SUBSIM Radio Room Forums - Decks awash: A scam?

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The official U.S. Navy Fleet Type Submarine manual contains basically the same info as the article the OP linked to.

To summarize, a surfaced submarine with dry tanks or a submerged submarine with flooded tanks is stable.

In between, a submarine with partially flooded tanks will reach a point where its "center of buoyancy" and "center of gravity" will coincide. At that point it is unstable and could potentially capsize. When a sub is diving or surfacing that point is passed quickly, so it is not a practical issue, but it could be dangerous in a "Decks Awash" situation:

http://www.maritime.org/fleetsub/img/fig5-07.jpg

Quote:

At A, the vessel is floating with all water excluded from the tank surrounding the chamber. The center of gravity is at G and the center of buoyancy, B, is found by intersecting diagonals of the displacement.

At B, water has been admitted to the lower section of the tank. Using the diagonals as before, it is seen that the center of buoyancy, B, is now coincident with G and the unit is unstable.

At C, the surrounding tank is flooded and the unit is submerged. The center of buoyancy is at B2, the intersection of the diagonals of the displaced water. The unit is stable, the center of buoyancy and the center of gravity are in the same vertical line. Any rotational movement about the center of buoyancy B2 immediately sets up a restoring moment arm

full discussion here at pp. 55-61:

http://www.maritime.org/fleetsub/chap5.htm#5B

In addition, when you look at the operation of the tanks during a dive, you can see the difficulty in maintaining a stable "Decks Awash" situation:

Quote:

The weight taken on is water, and it is flooded into tanks. The air, of course, is vented off the tanks as the water flows in. First, the large tanks, known as main ballast tanks, are flooded. These tanks hold 359 tons of sea water. (See Section 4A2.) The submarine now displaces 2,109 tons and draws approximately 22 feet of water. The main deck is not awash, since there are approximately 2 feet from waterline to deck. The ship still has plenty of positive buoyancy. Since the bow buoyancy tank vent has been open during this operation, allowing this free-flooding tank to take on ballast as the ship submerges, it is necessary to add to the displacement the weight of water taken on by the bow buoyancy tank (which belongs to the special ballast tank group).

This gives a new total displacement of 2,141 tons (2,109 tons plus 32 tons).

Simultaneously with the flooding of bow buoyancy, the safety tank also in the special ballast tank group, is flooded. This tank holds 23 tons of water, giving a total displacement of 2,173 tons and a draft of 24 feet. The decks are just awash, and some positive buoyancy is still retained, although the submarine is approaching a condition of neutral buoyancy. Two things remain yet to be done: 1) to take on additional weight, and 2) to distribute this weight so that fore-and-aft athwartship balance is maintained. This additional weight is added to the variable ballast tanks and distributed throughout the variable tanks by the trim system. With the ship in this condition, approximately 55 tons of water must be added to the variable tanks to submerge to a depth where the periscope shears are even with the waterline. The ship is not in a state of neutral buoyancy and is balanced both fore-and-aft and athwartship. At this point, any additional ballast taken on will cause the submarine to submerge; any ballast removed will cause it to rise (Figure 4-1).

http://www.maritime.org/fleetsub/chap4.htm#4B

To summarize, to get the sub to a "Decks Awash" state in a dive, the main ballast tanks, bow buoyancy tank and safety tanks are flooded. However, the bow buoyancy tank is located in the bow and causes the sub to become nose heavy to make a dive easier. To maintain level "Decks Awash", the bow tank would have to remain dry and certain other of the variable ballast tanks flooded so that the sub is balanced both along its fore and aft and side to side axis. It is certainly theoretically possible, but would appear difficult to do in anything other than calm waters.

Again, I suspect that when a skipper wanted to lower the sub's silhouette, he would just flood the main ballast tanks which would bring the deck down from 10 feet to 2+ feet above the waterline. In the open ocean, that would usually result in waves breaking over the deck so the decks are "awash".

Quote:

Originally Posted by troopie (Post 1979617)

@Bilge Rat: love your Avitar mate! Staunch, grumpy, alcoholic; I should have Captain Haddock as my Av. :haha:

It seemed the right persona for THIS forum....:ping:

waking up this old thread since I still don't have a clear answer on what is "Decks Awash".

I found photos of U-569 during the attack AVG posted about above (post #17). The Allied pilots had reported the sub was running "Decks Awash". That would have been a bit before the bottom picture was taken which shows the sub diving (no crew on deck) and 2 DCs dropped by the aircraft to the right of the photo.

You can see the deck was still flush with the level of the sea which would seem to indicate that the deck was a bit above sea level when it was running "Decks Awash".

http://www.uboatarchive.net/U-569DBs.jpg

Personally I never run decks awash as I can normally get close to merchants at night fully surfaced.

Quote:

Originally Posted by Bilge_Rat (Post 1980418)

It seemed the right persona for THIS forum....:ping:

DAMNATION! I think yer on to something here!:Kaleun_Salivating: I only use decks awash to facilitate a swifter dive against any and all unforeseen opponents, especially after the skyblue color change of ASW aircraft to sky blue which shortened the the U-Boot con lookouts warning time considerably- Black MAY. I do miss the 'pitter-patter' of crew's feet when crash dive is ordered (SHII) into the bows to increase bow-plane 'bite' in the maneuver.:hmm2:

Quote:

Originally Posted by Bilge_Rat (Post 2140809)

waking up this old thread since I still don't have a clear answer on what is "Decks Awash".

It is hard to pin down, isn't it.

I have always been skeptical that "Decks Awash" could be taken literally. I can believe that a sub with 8 ft. of freeboard might be flooded down to 4 ft. or even 2 ft., but it seems to me there would be problems with trying to cut it close.

Even setting aside questions of stability and the danger of capsizing, what is the main advantage of making a surface approach?
speed (clearly the biggest advantage)

better sighting?

"Decks Awash" can only reduce speed, and will not improve the sighting. It is thought that the submarine is much less visible, but it is not clear to me that this is so. Even if one is actually moving with the waterline at your deck, your tower is as visible as before, and you would have waves continually breaking over your deck so that it would be somewhat visible, to say nothing of your wake. I suppose you could sit still bobbing in the water, but without the ability to move, this is a very poor trade off.

Quote:

...and you would have waves continually breaking over your deck so that it would be somewhat visible

Your point is a good one. I make an effort to get into position long time in advance in order to cut my engines in a surface attack. I believe it is easier to see the water breaking than it is to see the subs hull.

I like to add this point to whether decks awash was used as an approach tactic. Any type of approach would have been practiced and drilled to perfection if it was ever to be consider in the bag of tricks. Do we have evidence of that being the case?

Quote:

Originally Posted by TorpX (Post 2141147)

"Decks Awash" can only reduce speed, and will not improve the sighting. It is thought that the submarine is much less visible, but it is not clear to me that this is so.

The only advantage I can see to "decks awash" is an improved dive time if you're spotted. Since you're already at least partially flooded the boat will get down quicker, but I don't see that big a difference between 45 seconds and 30 seconds, so I've never seen a point to it. You also aren't going to cruise in that condition, because fuel consumption is going to go up quite a bit.

Quote:

Originally Posted by Sailor Steve (Post 2141254)

The only advantage I can see to "decks awash" is an improved dive time if you're spotted.

I should have listed that, however, when I read "decks awash", I generally assume that they mean something more than just "riding the vents", but perhaps I shouldn't. Hard to really say without specific information. And, as you say, the difference between 45 and 30 sec. seems rather small.

Quote:

Any type of approach would have been practiced and drilled to perfection if it was ever to be consider in the bag of tricks. Do we have evidence of that being the case?

That's a good point. USN submarine attack doctrine doesn't seem to mention it.

I just skimmed the U-boat commanders handbook at hnsa.org, and there is nothing about "decks awash" there either. Note that the main danger of being sighted, is considered to be that of having the conning tower sticking up above the horizon, or being silhouetted against the moon/moonbeam. It's hard to see how flooding down 5 or 6 feet helps much in this regard. I guess it would make you harder to hit, if you are spotted, though.

Quote:

I. Danger of Being Sighted.
199.) As a matter of principle, the submarine commander should bear in mind that the submarine is always more difficult to see at night than any surface ship, unless the conditions of light are exceptionally unfavorable. The reliance of the commander on the invisibility of his boat at night increase with each new experience. Every contrary feeling must be overcome by the consideration that the enemy whom the submarine is attacking, being on the defensive, is in a weaker position, more especially as his lookout, in consequence of land gruelling periods of duty, is in no position to keep as good a watch as the submarine, which, at the moment, is concentrating all its energy and attention on the development of the attack.

200.) The difficulty of detecting the submarine at night on the surface due to its long and low silhouette, since it disappears almost entirely in the water, even including the conning tower. The conning tower can be most easily detected by the enemy, when it rises above the dip of the horizon, from the line of sight of the enemy. This is the danger zone for the approaching submarine. Against the background of the sea alone, the conning tower is very difficult to make out.

201.) The conning tower of the submarine always appears as a darker object, both in the dip of the horizon and against the background of the sea, and even on the darkest night. In our latitudes, the most suitable color for the conning tower, according to the experiences gained, is a light grey or a dull white-grey; in the Atlantic, a dark blue-grey.

202.) In view of the fact that paint, especially on the wet submarine, is liable to reflect the light (shine), care must be taken not to show the enemy the moon-lit side. If the circumstances make this unavoidable, the tapering silhouette should be turned toward him as soon as possible.

203.) Take care that the submarine does not appear in the track of the moon on the water; i.e., in the line between the moon and the target.

204.) Favorable conditions of attack, enabling the submarine to remain unseen:

a) Attack the enemy when he can be seen against the light horizon, or against the moon, and move toward him from the direction of the dark horizon, or the dark portion of the sea. In this case, the submarine itself is invisible even at the shortest distances from the enemy.

b) Go in with the sea, from windward, in order to reduce the head sea, which may well attract attention, especially if the sea is calm. For the same reason it is advisable to proceed at low speed when close to the enemy. This will also have the effect of reducing the stern sea, which is apt to betray the submarine if the water is smooth. In addition, the windward side has the advantage that it renders observation more difficult for the enemy, more particularly in a strong wind, or during rain.

c) During the attack it is always necessary and correct, to approach the enemy, up to the point of discharging the torpedo; i.e., of turning to fire the torpedo, in such a way as to show him the narrow outline of the submarine. The head sea and the stern sea then merge into one, and the form of the body of the boat itself, which might betray itself by contours of foam if it presented a larger surface to the enemy, is then sure to be invisible. A favorable method of attack is therefore to keep the submarine in the narrow position, and keep on turning, to show only the narrow outline of the submarine the approach by the "dog course" ("Hundekurve").
d) Caution should be observed during the transition from night to day, on account of the rapid alteration of the range of vision.

Quote:

Originally Posted by Sailor Steve (Post 2141254)

Realistically, I agree. I can it more running heavy ballast, not really decks awash, maybe 22 feet as not to effect speed, but get me down faster.

If you use one of the radar fixes in game, running decks awash may keep you under the radar.

two questions I saw are:

1. what is "Decks Awash"? In game, you can run at flank speed with the decks completely under water and just the conning tower showing. However, it seems that was impossible in RL as discussed above. "Decks awash" seems to be having the MBT flooded so the deck level is lowered, but still above water.

2. As to when it was used, the advantage would be making the hull and tower harder to spot and lowering diving time. As a tradeoff, speed is lowered making it harder to escape on the surface. This is modeled in game, if you order a depth of 25 feet which basically brings the deck to just above water, flank speed is cut from 21 to 13 knots.

In RL, I could see two situations where "Decks awash" would be used. Either a night surface attack or as in U-569's case above, in daylight where there are a lot of aircraft around.

Decks awash certainly cuts speed down in my set up. If I use it, I don't go as deep to cut the engines off and my crew is on stations. I know many go deep enough that the electrics cut on, but technically the sub is dived, just you can man the bridge yourself.

Seems my speed at flank is about 11 kts.