Trans RINA, Vol 156, Part B2, Intl J Small Craft Tech, Jul-Dec 2014 DISCUSSION
EXPERIMENTS WITH STEPPED PLANING HULLS FOR SPECIAL OPERATIONS CRAFT
M G Morabito and M E Pavkov, United States Naval Academy, US (Vol 156, Part B2, 2014)
COMMENT
Donald L Blount, Donald L. Blount and Associates, Inc, US
I am impressed by the quality and utility of the reported results with analysis of this experimental research regarding stepped planing hulls. It is a meticulously-planned and well-executed program, and I very much appreciate authors who publish their experimental data as it often provides a stimulus for others to extend the initial program with additional objectives as well as develop and use the data to validate analytical tools. Congratulations.
This program addressed four significant issues and found positive results supported by hydrodynamic analysis of test data studying step height over a range of speeds for constant hull loading and LCG. One of the purposes of this experimental program was to confirm that a single stepped planing hull
could,
without porpoising, significantly reduce high-speed, calm-water resistance with minimal hump resistance increase. This goal was validated with the test model attached to the carriage and towed free-to-heave and trim. This towing arrangement is acceptable for these goals as minimum resistance occurs at approximately four degrees trim angle, referenced to the straight afterbody buttocks. Thus, the curved buttocks at the bow were not likely to be wetted at high speeds.
Being personally interested in dynamic instability which becomes possible when curved bow buttocks become wetted at low trim angles, I require model attachment for tank testing to allow freedom in heave, trim and roll. In an operational environment, dynamic instability might occur when the operator trims the bow down with tabs or interceptors or as the bow becomes wetted in waves. With the addition of attachment being free in roll, I find that with little cost, a few more calm water test runs with ballast moved off center for a five to ten degree static heel
dynamic instability at lower trim angles [Blount and Codega, 1992].
A recreational craft of our design, near to the dimensions of your example, has been delivered with a step forward of the hull’s transom. As with your example, a design speed of volume Froude number of 5+ necessitated proper longitudinal spacing between the LCG and a hydrodynamic- flow-separating transom. Relocating LCG was not possible due to other requirements, so it was necessary to move the hydrodynamic transom forward.
Because of the perception around the waterfront that stepped hulls can have
characteristics at high speed, our
unpredictable handling design has
been
characterized as having an extended hull and deck overhanging a forward-placed hydrodynamic transom. Semantics can be as important as technology sometimes.
With a designer’s decision to mitigate hump-speed resistance by selecting step height and profile shape of the bottom surface of the overhang to be wetted or select an operator trim control system, I think that another paper with a bit more analysis of these data might provide guidance for making this choice. The allowable length of the overhang is related to craft speed, the height of the step, and whether hump-speed trim needs to be controlled, by the length of wetted-hull extension or some operator-controlled system, such as interceptors.
could have provided an indication of possible
Figure 14. Sketch of afterbody re-wetting of stepped hull at two speeds.
©2014: The Royal Institution of Naval Architects
B-119
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