MIT: Putting FastShip To The Test
Hydrodynamic tests come up positive for revolutionary ship design The proceeding article was excerpted from a paper created from test results of the FastShip design. The tests were conducted by the Massachusetts Institute of Technology's Department of Ocean Engineering, and the paper authored by Paul D. Sclavounos.
The Massachusetts Institute of Technology's (MIT) Department of Ocean Engineering recently put the TG 770 FastShip through tests to verify its hydrodynamic performance, and in the words of MIT, "the vessel... is outstanding in all aspects of its hydrodynamic performance." Test results also appeared to point to a promise that the vessel may be able to maintain a speed nearing 40 knots in extreme North Atlantic sea states.
The Test Over the past 10 years — under funding by the U.S. Navy and DNV — a three-dimensional panel method has been developed at MIT for the simulation of the free surface flow past realistic ship forms. A code has been written know as SWAN (ShipWaveANalysis) which is capable of predicting the Kelvin wave pattern and residuary resistance of ships in calm water and the motions and wave-induced loads in a sea state. SWAN was first used to evaluate the calm water performance of the TG 770 FastShip (F/S). Computations were carried out of the residuary resistance of the vessel over the speed range of 30 to 50 knots. The different components of the ship resistance were identified including the fractional resistance which is proportional to the ship wetted surface, and the residuary resistance which was found to consist of a wave, hydrostatic and vortex or induced components. The sensitivity of each resistance component upon the hull shape was identified in a way not possible to discern from a traditional tank test. Comparisons of the residuary resistance computed by SWAN with experiments carried out at SSPA was found to be very encouraging over a broad speed range. Moreover, a qualitative comparison was carried out with the residuary resistance of the well-established and popular semi-displacement British National Physical Laboratory (NPL) hull forms which resemble the F/S. For comparable length-displacement, beam/ draft and transom area ratios, the residuary resistance of the F/S design was found to be 15 percent less than any comparable semi-displacement hull form.
The hull form was then put to the test in a typical North Atlantic sea state, including the heave-pitch motions, wave-induced vertical bending moment and shear force distributions, relative motion and added-wave resistance in head waves.
The added-wave resistance is perhaps the most important seakeeping property from the point of view of speed performance in a high sea state. Computations of the added-resistance were carried out in a North Atlantic irregular sea state with upcrossing period of 10.4 seconds and significant wave height of 20 ft. (6 m) with the F/S advancing at 40 knots. It is worth noting that a significant wave height of 20 ft. does not preclude the occurrence of wave heights several times higher. The resulting increase in resistance was equivalent to 23,000 EHP, or just 5.5 percent of the installed power. The corresponding added-resistance index was found to be 0.15, which is lower than the corresponding index of naval or commercial ship forms, which typically have rough water resistance several times higher the F/S. The only hull form for which a smaller index was found is an America's Cup yacht advancing at a comparable Froude number. 1 The low added resistance of the F/S is perhaps its most remarkable feature, and it is attributed to two aspects of its design. First, its hull form is characterized by a fine bow and a wide shallow transom stern, which are responsible for the low heave and pitch motion amplitudes relative to a conventional cruiser stern ship. The second is the length of the F/S, which on the waterline is about 755 ft. (230 m) and quite larger than the typical wavelength encountered in typical ocean wave spectra.
It is known from oceanography that the typical period of the steepest wave encountered in ocean storms is very unlikely to exceed 10 to 12 seconds. The corresponding wavelength is less than 492 ft. (150 m), which is quite smaller than the F/S waterline length.
Wave-Induced Structural Loads & Relative Motion Computations were carried out of the vertical shear force and bending moment RAO distributions along the F/S length in a head-wave Pierson Moskowitz spectrum at wave upcrossing periods of 0 to 25 seconds. At the critical spectrum of 10.4 seconds period and 20 ft. significant wave height at 40 knots, two nearly equal maxima for the shear force were found to occur, one 10 percent of the F/S length upstream of the stern and the second 65 percent of the length from the transom. In the same spectrum, the vertical bending moment maximum was found to occur 45 percent of the F/S length from the transom.
The relative wave motion and velocity and the ship acceleration were also computed along the length of tbje F/S at 40 knots in the critical Pierson-Moskowitz spectrum. These quantities are needed in order to access the occurrence and severity of slamming and to evaluate the inertia loads on the cargo caused by the ship acceleration. Relative motion and velocity plots showed modest values near the bow and significant reduction of their magnitude near the F/S stern, indicating that slamming and white water are unlikely to occur near the aft end of the ship where the waterjet inlets are placed. Near the fore perpendicular, some slamming may occur, but the severity of the resulting loads is alleviated by the V-shaped bow sections. Comparison of seakeeping quantities were also conducted with independent computations of the same extremes carried out by Professor Tendrup Pedersen of the Technical University of Denmark (DTU) using more conventional strip theory. With exception of the relative velocity, which was traced to a difference in the MIT and DTU definitions, all seakeeping quantities were found to be in good agreement despite the disparity of the methods used.