The sailing world has lost a pioneering multihull designer and boat builder. Derek Kelsall, the man who during his life created the modern day trimaran, who famously introduced Eric Tabarly to multihulls and who was one of the first to champion foam sandwich construction, died on Saturday 11th December in hospital in Thames, New Zealand, aged 89.

Derek was born in rural North Wales on 15 May 1933 into humble surroundings. Son of a farm labourer, his mother a school mistress, the Kelsalls lived in a house with no running water or power. Despite this Derek studied engineering at Bristol University, but was unable to complete the course when he ran out of finances. After completing National Service in Kenya, he entered the oil exploration business working for BP in Libya.

His career in the oil industry took to Texas, but life behind a desk there didn't suit him. By this time his passion for boating was growing along with a fledgling interest in multihulls, both sailing and building them in the Caribbean. This built to a head culminated in a spur of the moment to enter the second edition of the Observer Singlehanded Transatlantic Race (OSTAR) in 1964 racing Folatre, a 35ft plywood Arthur Piver-designed trimaran which Derek had prepared for the race in a mere two months. The boat was special for the era for being ketch rigged, having an early wind vane self-steering system controlling the sails but there was much concern surrounding Folatre and her young skipper for she was the first multihull ever to enter the race without ballast (two other catamarans in the race both were fitted with twin ballast keels). Sadly five days into the race while lying joint second, Folatre suffered a collision with an underwater object destroying her rudder. However Derek returned to Plymouth, fixed the rudder, restarted and ended up making the crossing to Newport in the, for the period, respectable time of 34 days.

Aside from the experience of racing upwind across the North Atlantic singlehanded, the design of the multihull had piqued Derek's inner-engineer enough for him to attempt his own trimaran design. The fruit of this was the 45ft Toria, named after his newly born daughter. Toria was one of the most influential multihulls ever, establishing the fundamental concept of what a racing trimaran should look like, subsequently adopted by ORMA 60s and even most recently Ultime and Ocean Fifty trimarans. She had twin crossbeams attaching the relatively high volume, fine bowed floats to the centre hull, their geometry such that at rest only two hulls ever touched the water. Significantly she was the first boat to be built in the UK using the new technology of 'foam sandwich construction' - a step on from monolithic GRP construction, using AIREX foam as a core material, resulting in robust, high strength, lightweight panels.

Toria sailing on River Camel

Aboard Toria, Derek and SAS Captain Martin Minter-Kemp competed in the Royal Western Yacht Club's first two handed Round Britain and Ireland Race in 1966, comfortably winning it - the first victory in a major offshore race for a multihull. This success created considerable interest in offshore trimarans, including from Eric Tabarly, who two years earlier had won the OSTAR. The French legend helped Derek deliver Toria from Cornwall to London where it was displayed at the 1967 London Boat Show. However the experience caused Tabarly to return to France to work on his first trimaran not with Derek but French naval architect André Allègre. Had Derek been French how history might have been written, for Pen Duick IV, while a ground breaking design - a relatively giant boat at 68ft LOA and ketch rigged with twin rotating wingmasts - was agricultural in its aluminium construction and tubular crossbeams compared to what Derek was able to build at this time in foam sandwich. Tabarly was unable to defend his title in the 1968 OSTAR when his trimaran was damaged in a collision and subsequently suffered autopilot failure. However from designing racing winning trimarans, Derek enjoyed success in that transatlantic race having built the winning monohull Geoffrey Williams' 60ft Sir Thomas Lipton, designed by Robert Clark. This large for the time foam sandwich yacht was constructed - appropriately enough - in Sandwich, Kent.

Derek continued to enjoy further success with his small trimaran designs into the 1980s, including his Toria development Trifle, built for Royal Yacht Squadron Commodore Major-General Ralph Farrant, while in the hands of American Phil Weld his 44ft trimaran design Trumpeter came home third in the 1970 Round Britain and Ireland, another gale-ridden race won by a large monohull.

Derek's greatest series of race boats were the various 'Three Legs of Mann' trimarans built and sailed by Isle of Mann-based Nick Keig. The most successful of these was the 53ft Three Legs of Mann III, which Nick raced to second place in the 1980 OSTAR, however the most innovative was VSD, launched in 1982, a hybrid cat-trimaran with a flying centrehull. Although VSD achieved little on the race course largely thanks to its sub-standard rig, its concept was subsequently adopted in various forms by future catamarans ranging from the D35s to Alinghi 5.

On the larger stage Derek achieved perhaps greater success from his pioneering use of, and experience in, foam sandwich construction. This attracted some of the very top campaigns. Sir Thomas Lipson's 1968 OSTAR victory led to Derek building the Alan Gurney-designed Great Britain II for Chay Blyth, at 78ft long the largest 'composite' boat ever to have been built when she was launched in 1973. In the first Whitbread Round the World Race in 1973-74 she set the elapsed time record and went on to compete in five further editions of the race.

Derek Kelsall and Clare sailing Toria on River Camel

Great Britain II led to Derek designing and building two large trimarans for Chay Blyth, the 80ft long Great Britain III and the 53ft Great Britain IV aboard which Chay and Rob James won the 1978 Round Britain & Ireland Race.

While thanks to the exploits of Eric Tabarly and the likes of Alain Colas, the large racing multihull scene was going supernova in France during the 1980s, Derek never managed to break into this rich market although he came close. Eugène Riguidel and Gilles Gahinet won the 1979 Transat en Double Lorient-Les Bermudes-Lorient in their 52ft Kelsall tri VSD overtaking Eric Tabarly and Marc Pajot aboard Paul Ricard in the last few miles. Derek continued with Riguidel designing for him what was effectively a predecessor to the present day 'Ultime' class in the giant William Saurin. At 93ft LOA, she was the world's largest trimaran when she was launched in 1982 but was unable to defend her title, Riguidel and Jean-Francois Le Mennec finishing second in the 1983 edition to the catamaran Charente Maritime.

From the mid-1980s for the rest of his career, Derek's work as a yacht designer mainly focussed upon cruising catamarans, accompanied by further developmental steps in boat building techniques. KSS, the Kelsall Swiftsure Sandwich technique, cleverly used flat panels that could be laid up rapidly on a table, enabling construction time to be dramatically reduced, a process Derek first used in 1973 and constantly evolved over subsequent decades. This technique has been used successfully to build everything from day boats to giant passenger ferries. Among them too have been limited production run catamarans such as the Space 55, Suncat 40 and Islander 39 and larger one-offs such as the 70ft My Way and the 77ft light weight motor sailor, Mannanan.

After his wife Clare succumbed to Parkinsons, Derek was involved in the build of one of his 72ft catamaran designs for John Tucker in New Zealand. Derek loved New Zealand so much that following that project in the mid-1990s, he chose to make it his home. With new partner Paula Henderson he made his home in Waihi, just north of Tauranga, from where he continued to design yachts and motorboats and promote the KSS for the rest of his life.

Derek & Paula

Derek is survived by Paula and his daughter Victoria Liepins and son Liam and Liam's two children Elena and Libby. Meanwhile Derek's designs, including many of his early work from the 1960s, thanks to their foam sandwich construction, have survived him and are to be found in every corner of the globe.

A memorial service will take place for Derek in New Zealand on 25 January 2023.