Bilge Pump Max Depth

[tazdevl77]

Any idea as to the approximate max depth using the modified bilge pump thruster approach??

[Forest]

i think the max depth with a bilge pump would be 50ft i think the pressure of the water would break or slow down the bilge pump after 50ft.

[tazdevl77]

I had an idea after reading up on this concept on several websites. It seams too easy to actually work, but I'll throw it out here anyways and see what you all have to say. I was thinking you could add a 90 degree pipe fitting (not sure on sizes) to the front and rear end of the bilge pump housing so the fittings face each other (after it's been modified to turn a prop). Then fill the motor with oil (mineral oil?). You then attach a tube(Vinyl?) to the end of each fitting (use a fitting that can handle a couple hundred PSI) and make sure that's filled with oil too. You fill the entire assembly in such a manner that you have no air bubbles. My theory is that as outside water pressure compresses the tubing filled with oil, the oil in the tube will become equally pressurized and will pressurize the motor housing. So if you have 150 PSI of water pressure pushing from the outside in, you should have 150 PSI of oil pushing from the inside out, effectively neutralizing the pressure. In the event that some oil leaks out the seal from normal operation, the pressurized tube of oil also becomes a reservoir for oil. So the question then becomes, how much pressure can the motor effectively operate in? I was thinking that maybe this system may make a normal bilge pump good for a couple hundred feet. The maintenance would be that you'd have to change out the oil as it becomes contaminated and if you lose some oil in the housing which is replaced by pressurized oil in the hose, as you lose pressure you will get air bubbles and will have to check and make sure system is purged again before another use. I attached a basic photo of what I'm talking about. What are your thoughts? Tim

[tazdevl77]

After laying in bed thinking about how this design is too good to be true, I think I figured out why. One major potential flaw in my above mentioned design is surface area. Since PSI is Pounds per SQUARE INCH, would a smaller tube with less surface area be able to distribute enough counter force that is being pressed against the motor? Not sure of the math behind it..but... if the oil filled tube has a total area of 5 square inches and the water pressure is 100 PSI, then the total pressure on that hose is 500 Pounds... and if the motor housing is 20 square inches and the pressure is 100 PSI, would the total pressure on the motor housing be 2,000 Pounds? Obviously 500 lbs will not neutralize 2,000 lbs, meaning my above design is crap if what I've said here is true. Then there's always the concept of using pressurized gas (CO2 tank commonly used for paint ball) that you can pump into the thruster housing. This would require some kind of sensor and solenoid set up so it automatically adjusts. What if I just simply fill the thruster with oil and get all the air bubbles out? My goal depth for my ROV is 300' max, but I don't want to be constantly tearing apart bilge fans or pumps if I can help it Thoughts?

[sthone]

The problem with most bilge pumps is they use a lip seal to seal the shaft. In the tests I have done the problem with the seal is not that it leaks but the added pressure at depth causes the seal to actually seal to seal harder (in a sense) and this causes more drag on the shaft and in turn causes the amp draw of the motor to increase. You can see my results on this page.

http://www.homebuiltrovs.com/mayfair750test.html

-Steve

[Bored_Engineer]

I may be wrong, but I think that as the oil in the above mentioned apparatus compresses, the oil would be denser; so there would be more drag on the motor assembly on top of the drag being applied from the seal, therefore more amps. In professional ROVs I think they use hydraulics to power their thrusters, but Im not sure how to do that in a economically friendly...

[tazdevl77]

it seems the thruster is the most pain in the A$$ part of an ROV, are there any commercially available shaft seals out there?

[SoakedinVancouver]

The hose/oil trick does work, I remember seeing it being used on the low depth ROV's (<300 feet). They saved on fittings by connecting only one end of the hose and crimping down the other. About oil, on the hydraulic system (where the reservoir had a "window" of translucent rubbery material, subject to outside pressure), the oil was a "clear" red color, which would turn milky if contaminated by sea water. You would purge a bit from the system into a bucket and determine if you had to purge and replace.

About pressure on seals, I did hear stories that indeed it would cause problem re. shaft being braked and current going up. More or less a problem really if you are running from a genset or inboard power and working on a paying project! (which means you have SPARE thruster assemblies).

But these points are why I'll be limiting myself to 30 feet operational depth, 50 feet max depth rating, (crushing depth? Naaayyy, that'll never happen!...)

Stephen

[ponynicker53]

Commercial thrusters:
http://www.tecnadyne.com/thrusters.htm
http://www.crustcrawler.com/products/ur ... p?prod=300 (yes, I know expensive)
or
http://www.tsltechnology.com/marine/thrusters.htm

If somebody gets a price quote for any of these thrusters, please post here!
Thanks

[martinw]

Pressure compensation using oil has been used on ROVs for a long time.

http://www.sub-atlantic.co.uk/product/c ... 0cc-2700cc

Compensating thruster shaft seals and thin walled boxes at depth are all achievable with a compensator.

The flexible tube with oil in the picture "Thruster Compensator.jpg" is a simpler version of the commercial ones available.

The down sides of only using a tube are mainly to do with volume and effects of temperature changes on the oil.

The commercial ones use a bladder and a spring, the bladder holds the required volume of oil and the spring means if there is a leak oil flows out rather than water flowing in. There is normally a sensor on the compensator to indicate the level of oil left in it to signify how fast leaks are occuring (if at all).

Earlier this year I filled my thruster with oil and it actually gave more thrust, probably due to lubrication of the shaft seal.

Hope this helps.

Martin