Transverse Thruster

[John_G]

Dear All,

Please help me out!
How can I make sure that my thruster is going to work properly with a specific electric-motor power [kW] and RPM when i only know the propeller diameter? I can find out the pitch ratio maybe as well.

Please Help
Thanks
John

[John_G]

Maybe i shall revise the question and ask how much effect is necessary to rotate the propeller with a given diameter at a certain RPM. I want to make sure that the thruster (propeller) will be able to operate for the given RPM.

thanks

John

[Guest625101138]

As a first approximation the ideal power as a function of thrust can be determined as:
Power = (4/pi/9/rho)^0.5 x Thrust^(3/2)/D in Watts

rho = 1025 for salt water
D in metres.

To check you have it right, ideally a thrust of 1000N will be produced with 928W.

In practice an open water prop will perform quite a lot worse than this.

I have attached some screen images for a 0.4m 3-bladed prop designed for thrust application. With 1000W it will give 530N. Considerably less than the ideal case.

The performance will depend on the actual prop. Diameter is most important. Bigger the better. The RPM has to be chosen to suit the prop. Thick blades are better than thin blades and not just for reason of strength. Low pitch is better than high pitch.

If you have a prop in mind I can do an analysis to give an idea of what is possible.

Rick W

[John_G]

Hi,

Many thanks for your reply. How is this formula derived? I am not an engineer but i was thinking in terms of that a thruster is like a axial pump. So when i was surfing around trying to find some basics about axial pumps i found the similarity rule. So if i know for example Px and RPMx (i.e. power and rpm for what the thruster is designed for) i can check if another motor with different effect and rpm is working e.g. Py and RPMy.

Py/Px=(RHOy/RHOx)^3*(RPMy/RPMx)^3*(Dy/Dx)^5

So if the diameter is constant and density as well I can plot a curve to check if the new RPM and Power is enough or too much.

Am I totaly wrong or?

/John

[Guest625101138]

Originally Posted by John_G

Hi,

Many thanks for your reply. How is this formula derived? I am not an engineer but i was thinking in terms of that a thruster is like a axial pump. So when i was surfing around trying to find some basics about axial pumps i found the similarity rule. So if i know for example Px and RPMx (i.e. power and rpm for what the thruster is designed for) i can check if another motor with different effect and rpm is working e.g. Py and RPMy.

Py/Px=(RHOy/RHOx)^3*(RPMy/RPMx)^3*(Dy/Dx)^5

So if the diameter is constant and density as well I can plot a curve to check if the new RPM and Power is enough or too much.

Am I totaly wrong or?

/John

My formula was derived from first principles. It is basic physics. The actual data provided was straight from JavaProp.

Power is approximately a function of RPM^3 as it is for a pump. However you asked for force and it is not linear with power. It is related to power as Power^(2/3). So if you doubled thruster RPM you would increase power 8-fold while force would only go up 4-fold. If you increased RPM by 50%. power goes up 3.4 times while force goes up 2.2 times.

Force for a given power level is an inverse relationship with diameter. So you can get more thrust for the same power simply by making the diameter bigger. This is the reason tugs have very large props with low pitch. Fundamentally it is getting traction. Operating on a very large area but not putting a lot of energy into the water by moving it at high velocity.

For a set diameter, increasing the power by increasing the RPM will increase force in relation to Power^(2/3).

The RPM term in your pump power equation is correct in that power is proportional to RPM^3. The rest of it does not have unit consistency so cannot be correct. I think you will find the density term should not be cubed.

The problem with the pump equation is that it is not telling you anything about force and that is what counts with a thruster. You can produce a very large force for little power if you have a large diameter.

Rick W.

[John_G]

Hi Rick,

Many thanks for your reply. I am really impressed that you know all this. Can you suggest any good reading about the topic?

Thanks

/John

[John_G]

Hi Again,

Can the same laws (relationship) as above be used for a ordinary stern propeller?

/John

[Guest625101138]

Originally Posted by John_G

Hi Again,

Can the same laws (relationship) as above be used for a ordinary stern propeller?

/John

The principles for jets, thrusters and open or closed propellers are basically the same.

The simple equation I provided earlier is an ideal case that makes no account for blade losses. It cannot be applied to a moving propeller because there is no velocity term.

The easiest way to do reasonable prop analysis is to get familiar with JavaProp. The default option is for an air prop but it can be set up for water by changing the Options page to water parameters.

http://colaco.freeshell.org/mhepperl.../jp_applet.htm

You can look at the performance of thrusters. There is a thread here on a hose thruster that makes reference to a thruster supplier. You could google jet engines, newton's law, rocket engines, dig out an applied maths book.

Rick