DECKS AWASH (DA), as used here, means to operate at the deepest depth that the diesels will run continuously . The advantage in doing this is that, when compared to being fully surfaced, much less time is needed to disappear below the waves. If a threat appears, go to DA early and avoid being pounded (or sunk) as you wait, and wait, and wait, to submerge.

Running DA is mainly useful to avoid air threats. If battery recharging is critical and aircraft might be near you can safely recharge until the aircraft are spotted before quickly diving. Or, if you have radar, you can surface to DA and check for enemy aircraft before coming completely up with the comfort that if aircraft are present, they can still be avoided with a quick dive. With practice you will find other times DA is a lifesaver (rescuing downed airmen with enemy guns nearby, etc).

Two important points must be remembered, to use DA successfully. First, the critical depth for running at DA is often different between classes of submarine. Second, the critical depth for the diesels is usually deeper than that for CO2/O2 exchange and using the deck gun. This means CO2 can build up inside the sub. So there is a second DA to remember, DA(CO2). It, too, varies between sub classes but as a rule, it's 2-3 feet less than DA (diesel). DA(CO2) is always the same as the critical depth for using the deck gun, or DA(Gun), so the two can be combined.

Below is a guideline which lists the critical depths for DA(diesel) and DA(CO2/Gun) for the different classes of sub in SH4. [Measurements were made at flank speed, in calm seas, with 100% battery charge.]

DECKS AWASH DEPTH IN FEET FOR SUB TYPES

TYPE BOAT.........DA(diesel)........DA(CO2/GUN)
GATO.....................28...................26
BALAO....................28...................26
GAR.......................32...................26
PORPOISE...............28...................23
SALMON.................31...................26
NARWHAL...............30...................28
TAMBOR.................33...................25
SARGO...................31...................28
S-42......................26...................26
S-18......................26...................26

As a rule, 28 feet is a good rough guide for DA(diesel) (unless you're in an "S" boat or "TAMBOR", where it's 25 to 26). "SALMON", and "NARWHAL" classes can run slightly deeper (about 30). "GAR" and "TAMBOR" can do 32 feet.

For DA(CO2/Gun), 26 feet is a good guide for getting rid of CO2 for all boats except the "PORPOISE" class, where you must be at 23 feet, and the "NARWHAL", "TAMBOR", and "SARGO" classes, where it's 28.

The time needed to reach periscope depth is affected by 1) the depth at which the "dive" order is given, 2) the dive depth ordered ("periscope depth" vs "CRASH DIVE!"). "CRASH DIVE!" is always faster. But to avoid suicide, remember that ordering "CRASH DIVE!" takes you to 160 feet. Without that much water under you, you will die, unless you can successfully change the order (after submerging) before hitting bottom. Use "CRASH DIVE!" with caution.

Below are dive times (in seconds) between classes of boats for the order "periscope depth", fully surfaced and at DA(diesel). Times to reach periscope depth on ordering "CRASH DIVE!" will be faster. [Times were obtained at flank speed, in calm seas, with 100% battery charge.]

TIME TO REACH PERISCOPE DEPTH

TYPE BOAT......SURFACED............DA(diesel)
GATO..................120....................32
BALAO..................80.....................29
GAR....................120.....................23
PORPOISE.............85.....................24
SALMON..............107.....................26
NARWHAL............125.....................24
TAMBOR...............123....................26
SARGO.................123....................24
S-42....................72.....................47
S-18....................72.....................47

The difference between dive times is dramatic, and clearly shows the advantage of running at decks awash when a fast dive is important. It should be remembered that the DA strategy for recharging doesn't work in rough seas, where the electric engines will cut on and off, discharging the batteries.

Also remember that you get poor fuel efficiency (fewer MPG) and lower speed when running at Decks Awash since you are, after all, partly submerged.

(Incidentally, "CRASH DIVE!" from DA(diesel) will reach periscope depth for all boat classes in 17 seconds, except the "TAMBOR" class which makes it in just 15. Now that's a fast dive!)

Practical Tips:
To get to a specific depth, use the shallow depth meter (the one with the red needle at periscope depth), put the point of the cursor over the where tick mark should be for the desired depth, and left click the mouse. The "new depth" order will appear in the notification window at top of screen. (A voice will state the "new depth" order too, but often it isn't the same depth as you see printed in the window. ALWAYS go by what's in the window.) You may find the "new depth" you see ordered is not the one you thought you clicked on. If so, just do it again, this time pointing the cursor a tiny bit deeper/shallower on the depth meter from before, and notice the result. Keep doing this in tiny changes until you get the feel of it. Once you do, you should be able to "dial in" 26 feet and get it, or at least within a foot either way.

Again, to successfully run at Decks Awash you must have a sea-state that allows you to maintain constant depth, since you're running the diesels at (or near) the lowest depth they can get air. Boat speed also affects depth control. Faster is steadier.

Overall, it's probably safer to not push the limit, depth-wise, in using DA as a strategy. Give yourself a margin of error. Dive times should not be much affected.

Use the numbers as a guide and remember, they are based on the depth you see ordered as "new depth" in the notification window (top of screen). The depth seen at the bottom right of the controls (next to the "+" of Time Compression) is usually NOT what you dialed in, neither is the audio "new depth".

Versions:
This study was made using Silent Hunter4 (1.4). The later SH4 (1.5) data may, or not be different, but the concept is the same. Modding to Trigger Maru Overhaul (TMO) does not change the depths for DA, but dive time measurements from full surface ARE faster in TMO. But in all cases, starting from DA gives you a much quicker dive.