RE: [PSUBS-MAILIST] Does size really matter?

["Roger June" <rjune@fuse.net>]

Hope you-all don't mind me putting in my 2-cents worth.

I guess first of all Motor Guide's 107/36 thruster uses from 0 to 48 Amps.
And it will pull the max amps any time the motor sees high torque regardless
of Voltage.  The flip side is that as the voltage goes down (i.e. 12 VDC and
24 VDC) the torque produced decreases also.  This is an important concept to
understand.

Thrust verses speed because of two factors increasing thrust does not
necessarily mean an increase in speed.  Factor number one is that the
standard props on the thrusters are designed for high thrust at low speeds,
the prop pitch is fixed and as a result will only go just so fast.  Factor
number two is that when you are underwater drag has more to do with speed
than anything else.  Without going into the formulas for drag verses speed,
this means that if you are traveling at 5 miles per hour (mph) and you drag
is equal to 40 pounds, and you increase your speed to 10 mph then your drag
equals 121 pounds.  This is sample-a-matic of the actual formulas but again
you get the idea.  Because of this I concur with Rob, a 107# thruster is on
the large size for a thruster.  Having built my own DPV and tested different
size thrusters and as a supplier of (modified) Motor Guide thrusters I
recommend the 67#/24 VDC thruster for your DPV, but of course maybe you plan
on pulling two divers, or other equipment or just want the extra thrust.

Next lets consider the batteries; When picking out you batteries make sure
that the amp hours is rated above the required amp draw.  Most batteries
have a drop in amperage after the battery is used for a period of time.  Let
me see if I can explain this better.  A new battery fully charged puts out
60 amps, but after 10 min of continuous use will only supply 58 amps, after
30 min use only supplies 48 amps, etc.  The same battery after six months
use, fully charged puts out 54 amps and after 10 min of continuous use only
puts out 48 amps, after 30 min use puts out ... you get the idea.  Based on
the brand, kind, number, and size of the batteries that you use, the values
in the above scenario will change, but the general scenario is the same.

Next you need to be aware of the charging time, (unless you can afford to
have multiple sets of batteries).  Charging times can very from very short
(1 hour) to very long (48 hours).  This is important if you plan on doing
multiple dives in one day. I will touch more on this subject a little lower
in the email.

Look for continuous amp draw ratings as well as total amp hours for the
batteries.  Ideally you will want batteries that give you more than 48 amps
continuous draw with an amp hour rating high enough to meet you dive
profile/time requirements with a reasonable time/safety factor.  Hate to
have your DPV batts die...makes for a long surface swim, towing the DPV.
Not to mention all the ribbing you'll get about you being a DPV propulsion
vehicle.

When considering your dive time requirements I suggest the following; This
is typical for me.  A typical dive is about 1 hour and I do 4 dives a day (2
in the morning, take a long lunch and 2 in the afternoon/evening.  In my
case I'm using a 67#/24V thruster and a optima Troll Fury (two "blue tops"
packaged together.)  If the batteries are at full charge at the beginning of
the day I have 55 amp hours (Ah) available.  During the dive I run at an
average speed of 1/2 throttle, this results in a use of 24 Ah, this leaves
me 24 Ah. After the dive the DPV goes on the charger at a recharge of 24V at
10 amps max. input.  After 1 to 1-1/2 hours of charging time the batts are
up to approximately 39 amp hours. Second dive uses another 24 Ah leaving me
15 Ah. again the DPV goes on to the charger, and I take a long lunch say for
3 hours.  At the end of the lunch the batts are now up to around 45 Ah.
Third dive uses another 24 Ah, leaving the batts at 21 Ah, again the DPV
goes on the charger for about 1 to 1-1/2 hours putting the batts at 36 Ah.
Fourth dive uses another 24 Ah leaving 12 Ah.  In all cases I have
sufficient Ah at the end of the dive as a safety margin. (running the DPV at
half speed, 1 Ah = 2.5 min of run time)

If you are using a larger motor than you will need more batteries to
increase your total Ah.  If your battery charger is rated at 12V/15amps
verses 36V/5amps your charging time will change accordingly. If you tend to
run at full throttle then your amp usage will go up.

I hope the above profile gives you some incite in to all the things that
need to be considered, and how they all tie together.

Best of luck on your project, and If you come across a 36V/200Ah battery
that weighs less that 10 pounds, CALL ME!

Roger June
REJ and Company
rjune@fuse.net
513-300-1189

> -----Original Message-----
> From: owner-personal_submersibles@psubs.org
> [mailto:owner-personal_submersibles@psubs.org]On Behalf Of Rob Innes
> Sent: Saturday, November 09, 2002 04:20
> To: personal_submersibles@psubs.org
> Subject: Re: [PSUBS-MAILIST] Does size really matter?
>
>
> Herve
>
> Thanks for clearing that up for me. Also I'm wondering what type
> of battery is
> preffered for use in DPVs; Dry cell, gel cell, AGM, Lead acis? I
> like all the
> benefits of the Deep cycle Optimas except the price and the
> weight (40#+ each,
> would make for a heavy son of a bitch) I run a little gel cell in
> my dolphin
> which has been faultless and has been very well abused. Even
> though they are
> totally sealed, they say that they should not be contained in an airtight
> container. I presume they only gas off while being charged. I'd
> like to seal
> the batteries in semi-permenently and charge them through a
> charging port. Any
> problems there? Thanks
>
> Rob
>
>
>
> Herve wrote:
>
> > Rob,
> > 107/36 is pretty oversized for a DPV, however 3 batteries 12V
> 40Ah in serie
> > will give you the max thrust expected for that motor but for no
> more than 30
> > mn full power run.
> > the technician was right, 200Ah rating batteries is what should
> be the right
> > choice for that motor. with a good 3 hours capacity
> > power is the same regardless the Ah of the batt, but 36V : full
> power, 24V:
> > reduced power, the more Ah the bigger the battery size the
> longer will run
> > the motor. the Ah of a battery represents the "amount " of
> energy available,
> > when the voltage "represents" the power
> > Herve
> >
> > ----- Original Message -----
> > From: "Rob Innes" <innespace@jett.net>
> > To: <personal_submersibles@psubs.org>
> > Sent: Friday, November 08, 2002 11:20 PM
> > Subject: [PSUBS-MAILIST] Does size really matter?
> >
> > > I just bought a Motorguide 107# 36V trolling motor that I
> want to use to
> > power
> > > a DPV. One supposed expert told me if I discard the circuit
> board and run
> > power
> > > directly to the motor I could get much the same amount of thrust using
> > only 24V
> > > as long as I had enough amperage out of the batteries. Another , so
> > called,
> > > expert tried to tell me that the amperage or (Ah) was only
> important for
> > > running time and to run as much Voltage as the motor could
> take. As I'd
> > only be
> > > under water for short durations, I'm not so concerned with
> how long the
> > > batteries run for, as much as I am concerned with reducing
> battery size
> > and
> > > weight. My question is, Can I maintain maximum thrust by using smaller
> > output
> > > batteries, i.e. hooking up 3 small 12V 40 Ah batteries in series. A
> > technician
> > > from motorguide said I should run batteries of at least 200 Ah. Which
> > battery
> > > rating is most important for maximizing power output or is 36
> volts the
> > same,
> > > regardless of battery size. Any well informed advice greatly
> appreciated
> > >
> > > Rob
> > >
> > >
> > >
> > >
> > >
>
>