Trimaran design takes to the air

The Dynamic Assisted Trimaran or DAT is a unique combination of super slender monohull, multihull and hydrofoil technology. Originally developed by TechMan, the key features of the design have each been carefully selected to maximize the DAT?s potential as a passenger vessel. Potential applications also exist as a ro-pax vessel, a patrol boat and a super yacht.

Designed to operate with as much as 80 per cent of the vessel?s displacement supported by the foil system, only a portion of the slender centre hull remains submerged when operating at design speed. With only a fraction of the vessel?s centre hull remaining submerged at design speed, the vessel uses conventional flush inlet waterjet propulsors, rather than propellers or the more complex systems sometimes found on foil-borne hydrofoils.

The DAT is designed to transport large numbers of passengers in comfort in an environmentally sound manner, with reduced wake and wash, and with a high level of safety. In this respect, the centre hull and side hulls have a lower, watertight deck above the waterline.

Even if the centre hull is damaged, notes TechMan, the vessel will not heel because the centre of gravity is in the centre hull. The side hulls meet IMO stability requirements, and being well aft of the main hull (sharing a common transom) are protected in the event of grounding. Another major advantage of the design lies in its favourable drag characteristics which, TechMan believes, will offer significant fuel savings compared with conventional monohulls or catamarans.

Island Engineering?s ceo Bill McFann (who became involved in design and fabrication of a demonstrator) explains that unlike planing vessels, air-supported ships or conventional hydrofoils, the ?take-off? sequence for the DAT ? the transition from being hullborne to being foil-borne ? is gradual, with little to indicate that the transition has occurred.

Once ?in flight?, as McFann puts it, the foil lift fraction and centrehull immersion can be varied according to sea conditions in order to control the immersion of the waterjets. Under normal ?flying conditions? however, the outer hulls remain out of the water. The ?super slender? main hull ? which has a low waterplane area ? and active foil stabilisation combine to produce a very ?soft? ride with low amplitude motions and accelerations in pitch and heave. Rolling motions are almost eliminated.

All of the machinery for the craft is located in the centre hull. The wide beam of the trimaran hullform provides additional space for passenger accommodation or for vehicles on the vehicle deck in the ro-pax version of the design.

The 14m DAT demonstrator (shown here) was built as a quarter scale model of a 56m, 50 knot passenger/car ferry (dubbed the MM56CX), and fabricated in aluminium with a composite superstructure, although a range of geometrically similar vessel designs have been developed, from a 23m passenger-only ship to a 56m passenger and car ferry.

Propelled by twin waterjets, the DAT demonstrator is supported by hydrofoils mounted beneath the hulls, and has a digital, ?fly-by-wire? steering and stabilisation system. The maximum speed of the 6t model is approximately 30 knots. The Island Group, of which Island Engineering is a part, completed the design and fabrication of the demonstrator in May of last year, based on the original design from TechMan.

In November of last year, trials re-started using a new Blended Wing Lifting Body (BWB) developed specifically for the demonstrator by Navatek Ships in Hawaii using funding from the Maritime Administration (MARAD) and the Office of Naval Research (ONR).

McFann and his team at Island Engineering have tested the performance of many advanced vessels over the years, and say that the combination of speed/resistance and seakeeping which the DAT demonstrator has exhibited are “unmatched” by any other concept. “The motions are like those of a SWATH (Small Waterplane Area Twin Hull) and the speed/resistance characteristics analogous to a Surface Effect Ship (SES),” says McFann.

Ironically, such is the high performance of the newly developed hullform that McFann has found there is what he calls a “lack of acceptance, even resistance” to the concept, largely, he believes, because it has foils underneath it, and because the extremely high performance is difficult to accept at face value. “It sounds too good to be true,” he explains.

McFann says he would like to be able to reveal more details about the underwater features of the DAT?s hull and foils, but has agreements with TechMan and Navatek preventing Island Engineering from doing so.

Having developed the DAT concept (which is fully patented) this far, TechMan is looking for worldwide licensees, focusing initially on the fast ferry market, where the DAT?s ability to provide a ?soft? and stable ride will reduce the incidence of seasickness and provide a high level of passenger comfort.

Versions of the DAT proposed by TechMan and Island Engineering include the 56m ferry, which could carry 438 passengers and 64 cars at speeds of up to 47 knots; a 40m ferry, capable of carrying 400 passengers at 45 knots; a 29m ferry capable of carrying 120-186 passengers and a small number of cars (or 240 passengers without vehicles) capable of speeds of up to 45 knots; and a 23m ferry that could carry 50 passengers and four cars at speeds of up to 45 knots.