Page 148 - Practical Design Ships and Floating Structures

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First, let's examine what has become the norm for planing and semi-planing craft designed for good
motions in waves. Motions in head seas dominate the problem of seakeeping for fast craft, because at
high speeds, all seas are head seas. Offshore racing craft, and "wave-piercing", catamarans both
approach the problem of reducing motions in head seas in the same way, by moving the sensitive load
as far aft as possible and by reducing the rate of lift force with respect to immersion of the forward
sections, usually by making them narrow, with high deadrise. To a certain extent then, seakeeping and
planing efficiency must be traded off, paying for one with the other.

Also, if a planing hull strikes a wave, the force induced on the hull by the wave is still primarily at the
intersection of the hull and the instantaneous water surface. As the hull travels, this intersection moves
aft, and the force becomes larger as the hull gets wider and deeper and the hull both rotates backwards
and heaves up, thereby producing large combined accelerations. In contrast, a stepped hybrid hull will
initially rotate, but the rotation will increase the angle of attack of the aft foils, which lifts the vehicle
bodily upwards from the rear and reduces pitch acceleration. The hull is therefore "anticipating" the
oncoming wave and goes over it. This motion has to be carefully tuned to the anticipated wave
environment for optimum performance, but it is clear that a properly designed stepped hybrid
hydrofoil would have excellent motions. Since this behavior is enhanced by high lift in the foil, good
seakeeping is associated with good lift efficiency, rather than degraded by it.

Second, with the wide range of parameters available to the designer, it is clear that there is
considerable latitude to optimize for motions. A hull form with very high deadrise, low freeboard
planing hulls forward and foil support ail could be developed with very good motions because the foil
would bear the majority of the load and the hulls could be relatively inefficient, hence relatively soft
riding. In a pure planing hull, the designer has to lose efficiency by accepting a high deadrise, soft
riding hull. The cost of non-optimum lift production for the sake of seakeeping would be much less
for a hybrid hydrofoil.

7 PROPULSION

A problem of hybrids is that of propulsion: Getting the force into the water often requires passing it
through the struts which is costly in terms of money, appendage drag, complexity and efficiency.
Hydrofoils use mechanical, electric and hydraulic drives to props on foil pods, jets taking suction
through the foil, and shafts from the hull. Each of these methods has problems. There is some
consolation that the struts of a hybrid are somewhat shorter, but this is only important for through-strut
jet drives, and jet drives require higher flow rates for efficiency at the lower speeds of a hybrid.

However, unlike a pure hydrofoil, a hybrid can be propelled by hull mounted components. A jet drive
could be mounted in the forward planing hull and discharge at the step. A prop shaft could penetrate
through the step as well or surface piercing props could be mounted on or below the raised tail and dip
down to the water. This gives some added versatility to the hybrid concept that a pure hydrofoil
doesn't have. The choice of propulsion method is economic and operational and will be determined by
the mission. The hybrid offers wider latitude for less costly methods than a pure hydrofoil, but requires
an innovative approach to the issue.

8 EXPERIMENTS

The authors have had limited funds and time to explore this concept, but with the help of those
acknowledged have been able to experiment with a few small unmanned and manned models. The
latter experiments have produced two final significant insights. First, one manned model exhibited
severe, uncontrollable broaching instability due to the combination of a narrow planing surface and