Trans RINA, Vol 156, Part B2, Intl J Small Craft Tech, Jul-Dec 2014
the proposed configuration has been completely ignored and is very important to the safety of high speed boat operators. Given the limitations of the study presented in the paper, the conclusion that
"this configuration of
stepped hull may be an attractive design choice, and should be considered as a potential alternative in early design stages of future special operations craft" is an over-reach. At best, the authors could conclude that the fundamentals studied within the paper are worthy of further study. The phenomena studied in the paper are too immature to be incorporated into
future special
operations craft without incurring significant technological risk. The parameters studied within the paper lie well outside of configurations available within the recreational boat market. Typical step configuration s and loading distribution in recreational boats are not represented in the paper. The 90%-10% load distribution is only representative of the dynaplane which needed hydrofoils at the stern to maintain overall control of the boat. Recreational load distributions in my experience fall more towards a 60%-40% load distribution. The step configuration presented in the
paper is more
representative of incorporating a hydrodynamic transom forward of the geometric transom; however, at 25% LOA, the location is an extreme example of this application.
Is there any follow-on research planned to
address directional stability, seakeeping or any of the other myriad of design factors needed to turn this phenomena into an actual boat?
AUTHORS’ RESPONSE
The authors would like to thank the reviewers who provided discussions for this paper. They raised important
thought-provoking questions, and made
specific suggestions that were used to improve the clarity of the final published paper.
Donald L Blount suggested future research to explore the correlation between the rewetting of the afterbody with the method suggested by Savitsky and Morabito (2010). The authors are currently in the process of collecting recent stepped hull wetted length data from a variety of tests for that specific purpose.
Another key
suggestion from Mr. Blount is to estimate the afterbody pitching moment based on the wetted area from the photographs. We would suggest taking that process one step further by instrumenting the joint afterbody and forebody model sections
between the to directly
measure afterbody lift and pitching moment. This test would directly correlate the wetting patterns of the afterbody to the lift and center of pressure.
Dr Prasanta K Sahoo asked the authors to comment on whether the flow would be turbulent at the speeds tested, and also to comment on the significant changes in wetted surface area at higher speeds. Because of the large variation in wetted length that occurs on planing craft it is not feasible to locate turbulence stimulators on the hull.
Therefore the model size and towing carriage speeds must be high enough to ensure turbulent flow. When planning model tests of this type, the authors typically follow the advice of Savitsky and Ross (1952) who suggest maintaining a minimum Reynolds number of 2 x 106.
In order to expand the data to full scale, it is necessary to use the wetted surface area corresponding to each particular speed.
The total wetted area from the tests has been tabulated in the data tables, normalized by the square of the
beam as follows: /. For non-stepped planing hull tests, the Reynolds number is typically computed using the mean
wetted length of the planing surface, (Savitsky, 1964). Since the hull in this
/
Dr Sahoo asked if similar results are to be expected for round bilge hull forms. Typical round bilge hull forms do not achieve high enough volumetric Froude numbers to require a stepped hull because of problems with spray and dynamic instability. The authors do not suggest adding steps to round bilge craft.
Lawrence J Doctors asked why planing boats are used when slender displacement monohulls offer significantly lower relative resistance. It is true that slender displacement monohulls have much lower relative resistance, but only when operating at much lower Froude numbers. At these low Froude numbers, very long hull lengths are needed to achieve the speeds that a much shorter planing hull can. Length is often limited in design, even though longer non- planing hulls will have better seakeeping and resistance characteristics.
When slender displacement monohulls are
run at the same Froude numbers as planing craft, they have much larger resistance-to-weight ratios and suffer from dynamic instabilities.
Lawrence Doctors suggested a possible correlation between the height of the step, the length of the step and the change in trim. A hull form characteristic that comes to mind from seaplane nomenclature is the “Sternpost Angle,” defined as the angle formed between the extension of the forebody keel and the line connecting the step to the transom at the centerplane (see Figure 15).
For hulls with parallel forebody and afterbody keels, the sternpost angle is simply:
tan
Table 4 and Figure 16 were prepared to show the correlation between the changes in sternpost angle with step height and the change in trim at three different speeds. The data show a definite correlation between increased sternpost angle and increased trim.
©2014: The Royal Institution of Naval Architects B-121 study has a constant beam, the
following relation can be used to determine from the surface area data given in the tables:
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