Marine Propeller Thrust: How Do I Calculate It?

[Marco97]

Hello folks! I hope all of you are having a good day.
Here is my question:
How can I calculate the thrust needed by a marine propeller to move a body of about 120 kg? Also, as long we are talking about marine propeller, our propulsion is in water and not in air, so what about the rpm? Are they different?
Thanks for your answer, I'm working on a school project and I'm having this problem because I didn't really study those arguments yet. :)

 

[Drakkith]

Hi Marco97! Welcome to PF!

Unfortunately we need more information if we are going to help you. The thrust required to move an object through water depends on the speed at which you want to move it, the shape of the object, and a variety of other properties. The required RPM (of the propeller I assume) depends on the size and shape of the blades and how much thrust you want out of it. And yes, the properties of a ship's propeller will be very different from an aircraft's. I can't say I know much about this topic, but if you can provide a bit more information I'm sure someone here can assist you.

 

[sophiecentaur]

The actual mass of the body will probably have less effect than the length (very important) and the shape of the hull (I assume it is a boat of some kind - or are you planning self propelled water skiing? lol)
I could start you in the right sort of direction by saying that a boat's propellor behaves very much like a screw, as it passes through the water. The pitch of the propellor (distance advanced per turn in m - as with a normal screw thread) times the number of revs per minute will tell you the speed through the water in m/minute. The force needed times the speed will give you an idea of the actual power needed to achieve this speed. All this ignores slippage and various other factors but it gives a fair idea of what goes on.
People spend Megabucks on getting the right propellor design for their particular engine and their particular hull design. So a few more design requirements would come in handy if we are going to be able to help (point you in the right direction).

 

[rcgldr]

The force needed times the speed will give you an idea of the actual power needed ...

The force is also related to prop diameter, so that needs to be taken into account. A boat propeller operates in water that is disturbed by the boat, so that is another factor.

 

[Marco97]

I am so thankful for all of your answers, and really sorry to answer just now (few days after the thread was posted, I had my internet cut for a few weeks, needed to change provider).

I was thinking about a small propeller, to be put on a small boat. So I wanted to design one but as far as I am seeing from you answers, I need to answer more questions for my project. I'm so excited to build it!

So, let's switch to another question, what are the factors I should take in account? For the moment I was just thinking about the speed, the mass of the boat (But now I know that the length is more important, thanks :D), the fact that the propeller is in water and not air, and the trusth needed for a certain weight. I have to think wider, don't I?

Thank you again, this forum has such a great community!