Trans RINA, Vol 156, Part B2, Intl J Small Craft Tech, Jul-Dec 2014 EXPERIMENTS WITH STEPPED PLANING HULLS FOR SPECIAL OPERATIONS
CRAFT (DOI No: 10.3940/rina.ijsct.2014.b2.162) M G Morabito and M E Pavkov, United States Naval Academy, US SUMMARY
Many of today’s special operations craft and high-speed patrol boats can operate at volume Froude numbers high enough to justify the use of a stepped planing hull; however most of these boats are un-stepped. This paper describes experiments to determine the effect of adding a single step into the bottom of a planing hull with loading parameters consistent with modern special operations craft. In contrast to multi-step pleasure craft, which can operate at volumetric Froude numbers of 10, special operations craft often operate at volumetric Froude numbers of around 5, owing to the increased payload and decreased speed. Previous towing tests on a variety of stepped hull configurations indicated that a promising configuration for special operations craft may be a single step located near the transom (alternatively known as a hydrodynamic transom forward of the stern). The present experiments investigate this configuration further, by testing a single-step hull, with step located at 25% of the length forward of the transom. The step height is systematically varied to observe the effect on resistance, trim, wetted surface area and porpoising stability over a wide range of speeds. Of the configurations tested, the best reduced model resistance by 25% at the highest speeds tested, while increasing resistance by 10% in the hump speed regime. The stepped hulls tested had porpoising limits similar to conventional planing monohulls. A short method is provided to illustrate when a stepped hull may be advantageous for a given design.
NOMENCLATURE
Beam (m)
∆
Friction coefficient, Speed coefficient, Load coefficient, ∆
Volumetric Froude number, Length Froude number
Gravitational acceleration ( m s-2 ) Step height ( m )
Hull lift force (N)
, Chine wetted length of hull aft of the step, measured from the transom forward ( m )
, Chine wetted length of hull forward of the step, measured from the step forward ( m )
, Keel wetted length of hull
Waterline length (m)
aft of the step, measured from the transom forward ( m )
, Keel wetted length of the hull forward of the step, measured from the step forward ( m )
Transverse wetted breadth of the hull at the transom in horizontal plane ( m )
Transverse wetted breadth of the hull at the step in horizontal plane ( m )
S
Total resistance ( N )
Wetted surface area - varies with speed (m2)
Pressure resistance ( N ) Frictional resistance ( N )
Free stream velocity (m/s) Figure 1: Simplified Single Step Planing Hull
/
Weight supported by hull ( N ) β
Mean bottom velocity (m/s) Deadrise angle (deg)
Kinematic viscosity ( N s m-2 ) Density of water (kg m-3)
Trim angle measured from horizontal to keel ( deg )
,Static trim angle measured from horizontal to keel at zero speed ( deg )
1. INTRODUCTION
Stepped hulls have been in use for over one hundred years on high speed planing craft. A typical stepped hull consists of a deadrise planing hull with one or more transverse breaks in the bottom, allowing the flow to separate from the step and reattach further aft. Figure 1 shows a sketch of a simplified single step planing hull.
has been shown in experimental, computational, and theoretical studies that these steps can reduce resistance in certain conditions. [1-5].
It the
©2014: The Royal Institution of Naval Architects
B-87
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88