So how did a captain decide the tonnage of a sunken ship?

[Erich dem Roten]

I'm sure that ID books could be used to get a tonnage estimate, but to write down "5083 tons" seems a bit precise. I mean, if it was an attack at night or in bad weather, unless they could positively identify the ship it seems like it would be a best guess.

Looking back to a couple books I've read recently about subs, including Werner's 'Iron Coffins' and a book about the U.S.S. Tang (Richard O'Kane's boat) it seems many times firing solutions were made quickly and under tremendous stress. How historically accurate are the numbers we see today?

[AG124]

From what I've read, US tonnage estimates for sunken Japanese merchants in the Pacific were often wildly innacurate, usually being too high (possibly because US commanders were thinking in terms of Western merchants, whereas many Japanese merchants tended to be smaller than the Western norm, but I'm not sure).

In the Atlantic, as you noted, recognition manuals often provided identification of the ship sunk, and in many of the books I read, identification of a particular ship sunk in the Atlantic was quite often correct. However, you are correct in pointing out that in hurried actions and rough weather, identification was not always possible; I have always received the impression that identification during such incidents tended to be less precise and more inaccurate than normal (although I may be incorrect on that - I would myself be interested in hearing more on the subject).

As well, remember that although belligerant merchants ships maintained radio silence under regular conditions, once torpedoed, many that were still capable of operating their wireless sets sent out distress signals identifying themselves. Upon receiving the name of the vessel, U-Boat commanders or watch officers could check Lloyds or recognition manuals to find the ship's exact tonnage.

Hope this helps a bit.

[Dietrich]

If the "class" of ship was identified, then the "tonnage" was indeed a precise number. It is the registered cargo capacity of the ship. For some ships (such as the Liberty cargo ship), the registered tonnage was fixed and that type of ship was mass produced and easily recognised. However, there were certainly small and odd ships that would not have been identified, or for which exact recognition would be nigh impossible.

Oh... and just as a reminder... "tonnage" is a volume, not a weight nor a mass. Ref: http://en.wikipedia.org/wiki/Tonnage

[Thomen]

Originally Posted by Erich dem Roten

I'm sure that ID books could be used to get a tonnage estimate, but to write down "5083 tons" seems a bit precise. I mean, if it was an attack at night or in bad weather, unless they could positively identify the ship it seems like it would be a best guess.

Looking back to a couple books I've read recently about subs, including Werner's 'Iron Coffins' and a book about the U.S.S. Tang (Richard O'Kane's boat) it seems many times firing solutions were made quickly and under tremendous stress. How historically accurate are the numbers we see today?

They 'guesstimated' more often then not..

Originally Posted by Hardegen

The bridge was burning and it was therefore not possible to use the radio. He seems bigger than NORNESS, especially the superstructure on the stern. Amazingly long. He is surely 10,000 grt.

As Dietrich said, once the type and the dimensions are roughly known, they were usually not too far off.

[TarJak]

Mostly guess work and it is considered opinion of most historians that tonnage figures were fairly inaccurate and certainly inflated if nothing other than for propaganda reasons, let alone the rewards of medals etc.

one way of positively identifying a sunk ship though was if identifying debris (life bouys, rafts, boats or name plates etc,) were found after the fact. Additionally when survivors were either picked up or questioned by the sub crews information was gleaned as to the name, cargo and tonnage would have been fairly simple to get and there are plent accounts of encounters with survivors.

[Hitman]

All the above is correct:

1.- Ships were sometimes identified using recognition books, but more often through the last distress signal or debris that has the ship name (There are lots of pictures showing U-Boat commanders carrying life preservers of ships sunk) and finally also by asking the survivors.

2.- That said, the main method of constructing firing solutions did not involve a certain identification -unlike what you are forced to do in SH3 if you want to have a usable mast heigth for the stadimeter- but instead a guesstimation of the ship's dimensions. Here's how it was done for merchants: Cargo bays were counted (They can be seen as spaces between loading cranes) and then a certain figure was applied for each type of ship: F.e. small ships 1000 GRT per cargo bay, Medium ships 1250 GRT per cargo bay, big ships 1500 GRT per cargo bay. That gave a quick figure for tonnage, and also for ship's length, because 99% of ships have a certain proportion or ratio between length/width/keel depth/tonnage that is considered by naval engineers as ideal for stability and hydrodinamics. Ship length then allowed quick estimates of ship speed, but the primary and preferred method of getting speed and course was to follow a parallel course for some time, not necessarily at 90º AOB, but simply paralell, looking for a constant bearing.

I could write a lot more about this, and I really should do it somewhen because I'd like to share with everyone all the info I have been collecting for many years, but I lack the necessary time and have other priorities now.

[Jimbuna]

In addition to the above......if the victims name was uncovered there was always recourse to the Lloyds Register.

[deepboat]

Originally Posted by AG124

From what I've read, US tonnage estimates for sunken Japanese merchants in the Pacific were often wildly innacurate, usually being too high (possibly because US commanders were thinking in terms of Western merchants, whereas many Japanese merchants tended to be smaller than the Western norm, but I'm not sure).

In the Atlantic, as you noted, recognition manuals often provided identification of the ship sunk, and in many of the books I read, identification of a particular ship sunk in the Atlantic was quite often correct. However, you are correct in pointing out that in hurried actions and rough weather, identification was not always possible; I have always received the impression that identification during such incidents tended to be less precise and more inaccurate than normal (although I may be incorrect on that - I would myself be interested in hearing more on the subject).

As well, remember that although belligerant merchants ships maintained radio silence under regular conditions, once torpedoed, many that were still capable of operating their wireless sets sent out distress signals identifying themselves. Upon receiving the name of the vessel, U-Boat commanders or watch officers could check Lloyds or recognition manuals to find the ship's exact tonnage.

Hope this helps a bit.

A while ago I copied some pages from 34thflotilla.com of some ship regognition profiles. Some warships and merchants. I found this handy to look at as some times there wasn't enough time to call up the manual when the scope was up and I was surrounded by targets. Is there a pdf file somewhere that I can download which has more ship's profiles and tonnages as the link the 34flottilla.com doesn't seem to work.

[Erich dem Roten]

Thanks for all the replies, I figured if the submariners knew the name of the vessel they sank then an appropriate figure could be easily obtained. I guess what bugs me is when an attack is made under duress, where a positive identification is impossible and the sub immediately (or shortly thereafter) submerges and begins evasion maneuvers yet often reports a "solid" number in terms of tonnage sunk. I would expect the majority of the reports to be estimations that ended evenly (10,500, etc.) rather than the precise numbers I've seen on places like uboat.net. Everyone brings up very valid points though; I just enjoy the discussion.

[Klaus_Doldinger]

Just one more question about ship identification, concerning nationality:I suppose that the recognition books included the nationality of the ship, i.e., you didn´t need to observe the flag to determine if your potential target was belligerent or neutral. In fact, this is simply impossible in the game most times at night or at great distances. Am I right?

[Ping Panther]

I had read a very detailed book from a Pacific fleet submariner a while ago that indicated that it was very common to surface right after a merchant sinking. Primarily, this surface contact was made for establishing a very accurate ship-kill report while on patrol. The sub captain would verify tonnage, cargo, and in particular the ship name/I.D, route from port-to-port info., etc. The author indicated that most of the estimates of sub captains of ship tonnages that were not checked on at sea, were often greatly exaggerated in the initial sub patrol records. Usually, many post-war verifications of war-time shipping logistics records from merchant ship home port offices uncovered many of these inflated battle statistics.

[Sailor Steve]

Originally Posted by Dietrich

If the "class" of ship was identified, then the "tonnage" was indeed a precise number. It is the registered cargo capacity of the ship.

Exactly so. Ships could be identified from the ID books available. I actually copied this note from a cargo ship I was researching:

Empire Explorer: Converted from liner Inanda. What did it look like? U-boat captain who sank her reported sinking Inanda, which means that A) his ID book contained precise information, and B) the ship still looked enough like her old self to be recognizable. Details - 75 survivors were rescued by a single MTB.

For some ships (such as the Liberty cargo ship), the registered tonnage was fixed and that type of ship was mass produced and easily recognised.

Actually that's not quite true. The GRT (Gross Registered Tonnage) was actually the number registered with the insurance company, and indeed it never changed. However, even the mass-produced Liberties varied a lot, and I'm still not sure why. American Liberties ranged from 7176 tons all the way up to 7240, and the British ones went from 7219 to 7255. That said, they were confined to a few specific numbers within those ranges, and were not random.

One other thing to remember is that a lot of reported tonnages were indeed wrong. The exact tonnages we get in the game should probably be considered the number arrived at after BdU confirmed the sinking via records obtained before the war, or the number confirmed after the war. Unfortunately, as with 'Weapons Officer Assistance', the numbers are probably a lot more precise than what they knew at the time.

[harzfeld]

Whenever there were two same class/type ships sunk but reported both of them in different tonnages, I assume U-boat captains measured waterline between target's draft marking of their the hull. Here's link, & look at picture of draft marking.

http://reference.findtarget.com/search/draft%20(hull)/

So you see if a ship carry nothing, then waterline would be much lower, but if carry full of oil, then waterline would be much higher. But for storms, captains can still estimate close enough since storms usually rock ships with empty or light cargo more compare to ones with heavy or full of cargo.

[Sailor Steve]

I highly doubt that draft markings would be visible through a periscope, and especially not at night. The ID book would give the official tonnage for the class, and specific tonnage could be determined from loss reports.