Didi 120
Radius chine plywood racer/cruiser

Didi 120 radius chine plywood boat plans

Modern Performance

~ Didi 40cr brought up to date

~ Proven construction methods

~ Detailed drawings for amateur builders

~ Long waterline for maximum performance

~ Racer with comfortable accommodation

~ Cruising rig and keel options will come later

~ Flickr Album



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Didi 120 radius chine plywood boat plans

Our Didi range of radius chine plywood designs started with the Didi 38/40, with 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 38/40 design remains competitive after nearly three decades but yacht design has moved on. The builder of a Didi 40cr2 who is a serious racing skipper commissioned an update to that design. He wanted a more powerful boat than the slim downwind-optimised Didi 38/40, as well as maximum waterline length to maximize speed potential. The increased power is to carry a larger sail plan and comes from 400mm more beam, deeper bulb keel with lower CG and firmer bilges in the middle third of the hull. Those firmer bilges soften progressively further aft to create a broad but clean stern.

The deck configuration and interior layout of the Didi 120 are not much different from the Didi 40cr2 but that is intentional. Those aspects of the design worked so well for the builder and his crew that the only changes made were to improve on them rather than to change them.

Didi 120 radius chine plywood boat plans

The interior has benefitted from the increased volume of the new hull, from greater beam and waterline length. There are some minor rearrangements of the interior areas to fit in with the mast and keel locations. It is mainly aft of the mast that the increased width can be seen, which gives a bit more space to saloon, galley, navigation station, heads, aft cabin and the lazarette. The massive aft berth was a special request, to allow sleeping either fore/aft or transverse. For use at sea a longitudinal lee cloth can be fitted to create a comfortable and secure seaberth.

Construction is designed to current ISO requirements and uses stringers over bulkheads to support the hull skin. The stringers are increased in strength by adding doublers onto the inner faces where needed to satisfy the rule without adding weight elsewhere. Ballast keel loads under sail are distributed into the hull by laminated floors and grounding loads are carried by a substantial centreline girder.

The rig drawing shows both symmetrical and asymmetrical spinnakers. The symmetricals will be useful for windward/leeward racing and the asymmetricals for reaching legs and coastal racing. The asymmetricals are tacked to a removable bowsprit that stows on deck. The bowsprit is off-centre to port to clear the hinged lid of the anchor locker on starboard, as well as an optional vertical anchor windlass.

Didi 120 radius chine plywood boat plans

The deck plan shows three options for mainsheet system. The owner favours a bridle over the companion hatch garage, with the sheet led forward to the base of the mast, then aft to a winch on the aft end of the cabin roof. The other two options have tracks and cars in the cockpit with coarse/fine tackles, one of them full cockpit width at seat level and the other footwell width on the cockpit sole.

The deck plan also shows options of either tiller or wheel-steering. Both options have the rudder shaft exposed below the cockpit, with a gland to seal it against the hull and an Edson radial drive wheel keyed to the upper shaft. The linear drive autopilot connects to the radial drive wheel for both steering systems and the cables of the wheel-steered version connect to it as well.

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.

A cruising version will be added when a future builder requests it. It will have a smaller fractional rig, as well as shallower keel and rudder.

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

Didi 120 radius chine plywood boat plans
Didi 120 radius chine plywood boat plans
Didi 120 radius chine plywood boat plans
Prototype taking shape in Sydney, Australia.


LOA 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 5097kg (11234lb)

Displ Loaded 7324kg (16142lb)

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) 84.1sq.m (905sq.ft)

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

J 4.50m (14'9")

P 16.00m (52'6")

E 5.90m (19'3")

Mainsail (excl roach) 47.20sq.m (508sq.ft)

Foretriangle 36.80sq.m (397sq.ft)

130% Genoa 49.09sq.m (528sq.ft)

Powering Yanmar 3YM30

Headroom aft 1.88m (6'2")

Headroom at mast 1.79m (5'10")

Fuel 200 litres (53 US gals)

Water 700 litres (185 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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This page was updated 26 September 2022

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