Gérard Delhommeau

Boundary between unsteady and overturning ship bow wave regimes

Measurements of the bow waves generated by a rectangular flat plate, immersed at a draught D = 0.2 m, towed at constant speed U = 1.75 m s −1 in calm water and held at a heel angle 10° and a series of nine yaw angles α = 10°, 15°, 20°, 25°, 30°, 45°, 60°, 75° and 90° are reported. The measurements show that bow wave unsteadiness is significantly larger for the flat plate towed at yaw angles 30° ≤ α ≤ 90° than at 10° ≤ α ≤ 20°, which are associated with the unsteady and overturning bow wave regimes, respectively, separated by the boundary with g ≡ acceleration of gravity. These measurements of bow wave unsteadiness provide preliminary experimental validation of the foregoing simple theoretical relation for the boundary between the unsteady and overturning bow wave regimes for non-bulbous wedge-shaped ship bows with insignificant rake and flare. Extension of this relation to more complicated ship bows, notably bows with rake and flare, is also considered.

CALCULATION AND MEASUREMENT OF FORCES ON A HIGH SPEED VEHICLE IN FORCED PITCH AND HEAVE

Abstract An experimental apparatus to measure added mass and damping coefficients in pure heave or pitch or the coupling coefficients in combined motion on a surface effect ship (SES) sidewall with a forward speed in the small test section of a hydrodynamic water tunnel has been built. To check the results obtained, the whole experimental set has been modified to be used in a towing tank with a 3·3 larger model with same values of reduced parameters. Comparison with both kinds of measurements is satisfactory. The influence of Froude number, amplitude of motion, frequency and phase lag between motions for combined motion has been studied; some tests have been also performed with two symmetrical sidewalls (catamaran). Measured coefficients have been compared with computed ones obtained with the code AQUA + from the ECN. In spite of the large motion amplitudes, agreement between tests and linear computations is quite acceptable.

Etude expérimentale et théorique de la vague d'étrave des navires

ABSTRACT An experimental study of the bow wave generated by a ship in steady motion performed in a towing tank on a flat plate at different locations (simulating the half entrance angle αE and the flare angle γ of a classical hull) is reported. The measurements obtained from digital photographs give the influence of the different parameters, particularly on the shape of the contact curve between the wave and the plate. The results have been compared with reasonable agreement with analytical expressions; it had been too shown that these expressions are also valid for real ship bows. Furthermore, as a ship in steady motion is shown to generate an unsteady bow wave if αE 12° and if the ship speed is smaller than a critical speed defined in terms of αE by a simple analytical expression, we verify by experimental observations that a ship in steady motion generate an unsteady bow wave.