No speed gain after stopping battery recharge

[TorpX]

I was testing my mod set-up, to make sure everything was kosher, and did a little test with the battery charging speed.

class ____________ max. speed __________ w/ battery charge
S-18 ____________ 13.5 kn. ______________ 10.5 kn.
S-42 ____________ 12.75 ________________ 10.0
Porpoise _________ 20.0 _________________ 19.25 to 19.5 *
Gato ____________ 21.5 _________________ 20.5 to 20.75 *

* batteries charged at very slow rate

The speed for the battery charge was checked at 50% battery capacity, and again at 90%. That is why there are two figures for the diesel-electric boats. Note that while going at flank speed, charging was very slow for these boats (as we would expect). I tried the Gato at 1/3, and it seemed that charging didn't reduce that speed (as it shouldn't). This may explain the confusion over the matter.

The difference between 20 and 19.5, or between 21.5 and 20.75 may seem trivial, but it takes 8 times the power to obtain 2 times the speed, so the difference is greater than it appears at first glance.

[Sniper297]

"Typically there were 4 Diesel Generators on the boat (2 for the P Class)."

Can't believe everything you read on the internet, the Porpoise class had 4 main diesels, not 2. At some point they had a single long engine room for all four main diesels, later went to the forward and after engine room design, with a small auxiliary below the floorplates for topping off charges. One of the HOR equipped boats had all four main engines fail (drive gears were the main problem on the HORs) and limped home at 3 knots using only the auxiliary diesel to power the electric motors.

First batch of SARGO boats had a hybrid drive, forward 2 diesels standard generators, aft two direct drive. That proved to be so clumsy it actually reduced top speed while making maintenance a nightmare, so the last batch of SARGO class and every class after that were straight diesel-electric.

[ETR3(SS)]

Originally Posted by Sniper297

"Typically there were 4 Diesel Generators on the boat (2 for the P Class)."

Can't believe everything you read on the internet, the Porpoise class had 4 main diesels, not 2. At some point they had a single long engine room for all four main diesels, later went to the forward and after engine room design, with a small auxiliary below the floorplates for topping off charges. One of the HOR equipped boats had all four main engines fail (drive gears were the main problem on the HORs) and limped home at 3 knots using only the auxiliary diesel to power the electric motors.

First batch of SARGO boats had a hybrid drive, forward 2 diesels standard generators, aft two direct drive. That proved to be so clumsy it actually reduced top speed while making maintenance a nightmare, so the last batch of SARGO class and every class after that were straight diesel-electric.

Correct, got the facts mixed up in my head. P class was when they went to the 4 engine lineup. Cachalot was a 2 engine design. There was a lot of experimenting going on with propulsion in those days.

[Sniper297]

Yeah, when I first read about the diesel-electric concept (actually in a book about railroad locomotives) it made no sense, you're converting mechanical motion into electricity then converting electricity back into mechanical motion?! Oddly there isn't as much loss of energy as you would think, it's a LOT more reliable and durable than all those u-joints and clutches, and cuts way down on maintenance. Why they attempted the dual drive abomination on the first SARGO boats is a mystery, to me it should have been obvious after the success of the PORPOISE and SALMON class boats.

[TorpX]

Originally Posted by Sniper297

Why they attempted the dual drive abomination on the first SARGO boats is a mystery, to me it should have been obvious after the success of the PORPOISE and SALMON class boats.

I believe that there was a fear that the electric motors could be flooded out, rendering the boat immobile. But, as you suggest, once the diesel-electric proved itself, they embraced it completely.

Originally Posted by swdw

Actually, this is how it should be.

....

Surprised that, with all the other physics left out of the sim, this effect is seen at all.

Yes, I think this part works pretty well.

I don't remember the details, but in the past I tested the rate of recharge and fuel consumption. The conventional wisdom was that half the engines run at flank speed, so fuel is gobbled like crazy. While the engines may run at 'flank', I found the rate of recharge per hour, and the fuel consumed per hour, go down as the level of charge increases. Meaning less power and fuel is used to top off the charge than to start it. And, roughly speaking, the fuel consumed is proportional to the depletion of the battery. Meaning that charging a battery at 50% will take about twice the fuel, as charging one at 75%.

[swdw]

Originally Posted by TorpX

The difference between 20 and 19.5, or between 21.5 and 20.75 may seem trivial, but it takes 8 times the power to obtain 2 times the speed, so the difference is greater than it appears at first glance.

Actually, this is how it should be. Even though the boats were diesel electric, the load on the generator will affect the RPM. On a VERY basic level it's effect is similar to friction. Increased load makes the engines work harder to maintain RPMs. There's a point where the load is great enough, the RPM's drop off slightly. The harder you are charging, the more load you are generating, so the engines have to work harder to run the generators. This will drop your speed. Think of top speed difference in a car going flat and level and driving uphill. You can't reach the same top RPM going uphill, and it takes more fuel to hit the max RPM you can achieve under the increased load. Surprised that, with all the other physics left out of the sim, this effect is seen at all.