Boundary Water Layer - Simrad 38-200 COMBI D - INSTALLATION REV A Installation Manual

Simrad 38/200 Combi D Transducer
4

Boundary water layer

When the vessel forces its way through the sea, the friction
between the hull and the water creates a boundary layer. The
thickness of the boundary layer depends upon vessel speed and
the roughness of the hull. Objects protruding from the hull, and
dents in the hull, disturb the flow and increase the thickness of
the boundary layer. The flow in this boundary layer may be
laminar or turbulent. A laminar flow is a nicely ordered, parallel
movement of the water. A turbulent flow has a disorderly
pattern, full of eddies. The boundary layer increases in thickness
when the flow goes from laminar to turbulent. The figure below
illustrates the boundary layer of a vessel moving through the
water.
Boundary water layers:
(A) = Turbulent flow
(B) = Laminar flow
(C) = Air bubbles in the water
Furthermore, air bubbles in the sea water are pressed down
below the hull and mixed into the boundary layer. The boundary
layer is thin underneath the forward part of the vessel, and
increases in thickness as it moves towards aft. If the sides of the
hull are steep, some of the air bubbles in the boundary layer may
escape to the sea surface along the vessel sides. It is our
experience that a wide and flat bottom, with a rising angle less
than around 13 degrees, is prone to giving air problems for the
transducer. In any case a transducer location in the forward part
of the hull is preferred in order to minimise the influence of the
boundary layer.
851-165215 / Rev.A