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Re: [PSUBS-MAILIST] ETEK motor (speed controller for this?)
Paul,
That's an impressive controller!
I'm no expert at propulsion in water but if you look at it from a hands on,
grass roots prospective, you'll see what I mean.
Imagine a surfboard to represent a surface craft and a watermelon to
represent a sub. Place one index finger on the tail of the surfboard and
your other on the end of the watermelon. Push them slowly through the
water. They're about the same mass and take almost equal force to get
moving. Now try the same thing at faster and faster speeds until your
finger gives out. Which finger will give out first? The one on the melon!
There are limits to this of course, but in the horsepower and speed your
talking about, five horsepower in a Psub, will move you along fine and the
extra thirty won't make it go that many times faster, but in the surface
craft, you'll get a better return on the additional power you put into it.
If I can use my folksy water melon and surfboard comparison again I could
explain the prop RPM. Our surfboard and melon both have props on them but
there only spinners like that of a pinwheel. The faster you push the faster
they revolve.
This time you push with five pounds of force on the surfboard. As the
surfboard is cruising along on the surface it's prop, being a screw, screws
it way through the water at pretty high RPM's. Push the melon with the same
five pounds and of course as the melon doesn't travel as fast, it's prop
doesn't turn as fast. Obviously to use props for propulsion, they have to
turn faster then the water they are traveling through. The surface craft
needs a faster prop speed just to travel through the water and additional
speed to produce thrust. The slower moving sub needs less prop speed to
screw through the water and additional speed to produce thrust.
By now I'm sure most of you are thinking, but the surface boat uses a
different pitch prop. True! A faster moving boat uses a prop that has a
long lead, more pitch. For one turn of the prop it is designed to screw
farther through the water then a slower moving boats prop. This is taken
into consideration with propelling a sub. Any slow moving craft has a
flatter prop then a fast moving craft. If the pitch of the prop were all
the same the fast boat wound need a zillion RPM and a subs props would be
turning extremely slow.
It's the combination of RPM, prop pitch, prop diameter and the speed of the
craft through the water that have to be considered when sizing a prop. A
big prop with a big bit will transfer more engine horse power per revolution
into moving water. But the point of diminishing return is reached sooner
when powering a short plumpy sub then a sleek long surface craft. A fast
turning prop, slowly moving through the water (slow boat speed), will
cavitate sooner then a slow moving prop or a prop moving faster through the
water (fast boat speed).
I found the slowest motors I could find for my sub "Persistence", matched
the props to them and later fine tuned the props monitoring the motors
amperage.
A speed controller is a way of lowering the RPM but a 30 HP at 3600 RPM
motor turned down to half speed with a speed controller is no better the a
15HP at 1800 RPM. If you need 1800 RPM then buy the smaller motor. A gear
reducer is another option but get cumbersome for a Psub. A motor of 30HP
will still retain most of it's 30HP on the output of the gear reduction. If
you half the speed with gears, you double the torque, minus that converted
to heat by the gear reducer. A gear box is clumsy and won't fit into a pod
very easy but within the hull it's more useable.
I think you got your battery assumptions correct. You only store so much
power,be it in a gasoline tank, diesel tank or battery. In a Psub you
seldom travel for a long period of time. It's mostly short bursts and slow
speeds keeping the total power consumed in a hour of operation to far less
then an hour in a surface craft. Contact some battery people that do the
calculations. Figuring out how much operating time you have in a given size
tank of electricity is just math.
If you read to the end of this book and haven't read anything you didn't
already know, I apologize. ;-) Dan H.
----- Original Message -----
From: <Deeperdowndude@aol.com>
To: <personal_submersibles@psubs.org>
Sent: Tuesday, February 15, 2005 8:19 PM
Subject: Re: [PSUBS-MAILIST] ETEK motor (speed controller for this?)
> Dan,
>
> Thanks for the advice and for the link. Here's another controller link I
got from the company that sells the motors.
>
> http://www.evparts.com/shopping/product_details.php?id=271&product_id=3317
>
> Yes I am thinking of using it in a surface boat. Some of your comments
lead me to think about it from the standpoint of a sub, though. Could I ask
you a couple more questions about that?
>
> You said 34 horsepower was a lot for a sub and that 3600 rpm would be fast
for the prop. About that, wouldn't more horsepower simply mean the ability
to go faster? I understand the visibility-collision problem underwater, but
other than that is there some reason why a fast psub isn't a good idea?
>
> About the prop RPM: I've also read that fast props are good for boats and
subs do better with slow props. Is it because the sub prop would cavitate
at the higher RPM? And if so, couldn't that problem be avoided with a gear
reduction drive or by simply running at a slower power setting and RPM?
>
> But where you started me wondering was what you said about the batteries,
though. Unless I'm wrong, if I need 72 volts and 400 amps, and if I'm using
12-volt batteries wired together, that means I'd need at least six batteries
rated for at least 66.6 amp-hours each? So then, does your "starter"
comparison mean it's only going to run a very short time: in this case less
than one hour? Am I right so far?
>
> Paul
>
>
>
>
>
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