On transom clearance

• Transom clearance of a planing hull is studied experimentally and numerically.
• A pronounced peak detected in the resistance coefficient curve at clearance speed.
• Non-dimensional transom submergence and cpcp at the transom edge are minimum at Fn⁎Fn⁎.
• A RANS code is employed for modeling the flow around the hull.
• Computed resistance components revealed the reasons for the peak in CTCT curve.

This paper deals with transom stern flow phenomena around the Froude number where the transom clears the water. Experimental and numerical results for a high-speed hull exhibit a pronounced peak in the resistance coefficient at the clearance speed. This is associated with a maximum in the non-dimensional transom submergence, a minimum in the pressure coefficient at the transom edge and a maximum in the curvature of the flow leaving the edge. Just before clearance, the sum of the hydrodynamic and hydrostatic pressures on the transom yields a force pointing forwards, thereby reducing resistance. This pushing force diminishes abruptly as the transom becomes dry. The total pressure resistance of the rest of the hull increases rapidly just before clearance, mostly due to the increase in hydrostatic pressure resistance, caused by the sudden increase in aft draft. It is the rapid loss of the transom pushing force, together with the rapid increase in pressure resistance of the rest of the hull just before clearance that creates the peak in the total resistance coefficient.