Page 147 - Practical Design Ships and Floating Structures

122

The foil section is the NASA GA(W)-1 section (McGhee and Beasley, 1973). The foil dekalage is set
for several values. It is important to note that an incorrect dekalage angle results in larger drag than
the comparable planing hull, In fact, further investigation of this configuration shows that the
maximum proportion of foil lift is not associatcd with the minimum total drag. This is thus not an
optimum configuration in some more fimdamental way, though it is superior to the planing (only) hull.
This shows the first lesson we have learned: The numerous parameters available in this concept allow
many configurations, few of which are optimal and some of which are quite poor. Computer
simulation is vital to optimizing a hybrid hydrofoil, even at the most basic level.

4 FOILS

The use of the NASA GA(W)-I is worth discussing further as we believe the use of this type of foil is
another issue critical to the success of the concept. This section is one of a class of supercritical
sections that are designed by computer to achieve maximum lift coefficient (above 2.0) with minimum
possibility of stall. They achieve this by reducing the peak suction near the tip and “filling in” the
suction aft, so that the total area of high suction is increased without severe peak. This is achieved by
retaining significant thickness aft, often terminated in a reflex curve, hence the term “barn roof’
section. Since hybrids will operate at high angles of attack and low speed during take-off, foil stall is
a significant problem and makes takeoff difficult (especially considering propulsion effects).
Cavitation is also reduced, at least for the relatively low speeds that hybrids operate.

5 DYNAMIC STABILITY
Though it seems unlikely that a stepped hybrid will develop pitch dynamic instability, there is a
definitive criteria. Martin (1 978) developed the theoretical methods of determining stability for high
speed planing boats, producing two equations, one in pitch moments and one in heave forces, each
with various cross products wherein pitch and heave kinematics produces heave forces and vice versa.
Extending these equations to the case of a hybrid hydrofoil only requires adding the foil dependent
terms. These terms are available from standard ship control methods. For example, pitch moment due
to pitch angle is simply the lift curve slope of the foil times the stagger and pitch moment due to pitch
rate is lift curve slope times stagger squared. Such a set of equations has to be numerically solved, so
no insight can be gained directly by examining an analytic solution. Instead, numerous systematic
variations have to be examined. However, the two exemplar terms are the main damping effects, and
they will be very large in any practical stepped hybrid, so smooth water stability is almost guaranteed.

6 SEAKEEPING
Many high speed craft are limited by motion in waves rather than power. Methods to analyze motions
will be required to determine limiting conditions for crew and passengers, and structural loads. Martin
(1978) demonstrated how this proceeds for pure planing craft by extension of the stability method.
This can be extended in a similar fashion by adding the foil terms for forces and moments from waves,
but is worth noting that the foil excitation due to waves is relatively small because the foil is effected
only by the orbital velocity of the waves and very slightly by the elevation of the foil beneath the
waves. The velocities are small compared to the vehicle speed and the effect of elevation is minimal if
the foil is in submerged below a chord length. The particle velocity effects and wave height effect also
are opposed, so the net force is even smaller. It is difficult to make general predictions about the
seakeeping of stepped hybrid hydrofoils because this is profoundly affected by optimization, but there
are two important points that suggest good seakeeping is possible: