Trans RINA, Vol 156, Part B2, Intl J Small Craft Tech, Jul-Dec, 2014
Stepped hulls have become increasingly popular for high-speed pleasure craft due
to the availability of
lightweight, high-speed engines and surface-piercing propulsion. These craft can operate at volumetric Froude numbers above 10, necessitating the use of an advanced hull form.
Many of today’s high-speed naval pleasure
craft. combatant Volumetric craft
operate at comparatively lower volumetric Froude numbers than
Froude
numbers of up to 5 are typical for the fastest naval patrol boats.
conventional planing monohulls are possible alternatives.
Recent studies by White [3] and Lee [5] have indicated that large reductions in
resistance are possible by
utilizing stepped planing hulls on these craft. Lee [5] tested a variety of twin-step configurations, with step locations and geometry similar to modern pleasure craft, but displacements typical of naval patrol boats. One of the more favourable conditions for resistance was a very large aft step and small forward step, which resulted the highest running trim of all of the configurations. This trim was closest to the optimum 4-degree trim for minimal resistance of typical planing craft (Savitsky, 1964). Although no single-step configurations were tested during Lee’s series of
tests [5], the results
indicated that a single-step hull with large aft step may also be capable of attaining a 4-degree trim and minimal drag, without the resistance penalty associated
with
separation off of the forward step at low speeds, and potentially a more simple design.
In the present study, a configuration with a single, large step located at approximately 25% of the length forward of the transom is explored in greater detail, both experimentally and analytically. First, the limits of planing monohulls
are
single-step hull with an aft step to address the following questions:
What is the effect of an aft step on resistance and trim at high speeds?
How does step height influence resistance and trim?
What is the resistance penalty at low speeds?
How do steps affect the porpoising stability of planing craft?
The step location of 25% has been chosen for illustrative purposes and because of the availability of the model tested by Lee [5]. The effect of step location (i.e. why not 20% or 30%) has not been addressed experimentally due
It
Figure 2: Pressure resistance of a flat planing plate (neglecting friction)
calculation for a generic naval patrol boat is provided. Second, the stepped hull naval patrol boats.
discussed and an example is discussed as it applies to
Experiments were conducted on a Adding the frictional resistance, ½
total resistance becomes: ½
Where , the mean bottom velocity, is slightly less than the free stream as a result of the dynamic pressures (Savitsky
[6]). The casual observer will see that at high speeds, the frictional resistance, which increases with the square of velocity, rapidly becomes the dominant component of resistance. The only apparent way to reduce the friction component is to reduce wetted surface area.
to time and cost limitations; however, in the
following section of this paper, a discussion is provided about the influence of LCG and step location on the running trim and lift-to-drag ratio of planing surfaces.
B-88
Wetted area typically changes with speed during to the transition from displacement to planing; however at high speeds it becomes constrained by the longitudinal center of gravity (LCG). In the absence of large external moments, the center of lift of the hull must coincide with the center of gravity. This establishes a lower bound on the mean wetted length to around 30% greater than the distance from the transom to the LCG. The minimum wetted surface area is approximately:
1.3 cos
©2014: The Royal Institution of Naval Architects , the In this speed regime, both stepped hulls and
To explore whether a stepped hull should be considered in a design, it is necessary to understand the limits of conventional planing monohulls. The majority of the resistance of a planing monohull is the sum of the pressure resistance (primarily wavemaking and spray) and frictional resistance.
The pressure component of resistance, , is illustrated in Figure 2 for a flat plate.
, shown in the figure, holds for prismatic planing craft (hulls with parallel buttock lines), operating with
The approximation,
the transom dry and the curved portions of the bow out of the water (Savitsky, 1964).
2. LIMITS OF PLANING MONOHULLS 2.1 RESISTANCE CONSIDERATIONS
will be shown that step location for minimum resistance is primarily a function of displacement, longitudinal center of gravity, and speed.
For this class of stepped
hulls for special operations craft, the optimum step location appears to be farther aft than on lightly loaded, high-speed pleasure craft.
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