Page 170 - Mechanical Engineer's Data Handbook

FLUID MECHANICS 159

occurs when the boat speed is half the jet speed and
maximum power is attained. When the water enters
the front of the boat, maximum efficiency occurs when
the boat speed equals the jet speed, that is, when the
power is zero. A compromise must therefore be made
between power and efficiency.
Let:
V=jet velocity relative to boat
U = boat velocity

m=mass flow rate of jet

Water enters side of boat

Thrust F = m V( 1 - r)
VZ
Pump power P = m -
2
Efficiency q = 2r( 1 - r);
q,,=O.S, at r=0.5.

U

F,=mV ( 1-- ?i:)sina

sina cos8, Water enters fiont of boat
sin 8,
Thrust F=hV(l -r)
sina cos8,
(vz-U2) vz
sin 8, Pump power P=m =m-(I -9)
where: V=jet velocity, a=jet angle, 8, =vane inlet 2 2
2r
angle, O2 =vane outlet angle. Efficiency q = -
(1 +r)
4.5.3 Water jet boat q=0.667, for r=0.5.
q= 1.0, for r= 1.0.
This is an example of change in momentum of a fluid
jet. The highest efficiency is obtained when the water Output power (both cases)
enters the boat in the direction of motion. When the
water enters the side of the boat, maximum efficiency P,=mitVlr(I -r)