An Approximate Value of Planing Boat Thrust
This paper is concerned with presumptive planing boat thrust at the top speed. The nomogram adapting a pair of charts will be applied to obtain a set of propeller items, which is useful to select the best set of propeller items among many candidate sets of ones quickly at first design phase.
Thrust of PT at the top speed was obtained and discussed against resistance of PT. Difference between thrust and resistance will be checked on the basis of the trial test data up to the top speed. Reasonable thrust is obtained.
Key Words: Planing Boat, Power Design, Propulsion, Thrust, Screw Propeller, Cavitation, Resistance
The method of overlay system  is used to analyse PT's propeller items.
The outline of the system is as follows. Inflow velocity into propeller Va and diameter of propeller D are logalithmically scaled on abscissa and ordinate respectively. Straight lines power (ps) and revolution (rpm) are put on the co-ordinates, which is called A-chart. Curves propeller pitch ratio p/D, propeller efficiency η and critical inflow velocity into propeller against origination of cavitation Vac' together with a base point are written on B-chart, which are based on experiment data of a series of propeller tests such as Gawn's ones , . A set of B-charts for blade area ratios BAR = 0.50, 0.65, 0.80, 0.95 and 1.10 is provided. This system has been proposed in market since 20th Tokyo international boat show (1982). The first author of reference  Hirotsugu Tanaka has presented about fifty examples about propeller analysis and design on Japanese monthly magazine  after this event, using this system.
This traditional method will be used to analyse PT's propellers. PT  has a propeller at center driven by gas turbine together with two side propellers driven by each Diesel engine, which rotate clockwise and counter clockwise viewed from bow respectively. PT having 35 m in overall length, 9 m in width and 133 tonf (1304.3 kN) in mean displacement planes with 41.19 kt at the top speed. A series of trial tests of PT was done in Setonaikai sea Japan ( E 131° 15' 14" , N 33° 57' 9" ). Speed Vs (kt) together with propeller shaft revolution N (rpm), propeller shaft horsepower SHP (ps) etc. at each engine rate were measured during the test.
2. CENTER PROPELLER AT THE TOP SPEED
Center propeller's BAR equals to 0.60. B-chart having BAR = 0.65 is selected to analyse, which is close to 0.60. A-chart is overlayed by this B-chart, which is shown as Fig.1. A-chart's horizontal lines are put in parallel with B-chart's ones, consequently A-charts and B-chart's vertical lines are put in parallel. A- and B-chart have been put in right position. The intersection of SHP = 4680 ps and N = 1704 rpm lines is plotted by "○" mark on A-chart, on which the base point of the B-chart "○" mark is superposed. Then the superposed point is seen as "◎" mark. The intersection of horizontal D = 1.040 m and curve p/D = 1.196 lines is plotted by "△" mark. Va = 67 kt by this "△" mark, exceeding Vs (= 41.19 kt). This means that center propeller has cavitation. During analysis wake fraction ω is taken as zero because of high speed.
Reforming propeller items is necessary to get new center propeller.
3. PROPELLERS OF SIDES AT THE TOP SPEED
About one of these propellers discussion will be done.
A propeller's BAR equals to 1.10. B-chart having BAR = 1.10 is selected to analyse. A-chart is overlayed by this B-chart, which is shown as Fig.2. A- and B-chart have been put in the right position. The intersection of SHP = 2780 ps and N = 1706 rpm lines is plotted by "○" mark on A-chart, on which the base point of the B-chart "○" mark is superposed.
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