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Old 30 August 2012, 04:31   #1
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Calculating prop thrust

Just wondering if anyone knows how to calculate prop thrust if the vessels displacement, engine power and max spepd are known? The specifics are 6t, 260Hp and 20kn.
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Old 30 August 2012, 04:57   #2
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Huge anchor on the shore & a big spring balance?

Putting my serious hat back on, the hull weight is irrelevant to THRUST - the thrust is entirely down to the blades interacting with the water. So you would need to know the shape of the blades, engine torque & power at a given RPM as a minimum to get a meaningful number.

Whether that thrust goes into moving the boat or water is a whole other set of equilibrium calcs.

You still get thrust if tied to the pontoon at the marina- the only difference between that & the boat zipping along at 20 odd knots is that when tied to a pier the warps are taking the strain & you are moving water with the available energy rather than the boat.

Hence my bollard & spring balance comment.
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Old 30 August 2012, 06:12   #3
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Keith the only way to know for sure what your getting in the way of bollard pull is to do as 9D280 says.
Too many variables come into it when trying to calculate like cavitation, prop surface finish and condition etc....

We test our vessels periodically (about every 5 years or so) by hiring a load cell and pulling back on a fixed immovable object.

We use Straightpoint in Havant who will hire you any remote controlled load cell from 1 tonne upto 300tonnes.

To give you an idea, a 12 tonne remote readout cell will set you back about 175 per week ex vat.

Simon

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Old 30 August 2012, 06:24   #4
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And to consider thrust will change depending on if you are moving and the water is moving towards you etc ( hence why you see 'static' thrust quoted) .

I did in the past do an assignment at Uni to calculate how the trust of jet changes relative to the speed of it throught he air and how this then impacts the drag/lift on a wing etc ..so as said you end up with some properly long, nerdy equations .....I've grown out of it now
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Old 30 August 2012, 08:08   #5
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Thanks gentlemen. I fully understand the differences between static and moving water etc, but kind of hoped that my old schoolboy equation of F=MA would come into it somewhere. Surely the thrust imparted by the prop up the shaft to the engine is directly related to the mass of the vessel and friction (resulting in a given speed for the available power). The size of prop blades and design is irrelevent is it not?
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Old 30 August 2012, 08:15   #6
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Thanks gentlemen. I fully understand the differences between static and moving water etc, but kind of hoped that my old schoolboy equation of F=MA would come into it somewhere. Surely the thrust imparted by the prop up the shaft to the engine is directly related to the mass of the vessel and friction (resulting in a given speed for the available power). The size of prop blades and design is irrelevent is it not?
As said the mass has nothing to do with thrust ...it will of course affect accelaration.

Thrust imparted by the prop to the shaft will be a near constant ( for a given set of numbers - (I'd guess at water temp/ aeration) - but you would also have to consider the trim angle of the prop to the water unless you assume a totally paralell approach of water to the prop blades - which in theory you could maybe calculate but given all of this your theory number wil lbe a long way from reality.

All of this is why there is no substiture for a big rope and a big/special spring balance as has been said........

Are you trying to find out the accelaration of a boat or the thrust of a prop? ...they will need very differing things ( drag , volumes etc etc )to work out , but one ( thrust) will lead to the other (accel) in theoretical maths terms.
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Old 30 August 2012, 08:22   #7
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Quote:
Originally Posted by Erin View Post
.... but kind of hoped that my old schoolboy equation of F=MA would come into it somewhere. Surely the thrust imparted by the prop up the shaft to the engine is directly related to the mass of the vessel and friction (resulting in a given speed for the available power). The size of prop blades and design is irrelevent is it not?
If you are doing a constant speed then F=MA is not applicable as there is no acceleration.

At constant speed the thrust from the prop (whatever size and pitch) will equal the drag (resistance) from the water and air. This I imagine would be much harder to calculate than using RPM/Prop size/Pitch (consider what the F1 teams do to improve their aerodynamics and it still goes wrong)
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Old 30 August 2012, 08:43   #8
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Are you trying to find out the accelaration of a boat or the thrust of a prop?
Nope, it's all to do with selecting the right size Aquadrive or Pythin Drive for a shaft drive motorboat. I need to know whether the thrust bearing on said unit can cope with our particular setup. I'm trying to see if 18kN would be within limits or if I need the next size up..
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Old 30 August 2012, 12:33   #9
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The actual mass and final speed of your boat aren't relevant, you can assume those are constants. F = ma (m is mass and a is acceleration)

Power is a unit of speed (better to say acceleration) * a unit of force.
1 N force * acceleration of 1 m/s = 1 watt (by definition)

So your thrust bearing can accept up to 18kN force. Translated into watts:
18 kN force * 1 m/s = 18,000 watts
(20 kn is about 10 m/s fyi so 1m/s is a fairly rapid acceleration but plausible)

However 260 hp is about 19,000 watts.

So if you ever plan on using all 260hp you would be wise to upsize the thrust bearing. You might want to confirm with the manufacturer based on your hp and shaft size.
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Old 30 August 2012, 12:57   #10
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Hi Keith

What size/pitch prop are/will you be running?

My initial thoughts are that you will be fine as to overload the thrust bearing you will need to be generating roughly 1.8t of thrust/pull. I doubt you will have the diameter of prop/pitch and gear ratio in the drive to be able to generate enough force to do this. (especially not if you say the top speed is 20kts).

As Captnjack sais, even on paper you are only just over the limit. Your engine will not be generating 260hp ( or194 KW) anyway, plus you will have quite a loss as you transmit that power through the G/box to the shaft.

Simon
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