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

[Rob Innes <innespace@jett.net>]

Thanks again Roger for sharing your knowledge gained through experimentation
and concise record keeping. I wish we could have been so methodical when it
came to our own experimentation. I believe you're somewhat familiar with the
design I'm working on and therefore may understand why I chose the larger
thrust motors. I would have preferred to have a high output 24V motor but, as
you know, their price often exceeds that of the bigger 36's. I'm lucky if I
dive 4 times a year let alone 4 times a day, so duration is not too much of a
concern. My design, I guess, is less of a DPV, and more of a highly
manouverable sled. The reason I wanted more thrust than was recommended, was
mostly because of the exponential amount of thrust needed to gain every small
increment of speed.  Also, if it turned out that I had power to spare, I would
like to experiment with pitching the props up a bit. I know all stock rolling
motors max out at about 5 knots, they only differ in the size of boat they can
push. But I'm guessing the drag that my vessel would incur, would be close to
that of a 20 foot fishing boat so I opted for more rather than less thrust. I'd
like to keep the entire craft around  200 lb, so final battery selection will
come down to the necessary weight to become neutrally buoyant divided by 3 (the
number of batteries). Not the most calculated way to put a craft together, but
trial and error hasn't lead us too far wrong in the past....Oh, and before
anyone jumps on me for being a reckless suicidal wannabe submariner, I'm very
much aware of all the many ways I can be killed or maimed and I have a near
obsessive level of self-preservation!
I'd be interested to hear what other people have used to power their wet subs
and DPV's and whether they had too much or too little power to meet their
expectations. Thanks

Rob

Roger June wrote:

> 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
> > > >
> > > >
> > > >
> > > >
> > > >
> >
> >