Sailing hydrofoils are not new. Gordon Baker's Monitor was sailing in the 1950s and Don Nigg's Flying Fish in the 1960s. Most craft designed or retro-fitted with lifting foils have been created with the aim of increasing maximum speed, many having competed in time-trial events such as the Weymouth (UK) "Speed Weeks" since 1972. Several hydrofoils have held 500m speed records in various sail area classes, including Hansford's Mayfly, the Grogono brothers' Icarus, Sam Bradfield's (NF)² and Greg Ketterman's Longshot. However, outside the specialised speed-sailing world little use has been made of the potential of lifting surfaces to improve all-round performance until recently. Hydrofoil activity in the International Moth and 14' classes, together with the commercial availability of the WindRider Rave and the Longshot derivative Hobie Trifoiler has increased interest so that the authors feel it timely to share their experiences in this field.

While many competitors’ craft at Weymouth displayed short bursts of speed, few managed to traverse the 500m course without a "crash" of some kind. Of the small craft, the best results were from those who adopted the classic aeroplane surface piercing hydrofoil configuration. Attributable to Philip Hansford in 1971 (Alexander et al, 1972), the use of two surface piercing main foils supporting most of the weight of the craft plus a fully submerged inverted "T" rudder for pitch stabilisation became known as the Mayfly configuration after the small catamaran to which it was fitted. Copied by others, most notably James Grogono and the Icarus syndicate, this arrangement yielded a string of world records for both boats throughout the 1970’s (Grogono, 1987). Unfortunately, this system suffers from the close coupling between lift and side force on the main foils as a consequence of their dihedral. Often exacerbated by the helmsman attempting to compensate, the ensuing motion of skying and diving may lead to a "crash" – a dramatic reduction in velocity as one or more hulls hit the water – and a reduction in average speed. To physically isolate the lift- and side-force producing functions of the appendages necessitates geometrically orthogonal hydrofoil surfaces. With the absence of the inherent passive height control of the surface piercer, either a ladder foil arrangement or an actively controlled incidence fully submerged lifting surface is required. Active control held the promise of greater potential, particularly in waves.

In 1976 the system was adopted for use on the sailing trimaran Force8 (Pattison and Wynne, 1980). It comprised a forward reaching arm with a small planing shoe at its front end working a fully moving foil via crank arms and push-pull wires. A change in boat height above the water caused the arm to rise or fall relative to the boat, which in turn changed the incidence of the foil such that the lift generated opposed the initial disturbance from equilibrium. As such it was a simple proportional system, but Pattison also incorporated a manual input for roll control, similar in function to the ailerons on an aircraft. Force8’s main shortcoming was with the foils sticking: acetyl journal bearings were used rather than balls or rollers, resulting in occasional broaching and crashing. Greg Ketterman’s Longshot (Ketterman, 1994), avoided these problems by using a flexible structure to allow the incidence of submerged foils to be changed rather than crank arms and bearings.

Both Longshot and Force8 had forward reaching height sensors, a feature we considered unsatisfactory because of their risk to nose-dive or collect floating debris. The alternative, a trailing planing sensor arrangement, first appeared at Weymouth in 1977 on the modified A-class catamaran Rampage, by Mark Simmonds. This boat only had one controlled T foil under the port hull and it was not until Hansford adopted the system on both sides of his trimaran Dot (later Philfly) in 1985 that truly stable, level flight on a variety of headings was seen.

The authors’ own Bandersnatch, a 14’ ultra-light catamaran served as a test platform for a variety of configurations, finally converging on the trailing planing sensor and trailing edge flap inverted T foil system. As such, this craft demonstrated excellent stability foilborne. However, with low displacement hulls of almost zero rocker and large surface piercing additional lifting surfaces, low speed manoeuvrability and displacement windward performance were poor.

To overcome the shortcomings of Bandersnatch, a 4.9m single-handed catamaran was designed with the following aims:

• Retain the excellent flying characteristics
• Improve windward performance
• Reduce time to deploy and stow the foils
• Be operable single handed from a specific boatyard in Plymouth, UK.

These objectives were fully met with Calliope, a 4.9m long by 2.8m wide catamaran built in 1992, Figure 1. Essentially a wide beach-cat, the hulls were built from tortured 3mm plywood to give a semi-circular section and modest rocker - the intention being that they should offer low drag at low speed. Above 10-12 knots the hulls would be clear of the water and their shape would become an aerodynamic concern!

Initially fitted with trailing edge flap foils controlled by bow-mounted trailing sensor arms and a fixed incidence inverted T rudder, one major problem limited performance: ventilation of the main foil struts. Over the course of an 18 month period the two principal reasons for strut ventilation were identified and remedied. The first was the disturbance to the water surface by the trailing sensor ahead of the strut and the second was the strut section. Moving the sensor arm inboard and adopting highly polished and rounded strut leading edges was the cure (Chapman, 2000). As a further improvement the foil system was changed from trailing edge flap to fully moving, balanced, inverted T, which had the great advantage of significantly reducing the control forces involved. The final specification of the struts and foils and other relevant details are given in Table 1.

As part of the development program, an instrument system was employed to record the basic parameters of boat speed, apparent wind speed and apparent wind angle. The data gathered over many hours of flying time was used to verify a simple velocity prediction program which served to increase our understanding of the factors that affected the performance of this craft.

Calliope in 1995.

By 1997 it was considered that to fully exploit the progress made with Calliope it would be desirable to build a larger, 2-man version. Just as Calliope had not been intended to be an out-and-out speed sailing machine, nor would her successor, which was given the name Ceres. Instead the aim was to produce a craft that would be "larger, stronger, lighter, faster" and capable of competing against the new generation of "formula" beach cats in coastal waters while demanding less athleticism from the crew.