Yacht News

Boat Propulsion Formula and Efficiency

May 1st, 2010 molitva leyli

Propeller efficiency for any particular installation, without having to go through a super detailed analysis of fluid dynamics. Particular planing boat with one particular motor.

Let’s say these are the parameters for this installation:

Motor RPM: 5,750
Gear Ratio: 2:33 to 1
Propeller RPM: 2,468 for both props
HP at prop at WOT: 74
Prop: SS 3 Blade 36.9 MPH 13″ Diameter 17″ Pitch AND a
Prop: SS 4 Blade 35.1 MPH 12 3/4″ Diameter 16″ Pitch
Boat Weight: 2,000 pounds
Boat Length: 18′

With a Speed Length Ratio of 7.8
The 3 blade has a crouch number of: 192
The 4 blade has a crouch number of: 182

the answer;

There are some method, but they are not very percise;

17″ pitch x 2468 rpm = Advance of 58.3 ft/sec
16″ pitch x 2468 rpm = Advance of 54.8 ft sec

58.3 ft/sec – 36.9 mph = Slip of 4.2 ft/sec
54.8 ft/sec – 35.1 mph = Slip of 3.3 ft/sec

Thrust EHP= .5*1.990*((13/12)^2/4)*pi*4.2*54.12^2 = 20.5 hp
Thrust EHP= .5*1.990*((12.75/12)^2/4)*pi*3.3*51.48^2 = 14.2 hp

20.5 hp / 74 hp = 28%
14.2 hp / 74 hp = 19%

Both wheels are over bladed and under pitched for top speed with the 13″ wheel doing better, nearer to typical for small speed boats.

Posted in PROPULSION boat ship yacht vessel watercraft Tags: Blade, Boat Length, boat propeller, Boat Weight, Diameter, Efficiency, Fluid Dynamics, Gear Ratio, Hp 19, Hp 20, Length 18, Length Ratio, Pi, Pitch, propeller choice, Props, propulsion, propulsion efficiency, propulsion formula, Rpm, Speed Boats, Top Speed, Wot

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