Surface Wave Contours Associated with the Forebody Wake of Stepped Planing Hulls

Abstract

Results of an extensive series of model tests that define the longitudinal surface wake profiles aft of prismatic hulls having deadrise angles of 10º, 20º and 30º are presented. Empirical equations are developed that quantitatively define these profiles and are in a form that can be easily applied by designers of stepped planing hulls. These equations are applicable for an expected range of variations in trim angle, speed coefficient, and loading coefficient typical for these hulls. A brief introduction to the concept and to the hydrodynamic advantages of stepped planing hulls is presented to orient the reader as to the importance of wake data in their design. Examples are presented that illustrate the application of these wake data for stepped planing hulls with wetted forebody chine to achieve maximum hydrodynamic lift/drag ratios. Finally experimental results are presented that illustrate the potential resistance penalty associated with the operation of chines dry forebodies where the stagnation line crosses the step.

Numerical Analysis of the Take-Off Performance of a Seaplane in Calm Water

“…The hydrodynamic resistance characteristics of fixed trim angles ranging from 3-6 deg at a velocity of 10 m/s are plotted in Figure 19. Since the wetted surface area varies inversely with an exponential power of trim [9], the friction resistance increased rapidly as the trim angle decreased. Whereas, the pressure resistance increased linearly with the trim angle.…”

“…The water flow is forced to separate at the step giving room to an air-filled cavity, which could reduce the wetted surface on the afterbody and prevent unfavorable suction that keeps the afterbody from leaving the water [8]. A recent study by Savitsky expanded the empirical methods to estimate the hydrodynamics of stepped hulls through predicting the wake profile of forebody [9]. But the application of the methods is still limited to the range of parameters which are not applicable for seaplanes.…”

Experimental measurement of the nearfield longitudinal wake profiles of a high-speed prismatic planing hull

“…For these reasons, establishing a method of reliably and accurately modelling the nearfield wake is crucial to the performance prediction and design of stepped planning hulls. To this end, Savitsky performed an extensive model-testing program in 2010, measuring the wake profiles of prismatic planing hulls in a number of test conditions (Savitsky and Morabito, 2010). The data was used to develop empirical formulae that calculate the longitudinal wake profile at the Centreline (CL) and the Quarter Beam (QB) longitudinals of a planing hull, given that it fits within certain parameters.…”

“…The focus of this study was once again in attempting to model how and when ventilation of the transom stern occurred. Savitsky and Morabito (2010) conducted the only investigation into the nearfield wake of a planing hull to date. The aim was to develop empirical equations that quantitatively defined the longitudinal surface wake profiles aft of a prismatic planing hull.…”

“…There is a noticeable scatter in the results and it is clear that while an empirical equation may be capable of providing a good representation of the wake profiles they will not be entirely accurate for all cases. (Savitsky and Morabito, 2010) A number of studies makes use of Savitsky's Wake Equations in conjunction with numerical tools, primarily as one-off validation cases of the CFD (Faison, 2014;Ghadimi et al, 2015). In most cases Savitskys empirical equations are shown to have a reasonable correlation with numerical solution, however, in some cases there is a significant difference.…”