Pressurized motor pod


#1

I’ve been thinking about the possibility of using a small air pump to create a slight pressure in the motor pod. This air pressure might do a couple of things:

  1. Reduce friction. Instead of a lip seal, I might be able to use a simple nylon bushing with far less friction. With the pod pressurized, air will leak out instead of water leaking in through the bushing.

  2. With a simple check valve/pressure relief valve on the bottom of the pod, the pressure will automatically clear any water that may get in the pod.

A small, DC aquarium air pump like the one below draws far less power than the lip seal consumes. This one operates at 1.8 Watts.


#2

Sounds like a very interesting idea.

Where would you put the pump? Suppose it needs to be in a dry place and you don’t want the intake valve to be flooded. So maybe needs a mechanism to protect it.


#3

The pump would be up in the board. A small tube would run down the mast along with the wires for the motor.

Some sort of baffle cahmber will likely be needed to avoid getting water in the pumps inlet.


#4

Interesting concept - but some hurdles to overcome.

  • You need a stronger pump. The pressure at 0.5m below water is already around 800mmHg *. More than this pump can deliver.
  • absolutely no water in an air pump as it will damage it.

Maybe it would be easier to pressurize the motor pod before every ride with a really high pressure? Hmm, doesn’t’ really solve the problem of needing power eating seals …

*e.g. according to this: http://www.calctool.org/CALC/other/games/depth_press)


#5

I think you may have made a mistake in your calculation. At .5m depth, the water pressure is about 37mmHg; the pump is rated for 500mmHg.

ps I checked the online calculator you used, it is in error. Try this one:

http://docs.bluerobotics.com/calc/pressure-depth/


#6

Aah, thanks for your correction.
One calculator shows with the ambient air pressure, the other without ambient air pressure.
So yes, your pump should work.


#7

No worries…now on to figuring out a good baffle system to remove water if rquired.

My old stand up JetSki had some kind of baffling system to keep water from being sucked into the motor.


#8

Could be good to use a 3d printed part.


#9

found this chart from SKF (bearing producer) on friction loss from shaft seals, it might not be very accurate but the loss seams to be quite minimal.

just for fun a manual calculation, the friction and contact pressure, is a bit of calculated guesswork, based off some tables I found online. Found about 50w of loss. But I think it is a bit less.

in my opinion, power loss from a shaft seal is acceptable. Having pressurized air to prevent leakage might work but way to complicated. Just fill the compartment completely with grease and no water will ever enter. it also reduces noise, keep it simple.


#10

How come you use these rubber seals? Would a mechanical seal not be better? Its the one where two precision ground spring loaded faces are rubbing against each others and forming the seal.


#11

Max, do you have a link for this type of seal?

I’m wide open for better ideas on sealing the shaft. I’m looking for the lowest friction seal to keep water out of the motor pod.


#12

I have no experience with it except that it got stuck at University. Something like this: https://www.mcmaster.com/#9281k87

With a rubber seal, the shaft rubs against the stationary seal which needs lubrication and a causes wear since shafts are not polished. Mechanical seals have two parts where one part is stationary and the other one is mounted to the shaft. The shaft is in contact with a rubber seal, but they don´t move relative to each others. The motion happens where both parts of the seal touch. Those surfaces are super polished and have very little wear. So you have metal rubbing on metal. (or ceramics) If I remember correctly.


#13

Thanks Max! I’ll give this a try.


#14

One more thought, with good seals, you could pressurized the pod only occaisonally. Help keep water out, also help purge any water that happened to get in the pod. Electric pump or hand pump could do this occaisonal job.


#15

did you try this, and results?


#16

Still working on it. I’m hoping to have the motor/pod/mast assembled for tank testing in the next couple of weeks.