Re: [PSUBS-MAILIST] Pressure Compensated Trolling Motor Question

[Michael Wright <mwright@smallip.com>]

Chris Nugent wrote:

> Ok, I'm tired and I have the flu, and on top of that know nothing much 
> besides the basic idea behind pressure compensation, so don't hold this 
> question against me.  But, it occurred to me, and here it is:  What 
> about using the trolling motor itself to drive a pump to move the 
> compensating fluid through the filter? 
>     Too much power loss?  Too complicated?

Nice idea but I was hoping for a system that required minimal mods to 
the trolling motor.  If the system were being designed to operate 
indefinetly then running an oil pump from the motor directly would make 
a lot of sence (rather than having to compensate a second pump motor). 
I think going to brushless DC motors would be a better choice in that 
case though.

So I'd say more complex than necessary.  Which a filter and pump 
approach may be as well.

My thought was that the cost for a trolling motor based solution is 1/10 
to 1/20 the cost of an equivelant off the shelf brushless thruster.  If 
the project can afford brushless thrusters, go with Cyvect's, they're 
beautiful, efficient, small, and virtualy silent.  I know given the 
choice between getting 10 additional days diving or having brushless 
thrusters I'll take the ten days diving and accept pumping the oil (or 
even changing it every time).  My goal is to make a repurposed Minn-Kota 
only slightly more hassle to use than the best thrusters on the market, 
at least for our design goals.

My assessment is that any small sub is going to require at least as much 
prep and battery charge time as it provides dive time (likely 
significantly more).  Having the thruster tests take 10 minutes rather 
than 2 minutes doesn't seem like a big deal.  My design target is 4 
hours operation per dive with 72 hour rescue window.

The goal is to be able to use parts that have their design cost 
amortized over tens of thousands of units rather than parts with a total 
market counted in the hundreds.  The downside of this approach is that 
if the primary market for the part doesn't have the same requirements we 
do then we have to adapt the part to our needs.  In this case we need a 
trolling motor to go 10 times deeper than it was designed to, so we 
convert it from a 1 atmosphere pressure vessel to a fluid compensated 
ambient pressure vessel.


Thanks for the thought Chris.

mike
>  
>    Nuge
> 
> */Michael Wright <mwright@smallip.com>/* wrote:
> 
>     Hi Everyone,
> 
>     I have a question for anyone using Liquid Compensated Trolling motors.
> 
>     I've heard that the drawback to them is that the carbon dust from the
>     brushes goes into suspension and eventualy causes a ground fault to the
>     case (or direct short if no ground fault detection is used). This seems
>     quite plausible with the simple solution being to change the
>     compensating fluid regularly.
> 
>     The quesitons I have are: Is this a problem worth solving in a better
>     way? Is the particle size large enough for an automotovie oil filter to
>     catch them?
> 
> 
>     The trick I think is to use a drill powered oil change pump to
>     circulate
>     the compensating fluid through the filter. Since the pump has no motor
>     and no air spaces it is pressure compensated by the thruster fluid.
>     Cycling the full volume of oil through the filter would take less t!
>     han a
>     minute and could be done with a cordless drill (safe and expendable).
>     Since the compensating system stays closed there is no mess and no need
>     to bleed the air out of the system.
> 
>     So that's the idea.
> 
>     mike
> 
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