Report 4 outlines the methodology and results of the full-scale research tests investigating candidate hull forms.  An outline of the approach to evaluation and optimization of the Teknicraft foil-assisted catamaran (FAC) design by means of computational fluid dynamics modeling is also provided.  This report is released as a draft interim report.  A final report shall be issued following completion of computational fluid dynamics (CFD) optimization studies scheduled for January 2008.

Vessel wake trials have been conducted in this study with two FACs designed by Teknicraft Inc., NZ and by two air cavity/surface effect type vessels designed by M Ship Co. of San Diego, CA.  The vessel wake trials are providing data for calibration and validation of wake transformation models (described in Report 2). 

Vessels tested in wakewash and performance trials, clockwise from top left Spirit, 1060, M80 Stiletto, M38

Shore impact studies conducted in Rich Passage with the FAC Spirit indicated that the vessel design shows considerable promise as a low-impact POFF alternative.  However, the results suggest that further benefits could be gained by the use of an adjustable foil.  Therefore, trials were conducted with the vessel 1060 in February 2006 to assess the potential benefits of utilizing an adjustable-foil catamaran design for a low-wake passenger ferry.  A new research vessel (RV2), presently under development as part of the research program, will be based on Spirit and 1060, but optimized to minimize resistance and wake generation.  A CFD modeling study is being conducted to assist in the design of this second research vessel.

A comparison of the maximum wake heights between vessels indicates that both the Spirit’s (with interceptors) and Stiletto’s wake heights are among the smallest of those measured at speeds above 30 kts.  A comparison of maximum wake energies from different vessels showed that Stiletto produced the highest measured wake energy during its transition from non-planing to planing conditions. However, as vessel speed increased above 20 kts, Stiletto’s wake was among the lowest in energy among tested vessels.  It should be noted that these comparisons do not include the spectral distribution of wake energy and larger and faster vessels will generally produce wakes with a higher percentage of energy at lower frequencies.  Energetic low frequency waves from fast POFFs are associated with beach flattening on the mixed sand and gravel beaches in Rich Passage (Report 1 and 2).  It is interesting to note that for the Chinook-class vessels, the maximum wake energy peaked at a higher velocity than for other vessels, and wake energy was as high at 30 kts as it was at 13 kts.  This is partly a function of the substantially larger size of Chinook/Snohomish (350 passengers) relative to the other vessels.

M Ship Co. is a California based company specializing in innovative design of high speed, low wake vessels employing surface effect (air-cavity hull) design elements.  Several of their designs have been identified as having a very high potential for low wake generation at high speeds.  A series of full scale wake wash and vessel performance tests were conducted in San Diego in November 2006 with the two available prototype scale M-Hull vessels: specifically the M80 Stiletto, and the M38 prototype.  It was found that wake energy decreased significantly with vessel speeds above the hump speeds.

Comparison of the maximum wake height  H_max and energy  E_@Hmax variation with vessel speed

The impact of detailed hull form design on wake generation and resistance is being assessed using CFD techniques.  After review of available modeling approaches, it was decided to pursue two lines of investigation: viscous flow models based on the Reynolds-averaged Navier-Stokes equations (RANS) and inviscid or Potential Flow (PF) modeling approaches.  The CFD code CFDShip-Iowa and the INSEAN PF model, WAve Resistance Prediction (WARP), were selected by the POFF Study team as the most appropriate combination of RANS and PF codes respectively, for wake and resistance optimization studies.  The CFD studies are still underway and will be reported upon in the final version of Report 4.

CFDShip-Iowa simulation of water surface elevation results for Spirit Case 1 (Fr=0.61, Displacement=56410 kg) side view (top), top view (lower left), and bottom view (lower right)

CFDShip-Iowa simulation of water surface elevation results for 1060 Case 1 (Fr=1.32, Displacement=26161 kg) side view (top), top view (lower left), and bottom view (lower right)

Example of wave profile comparison (optimization studies):  view of optimized wave form from bow (top left); view of original wave form from bow (top right); longitudinal cut of water surface elevation along the centerline (lower left), outboard longitudinal cut at a distance of 0.25 L from the centerline (lower right)

An extensive sensitivity analysis and preliminary hull form and foil shape and position optimization study has been conducted with the fast and efficient potential flow computer models.  The team anticipates that detailed optimization with the more computationally intensive flow models will get under way in September.  The optimization work involves application of an automated system based design environment that involves several independent computer programs that are run in sequence.   Starting from a set of three initial designs, a new set of hull shapes and foil configurations (shape and position) is automatically generated. The resulting simulation output is post-processed to arrive at optimal values of wave and total resistance and design variables such as trim, displacement, and hull geometry.

3 different initial shapes of the demihulls and of the foils were adopted as a starting point for the preliminary design optimization

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Wake Trial and Low Wake Vessel Optimization