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


optimized. When the load is split between two running surfaces, it is difficult to optimize both, because the afterbody is


running in a disturbed wake. Further


investigations are currently underway to determine the effects of step configuration on dynamic stability.


As loading is reduced and speed is increased (approaching the conditions of recreational stepped hulls), the effective LCG must be smaller and the ideal position of the step is farther forward, closer to the LCG. Many high performance craft are designed such that 80-90% of the load is carried by the forward step, and 10-20% carried aft, to provide for pitch damping.


model’s centerline and at its LCG. The towing dynamometer can be seen in Figure 5.


Figure 5:


Photograph showing instrumentation in stepped planing hull model


The model was connected to the force block and the force block to a pivot located 13.3 cm (5.25 inches) above the model’s baseline. Running trim was computed from data gathered with the pitch potentiometer. The potentiometer was located on the pivot. Heave was measured


with potentiometer


Figure 4: Stepped Hull Configuration for Moderate Speeds


3. RESISTANCE MODEL TESTING


Model tests were conducted on a hull with a single step located at 25% of the overall


length forward of the


transom, at a range of step heights, to determine the effect of this configuration on resistance and trim of the boat in calm water.


3.1 FACILITIES AND INSTRUMENTATION


Model testing took place in the Naval Academy Hydromechanics Laboratory’s 380 foot Transducers were used to measure:


Carriage speed – tachometer Drag Pitch


Heave – force block


– rotary potentiometer – rotary potentiometer


Each measurement transducer produces a DC voltage proportional to the measured value. The voltages are sampled by a 16 bit analog to digital converter at 50 hertz.


The tests were performed with the model attached to the high speed towing carriage. The model was free to heave and trim, but was locked in surge, sway, roll, and yaw. Drag was measured using a force block located on the


B-90 towing tank. 3.2 MODEL


The model was built by the Naval Surface Warfare Center’s Combatant Craft Division and is identical to the one used by Lee [5]. Table 1 lists the principal characteristics of the model. Figure 6 shows the body plan of the model. The hull is prismatic aft of station 4. The hull is made of carbon fiber cloth and epoxy resin.


Care was taken in the construction process to


ensure the sharp chines for proper flow separation from the chine.


The model was built in three sections that could be shifted vertically to create transverse steps. The steps are located 508 mm and 1016 mm forward of the transom.


located on the towing post positioned at the model’s LCG. During testing, the various data were acquired as time histories, displayed on the screen, truncated at the discretion of the test engineer to eliminate starting and stopping transients, and averaged to find the mean steady state values. An underwater camera set up was used to capture wetted lengths of the chine and keel.


Figure 7 shows a profile view of the


model with the locations of the adjustable steps and the towing dynamometer. The hull was painted with yellow paint and then striped along the chine and keel to allow wetted area to be read from underwater photographs (figure 8). Due to the propensity of the model to take on water during acceleration, spray deflectors made of thin rigid foam were made to cover the first section of the model. These features


©2014: The Royal Institution of Naval Architects


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