# Thrust power

Discussion in 'Props' started by Noel McGettigan, Dec 16, 2018.

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### Noel McGettiganNew Member

Good afternoon
My name is Noel McGettigan and I am a Marine Surveyor. I am currently doing an online course with the International Institute of Marine Surveyors and there is a question on propellers / thrust power etc. and I was hoping if possible someone could help me on solving the question.

The question is as below:

Considering another vessel, if the speed of the water steam passing through the disc of the proposed propeller for this vessel is 10.5kts with a thrust of 35KN, what is the value of the Thrust Power,

Also calculate the propeller thrust, efficiency, effective / delivered power

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### Fran kellyJunior Member

Hi noel,
I am doing the same course myself and this question has me stumped. I am wonderimg did you ever find the formula or the answer.
Kindest regards
Fran

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### BlueBell. . . _ _ _ . . . _ _ _

Okay, I'll have a go at this.

What is the "proposed propeller" they make reference to?

Could this be as simple as thrust times velocity?

They ask for propeller thrust... well they've already said it's 35KN... at 10.5 knots...

And what are the propeller dimensions?

I believe there is a lot of information missing, including Shaft HP.

And 10.5 knots of water "flowing through the disk", is this entering the prop disk or exiting?

Last edited: Apr 16, 2019
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### BlueBell. . . _ _ _ . . . _ _ _

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### Fran kellyJunior Member

Hi Blue bell,
Many thanks for a reply. The complete question is as above in Noels post. I'm not really sure what they are looking for it is possible there is some information missing. Or as you said it could be as simple as thrust times velocity.
Kind regards
Fran

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### DCockeySenior Member

Mass continuity, ie the same mass flow rate out as the mass flow rate in, and constant density means the speed normal to the disk the same exiting as entering.

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This is more straightforward than you realise. The reason being is the terminology used. Deliberately used to check your understanding of each.

The value of Thrust Power, P= V.T
V = water flow through the prop (which is different from vessel speed)
T = Thrust by/at the prop

That's it. ...But make sure you use the correct units

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### BlueBell. . . _ _ _ . . . _ _ _

Ad Hoc, you missed this part...

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Nope...i saw it.

One step at a time.
And he needs to understand what he's doing first.

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### Fran kellyJunior Member

Ok guys, so it is as simple as 10.5knots (which I presume has to be converted to m/sec) multiplied by 35kn?
Thankfully my question doesn't include the second part "Also calculate the propeller thrust, efficiency, effective / delivered power"

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Is that a statement or a question?

What are the units of power? Power = kg.m^2.s^-3
What are the units of thrust =? and Speed = ?

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### Fran kellyJunior Member

Ad Hoc, that was a question. I am completely lost. Thrust is measured in newtons and speed in this case is knots??

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Speed = m/s
Thrust = N = kg.m/s^2
Power as noted above.

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### Fran kellyJunior Member

So, would I be right in saying:

converting the water speed from 10.5 kts to metric = 5.4 m/s
And the thrust of 35 kN = 35000 kg.m/s^2

Multiplying both 5.4 x 35000 = 189000 kg.m^2.s^-3 is the answer for the Thrust Power (Pt)

Is this correct?

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