# Calculating thrust

Discussion in 'Propulsion' started by swashdrive, Sep 11, 2012.

1. Joined: May 2012
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### swashdriveJunior Member

G'day

Can anyone please provide formulas and some explanations of them, that are used to calculate an efficiency of propellors and different propulsion systems.

Is there one to calculate thrust in a dynamic state rather than the static state or bollard i think its called. Can Thrust be determined at a said speed ?

Is this a Law of hydrodynamics and can it be violated ?

regards
Craig

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### gonzoSenior Member

You can calculate the drag of the hull and that is the thrust produced by the propeller if you are trying to find out for an existing vessel. Otherwise, you can use one of the many formulas, like bp delta.

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### ldigasSenior Member

The problem of propulsion is usually approached from two directions. The first one is when the resistance characteristics of the hull are known (with all that that entails), and then the (let's say in this case) fixed pitch propeller is designed which at certain combination of shaft power, rotations and so on, can deliver the necessary thrust.

The other is the reverse way, when the characteristics of the propeller are known, then at certain rotations per minute/speed/etc. we try to calculate what is the power/thrust that that propeller can produce given its geometry.

Both of these are problems that propulsion experts deal with, and there are many known approaches. None of these can be summed to a single formula, though.

Different propulsion systems require different approaches. There are some similarities between some of them, as well as differences.
I know I'm putting it vaguely, but it is a very wide question.

The state of bollard-pull is simply a state at which the speed is zero (imagine the ship tied to the pole). Usually, when the "ship is free" he needs very little thrust at low speeds "to counter" the resistance, and to move.
In this state the ship is let's say towing something, so he needs to produce a larger amount of thrust to counter its own resistance plus the resistance of a object in tow.

I like to think of it more as a case of Newton's law, by which action equals reaction. To move a hull at a certain speed, which ... produces a certain force of resistance, one must apply the same force in a contrary direction.
But then again, I never studied law.

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