Stern
Lines
Stern drive tugs should be designed with a "buttock
flow" or - also called - "pram-type" stern, where
the water inflow to the propellers is mainly along
the buttocks, not from the sides. The angle between
the baseline and the buttocks in the stern should
be kept as small as possible. A good rule-of-thumb
for maximum recommended angle is 13º + 1º for each
meter of immersion (hull draft). Thus, on ASD tugs
the angle should normally be kept less than 17º
…17.5º (see fig. below)
Profile of a buttock flow stern. Maximum recommended
angle
is 13º + 1º for each meter of draft
Larger
angles will cause the water flow to separate as
well as water inflow from the sides, which is prone
to decrease propeller performance drastically.
The
stern profile need not to be S-formed, as there
is no need to, or benefit in straightening the buttocks
to be almost horizontal in way of the propulsors.
The
stern should have a slight V-angle all the way to
the transom. There is no need for flattening the
area in way of the propulsors. A V-angle - even
a slight one - will reduce the risk for stern slamming
in waves.
The
propulsors may protrude below base line, as on some
of the finest tug designs they do. The skeg dimensions
ought to match.
Hard Chine
Hard chine designs are possible, but only double
chine type is recommended. Alignment of the chine
needs attention. Flow separation may occur where
flow-lines cross the chine. This increases the resistance
and deteriorates the operating conditions for the
propeller.
The
double chine should extend all the way to the transom.
Round
Bilge
From hydrodynamics point of view the best bilge
form is a round bilge with the radius growing towards
the stern. A hull form with a narrowing stern is
also advantageous.
Transom
The
transom should be designed with as little immersion
as possible, as the water "trapped" behind the transom
causes a large increase in resistance.
