Guide to fuel conservation

[sqk7744]

Ducimus many thanks for posting, this is great!

(so where's the Fuel Flow meter? ) I guess the RPM gauges are not live.

Cheers!

[Ducimus]

RPM gauges are at the helmsman station in the control room (forward of the chart table), and behind you in the conning tower. Unfortunatly their not very accurate. Theres 4 of them and they range from 0 to 250. The needle starts measuring before it even hits 0. And then turns totally around ,and pegs soemwhere at what im guessing is around 540 (5XX soemthign) because that is what the max RPM in SH3 was. Im assuming its the same here, despite what the listed RPM in the sim file is saying. Anyway, point is, RPM gagues exist, and the scale on them is way off.

[Frederf]

Originally Posted by PepsiCan

There's also plain physics involved. If you burn diesel to charge your battery for the energy value of 100 kilowatts, then you will not charge your battery for 100 kilowatts. A lot of energy (as much as 50%) will be lost in the form of heat, friction (resistance in the .

So, the cycle looks like this:
- you start with a full battery and you use up 50 Kilowatts to transport yourself 50 miles.
- you then need to burn a 100 kilowatt to recharge your batteries (50% energy loss)
- so, you actually used up twice the amount of diesel to cover those 50 miles then you would have had to use if you'd run on the surface.

Conclusion: running submerged is very inefficient due to the way the recharge process works.

On the surface, applied physics would seem to indicate that running submerged on batteries would have less overall range than running on diesels alone, but this is not neccesarily the case.

While it's true that the energy in the batteries used for submerged travel is not "free" and has to be gotten from fuel reserves and that the fuel-diesel-battery-motor energy chain is not 100% effecient, it's still possible for battery / electric engine use to increase, not decrease the range of the submarine. I will try to explain using the most extreme case of diesel/electric mixed propulsion.

Version A: A stopped sub with 0% charged batteries will run the diesel engines strictly as a generator to charge the batteries. Once the batteries have a charge, the sub uses its electric motor to move 10nm.

Version B: A sub uses its diesel engines to drive 10nm.

Which version uses more diesel? You may be tempted to say Version A uses more diesel fuel since the recharging, eletric motor process has more steps and thus more chances for energy to be lost due to heat, friction, 2nd law of thermodynamics, etc but it is not neccesarily the case.

It is because the diesel engine does not have the same effeciency at all RPM! It is possible to charge the batteries at the RPM that is the most effecient for the diesel engine while maybe the best RPM for the diesel engine/ boat hull is not so effecient for the diesel engine. The convoluted fuel-engine-battery-motor process, despite having more steps CAN (in theory) be more fuel effecient than the diesel engine alone because of the variable effeciency of the diesel engine under various loads.


Now I am completely uncertain about the following two concepts:

1. Were real life WWII submarines more effecient under mixed diesel-electric propulsion compared to pure diesel? It's theoretically possible but was it actually the case? Unknown. I thought German U-boats benefit from the mixed propulsion.

2. Are WWII submarines as modeled by the game (vanilla, modded?) more or less effecient under either method? Again unknown.

MORE INFO AND CITE:

http://www.ossapowerlite.com/tech_li...efficiency.htm

Not all of the points made in this article apply to WWII submarines as they are designed, but there are plenty of valid points made that do.

[Tom C]

Frederf, I don't know about SH4 but in RL I think you're on the right track. Not sure about the S-boats, but AFAIK all the fleet subs had the diesels driving the electric motors, i.e. the diesels never drove the propeller shafts directly. Any difference in efficiency must lie in charging efficiency versus motor efficiency, and I don't know which was the bigger loss-- as you said, if they were different the diesel rpm could be optimized in either direction.

But, the US WWII sub hull and propellers were optimized for surface running, not submerged running. IMHO if you're not losing efficiency in the drive train, you're losing it in hull drag underwater, and you still won't get the same mpg as on the surface.

[Sailor Steve]

Originally Posted by Tom C

Steeltrap, did O'Kane say whether he kept all or some diesels idling (instead of shut off), in case he needed to move in a hurry?

I'm still playing the archetypal over-cautious early-war skipper and I see Japanese periscopes under every whitecap!

I don't know what the great one actually did, but engines completely off should be no problem, as the electric motors can accelerate you pretty quickly and you'll want to be diving in that situation anyway.

[-Pv-]

When I'm eating time in a patrol area waiting for targets to show up and I'm also in an area agressively patrolled by the enemy, I will use 22ft deck awash and set 3 knots diesel speed. This lets me loiter a very long time without giving up mobility, fast acceleration and nearly instant diving time. The wake is minimal at this speed.
Conservation theory asside, it's my practice to conserve very aggressively on the way to the 1st patrol area so I can spend what I've saved being more aggressive and evasive in the patrol area.
-Pv-

[Torpex752]

Theres is one more variable that none of us has considered. The ballast tanks. There were 2 ballast tanks that were routinely converted to fuel tanks to extend the subs range capacity. This ability was never "advertised" when discussing the subs range, so most posted data leaves this out.
So I'll see what I can find on its capacity and then maybe that amount should be added to our fuel supply? Just a thought.

Frank "Torpex" Kulick
Subsim Staff

[Tom C]

Hi Frank, are you still involved with PTC? I haven't looked in on that in years.....

I just happened to be reading a chapter in Galantin's "Take Her Deep!" last night where he mentions the fuel ballast tank conversion added 24,000 gallons.

I don't know but that sounds to me suspiciously like an explanation for why the fuel capacity for Gato/Balaos is sometimes given as 115,000 gallons (more or less) whereas Tambors (with almost identical hull, only 2 feet shorter I think?) is usually given as 93,000 gallons (again more or less), around 22,000 difference. What capacities does SH4 use?

[Frederf]

What effects on diving performance would having ballast tanks full of fuel would there be?

[NEON DEON]

Originally Posted by Frederf

What effects on diving performance would having ballast tanks full of fuel would there be?

I suppose you could partialy flood negative in order to compensate for the difference in weight between oil and salt water.

I think the first bit of fuel to be used if part of the ballast was used to store fuel would be the oil from the ballast. So by the time you got into it the ballast should not have fuel left anyway.

Just guessing.

[Tom C]

What effects on diving performance would having ballast tanks full of fuel would there be?

Galantin says, "On the surface the ship rode a bit lower in the water, as it was without the buoyancy of an empty main ballast tank."

So if anything, dive time might be a bit faster.

Just before that he says that the fuel ballast tank was always kept full until the fuel was used up, which makes sense because blowing and venting that tank would blow away the fuel. "As diesel oil was consumed, seawater admitted to the bottom of the tanks took its place. This created the apparent anomaly of a ship becoming heavier as it burned up its fuel supply, since the fuel was lighter than the replacement water."

[GT182]

Great info as always D. Unless it was needed I always kept my speed to what was most efficient in SH3. That ment checking the range often as related to the speed you were using. If lowering you speed ment a great range, that was the way to go. Or stop and filler up wherever you had the chance at a friendly base or "tender".

And to what Torpex say, would it be possible to make a mod to use these ballast tanks for fuel?

[Ducimus]

Originally Posted by GT182

And to what Torpex say, would it be possible to make a mod to use these ballast tanks for fuel?

*shrug* use stock fuel settings! All boats save the porpoise and S boat in stock have a 15,000 NM range at 10 kts. When it should be in the neighborhood 12,000 @ 10 kts. In TMaru i fudged it and gave in the neighrborhood of 12,000 @ 10.95 kts (may as well say @ 11 kts). That little fudging of the numbers i did, in action increased max range by 1000 to 2,000 NM, which is still less then stock.

edit:

As anyone coud guess, im a big stickler on fuel. Its an important gameplay aspect to me. That said, if someone links some hard evidence of how much extra fuel was carried in the ballast tanks, ill adjust for that in TMaru 1.5.

[Ducimus]

Meh, this threads worth a revisit:
http://www.subsim.com/radioroom/showthread.php?t=114746

Im wondering if i should just stop counting rivets and bring it back to stock, but it really bugs me not having to worry about fuel.

[Frederf]

I thought that having fuel in a tank that was designed to help surface and submerge this submarine would limit the captain's ability to do certain things, lest he risk blowing out precious diesel like it was bilgewater.