Didi 38 & Didi 40
Radius chine plywood cruiser racers

Didi 38 radius chine plywood boat plans

Read about our experiences on a Didi 38 in the MTN Cape to Rio 2000 Race. It was rather different from what all competitors expected. Click here

Affordable Performance

This one was for me! For the first time in my professional design career I was the client. I drew her to be the type of boat that I enjoy - light, responsive, fast and fun.

She also had to be quick to build for only one or two people and fit a terminally ill wallet.

Launched in 1995, I have crossed the South Atlantic three times on her and experienced a run of nearly 250 miles in 24 hours and a top speed of nearly 22 knots.

~ More conservative version, with masthead rig and shallower keel. More info.

~ Plywood bulkhead kits. More info.

~ 40ft Cruiser Didi 40cr, with greater space and comfort. More info.

~ Advice, tips and build photos in Builders' Notes

~ Sailing photos ~ Flickr Album



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Didi 38 radius chine plywood boat plans
Innovative radius chine plywood construction

Construction of this yacht is radius chine plywood (click here for an illustrated article on the method), drawn for ease of building. I built her myself in my garden to prove the innovative methods of construction which I designed into her, to achieve a strong and light structure which could be built by people with average handyman skills and a very modest budget.

Offwind performance was a major priority with this design and on headings with the wind aft of abeam she will sail with most 50 footers. Efficiency upwind was not forgotten and here she can hold off challenges from most yachts of similar size.

Didi 38 radius chine plywood boat plans

Thoroughly modern

Her lines are thoroughly modern, with short overhangs, fine bow at the waterline and powerful stern. Sections are slightly Veed throughout the length to give a large planing surface, efficient when upright or slightly heeled. The forward underbody has been shaped to develop dynamic lift to keep the bow up at speed and promote surfing and planing.

The hull is fairly slim and well flared to give a narrow waterline, reduced drag and improved comfort at sea. Light and beamy boats are not comfortable in a seaway. After having crossed the South Atlantic twice on my DIDI 38 I can vouch for the comfort of a light boat of moderate beam.

Draft of the racing keel is 2,25m and we have two shorter cruising keels as well. All are fabricated from steel plate over a simple tube skeleton and are lead filled. They are through-bolted to the hull and supported by the internal I-beam backbone and laminated ribs. This results in a stiff and watertight hull/keel structure.

She can also be built with the stern overhang extended by 0,57m for better performance due to increased sailing length and cleaner wake. Overall length increases to 12.07m (39' 7").

Didi 38 radius chine plywood boat plans

Easily handled deck stepped rig

The mast is deck-stepped on top of the main bulkhead, which is reinforced with timber posts. This is structurally the most sound solution for a timber boat. It places everything below the mast in compression rather than trying to force the hull and deck apart, as happens with a keel-stepped mast. It also eliminates the problem of water going below through the mast and collar.

The fractional rig has double swept spreaders, giving a stable setup which does not require runners. This eases handling by a small crew.

Didi 38 radius chine plywood boat plans
Comfortable and practical interior

Down below, the layout is based on a feature found in most of my designs, the midship engine. It gives excellent access for maintenance, places the heavy weight at the optimum position to reduce pitching and allows a flatter after underbody because the weight is not having to be supported as far aft. It also allows a good seagoing U-shape galley to be built around the engine, with centreline sinks.

The forecabin has a full width double berth with loose tops which can be removed for racing, leaving a large sail locker.

The saloon settees are integral water tanks and the saloon table is a box in two sections. The lower is an epoxy coated plywood fuel tank and the upper is a storage compartment.

There are two double quarter berths aft, the starboard one in an optional private aft cabin. Right aft is the lazarette accessed from the port quarter berth via a bulkhead opening. This eliminates the possibility of endangering the vessel through a lost cockpit locker lid.

For a performance yacht she has a lot of joinery but it is light and strong. Drawn initially for the Cape to Rio Race, stores and gear must be carried for 5 crew for 4 weeks. A lack of proper stowage results in a disorganised boat which is bad for crew morale and dangerous in an emergency. Good stowage improves her all round with little weight gain.

Didi 38 "Black Cat" on launch day

Didi 38 "Black Cat" on marina

Didi 38 "Black Cat" interior looking forward

click on photos

Didi 38 "Black Cat" interior looking aft

to enlarge

Low-tech performance

This is not a hi-tech boat, rather being one which can give performance satisfaction to those who want to sail close to the hi-tech guys but without the high cost. She gave us 248 nautical miles in 24 hours, a top speed of 17 knots and an incredible period of an hour in pitch dark mid ocean when our boatspeed remained over 13 knots. She has also regularly featured in the prizes in local short-handed and coastal passage races. In the 2000 Cape to Rio Race she came close to 22 knots boat speed.

Follow this link to polar plots for the Didi 38.

My "Didi" (previously named "Black Cat") has lived up to all of my expectations in her design concept. I would recommend her to any potential amateur builder who wants a performance design which he can build for himself at reasonable cost. Follow these links to an article on radius chine plywood and other designs of similar construction, the Didi Mini, Didi 26, Didi MG30 and Didi 34.

Follow these links to print or download a material list for the structure, stability curves or a list of drawings for this design.


LOA 11,50m (37'9")

LWL 10,33m (33'10")

Beam 3,40m (11'2")

Draft 1,6m to 2,25m
(5' 3" to 7'4")

Displ to DWL 4750kg (10470lb)

Displ IMS trim 4000kg (8816lb)

Ballast 2000kg (4408lb)

Waterplane area 18.56sq.m (200sq.ft)

Immersion rate 190kg/cm (1064lb/inch)

Wetted surface 27.2sq.m (293sq.ft)

Sail area (main + 100% fore triangle) 66.6sq.m (717sq.ft)

Sail Area/Wetted Surface 2.45

Sail Area/Displ 23.8

Displ/length 120

Prismatic coef .56

Block coef .44

Fineness coef .69

Righting Moment @ 30 deg 3303kgm (23890ft.lb)

Righting Moment @ 60 deg 4856kgm (35124ft.lb)

Righting Moment @ 90 deg 3342kgm (24172ft.lb)

I 14,35m (47'1")

J 4,10m (13'3")

P 14,30m (46'11")

E 5,35m (17'6")

Mainsail (excl roach) 38,28sq.m (412sq.ft)

Foretriangle 28,30sq.m (305sq.ft)

150% Genoa 43,85sq.m (472sq.ft)

Powering 15-25hp diesel

Headroom aft 1.9m (6'3")

Headroom at mast 1.8m (5'11")

Fuel 190 litres (52 US gals)

Water 600 litres (165 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 30 July 2018

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