The abbreviation Hysucat stands for Hydrofoil-Supported-Catamaran which
describes a hybrid type of Boat consisting of a planning type catamaran
equipped with a hydrofoil system. A typical example is shown in the sketch of
Figure 1: Typical Hysucat with foil arrangement
A hydrofoil is a wing like structure which is installed under water and which
is similar to an aircraft wing. It has a characteristic streamlined profile
form which creates pressure differences on the upper and lower surfaces as
schematically indicated in Figure 2 when it runs in a parallel flow. The
pressure forces are negative on the top surface and positive on the lower foil
surface. The summation of all the pressure elements on both surfaces results in
a lift force L , (called “Lift”) vertical to the inflow and a dragforce D
(called “Drag”), parallel to the inflow.
Such wings are used on aircraft wings, propeller blades, pump blades and fan
and turbine blades and Hydrofoil Craft. In most technical applications the Lift
L is a desired force (lifting-up force!) and the drag D is an undesirable force
component opposite the flow vector because to overcome the resistance or drag
force considerable energy has to be fed in by the engines and propellers which
leads to fuel consumption with continuous costs.
The ratio of the drag D over the lift force L can therefore be considered as a
kind of efficiency indicator:
e = D/L
and is known as the drag-lift-ratio.
Figure 2: Typical Hydrofoil Profile
A planing craft running with high speed along the water surface maintains the
attitude and trim due to dynamic uplifting forces, so called planing forces and
therefore can be considered in a similar way as the above hydrofoil wing.
A comparison of the planing craft with the hydrofoil wing is schematically
presented in Figure 3.
The drag-lift ratio of the boat is much higher than the one of the hydrofoil
wing. Experience values show that the drag-lift-ratio of the foil is about 10
times smaller than the one of the boat.
Figure 3: Planing craft and hydrofoil drag-lift ratios
In short it means that the load carrying efficiency of the foil is much better
than the one of the boat.
The basic principle of the Hysucat uses an efficient hydrofoil system in the
tunnel between the two demi-hulls where it does not disturb view or operation
of the craft. The hydrofoil system consists of a mainfoil in height of the
keels and slightly forward of the centre of gravity of the craft and two
smaller strut foils in the tunnel near the transom as indicated in Figure 1.
At speed the hydrofoils create dynamic lifting forces which keep the catamaran
partly above the water level. This way the demi-hulls of the catamaran are
moving less deeply submerged through the water which results in smaller
buoyancy and lift forces which means that the hull resistance is smaller. As
the efficiency of the hydrofoils is considerably better than the one of the
hulls the total craft resistance of the Hysucat is much smaller than the one of
At speed the remaining hull parts in the water create sufficient transverse and
The arrangement of the foils is important to achieve sufficient longitudinal
stability and requires that all the force components of hulls, mainfoil and
trim foils are in balance around the centre of gravity at all speeds.
A special characteristic of the hydrofoil which is known as the
foil-surface-effect and which means that the lift and drag reduce gradually
when the foil approaches the water surface from beneath. This effect is used in
the Hysucat for an automatic trim stabilisation.
For example, if the rearfoils approach the water level their lift reduces which
means that the craft’s trim is enhanced which is desired as the planing hulls
reduce the trim at high speeds. At very high speed the rearfoils may surface
with their lift capability switched off.
In short the Hysucat maintains an efficient trim angle at all speeds which
results in smaller wetted hull area with corresponding lower resistance.
To use the trim-controlling capability of the trim foils efficiently they have
to be arranged somewhat higher on the inner tunnel wall near the transom as
indicated in Figure 4 which means they run similar deeply submerged as the
Figure 4: Foil Arrangement for Trim stabilisation
The mainfoil lift results in a down-wash with angle T and this also has to be
considered for the rearfoil setting.
For the right arrangement with the corresponding foil submergences HW2 an
automatic trim effect is created which gives the Hysucat a favourable and
efficient craft attack angle in the whole speed range. Adjustable foils are not
The resistance of the Hysucat is much smaller than for the comparable catamaran
and especially at high speeds. Around the year 1980 some tests on a Hysucat
model were conducted in the towing tank at the University of Stelllenbosch,
South Africa and the very first test showed a resistance reduction due to the
foils of 40%. This unbelievable improvement led immediately to the creation of
the Research Project “Hysucat” which now is active for more than 25 years and
which has produced many more improvements by systematical optimisation.
As the resistance of the craft is directly proportional to the required
propulsion power the Hysucat needs only smaller engines than the catamaran and
is more economical in fuel consumption, even than usual deep-V-planing craft.
Further improvements of the Hysucat were observed on it’s sea-keeping and rough
The foil system gives a strong damping effect of the vertical and pitch motions
which gives it a most friendly sea-keeping in rough water which does not exist
on usual mono-hulls or catamarans.
Figure 5: Hysucat in wave crest encounter
In Figure 5 the damping effect is demonstrated. If the Hysucat runs into a wave
crest head-on the demi-hulls will be submerged deeper with a larger uplift
force which pushes the forward part of the craft up.
This creates a change of the relative inflow angle towards the mainfoil with a
smaller or even negative attack angle a which reduces the foil forces
momentarily. In other words, by running into waves the hull forces are
increased but the foil forces reduced.
When running through a wave trough similar force variations result, so that the
total wave running behaviour gives much softer water in-and out motions. The
extreme and hard pitch motions of fast planing craft in rough water are absent
for the Hysucat.
Further details of the Hysucat development and applications can be traced on
the FASTcc webpage and be studied in the given Research Reports and
In the development phase of the Hysucat a great number of towing tank model
test series of applications were completed in order to optimize special craft
to be built.
Later the Hysucat theory was developed which was applied for a Mathematical
Hysucat Model in a computer program.
The finalized Hysucat product appears simple and is easily understood. The
Hysucat design, however, is relatively complicated and is more similar to a
Supersonic aircraft design.
Nowadays all Hysucats and foil system applications are optimised with aid of
the Mathematical Hysucat Model by FASTcc and only in special cases towing tank
model tests are applied (mainly for legal contract conditions!)
The Stealth – Hysucat series are built in South Africa and comprise Sport
Boats, Yachts, Police Patrol Boats, Navy Offshore Craft of 5,3m to 22m.
The largest Hysucat built so far is the 45m E-Cat of Halter Marine, USA, and
the largest design was for a 72m Buquebus car ferry with a displacement of 635
t. These larger semi-displacement type catamarans have a different foil system
which is optimal for these craft at relatively lower Froude Numbers and is
known as Hysuwac-System, see above webpage.
Figure 6 shows a photo of an early 6,5m Hysucat RIB at full speed easily
handled by the two ladies. Of this Sport boat type over 250 units were build so
Figure 6: An early 6,5m RIB Sport Boat
Copyright © 2003 Foil Assisted Ship Technologies cc