Decks awash: A scam?

[IonicRipper]

I was reading this article... Here's what they said about decks awash:

"If the center of gravity and the center of buoyancy are coincident and if a wave were to hit the boat from the side, capsizing could occur. Those Hollywood movies where a submarine is shown with decks awash, making an approach on the surface is mostly fiction. That is about where the two centers become coincident and it doesn't take much to roll the boat over."

Apparently (according to this article) they never used decks awash in WWII.
Can someone confirm? (Or refute.)

The article in question: http://www.subvetpaul.com/Flt_Class_Sub.html

[Cybermat47]

I'm not sure if the U-boat aces had decks awash, or just stayed on the surface. And is the article talking about USN Fleet Boats and U-Boats?

[Sailor Steve]

They're talking strictly about US boats, but the problems would be about the same either way.

I know that night surface attacks were common, but I have no clue about decks awash. It will be interesting to see what comes out of this.

[troopie]

I've read the same thing elsewhere from a (now forgotten) different source. I believe it, but can't get my head around the physics of it; surely the boat would be more top heavy the higher it is out of the water? Personally though, I never use decks awash; feels like a cheat to me. Surely the increased drag travling that way would've been undesirable? Also, I'm unsure If any in-game advantage is gained, I mean, do other units register that you're sitting a little lower in the water, or simply that you're surfaced?

[Sailor Steve]

Originally Posted by troopie

...surely the boat would be more top heavy the higher it is out of the water?

When the tanks are empty the major weight involved is the keel, which is designed to keep the ship upright. With the ballast tanks full the boat now has more than 350 tons of water on board, most of it in outboard tanks level with the boat's normal waterline. Once submerged the center of gravity is balanced, but surfaced it looks like any serious ocean motion could possibly be catastrophic. I don't know if that's true or not, but now that I think about it I see how it could be possible.
http://www.maritime.org/fleetsub/chap4.htm

Personally though, I never use decks awash; feels like a cheat to me. Surely the increased drag travling that way would've been undesirable?

Running all over the ocean with no penalty is certainly a cheat, since the game allows it to work when in real life fuel consumption would rise dramatically. The main idea would be to use it only to lower the profile in a night surfaced attack, and to facilitate diving quickly in the same situation. Also the game assumes you're surfaced, and since you have no control over which engines you use you are automatically running on diesels, when part of the point in a surfaced attack is not to be heard as well as not being seen you would certainly be running on batteries in that situation.

Also, I'm unsure If any in-game advantage is gained, I mean, do other units register that you're sitting a little lower in the water, or simply that you're surfaced?

I don't know that one either.

[Claves_Mortis]

Originally Posted by troopie

Also, I'm unsure If any in-game advantage is gained, I mean, do other units register that you're sitting a little lower in the water, or simply that you're surfaced?

For every surface attack I will go to a depth deep enough to just allows me to get on the bridge / UZO. I am doing this because obviously everything "on top" of a wave / the water is affected more by the waters movement than everything "inside" the wave / water is, meaning that if you are as deep as possible (while still being able to use the UZO) you can use the advantage of the x7 magnification while reducing the shaking part caused by waves, being able to do more precise usage of the UZO.

I do not know if that is historicaly correct, but in Silent Hunter 4 it does work (for the more steady view). If you do live at the sea you can easily try that yourself: get into a small, light (somewhere around 2-3 meters in length will be a size where you can easily realize it) without any equipment and only yourself in it - shaky. If you get your outbord motor, your gas tank, anchor, chord, some friends and maybe some water in your boat, resulting in lying deeper in the water the waves won't shake the boat as much anymore and the boat will also be less affected of your movement (leaning from left to right for example), it will still move, but slower and more predictable.

I also have no idea if it affects the ability of how easily the submarine is spotted by other ships in the game, but as how far your periscope does stick out of the water does affect the chance of being spotted (I am pretty sure about that, but it's only what I feel / experience, no written prove for that), I could imagine how far the tower is above the water does count too.

[IonicRipper]

I remember conducting a surface attack on a lone merchant on a fairly clear night with decks awash at approximately 1200 yards. My first shots missed (i think i miscalculated the torpedo depth) but he never spotted me or the torps so it gave me a chance to recalculate and ultimately, skin him. I should have saved the game and try the same thing fully surfaced to see if would have spotted me. I might try that with the mission editor some time today. I'll keep you guys updated.

[Bilge_Rat]

That is certainly an interesting question and one which has puzzled me for a long time.

Running "Decks Awash" in the sense of having the decks underwater with only the Conning Tower above water does not appear to have been used in attacks. Blair's three books on ww2 sub warfare does not mention an attack with "decks awash" (i.e. decks underwater). The Official manuals, i.e. the 1943 German sub commander's handbook and the 1946 U.S. Fleet submarine manual also do not discuss this attack tactic.

Without delving into the difficulty of maintaining a sub level in the open ocean with decks underwater, there would also appear to be little tactical advantage. A submarine is already hard to spot. The hull which is only a few feet above water is very hard to spot. The Conning Tower is also hard to spot from the lookout station/deck of a typical freighter since it usually is below the horizon. There would not appear to have been a big advantage in concealment in running with decks under water.

There also seems to be a confusion as to what "Decks Awash" means. Some submarine which were recorded as "running decks awash" had their decks above water, but had some seawater already in their ballast tanks so they could dive more rapidly. Their decks would be above water, but lower in the water than normal. For example:



HMS B11 running "decks awash".

http://en.wikipedia.org/wiki/File:HM...ecks_awash.JPG

I suspect most submarines which recorded in their logs that they were running "decks awash" actually had their decks above water.

My (admittedly limited) understanding of this is that the author may be correct in general he is probably wrong about the particulars. He seems to be talking about the effects of metacentric height (GM), essentially the vertical difference between Centre of Gravity (CG) and the Metacentre which is related to the interior volume of the vessel. Centre of Buoyancy (CB) will be below CG in a stable floating vessel. High values for GM generally mean stability in roll.

The author seems to imply that when a submarine is running decks-awash, the CG=CB and therefore roll stability is lost. But since the boat retains positive buoyancy, this is impossible as the interior and the unflooded surface areas of the ballast tanks remain free of water. The boat may be more sensitive to roll but sufficient reserve buoyancy and a positive GM should exist to prevent capsizing.

I recall that both Prien mentioned running decks awash in Scapa and Kretchmer used the technique in night surface attacks when conditions were favourable. It was common to flood down when loading external torpedoes into the boat as well, a difficult evolution in any sort of seaway.

Paul Schratz in Submarine Commander mentions his captain flooding down his Fleet Boat to facilitate rescuing pilots while life-guarding but I don't recall him mentioning the prevailing sea state.

For more on ship stability and metacentric height see:

http://en.wikipedia.org/wiki/Metacentric_height

http://www.gwpda.org/naval/gmdefn.htm

http://www.navweaps.com/index_tech/tech-009.htm

[Bilge_Rat]

I rechecked this morning, but in Blair's U-boat book, vol.1, Prien stated that he was on the surface when he went into Scapa Flow, no mention of "decks awash".

You can find many photos of subs with flooded decks when they were in a static state in calm water, i.e.:



http://www.history.navy.mil/photos/s...sh-s/ss105.htm


USS Chivo moored with decks awash:



http://www.navsource.org/archives/08/08341.htm.

I rechecked this morning, but in Blair's U-boat book, vol.1, Prien stated that he was on the surface when he went into Scapa Flow, no mention of "decks awash".

Gerald Snyder in The Royal Oak Disaster mentions Prien flooding down some to increase the effectiveness of U-47's rudders when entering Scapa (pg 87). Without context as to "normal" trim, it's impossible to know how deep this might have been or whether the decks approached being "awash".

This was the passage that I imperfectly recalled in my post.

Still, it demonstrates that trimming down the boat had uses and whether negative trim brought the decks awash or not is probably based on the specific situation and captaincy decision logic.

you also have to take into account the dive plans and forward motion anchor the stability of the boat so it wont want to tip over.

its just like the fishing boats that use outrigger planes to make them stable and not wobble around out there


as to running decks awash it was purely used to present less of a target in a situation where they would be spotted or come under fire so the idea is to get the best underwater speed you can by keeping the tower out of the water yet keep the stealth of being "mostly" out of sight.

i think it was a tactic that was rarely used and only under the right weather conditions

[IonicRipper]

I made a quick single mission to test if the game AI would see a difference with and without decks awash.

Fully surfaced, the enemy ship spotted my sub at around 2400 yards.
With decks awash they spotted my sub at around 1400 yards.

So the game does simulate this, which is a nice touch.

[Armistead]

As far as US boats, the term was used by some to define running heavy ballast, but mostly it was a term used in diving to denote decks were awash as the sub headed down.

I know I've read a few reports of Skippers running heavy ballast and remarking it lowered their profile some, course, they just wanted to get under quickly if needed.

In game, it does seem to have some effect on the height of enemy radar detection, but also depends on what mods you use. It does impact visuals as well if you dive deep enough where only you can man the bridge, but the reason is simple, if you're men aren't on the bridge with you, the game sees your sub as dived, electric engines on,..you're now a sonar target, but your scope can be spotted.

[AVGWarhawk]

I do not see anywhere in this thread conversation on the batteries as ballast. The batteries were as big as Volkswagen Beetles. There was fore and aft batteries. It would take quite a bit to roll the sub.