Jet Propulsion?


#43

How about to get more thrust from a jet use an air pump to pump air through a hose down the mast and out into the impeller? Something like this maybe? 50w, 12v and 300lpm of air flow. We could even use the air flow to cool the esc and batteries before going down the mast. Maybe connect the air pump to an esc as well and optimise the air flow for the jet depending on rpm etc.


#44

The thrust from jets is the “exhaust” mass moving in one direction and the body in the opposite direction. The density of air being lower than water means it would have to be many times faster to have the same effect. Water is a great, heavy material to be used for thrust.


#45

Had the same idea in mind, but i will try it without additional air first, and then compare the thrust with and without added air.


#46

My simplistic thought process is that air being compressible unlike water will get compressed as it goes through the impeller and so as it goes out the jet it will want to expand giving more thrust. Also the air and water mix being compressible will allow the impeller to get up to speed faster with less torque. Maybe there is an optimal amount of air to give more power to an underwater jet?


#47

@surfsup, I like the idea of pumping air down to improve jet efficiency, I suggested that before too here. Jet dives run more efficient with a water/air mix, all jetskis benefit from this as the intake is so close to surface with lots of air bubbles. We don’t have that 4ft below. You should build a prototype, I am really curious how this would help :smile:


#48

Great minds think alike huh lol. I’d love to do a prototype to test but I think others will beat me to it. I have bruno’s magical 100kv inrunner so will be testing that first with a prop but I’ll probably be slow to even get that done as I have very little time at the moment. Bruno told me the motor was 85amp, 90v which sounded like it might work but I see he has told you its 65amp, 120v. I wonder which it actually is? I’m losing faith in this motor working as I’d hoped so will try a jet if it fails…


#49

Hi
I had a quick try with this and did not see any improvements in thrust
yes it allows the motor to spin up.
might need to reduce the outlet diameter to compress the air more???


#50

and also I did not pump the air down I allowed the air to be drawn down by Venturi effect.
just a hose down and ending just before the water inlet


#51

Interesting. Thanks for sharing your test. Could it have sucked too much air?


#52

Hi
I had a valve to throttle the amount of air but felt that the out let nozzle was too big to compress the air
to give that explosion of energy as it exited.
I was running a 5mm internal bore silicone pipe.
the same size as I use for cooling the ESC

I might try again with a different outlet nozzle will need to print one

Thanks Nick


#53

I see. With that small 5mm hose and passive Venturi air I doubt that it was too much air then. More likely not enough to make a difference… but I am just guessing. Would be great if you could measure differences in thrust, amps and prop rpm at different air mixture settings.


#54

Thanks for doing the test and sharing… Interested if the smaller outlet will help


#55

Hi the amount of air was more than enough
the motor spun up to maximum rpm
but the thrust dropped off
so maybe a longer prop more like an air compressor
and a smaller outlet.

if you go too small it will load up the prop and you will not see the benefits I think.

lots of testing and printing needed.

Nick


#56

Because the dolphin tail does not require air for a highly efficient cruise, neither does the jet propulsion require air.


#57

I see that MHz has now got a jet propulsion system https://www.mhz-watercraft.com/shop/en/jets/549/hydrofoil-set-impeller-nozzle-stator-bearing?c=110
It is rather wildly priced!


#58

For such a high price - a nice design, high quality material, yet average efficiency. The solution exists: help find the manufacturer of impellers in China and I create high-performance impellers for all categories of e-foil!


#59

I have mills, CNC and 3D printers. if you create a design, I can build it and test it.


#60

He said he can design if we give out max torque measurement. But i am not sure which, static thrust torque or dynamic (which would be pretty hard to get).


#61

Same here. Why not make a prop for 4000RPM that we can all use?


#62

There’s prices are fu#king outrageous, same motors on Alibaba for half the price including shipping to Jupiter or mars !