canard evolution wing evolution
Configuration History

The Decavitator evolved over the course of its four year lifetime. Most changes were made below the water line, but the hulls and fuselage were also modified. The details of the final version of the boat are much different than those of its predecessors, but the basic layout did not change much.

Version I (The V-Foil)

The first version of the boat used a V-foil and air propellor. The concept of using an air propellor was initially introduced as a joke, but its advantages became apparent as the project progressed. The V-foil part of the design turned out to be less effective. The V-foil design included twin wooden hulls, a 10ft diameter air propellor, a triangular fuselage based loosely on the Daedalus layout, and a surface-piercing V-foil with a 10ft span.

Front view of the original V-foil vonfiguration. The main wing was a shallow V and the tail was an inverted T with rudder and elevator.
Side view of the original V-foil configuration. The center of gravity (CG) is to the left of the main wing in the diagram.

A water propellor was considered, but the air propellor and Daedalus-like fuselage were kept. In retrospect, the air propellor turned out to be Decavitator's greatest advantages as well as one of its greatest limitations. Use of an air propellor meant that fewer structures had to pierce the water surface. And dragging the drive train and propellor through air cost less in terms of drag than dragging an equivalent drive train and propellor through water. Unfortunately, the air propellor also made the Decavitator extremely susceptible to crosswind disturbances.

Versions II-III (Fully Submerged Ladder Foils)

The V-foil was replaced with two fully submerged foils located directly under the pilot. The original submerged foil configuration used a ladder foil. After repeated harsh landings (including structural failure at the Milwaukee event), the ladder was replaced with a twin wing, pop-up design. Instead of 'climbing' up a ladder foil as speed increased, a large, low-speed wing was pulled up out of the water at high speed leaving only the smaller, high-speed wing.

v4Front view of version II. The main wing was a two-wing ladder and a canard in the bow of the boat (the inverted T in the center) provided pitch control.


Side view of versions II and II. The left image shows the boat in high-speed mode with the large wing out of the water.

In the V-foil configuration, pitch was controlled through a rear-mounted elevator and rudder. After converting to fully submerged main foils, a single canard with rudder controlled the vehicle's pitch, but provided no roll stability. As a result, the single canard was soon replaced with twin canards, one at the bow of each pontoon.

v4Front view of version III. This version shared the same main wing configuration as version II, but instead of a single inverted T in the bow, this configuration used two inverted Ts, one on each hull.

The twin canard configuration provided both roll stability and a solid pivot point about which the pilot could control the pitch of the vehicle. The first fully submerged configuration used two main foils for primary lift. The idea was to use the larger, upper foil to bring the hulls off the water then to accelerate to the point where the smaller, lower foil could support the weight of pilot and boat. The pilot could then 'climb' out of the water onto the smaller foil. This configuration worked well in tests, but the resulting high center of gravity along with rather exciting 'landings' ruled out use of this configuration at higher speeds.

Versions IV-V (Pivoting Low-Speed Wing)

The solution to the ladder foil problems was a pair of submerged foils with an upper foil that pivoted out of the water at high speeds. With both foils in the water, the pilot could easily lift the hulls out of the water. Once foil-borne, the pilot then accelerated to about 16 knots. At this point, the upper foil was pulled up and the boat accelerated to its maximum speed on the small foil. This is the configuration which was used to set the world record.

v4Front view of versions IV and V. This configuration is similar to version III, but instead of a fixed ladder foil, these versions used a pivoting large wing.

Side view of versions IV & V. The left image shows the boat in high-speed mode with the large wing out of the water.

A lightweight kevlar fairing was added around the pilot in order to reduce drag. In addition, all of the bracing tubes were faired with lightweight foam. Although the fairing reduced drag, as it turned out, it also made control of the boat at high speeds virtually impossible.