120. But it is beefed up to far in excess of ISO requirements by adding another 6mm of plywood and a layer of biaxial fibreglass over the outside. It also has a layer of Kevlar/epoxy to bottom and lower topsides forward of the mast bulkhead, laminated over the inside of the skin and stringers.

The interior is broken up into compartments by 4 watertight bulkheads. These are placed in the bow to form a crash compartment, at both ends of the main accommodation and forward of the rudder shaft to form an aft crash compartment containing the steering equipment. The mast bulkhead has a watertight door and the others have double frame hatches for access. There is also an escape hatch in the transom, as fitted to Mini 650 boats.

The interior accommodation is concentrated amidships, with a centreline galley module in the saloon and the forward-facing nav station aft of that, with a radiused seat for comfort when heeled. Outboard of the nav station are adustable pipe berths over storage and tankage compartments. The toilet is in the sail locker forward of the mast bulkhead, as well as cave equipment lockers port and starboard. The wet locker is to starboard of the companionway, draining into the bilge.

The sailing rig has the standing rigging over-size by one increment, with the shrouds terminating on integral composite chainplates that are built into the hull sides. Below decks there are substantial semi-bulkheads aligned with the loads to resists hull distortion in this area. It has swept spreaders for a stable mast that needs no attention during tacks and gybes but it also has runners for additional strength under fractional spinnakers. The boom is long, to reach the mainsheet track mounted aft of the tiller, so it must be kept low during gybes to prevent contact with the standing backstay. The sail plan shows two mainsails, a conventionally-roached main that is intended for short-handed racing and a squaretop main for crewed racing and cruising.

The deck is laid out for single-handing, with all lines led aft to the cockpit. The mainsheet uses the German system, with both ends led forward along the boom, then aft along the side decks to dedicated winches on the cockpit coamings. Most of the control lines from the foot of the mast run to the cockpit coamings, with eight lines on each side running via tunnels through the pilothouse against the cabin sides, terminating in two banks of clutches on each side. To make this work without excessively wide tunnels encroaching too much on the interior, the lines are led through double-banked 4-sheave deck organizers at both ends of the tunnels. The remaining four control lines run over the pilothouse to clutches and a winch on the aft end of the dodger roof.

Steering is through a deep ballanced spade rudder on a shaft and tangs fabricated from Duplex 2205 aluminium. The shaft runs through Jefa self-aligning bearings, the lower one sealed with a boot and the upper one with double lip seals. The lower bearing is glassed into substantial gussets fore/aft and transverse. It has a tiller in the cockpit and double tillers below the cockpit for primary and backup autopilots. Allowance has been made in the transom layout for a Hydrovane self-steering mounted 200mm off-centre as well as substantial brackets to secure the required drogue sea anchor.

The ballast keel is a welded steel fabrication with the skins of the fin pre-shaped to a NACA section and welded over a framework of top and bottom plates and three internal pipes that are accurately spaced to define the NACA section. The top plate is wider than the fin, with a splayed plate each side to form a flared top for reduced loads on the keel bolts and the important welds between the top plate and fin. The delta ballast bulb is a separate steel fabrication, filled with lead and welded to the fin when both components are ready. The keel bolts are threaded rod that screw into the top plate and backing capnuts formed from round bar welded to the plate. This method allows individual bolts to be extracted for inspection or replacement.

There is interest in a cruising version of this hull/deck configuration, with a more conventional cruising interior.

Follow these links to print or download stability gaph or a list of drawings for this design.

LOD 12.00m (39'4")

LWL 11.60m (38'1")

Beam 3.80m (12'6")

Draft 2.63m (8'8")

Displ to DWL 6310kg (13907lb)

Displ Min Operating 5790kg (12761lb)

Displ Loaded 7847kg (17295lb)

Ballast 2437kg (5371lb)

Waterplane area 24.57sq.m (264sq.ft)

Immersion rate 252kg/cm (1411lb/inch)

Wetted surface 41.35sq.m (445sq.ft)

Sail area (main + 100% fore triangle) 88.66sq.m (954sq.ft)

Ig 16.40m (53'10")
Is 17.50m (57'5")

J 4.50m (14'9")

P 16.00m (52'6")

E 6.47m (21'3")

Mainsail (excl roach) 51.76sq.m (557sq.ft)

Foretriangle 36.80sq.m (397sq.ft)

130% Genoa 47.57sq.m (512sq.ft)

Powering Yanmar 3YM30

Headroom aft 2.0m (6'7")

Headroom at mast 1.79m (5'10")

Fuel 300 litres (80 US gals)

Water 600 litres (160 US gals)

This design was drawn in CAD in the metric system but the drawings are dimensioned in both metric and inches.

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