# How can I calculate the Torque needed to start a large prop turning?

Discussion in 'Props' started by jtpmail, Oct 6, 2018.

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1. Joined: Oct 2018
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### jtpmailJunior Member

I'm working with some large Ship operators. I have to specify a propulsion system and all I can see on existing specifications is Horsepower. I know how to convert HP to Torque at a given revolution but how do I ascertain the torque required to start a large propeller from standstill. For example https://gizmodo.com/this-five-bladed-behemoth-is-the-worlds-largest-cargo-1546432773 in this article it describes the largest prop ever made. It is apparently being attached to an 85,00hp power train. But I need to know what it would require in torque from a standstill to move the prop. Anyone any clue if there is a calculation based on the size of the blades etc?

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### BarrySenior Member

There are two components
The rotational drag caused by the prop moving the water. I would expect that the prop manufacturer would have this number
It does not matter if they give you torque or horsepower as the drag will be rpm dependant so easy to pick whatever you think that you need.
The second is the hp used to accelerate the propeller and inherent drive train, shafts, gear train, etc (angular acceleration) of the mass, but this might get very complicated as much of the mass is close to the shaft hence less velocity and hence less hp required,, and then a varying mass at each location at a distance from the inner hub diameter to the outer blade diameter.

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

You'll need the weight of the prop
and the approximate centre of gravity for each blade.
This will be your lever arm length from shaft centre.
Multiply it by double the mass and you have your
foot-pounds of torque to get it moving.... in water.
Add 10% to include drivetrain inertia and resistance.

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### jtpmailJunior Member

Many thanks Barry

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### jtpmailJunior Member

Excellent. Thank you.

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### BarrySenior Member

This does not consider the ANGULAR ACCELERATION of the propeller. In a frictionless situation, (out of water and frictionless bearings) you could turn a 10,000 pound 2 bladed prop with 10 foot pounds of torque but it would not accelerate (increase the rpm from zero to whatever rpm that you need) very fast

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### jtpmailJunior Member

We have an engine design that can run very slow and produces high torque at very low revs (less than 1) We can build the engine to specifications to produce the torque as high as say 5mil ft lb but it's the initial overcoming of inertia then the accelerating and maintaining the revs against the resistance of the prop that is alluding me. We don't know if the prop can be driven directly at 1:1 or if we will need a gearbox and if so what ratios. Or even if we can run the engine at a lower speed and use a multiplier gearbox for better fuel consumption. I have started to get the gist of it now. I have requested some more info from prop manufacturers which should help but any further insights are welcome. Of course we will employ marine engineers for the final design work but at this stage I'm tasked with setting a baseline spec and budgeting for the build. Out of my depth. Thanks again.

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Why...what is the objective?
Not questioning you per se, just the rationale behind it, since it is not a calculation that naval architects generally do.
The prop is designed for one application, max speed (or thrust). But, any application analysis that is off from this design input will always be less than ideal...hence the question.

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### jtpmailJunior Member

Well, it's probaby not relevant as you say. But I have used calculations from existing engines and props but we have a very different system and just needed to understand the forces involved in starting the propeller from standstill and also accelerating to and running at optimum running speeds. It's all about the new emission targets and exploring new systems. I'm going to take a marine engineer on contract to finalise the calculations but needed to ensure we had reliable horsepower and torque requirements before moving to the next phase. I have shared your responses, and those from others with our own designers and it has been helpful to put them on track.

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

So, it's electric.
You won't have any problem getting the prop moving in water.
Just be ready for a big jump in amperage when you do.
1:1 drive ratio should be fine.

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### jtpmailJunior Member

No getting anything past you. Thanks for the help.

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12. Joined: Oct 2008
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Then it is more about the engine management system profile versus the prop cube law profile. Since if the prop doesn't follow the cube law, it will try and still get its power up to the mcr curve, and this produces the black soot and terrible exhaust. If the master attempts to push too much power low down, that's what'll happen. Just look at all the black smoke coming out of exhaust stacks of tugs and others, upon start up and towing, as a prime example.
You need to match the prop with the engine profile.

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### jtpmailJunior Member

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### DogCavalrySoy Soylent Green: I can't believe it's not people

Torque converter. With lock up close to 1-1.

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