Trans RINA, Vol 156, Part B2, Intl J Small Craft Tech, Jul-Dec 2014


With the underwater photographs from these tests, such as illustrated in Figure 8, might it be possible to compare the rewetting the hull bottom aft of the step with surface contours


when lower relative resistance can be obtained with slender displacement monohulls?


predicted with Reference 2 [Savitsky and


Morabito, 2010]? With a reasonable comparison, it might be possible, using the 2010 reference, to relate length of afterbody wetting at hump speed with step height to develop some rough order of magnitude of bow-down trimming moment. See Figure 14.


Finally, I trust that the authors find continuing personal interest with new insight which extends the technologies of high-performance craft.


Dr Prasanta K Sahoo, PhD, FRINA, MSNAME, Associate Professor (Ocean engineering) Florida Institute of Technology


Let me take this opportunity to congratulate both authors in presenting a remarkable paper that is quite relevant to designers of stepped hull forms.


The authors have taken a typical chine hull form typical of combatant craft to illustrate effect of stepped hull in reducing


resistance and demonstrating the limits of


porpoising stability. The systematic variation of step height in conjunction with variation in trim angles and porpoising is a step in the right direction providing better insight for optimizing stepped hulls for combat operations in various speed regimes.


Here are some observations that I would like the authors to comment on:


1. 2. 3.


Is there any indication that with volumetric Froude numbers being as low as 5, the flow would be turbulent?


It appears that there is significant changes in wetted surface area at higher speeds and how has this been reconciled.


It is to be noted that typical combat vessels would be round bilge rather than chine hull form. Can designers assume that similar results are expected for round bilge hull forms?


Lawrence J Doctors, Emeritus Professor Naval Architecture, University of New South Wales Sydney, Australia


This interesting paper provides useful information on the effect of a step on the resistance and trim of a planing vessel; it will be of value to designers of such vessels. Reviewer is impressed by the methodical approach to creating the step - by means of relative vertical shifting of the segments of the model.


Figure 3 shows that typical values of the resistance-to- weight ratio approach 0.2 at relatively high speeds. Can the Authors suggest why planing boats are often used


With respect to Figure 9, which shows the resistance-to- weight ratio for different step heights, have the Authors compared these experimental results with the standard Savitsky method assuming, say, that the method applies to just the wetted part of the hull ahead of the step. That is, can one ignore the part of the hull behind the step - at least for the larger steps at high speeds?


The trim in Figure 10 shows the experimental trim. This trim is greater for larger steps, so one is tempted to assume that the change of trim could be related to the angle formed by the step height and the hull


length


behind the step. Have the Authors tested this idea? Dr Dean Schleicher, US


I would firstly like to thank the authors for their work in a hydrodynamic area of expertise that is often neglected by


the technical community fundamental study but which has been


extensively exploited by the recreational boat industry. The


presented in the paper is


applicable to any boat that requires military-type useful load fractions and is not specific to "special operations craft." The link to any particular mission is never fully established. Load coefficient is not studied as a variable within the test series. High speed boats with Fnv=10 have


very low useful


displacement-light ship displacement.


load fractions (Full load displacement)/full load While the utility of a high speed


requirement to military missions is somewhat debatable given the extreme sacrifices necessary to achieve high speed, the requirement to be able to carry useful military loads is not. While the paper examines step geometry, the data is presented for a single load coefficient which limits the usefulness of the data presented. Load, speed and step geometry are intimately related and further fundamental


study is recommended to provide design


parameters to support design trade studies. Is there a plan to expand the study to include the influence of load on the results?


In addition to carrying large useful load, for military missions these loads need to be carried in appropriate sea states and not only in calm water; rough water operation is another significant


discriminator from recreational


boats that (for the most part) remain tied to the dock when the weather turns rough. Military boats spend significant periods of time operating well below top speed and in sea states other than calm water -- analysis of the percent time operational for high speed in calm water would yield a very small number. Is there a plan to examine rough water characteristics?


This paper is fundamental research that has been


performed under constrained conditions (i.e. not free to roll or yaw and calm water only). Directional stability of


B-120


©2014: The Royal Institution of Naval Architects


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