Practical Ship Hydrodynamics

Resistance and propulsion 95
resistance tests or propulsion tests with non-cavitating stock propellers
are performed and combined with open-water tests in a cavitation tunnel.
– Surface-piercing propellers
Surface-piercing or ventilated propellers operate directly at the free
surface. Thus the suction side is ventilated and therefore the collapse of
cavitation bubbles on the blade surface is avoided. Due to the operation at
the free surface, Froude similarity has to be maintained in model tests. On
the other hand, thrust and torque, but more important also the side and
vertical forces, strongly depend on the cavitation number. The vertical
force may amount up to 40% of the thrust and therefore will strongly
influence the resistance of planing vessels or SES, ships where this type
of propeller is typically employed.
ž Waterjet propulsion
A common means of propulsion for high-speed ships is the waterjet.
Through an inlet in the bottom of the craft water enters into a bent duct to the
pump, where the pressure level is raised. Finally the water is accelerated and
discharged in a nozzle through the transom. Power measurements on a model
of the complete system cannot be properly correlated to full scale. Only the
inlet and the nozzle are built to scale and an arbitrary model pump with
sufficient capacity is used. The evaluation of waterjet experiments is difficult
and involves usually several special procedures involving a combination of
computations, e.g. the velocity profile on the inlet by boundary layer or
RANSE computations, and measured properties, e.g. pressures in the nozzle.
The properties of the pump are determined either in separate tests of a larger
pump model, taken from experience with other pumps, or supplied by the
pump manufacturer. A special committee of the ITTC was formed to cover
waterjet propulsion and latest recommendation and literature references may
be found in the ITTC proceedings.
3.7 Exercises: resistance and propulsion
Solutions to the exercises will be posted on the internet (www.bh.com/companions/0750648511)
1. A 6 m model of a 180 m long ship is towed in a model basin at a speed of
1.61 m/s. The towing pull is 20 N. The wetted surface of the model is 4 m 2 .
Estimate the corresponding speed for the ship in knots and the effective
power PE using simple scaling laws, i.e. assuming resistance coefficients
to be independent of scale.
2. A ship model with scale D 23 was tested in fresh water with: RT,m D
104.1N, Vm D 2.064 m/s, Sm D 10.671 m 2 , Lm D 7.187 m.
Both model and ship are investigated at a temperature of 15°.
(a) What is the prediction for the total calm-water resistance in sea water
of the full-scale ship following ITTC’57? Assume cA D 0.0002.
(b) What would be the prediction following ITTC’78 with a form factor
k D 0.12? Assume standard surface roughness. Neglect air resistance.
3. A base ship (Index O) has the following main dimension: Lpp,O D 128.0m,
BO D 25.6m,TO D 8.53 m, CB D 0.565 m. At a speed VO D 17 kn, the ship
has a total calm-water resistance of RT,O D 460 kN. The viscosity of water
is D 1.19 Ð 10 6 m 2 /sand D 1025.9 kg/m 3 .