DPV - cruise and then max speed - can you estimate?

[Basking Ridge Diver]

This is a very general OW not Cave question - I have seen the Tahoe Benchmark Reports - but I don't think this has been posed...

If you run a DPV at cruise speed for say 50% run time speed (arbitrary percentage by me) and then switch to or hit max speed - how long would you expect the DPV to run? Is this theoretical and can not be answered? Would it be close to 25% left of max speed run time- or less? If I said Lithium battery would this change the answer to the question?

I realize that most times you want to head into the current first but if the current switches and you had been at cruise speed and now you turn back and hit a stronger than expected current - what do you do? How much life do you have left or is there a "trick" to hitting the bottom and the current is not as strong as the surface and staying at cruise to keep the battery life?

If this is a dopey question feel free to tell me you never had to swim your scooter home...

 

[Ouvea]

If you have an older DPV with lead-acid batteries and you have to tow someone, you're in deep S _ _ t. Even some of the Li-Io battery powered DPVs may not have the torque to tow two divers with any rapidity. If you had to go up-current, then it may not be possible.

Most manufacturers publish their the run times at various speeds. You can extrapolate your remaining run time @ max speed, given your current trigger time. Logic Dive Gear has an en electronic module (Sentry) that will give you the DPV's battery status. I have been fortunate that my DPV has never failed, nor the the units owned by my dive buddies.

This is not a dopey question and one that should figure prominently in any dive plan where a DPV is used.

 

[tbone1004]

@Basking Ridge Diver slightly complicated question and is a function of efficiency and voltage drop. Unfortunately @fdog won't publish the actual graphs which makes this a bit misleading and isn't active on this forum any longer so won't see the request.

If we look at Tahoe and take the Cuda 1150, and assume you're in their standard gear, burning 1075w of power at 292fpm, you have a theoretical range of 4.1 miles. Cruising you are sucking 501 watts of power at 150fpm and have a range of 5.8 miles. Pack is "Rated" at 1291wh
Puts us at 18.1min/mile and 324.3w/mile on high, and 35.2min/mile on "cruise", and 294w/mile on "cruise". There is more voltage drop in the packs at full tilt boogie than there is in cruise so you will have more of the pack available, but you really don't want to use any more than 80% of the pack or you risk doing some serious damage to it over time.
So, say you have a 2 mile leisurely set out on "cruise", you're going to consume about 588w or 45.5% of the packs theoretical max.
Coming back on high you have to go the same distance so you'll consume 649w or 50.2% out of the pack.
We have to assume that 45% of the pack is 45% of the pack. Voltage drop is more significant on SLA batteries, but you assume your pack capacity in wh based on consumption at full speed so worst case if you claimed 45%, it may have only been 40% consumed.

As a diver, you have to use the data that you have available to you to plan those dives. If you have easy to see distance markers then great, if not use minutes of burn. Use a data logger if you don't have one to get the actual consumption of your pack and the actual capacity of your pack to plan dives. If I was diving in the ocean, I would not go any farther out than I could get back home on high regardless of how you got there.

I don't like or recommend using only 80% of the theoretical pack value. In this case that's 1030wh. Half of that is about 500w. 500w/324.3w per mile is 1.54 miles, so I would not take the cuda 1150 more than 1.54 miles out.

 

[Jack Hammer]

It can be a little tricky to answer and can vary widely by model. Simply put, some motors are less efficient at higher speeds.

My Sierra (with slightly higher voltage) pulls around 300 watts at cruising speed and twice that at max speed, which is only about 30% faster than cruise.

 

[tbone1004]

It can be a little tricky to answer and can vary widely by model. Simply put, some motors are less efficient at higher speeds.

My Sierra (with slightly higher voltage) pulls around 300 watts at cruising speed and twice that at max speed, which is only about 30% faster than cruise.

It's not exactly the motor, it's the power that it has to put out. I.e. spinning full speed with no load it should be about the same efficiency wise, but load is exponential so when it is turning the screw at full speed with twice the load, it will need exponentially more power. Think MPG in a car at high speeds.

 

[Jack Hammer]

Yeah, I was trying to oversimplify it without introducing hydrodynamics, increased drag, overheating, etc. I've got the Sierra with the old, old, old lower wattage motor and non programmable electronics. Unlike my XK1, it doesnt like long runs at top speed.

 

[DA Aquamaster]

There are a few things to consider. Whether you're working with aerodynamics or hydrodynamics, drag is a square function, so if you double the speed, you increase the drag by a factor of four. Since thrust has to equal drag, the power required to maintain a speed that is twice as fast also increases by a factor of four.

That also means you need a lot more power to go just a little faster.

I did some time and speed tests with our P1s in side mount CCR kit and the numbers held up well compared to the factory advertised numbers. My numbers over a measured course at 5 different power settings were:

230 fpm at 100%
204 fpm at 75%
185 fpm at 50%
165 fpm at 37.5%
148 fpm at 25%

In terms of duration the run time for the P1 at 100% power is only around 50 minutes, but at 230 fpm that's about 11,500 ft. However, at only 25% power it's still producing 148 fpm and the run time is around 120 minutes with a range of about 17,750 ft. In other words, zipping around at 230 fpm comes with a penalty of nearly 1/3rd less range compared to a 150 fpm cruise speed.

Generally speaking for maximum efficiency in flight you cruise faster into a head wind to reduce the time spent in the headwind, and you reduce the cruise speed in a tailwind. The same is true with a DPV. If you are going into a 25 fpm flow it makes sense to cruise a bit faster to limit the time in the flow. Using the numbers above, at 50% power you'd be moving 185 fpm, and making good 160 fpm over the bottom. On the way back, if you slow down to 25% power you'll be making good about 173 fpm over the bottom, letting the flow do the work with comparatively little drain on the battery.

As Tom indicates, the voltage drop with a lithium ion battery is not as great as with an SLA battery, but there is still some drop, and you still get more drop at higher current drain. For example, the P1 PTB's batteries produce 36 volts at full charge, and the factory set cut off voltage is 30 volts to protect the batteries from excessive discharge. With the P1, when that voltage is reached, it starts to rapidly cycle the motor on an off, with the result that it feels like it is running very rough, and it's running at significantly reduced power. However, if you are running at 100% or 75%, for example, you can shift down a gear or two and it'll go back to normal operation as with the reduced current drain it's not dropping to the cut off voltage.

In other words, you'll probably get a little farther if you use the higher power settings earlier in the dive, then backoff to a lower cruise setting for the last part of the dive.