Abstract
A small craft is considered in the planing regime when its Froude number is higher than 1.2, and under that condition its weight is mainly supported by hydrodynamic pressure acting on the bottom of the hull. It is also known that installing stern flaps at a certain angle from the bottom line will alter the trim angle and, as a consequence, the resistance exerted by the water. In this work, using the classical work from Savitsky, the resistance on a planing craft is estimated, including the effect of flaps, and then the influence of those appendages on the hydrodynamic behavior of a craft of local design was experimentally verified. The wooden model was 2.0 meters long and was side towed from an outboard powered boat, with a 3.2-m arm, in a small artificial lake. The tests were run between 5 and 12 knots, with uneven intervals due to the outboard control; the model was towed without and with flaps at 5 and 10°. Finally, experimental and empirical results for towing force and trim angle were plotted. In some of the experimental curves the presence of humps may be identified, but less pronounced than with the theoretical results. Experimental resistance values are lower than those obtained from Savistky’s formulation for no flaps; in the case of flaps at 5°, the agreement in trim angle was very good. Finally, the benefit of flaps on the performance of the planing model was corroborated, but it should be emphasized that this improvement is only valid for a certain velocity range.
Highlights
According to Brown (Brown and Savitsky, 1976), a boat enters the planing regime when its relative velocity, or velocity coefficient, is > 1.5; this parameter is equivalent to the Froude number, taking the beam at the chine as the representative length
The hydrodynamic pressure acting on the bottom lifts the hull, producing a significant change in draft, trim angle, and resistance (Faltinsen, 2005)
Flaps are appendages that reduce the trim angle of a planing craft and reduce the resistance exerted on the hull
Summary
According to Brown (Brown and Savitsky, 1976), a boat enters the planing regime when its relative velocity, or velocity coefficient, is > 1.5; this parameter is equivalent to the Froude number, taking the beam at the chine as the representative length. Under this condition, the hydrodynamic pressure acting on the bottom lifts the hull, producing a significant change in draft, trim angle, and resistance (Faltinsen, 2005). It is well known that installing a flap on the transom of the hull may help to reduce resistance; those devices having an angle with respect to the bottom develop a local hydrodynamic force, producing a change in trim and altering the longitudinal component of the normal force acting on the bottom of the boat
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