pump piston powered boat

Hello, we are building a small human powered boat. We are planning to use a simple piston system which just pumps air out of the back of the boat below water level.
We don't know how to calculate the thrust. Our piston is 60cm long, diameter 100mm ( approx 4.7 litres) The shaft then reduces to 32mm for the outlet pipe. We are planning on having 4 of these. The weight of the boat is approx 30 kilos and it should take two people so the total weight would be about 200kg. The boat is 3 meters long and fairly aero or aqua- dynamic. We would love to know if there is a formula we could use to gauge roughly what the thrust would be, or how fast it might go if say the piston completes 1 cycle per second. We would be very grateful for any advice thank you Liam

 

F=m*a
There will be losses depending on the design. The explanation for that would take several volumes. You are not likely to get the boat to move with that setup. The mass of air is very small, so your acceleration has to be very high.

 

I have no very clear that this formula is worth for something. Maybe talk about the momentum (m * v)?, what do you think?

 

The momentum of the water = 0 in the pump. It needs to be accelerated to produce a reaction force.

 

Gozo sorry, I have not explained. When I speak of "m * v" I mean mass times velocity. Excuse my clumsiness.

 

I'm afraid this isn't an easy calculation at all. It involves quite a bit of physics. Here are some introductory notes on compressible gas dynamics from Notre Dame. You can use the math in the document to design an appropriate exit nozzle.

http://www3.nd.edu/~powers/ame.30332/notes.pdf

(like I said, not an easy thing to do.)

The problem also suffers from trying to push a low density fluid into a high density fluid.

https://en.wikipedia.org/wiki/Rayleigh–Taylor_instability

Calculations are much easier when pumping dense, incompressible stuff like water towards a low density exit such as the atmosphere.

 

Found this, FWIW:

https://www.physicsforums.com/threads/compressed-air-powered-boat.564885/

 

Thank you. I suppose I realise that it would not be fast but we do only want to drift down an inland waterway at walking pace, there would be some movement don't you think?

 

Thank you all for your help. We will do a few experiments and if I have any results will post

Thank you all
Liam

 

I would guess that you would do better pumping water rather than air.

Steve.

 

I would think that you would get better results pumping water rather than air although still not very good results.

 

Why would you want to take this approuch, instead of a more conventional method, that might have considerably better efficiency? I'll assume this will be human powered, which makes this question more important, given the amount of "engine" you'll have to power this contraption.

 

Thrust, whether it is by a pump, a propeller or any other means, is caused by accelerating a mass. When a mass has constant velocity there is no thrust. Air has a very low mass compared to water, so you have to accelerate a very large volume at a much higher rate. That is why airplane propellers would not move produce enough thrust for lift-off if they turned at the same rate as a boat propeller.

 

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Maybe make the air come out in pulses like a toy putt putt boat. You might somehow have to convert your human power to pulses of water instead of air.

http://sci-toys.com/scitoys/scitoys/thermo/thermo.html

Off topic, but could magnetism somehow power a boat? Make the stern of iron and hang a magnet off the bow, like a horse and carrot?

.

 

That is, if the aircraft propeller drives a constant mass of air at a constant speed (acceleration = 0), the aircraft will not move. Did I understand correctly?