WI-. v. Scheepsbouwkunde
Technische Hogeschool
Deift
Contribution to the Discussion of "Supercavitating Propellers" by Tulin. Fourth Symposium on Naval Hydrodynamics-.
C,D. Lovstad, Aktiebolaget Karlstads Mekaniska Werkstad, Karlstad:
The question of SC-CPPs is often raised by hydrofoil designers be-cause the prime movers for hydrofoils are always high speed non-reversible engines. The CPP feature is then desired for slow
speed manoeuvring and backing. Alsolthe CPP will allow the utiliza-tion of full power'throughout the speed range and load range of the craft.
As Mr. Tulin mentioned in his lecture in Sweden in
1959
there is a possibility of adjusting the pitch of an SC-CPP so that transition from non-cavitation to supercavitation is achieved at some distinct propulsion point in order_to avoid part cavitation which is con-sidered dangerous from an erosion point of view. Whether the latter advantage can be utilized or not depends upon the engine characteristics, the hull resistance and the homogeneity of the propeller flow. Full scale results from several SC-CPP installa-tions on high speed planning crafts show that there is no erosion after 300 hours of service at different speeds. The material in the propeller was in these cases stainless steel of the 13 % Cr type.At the KMW Propeller Laboratory an SC-CPP series of propellers has been developed. The propellers in the series have BAR 0.40, 0.50 and 0.60 and three different maximum face camber values for each BAR. The camber lines are of the Virgil Johnson 3-term type. The
trailing edge is sharp at the outer part of the blade in order to give good performance at high cavitation numbers. The models are
tested at several adjusted pitch ratios. The test results from the cavitation tests at one pitch ratio are given in fig. 1.
The minimum cavitation number of 0.25, which can be tested in the KMW Propeller Cavitation Tunnel, corresponds to about 58 knots. For higher speeds the test results are extrapolated; but it is assumed that the propeller efficiency never drops below the values obtained at cavitation-number 0.25 as comparative SC.-tests have Shown that the KMW tunnel gives somewhat lower values, especially at low cavitation numbers, than other tunnels engaged in SC-testing.
-2
The difference in hub ratio between the CF and FP SC propellers will uàlly be small. Up to now" SC-CpPs with a maximum hub
ratio of
0.30
have been installed. The corresponding FPP would probably have a hub ratio of 0.25. The difference in efficiency between CF and FPSC propellers will be negligible.-To illustrate the utilization of power for a turbine powered hydrofoil craft fig. 2 and 3 are shown. The figures show pitch
and thrust for the CPP and BHP, RPM and thrust for the FPP as a function of speed of advance. It is assumed that the FPP is used with a free turbine, the maximum torque of which is tabled in the
diagrams. For the CPp the form of the maximum torque curve is unimportant as full power at full rpm can always be utilized. In fig. 2 the ratio CPP-thrust to FPP-thrust is given. It is seen that this ratio is somewhat higher than what corresponds to the ratio between absorbed horsepowers. The reason is that the FPP will operate at higher slip in the low speed range than the CPP which is then operating at reduced pitch.
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