Thanks for the info.

Good point with the short patrols on the XXIII. Makes me wonder if any progress was made on a better adhesive for the sonar coating from earlier attempts.

T

As far it can be read, some progress was indeed made but the issue has probably not been really fully 100% solved yet, going by the pictures of modern boats with missing/flabby tiles.

Radar
The Type XXI is fitted with FUMO-65.To be honest my experience is that it has been almost useless. I do not kwow if it is my mod soup to blame but I get an oscilloscope, not the most user friendly interface instead of the PPI it was supposed to have.
In terms of actual perfomances it seems to have never picked up anything that had not been already detected by visual or RWR except in the worst visibility conditions and even then it was non radiating targets, else the RWR would have probably picked them up first. Manual operation might yield better results but as noted the interface leaves a lot to be desired. It is also not clear what the range knob actually does in practice.
Basically it is of some use only in bad weather and likely only against surface targets at that.

Employment
Late war procedures for employment of u-boats can be found at http://www.uboatarchive.net. I post the sections relevant to the game.

Current Order No: 1. Issued Nov. 1944.


(Procedure when underway and in operational areas).



I. The Schnorchel has to a considerable extent, escaped enemy air attack and with the simultaneous decrease in U-boat losses, has enabled them to operate again, in areas strongly defended by aircraft.

Principles for Proceeding. The task of the Commander is to take the boat to and from the area of operations without any great diversions, to avoid skirmishes with the enemy defence, to utilize however, every opportunity for attack.

Principles for the operational zone. In order to make full use of the possibilities offered by the Schnorkel, approach close to the enemy shipping near the coast. Only there, are successes to be obtained.

II. Procedure of boats when underway and when in the operational zone depends on the strength put up by the enemy air force. Commander U-boats differentiates between areas with a strong, medium and weak defence. If a Commander establishes another aircraft position in an area, as described under A - C, he can intensify or modify his tactics according to the change.

A. Strongly defended areas. Squares AN, AF, AE, AM, BF, CG, DJ, AL, BE of the eastern third, further a strip of nearly 100-200 nautical miles off North and South America, including the Caribbean.

Procedure: on principle proceed submerged when underway and when on operations, recharge when Schnorkelling (total submerged warfare). Recharge only above surface, only when Schnorkelling is not possible for technical reason.

Execution.

1. On setting out and returning when Schnorkelling.

a). For more detailed instruction concerning schnorkelling times, boat routine, making best of weather conditions, see Current Order No. 20.

b). Normal cruising depth above 50 meters (162.5 feet) - At routine times and when taking hydrophone effects at periscope depth. Good hydrophone conditions in no way exclude periscope observations while proceeding.

2. When setting out and returning while submerged with charging above surface. Only in cases of a faulty Schnorkel or when other reasons (e.g. a too strong current) make it necessary.

a). Charging to be carried out, in conditions unfavorable for air attack, that is, in general, by night. Favorable - moonless, very dark, haze, fog, storm.

b). If at night especially great air and radar activity are observed, charging can be carried out in the daytime, in fog, or in clear weather with good visibility and good Ack Ack conditions. In case of doubt the proceeding by night is more favorable, since, in the event of an attack, better opportunity is afforded for submerging and for shaking off the contact, moreover, a simultaneous attack from several aircraft is difficult.


c). During charging, complete Ack Ack readiness. Commander on the bridge. Keenest lookout. All armament ready and unmasked, so far as weather conditions allow. Gunner in layer position with shoulder-piece in position. In weather conditions which enable accurate surprise attacks, if necessary man only the twin barrelled gun, in order to be able to submerge more quickly when located.

d). Set up the available FUMB. If there is FUMO on board, warm up well before surfacing, and use as follows:- Immediately after surfacing a circular sweep with the FUMO, then only Radar observation with "Wanze" and/or "Borkum and Tunis". If airborne locator has been observed, in general, submerge. If charging is urgent, when detecting a locator, D/F with FUMB, search by FUMO, and continue to observe the enemy. Until picked up by FUMO, a longer period (up to 30 mins.) can elapse. If no enemy is detected, do not return too soon to FUMB. If the "Hohentwiel" set is in perfect order and the servicing crew well-experienced, the apparatus, in continuous search alone may be used, when underway as an air-raid siren.

e). Do not remain on the surface longer than the time required for charging, as this endangers the boat unnecessarily. Exceptions: In especially unfavorable flying conditions and if the FUMO is functioning perfectly.

III. Procedure in the operational zone.

A. Submerged on principle and only while schnorkelling. On breakdown of the Schnorkel, in general, return to base. In this case carry out charging as under 11.

B. Keep close to traffic focal points and to coast routes, for it is only there that interception of traffic can be guaranteed. Lying on the bottom and taking advantage of the density of water which renders it more difficult for the U-boats to be intercepted by hydrophone and radar, facilitate, according to the latest experiments, a successful U-boat operation even at a depth of well under 100 metres (325 feet).

C. Initiate all measures for daylight attack, since this is most favorable. Submerged night attack promises success only at full moon.

D. Go frequently to periscope depth, do not rely upon the hydrophone. By night and when there is traffic, go down to the bottom, to save batteries.

IV. General Rules for "total submerged warfare".

A. Avoid surfacing by astronomical reckoning as far as possible. According to experiments so far to hand, extremely exact determination of position is made possible by very accurate dead reckoning, by taking the current into consideration and by making full use of the sounding device, electrosone and radio-beacons. When close to our own, or to enemy coast, make full use of every means of determining a position by land.

B. [...]

C. Even when proceeding without incident under water and when on operations, it is necessary to reduce the consumption of air and current to the minimum, in order to shorten the charging time and to have sufficient battery capacity for the attack when contacting the enemy, and further, in case of pursuit, to be able to run on it long enough without being forced to Schnorkel and/or surface.

D. Especially when the Schnorkel is out of order the boat must continue to run submerged in order to evade the shadowing aircraft and to be as faras possible beyond the range of location of the point of submerging, when it surfaces. Therefore when below water, not more than slow speed ahead, both engines. Use only the most necessary consuming plant, economize on light and air, the E-condenser consumes much current. Trimming and pumping with the auxiliary bilge pump. Carry out night routine according to Current Order No. 2.

B. Areas with medium air defence.

Areas not specified under A). Atlantic - North of 15 degrees South, coastal strips 100-200 nautical miles off the African coast, fringes of the Indian Ocean, including islands.

Procedure: Submerged by day, above surface by night.

a). In the open Atlantic between 30 degrees North and 15 degrees South, in clear, cloudless weather, with good Ack Ack conditions and with the FUMO switched on, proceed on the surface; without the FUMO, on principle, submerged. FUMB locating has been intercepted, dive and remain for a while submerged. If the enemy holds you on his Radar beam, he later sends out carrier planes, operating from sight.

b). Above water keep Ack Ack gun ready for firing and by day at least six look-outs on the bridge.

C. Areas with weak air-defence.

Atlantic - South of 15 degrees South, open spaces of the Indian Ocean.

Procedure: On principle, on the surface, FUMB continually switched on.



Current Order No. 2


Procedure when Schnorkelling. Issued Nov. 1944.



Many undertakings of 40-60 days duration were carried out without surfacing. The Schnorkel proved very effective and through it operations were again possible in areas heavily patrolled from the air

a). In General.

? Principle: Under all circumstances attempt to recharge battery completely every day. On account of enemy positions the Schnorkel is not to be shifted. Continued reduction of the battery capacity can finally cause the boat to be in especially dangerous situation.

(1). Chose favorable conditions for Schnorkelling, i.e., in general at night, in a swell, poor visibility. By day only in clear weather with good visibility, the emotion of the sea from force 2 and is the Schnorkel does not smoke. Best at night, once 2-3 hours at the beginning and end of the night.

(2) The Schnorkel is not undetectable by radar. If the Schnorkel is correctly manipulated, that is to say, with the exhaust under water, it can be scarcely or only inaccurately intercepted. The motion of the sea reduces the possibility of being located. If the Schnorkel is fully raised, it can be located and attacked, therefore it is wrong to let the Schnorkel protrude too far when there is phosphorescence, so as to avoid a trail of foam. Then interception through location by radar is possible at a much greater distance and therefore more often than by a trail of foam. The range of location of the camouflaged Schnorkel is about 10% of the boat when on the surface. In practice it is scarcely interceptable.

3). Air attacks on dark nights are generally only possible with the use of artificial light. If a searchlight is detected shining directly on the boat or a flare above it, sound the Schnorkel alarm, and dive to at least 80 metres (260 feet). If searchlights and/or flares are detected searching a long way off, or their location is above the Schnorkel dipole, only sound the "quiet-Schnorkel-alarm", that is to say, dive to 20-30 metres (65-95 ft. approx).

4). When the Schnorkel is in use, periscope manned by the commander and officers of the watch in watches of roughly 1-2 hours. In boats of type VII C, turn the main periscope continually by hand. In boat type IXC in general, only the periscope for air observation. The raising of both periscopes results in oscillations, and moreover increases the danger of being located by radar. Do not proceed at too high a speed (i.e., with both Diesels on screw (aufSchraube)) this endangers the periscopes. They swing, bend and become leaky and cloudy; slight oscillations are frequently corrected by very small alterations of periscope height.

5). Noise caused by Schnorkelling is about the same as when the boat is running on both E-engines at a rate of 200 revolutions per minute. In areas where there is hydrophone activity do not refrain from using the Schnorkel because of the fear of being heard. According to experiments, boats have proceeded unperturbed with the Schnorkel in use, with hydrophone bearings of weak signal strength and with "circular saw noises" of medium signal strength. "Circular saws" on shadowing vessels cut out other noises.

6). Circular acoustic sweeps for enemy positions every 15-45 mins. - (unreadable) - dive to a depth of 20 metres. It is possible that

the shadowing craft makes towards a U-boat which it has intercepted by hydrophone and in order to remain undetected, will stop when the boat stops her Diesels in order to make a hydrophone sweep. Therefore measures have to be taken to be ready for a circular acoustic sweep as soon as Diesel engines are stopped.

Execution.

a). Order to engine room, central control and listening room: "Ready for circular acoustic sweep".

b). Switch off charging, charging diesel engine, electric fan and condenser.

c). Lighten the boat in order to avoid further pumping, once the Diesel engine has been switched off. [...]
d). Engine room telegraph to "Dive". Cruising Diesel is to be turned off and the E-engine run, utmost silence in the boat, and keep a listening watch.
e). [...]
f). [...].

Methods of Proceeding.

a). Schnorkel in sea up to force 6-7 (according to type of boat) steer broadside on, 1-5 degrees, with the ballast forward, with as little flooding as possible, otherwise current and air unnecessarily consumed when pumping prior to the circular acoustic sweep. Choice of cruising speed depends on the motion of the sea, and the oscillations and angle of the periscope.

b). Whenever possible ventilate the boat at least once during the day, for 15 mins. with the Schnorkel; this saves potash cartridges....


Personal comments
The XXI does regain a part of the mobility lost by the II/VII/IX forced to remain underwater, though it cannot really match the capabilities of surfaced submarines.In actual terms it is feasible to pursue most contacts detected by hydrophone and to expand the area of operations to the western approaches. Going to let's say America is going to be a somewhat time-consuming affair however and chasing ships moving away at medium/ fast speed is unfeasible/risky.Personally I patrol just off che continental shelf, where chances of mines is less, I can make full use of depth and traffic is still enough to be able to pick and choose what to engage.

Antiaircraft firepower il less compared to conventional u-boats, while diving time is pretty good, so staying on the surface during air attacks is less appealing than ever. In real life getting in and out the antiaircraft positions would also be somewhat awkward, while the rear one had a direct hatch access to the hull leaving the forward one would apparently entail climbing through the turret hatch and then back in the conn. Considering three people (one gunner and two loaders) and the fact that the early version could dive in as little as 20 seconds (eventually increased to improve underwater performance but still fast) and it does not look good. Personally I use my guns only when close to the harbor where I cannot dive.

I Research as much as I can when I have a Keen interest in a subject and on the Elektro-Boots of Type XXI and XXIII are an area I scoured books and the Internet for Data. ( I changed my Type XXIII to Reflect Reality based on the Royal Navies Secret Evaluation conducted in Scotland on Surrendered U-2326 ) I did not change the Type XXI to reflect what was built and the one boat that went on a war patrol sinking no ships but testing its remarkable stealth before its surrender to the Allies. The Royal Navy had been slated for their own type XXI for Evaluation but after the secret tests performed on the Type XXIII U-2326 they told the Russians and the Americans they did not even want it as they found no useful technology or performance variables to suit the needs of the Royal Navy. They Concluded the Schnorkel was at best a Desperate Measure for a Beaten Enemy and their Interest in a Walter Boat was Furthered. ( they eventually got Dr. Walter Himself hiding him from the Soviets and whisking him back to Britain to Aid them in developing Futuristic Propulsion Systems ) They Found: The Pressure on the Eardrums and Discomfort of the Men Detracted them from Performing their Duties to the Utmost. The Elektro-Boats are a "Wet" Boat Moisture Seeps Everywhere even more so than a Hull of Standard Shape. The Type XXIII was a simple Boat designed for a Single Purpose to divert attention from the Atlantic for 6 months and then Hit Hard with the Type XXI's on the Long Abanandoned Convoy Routes of The Western Atlantic.

Type XXI Boats With main engines
Had to disengage the MAN Superchargers before Starting the Diesels for Underwater Propulsion with a Blocking Arrangement on the Exhaust Manifolds due to the Engines having a Higher Manifold Pressure then Specified by the Design. After The MAN Factory was Bombed Production of the SuperChargers Ceased for a time and Most Type XXI Boats went into Service without them reducing the surface speed at FF to 12-13 knots. Engines were mounted at Assembly Yards to the aft section and the SuperCharger was Installed Then and could not be Installed Later at all. ( The Majority of MAN Superchargers after Production Resumed were then sent to Yards Building Type IXD2 Boats )Halting Construction was out and as the Boats were designed to mostly stay submerged this was not deemed critical to its purpose. Radar was used as a Navigational Aid not as a Tactical Advantage in Practice. but in Theory one could in Weather not suited for Periscope or UZO Attack plot Torpedo Attacks for the LUT Pattern-Running Torpedos, however using Radar for this was Suicide. The Type XXI and XXIII could launch Torpedos from 20 meters Depth and by Sound and Sonar the type XXI could Plot Torpedo Attacks while Submerged without the Periscope into the side of a Convoy and Evade at Silent Speed Reload and Attack Again. This Stealth, quick Reload Time, and Fast Submerged Speeds was how the Germans were going to to Tip-the-Balance back to their Favor again with a New Atlantic Blockade. The Starvation of Britain, and the Allied Invasion Collapsing for Lack of Logistical Support and Great Britain and The United Staes would then Sue for Peace. This Link has Valuable Data for in Depth Study of the XXI as well as other valuable information found on the site.

http://www.uboatarchive.net/DesignStudiesTypeXXI.htm

There was no way the elektroboot would be able to force the british and the americans to ask for terms. By 1943 the americans alone were turning out a million of tons of merchant ships per month and while given the commitments the shipping situations was not always rosy all the sort of measures would be implemented to prevent Britain from going under, including delaying offensives if it became necessary. The new boats were indeed tough nut to cracks but large numbers of frigates/DEs carrying Squid and the latest ASDICs were still a threat to them, the occasional detection of the snorkel from the air was still possible and so on.
In addition the new u-boats had to be built, sheltered and supported by at least a minimum of surface and air forces. The components factories were of course liable to fall under the increasing threat of Allied air power but even hardened facilities such as Valentin assembly bunker or the Keroman shelters could hold only so much against repeated Tallboy and Grand Slam attacks. Sperrbrecher were needed in numbers to prevent the allies from simply paving the harbours access routes with air dropped mines and to provide air defense to the U-boats at least until they could leave the shallower area; unlike the U-boats or the E-boats putting them under concrete was just not feasible and losses were heavy, about 50% of the force.
Without at least a chance of the Luftwaffe showing up the facilities could thus be pounded, eventually with some success, and allied air/naval forces forces could come closer and closer to blockade the bases.
What the XXI could accomplish was enabling the Kriegsmarine to resume the battle of the Atlantic, instead of the naval guerilla war they fought in 1943-1945 but that is all. Best case scenario they delay the end of the war by a few months so that Germany is nuked or a greater portion of it ends under the soviets. Not so great outcomes for them.

As for the Type XXIII, it was essentially the elektroboot equivalent of the WW1 era UB I: small, railroad capable coastal boat built to operate in shallow areas. Unsurprinsingly they both ended up with a somewhat lacking armament (two bow tubes and no reloads) though of course the XXIII had an edge in speed/range. Had it been a mere stopgap instead of filling a different tactical niche they would probably have dropped the railroad requirement and specified at least a pair of reloads as for the Type XVII. The elektroboot/Walter equivalent of the Type VII never became ready of course.

As for the superchargers issue I do not know, Blair of course goes on rant about it as it can be expected but probably it is true that their specs were over ambitious and the lack of Diesel power might have some effects on charging times. In addition it is worth noting that U-2540 postwar had theirs replaced with a different type while the original electric motors and most others components not involved in tests were retained; it may have been simple convenience but perhaps there were real issues.

Oxygen settings
According to the sources I have found it could take 27 hours for oxygen in the air to drop from 21% to 17% though this was probably calculated on the basis of reduced activity on the crew part.
A renewable oxygen supply setting of 2.5 and silent run (entailing reduced crew activity) seems to get fairly close to that result in the game. It should be noted however that under the same conditions as above chemical scrubbing of CO2 would be initiated after 11 hours,well before adding bottled oxygen became necessary and since the supply of of CO2 absorbing compound cartridges was large (enough for 19-23 days of operation) but not unlimited a brief daylight snorkel run is still advisable for realism sake when feasible.
Non renewable oxygen supply is bit harder to assess.Installed flasks capacity could apparently vary from boat to boat, 1200 liters (at 150 bar) could be typical but up to 1500 liters were also possible. In addition a number of IG-brikett, each capable of generating 1500 liters of oxygen, were also carried, 25 could be a typical amount.
Total oxygen stored for the 1200 liters flasks + 25 brikett configuration would thus work out at something about like 217500 liters (1200 x 150 + 25 x 1500) if my math is right.
Human oxygen consumption at rest is about 15-16 liters per hour, increasing up to four times that for light work and up to eight times for hard one, though probably most of the typical activities aboard a would fall under the former category.

EDIT
While I had previously suggested 2.0 for non renewable oxygen supply 1.5 seems to square better with available data. I still have to try out 1.0 just in case.

What about battle tactics in 1944-45 with an XXI type sub.

Every time I go out in one I still get sunk even with the great underwater speed

Personally I use the maximum speed only in short bursts to get out of a hedgehog or depth charge pattern.Trying to outrun a convoy escort will only result in collecting a pack of DE on your back, and while they seemingly cannot shoot you while you run it is not a competition you can win. IMHO the main selling point of the type is not the absolute maximum speed but the sustainable cruising speed and the relatively high silent speed. At 100 RPM you can do more than three knots and the game standard silent speed is over four.

Now, following the procedures listed in the operational orders I quoted, in particular "total submerged warfare" and using the snorkel mounted RWR it is possible to largely avoid air attacks except for the bombing raids at the beginning of the patrols. Possibly non radar equipped planes could still suprise you but it never happened to me.
I also give a wide berth to task forces and I do not troll lone destroyers; in addition I treat all the continental shelf near Britain as a potential minefield and I stick to deep waters as much as possible.
In general in my favorite hunting areas off Britain most of the traffic consists of two ships groups sans escorts and while it is usually small fishes you can still bag a decent amount of tonnage with the XXI torpedo load. Against convoys what has worked for me is the diagonal approach at periscope depth and at quiet speed, getting inside the convoy, fire a salvo, go deep (always at quiet speed) and turn towards the rear of the convoy and trying to dodge the rear escorts as they run over me. I cannot however say that, unlike in dodging air attacks, I have enough experience in this area to be authoritative.

Tactical use of the XXI and self-seeking torpedoes
 

There were a number of reasons why the Kriegsmarine built the fleet it did in the 30's. Political constraints meant that a few capital ships could be built without triggering too many adverse british reactions, while on the other hand a large submarine fleet might not be tolerated to the same extent. A submarine required, ton for ton, a much greater quantity of copper/lead/rubber than a battleship and imports had to be economized even before the war; needless to say an elektroboot required an even greater amount of those materials. The capture of the Biscay ports could not be relied upon and without those the prospects of a proper tonnage war are dubious; in the event at least a minimum of a surface force turned out to be handy for the norwegian campaign.