Fuel Usage VIIC/41

[Osmium Steele]

U-297 Left Bergen on 25 November, submerged to snorkel depth fairly soon after leaving port and proceeded westward around the Orkneys then south to the north scottish coast. She was sailing westward when she was attacked and sunk on Dec 6th, by a Sunderland dropping 6 depth charges.

Here's the question. How much fuel would she have used to that point? And would she have used enough to empty one of the fuel oil tanks in the saddle tanks?

My model is going to depict the boat as she was just before the DCs hit, and I'm wondering if I should leave the lower saddle tank vents closed or open them.

Any help would be greatly appreciated.

Os

[TLAM Strike]

Aren't the vents always open? Since fuel oil is lighter than water dosn't it just sit atop the water as the water slowly fills the tank as the oil is used up?

I know the Russian Foxtrot had open fuel tanks.

[Osmium Steele]

Originally Posted by TLAM Strike

Aren't the vents always open? Since fuel oil is lighter than water dosn't it just sit atop the water as the water slowly fills the tank as the oil is used up?

I know the Russian Foxtrot had open fuel tanks.

I read an excerpt of a biography of an old uboat mechanic. He said when they decided to fill the tank with water after the fuel oil was gone, they opened the vents and heard loud clanging as the boat rocked. The screw (worm gear) that opens the vents had broken, and the vent was bouncing against the pressure hull and wall of the MB/FO tank. He had to crawl in the empty tank and link the loose ends together to keep the vent closed. I assumed from his account that the tanks were kept closed until empty.

The lower vents are huge and would likely cause some turbulence in the tanks as the boat was underway. This would inevitably cause some mixing of seawater and oil, especially in a tank filled with oil.

Is the foxtrot directly vented to the sea, or is it a rudimentary seawater compensating system like modern boats' diesel tanks?

On another note, every pic I've ever seen, except one, of a boat out of the water had the vents closed. I can think of myriad reasons to keep them closed while exposed that way, but still. You'd think there'd be a few of them lying around.

I tell ya, researching a boat of which seemingly no pictures exist, and precious few of her sister ships, is a pain in the butt.

[Penguin]

As I am at work atm, I am talking out my buttocks now - ok, would be the same at home since most of my books are still in boxes

The lower vents of the saddle tanks were shut as long as they were used as a fuel tank. The consumed fuel was replaced by sea water, which was pumped in from the diving tanks. The reason for keeping shut is simple: the boat movement. If she rolls during rough weather, the vents might come out of the water and fuel could get lost. When the tanks were empty, they were rinsed out, then used as a dive tank. I read a veterans report, that this could only happen when the sea was a little rough, sadly he didn't ste why this is the case. When used as a dive tank, the lower vents were always open, just as the lower vents of the "normal" dive tanks.

So let's check out the facts about fuel consumption: most easy to remember is the range at 12 knots: 12000 km or 6500 NM. At 10 knots we have a range of 8500 NM.
This interesting document gives us the RPM at whioch the engines run at different speeds: http://www.uboatarchive.net/Manual.htm. (page 14)
So we have 275rpm at 10 knots (langsame Fahrt), 340 at 12 (halbe Fahrt). The fuel capacity for the boat are 117 tons (metric), with 40 cbm or 43 tons in the saddle tanks.
So at 340 prm during surface ride, we have 1.8 tons per 100 NM, at 275 1,37 tons per 100 NM.

During a peaceful cruise the boat would never have burned 1/3 of her fuel during this short distance. As anybody who played SH in 1944 remembers, a Feindfahrt during this time is nearly constant snorkeling or constant dolphin moves.

The interesting question would be, at which rpm the Diesel engines run while snorkeling, though it is highlx unlikely that they operate higher than 340 (halbe Fahrt) - as one could not use the Schnorchel at higher speeds than 6,7 knots. At this rpm she could sail about 2000 NM alone with the fuel from the saddle tanks.

Overall I would say that it is highly unlikely that U297 burned all the fuel stored in the saddle tanks and had the time to rinse them out - so I'd leave the lower vents shut

Last but not least, I found something about U297: http://www.militaria-depot.de/u-297-...e=militaria_de
This was stuff from the 2.WO, though there seems to be no pic of U297, we find out that she was declared MIA on january the 3rd. There's also a document signed by Aldegarmann.

Oh and as the nitpicking guys we Germans are, I must also make a correction: U297 was stationed in Horten (near Oslo), not Bergen

[Osmium Steele]

Originally Posted by Penguin

Oh and as the nitpicking guys we Germans are, I must also make a correction: U297 was stationed in Horten (near Oslo), not Bergen

HAH! I caught that after I posted, but didn't think I'd get called on it.

Thank you for the rest, Penguin. And I just updated my build thread.

EDIT: That document sight was fantastic. AARRGGHHH I wish I had taken german in school.

[Bilge_Rat]

on fuel usage, remember the U-297 in late 44 would have travelled almost the whole way underwater, so the surface fuel consumption is not really applicable.

second, from what I read in Blair, U-Boats only used snorkels to recharge batteries, say around 4 hours out of every 24 and travelled the other 20 hours on batteries, so the fuel usage would be even less.

a similar boat, the U-299, left Trondheim 24/01/45 and came back 82 days later on 15/04/45. U-297 was only out about 12 days, so it may not have used up much fuel.

[TLAM Strike]

Originally Posted by Osmium Steele

Is the foxtrot directly vented to the sea, or is it a rudimentary seawater compensating system like modern boats' diesel tanks?

I think the Foxtrot is vented directly in to the sea, there was a Foxtrot during the Cuban missile crisis that in heavy seas off Kola lost most of its fuel because of the heavy rolls causing the fuel to leak out from the bottom of the tanks. The sub returned to base on one engine burning a mix of the remaining diesel, sea water and lubricating oil.

[Penguin]

Originally Posted by Bilge_Rat

on fuel usage, remember the U-297 in late 44 would have travelled almost the whole way underwater, so the surface fuel consumption is not really applicable.

That's why I tried to use the engine rpm as a crutch to calculate the consumption

Originally Posted by Bilge_Rat

o
second, from what I read in Blair, U-Boats only used snorkels to recharge batteries, say around 4 hours out of every 24 and travelled the other 20 hours on batteries, so the fuel usage would be even less.

You are absolutely right. As I am a sucker for battery capacity, I transfered my own gaming style to the real world. That's why I usually sink with a big hole in the hull, but full batteries

[Osmium Steele]

OK. Closed vents it is!!

Thank you gentlemen.

[Bilge_Rat]

of course, the other unanswered question is whether U-297 had full fuel tanks when it left port.

In late 44, Germany was desperately short of oil and the Luftwaffe and Panzers had priority over the Kriegsmarine.

[Osmium Steele]

AAaaarrrggghhhhh!!!!

Stop throwing monkey wrenches into my potato salad!!

Next you'll be telling me that paint supply problems mean she might not have been freshly chipped and painted after her final training cruise; thus, went to sea all rusty and cruddy.

[Bilge_Rat]

Originally Posted by Osmium Steele

AAaaarrrggghhhhh!!!!

Stop throwing monkey wrenches into my potato salad!!

Next you'll be telling me that paint supply problems mean she might not have been freshly chipped and painted after her final training cruise; thus, went to sea all rusty and cruddy.

I hope you will post pics when the model is finished.

[kraznyi_oktjabr]

Originally Posted by Osmium Steele

AAaaarrrggghhhhh!!!!

Stop throwing monkey wrenches into my potato salad!!

Next you'll be telling me that paint supply problems mean she might not have been freshly chipped and painted after her final training cruise; thus, went to sea all rusty and cruddy.

*monkey wrench inbound*

Good question actually. According to uboat.net U-297 arrived from its training cruise in 31 Oct and departed to its patrol in 1 Nov. I doubt that crew had time for anything else but critical maintenance.

EDIT: Ooops... my bad it says "front boat" not patrol. What that actually means?
EDIT2: I missed that 25 Nov date when I first read you original post. Found that date later myself too after little googling.

[Dowly]

Originally Posted by kraznyi_oktjabr

EDIT: Ooops... my bad it says "front boat" not patrol. What that actually means?

Put into combat duty, I think.

[TLAM Strike]

Originally Posted by Osmium Steele

Next you'll be telling me that paint supply problems mean she might not have been freshly chipped and painted after her final training cruise; thus, went to sea all rusty and cruddy.

I just added some rust to my Type XXIII and Type VII U-201 models and it does add "character".