Carbon Monoxide Poisoning and the forgotten Exhaust Shutoff Valve on the Type VIIC


Aaron S. Hamilton

This article is a combined excerpt from Hamilton’s two upcoming books: German Submarine U-1105 ‘Black Panther’: The Naval Archaeology of a U-Boat (Bloomsbury Press, 2019) and Total Undersea War: The Evolutionary Role of the Snorkel in Dönitz’s U-Boat Fleet, 1944-1945 (TBD). The author wishes to expressly thank Kai Steenbuck of the Deutsches U-Boot-Museum whose own research helped correct a number of inconsistencies and confirm that the above deck exhaust trunking was present before the completion of extensive carbon-monoxide testing of the Type VIICs U-235, U-236, and U-237 in the fall of 1943.

Since the end of World War II little has been written about the technical characteristics or operational impact that the introduction of the snorkel had on the U-Boat force during the last year of the war. This brief article will not detail the entire history of the snorkel, but discuss the configuration of the snorkel system on the Type VIIC and detail for the first time the function and importance of the starboard side above deck exhaust trunking that housed an important shutoff valve access in the Control Room.

The above deck exhaust trunking and shutoff was a critical component of all Type VIICs (to include the VIIC/41), but few know about it for two reasons. First, a completely fictional U-Boat design schematic known as the “Type VIIC 1944” is routinely published in U-Boat technical histories, but this design depicts no above deck exhaust trunking on the starboard side. There is no archival record to support that “Type VIIC 1944” ever existed, or was even considered as a possible future design in the Kriegsmarine. Second, this important feature, which was present on U-995 at the end of the war and during its time employed by the Norwegian Navy, was unfortunately removed when it was restored and put on display in Laboe. Due primarily to these two factors, no accurate depiction of a snorkel-equipped Type VIIC (to include the VIIC/41) has ever been made by a model company anywhere in the world. So, why was this seemingly insignificant pipe and shutoff valve so important?

On all snorkel-equipped U-Boats it served an important function to keep back sea water from entering the diesel engine room. This became acute during the transition from running the diesel engines during snorkeling to employing the electric motors on Type VIIC’s due to their weaker diesel engines that were not able to counteract the increased pressure in the exhaust line, for even a few seconds. This was not a problem that existed in the various Type IX snorkel equipped U-Boats that had a more powerful MAN diesel engine. While technical modifications were introduced into the Type VIICs to overcome the challenge of lower diesel engine power output to keep the exhaust line clear, not every Type VIIC received the required fixes. Leaving U-Boat crews to function the best they could or devise their own solution to the problem. This is statistically significant given the fact that out of 353 diesel powered U-Boats that received a snorkel install during the war (not including Type XXI or XXIIs) 275 were either a VIIC or VIIC/41, representing 78% of the total operational U-Boat force in the last twelve months of the war.

In the fall of 1943 the U-Boat force introduced the snorkel as a means of allowing a U-Boat to remain submerged while crossing the Bay of Biscay and reduce the risk of being identified and attacked by Allied radar-equipped aircraft. At the time initial tests were made no one ever thought the snorkel would become as successful as it did and allow a U-Boat to remain submerged for months at a time.

Each diesel U-Boat type required a slightly different snorkel configuration but the main characteristics were similar. On diesel boats the snorkel apparatus was a streamlined cylindrical mast that measured 9 meters (29’x6″) in length. It contained two air conduits. One brought air into the engine compartment of the U-Boat through a float valve at the top of the mast designed to stop water from entering the system if submerged. This float valve, also termed the snorkel head, sat about 1 meter above the water during normal operation. The other line vented the carbon-dioxide (CO2) and carbon monoxide (CO) exhaust, oxyhydrogen from the batteries, and other internal gasses from the diesel engines at a point 1 meter below the top of the snorkel float valve at a 90 degree angle. The design expelled the gas underwater while snorkeling to minimize not only detection, but interference with periscope vision. The snorkel was raised from a submerged depth of about 25 meters using a hydraulic piston (earlier versions employed a pulley). Once raised, the snorkel was held in place by a staying rod that locked on the mast by a mechanism in the control room. When lowered, the mast was locked in a recessed well on the deck on diesel boats. This was the basic configuration of the snorkel. How it was installed onto each U-Boat Type was different.i

There were two types of snorkel installs on the Type VIICs. The initial design known as the Type I, utilized external intake trunking that ran along the portside of a U-Boat into the diesel engine. A spring-loaded flange at the opening was seated against another flange on the snorkel mast when it was raised. The very first Type I masts utilized an exposed intake and exhaust line that were clamped together and raised and lowered by a tension based pulley system. The Type I was improved upon when it went into full production by encasing both the intake and exhaust lines into a single streamlined mast that was raised by a hydraulic piston. In the summer of 1944 a new improved version known as the Type II eliminated the problematic flange and ran the induction trunking below deck through the snorkel boot heel. By way of comparison, the Type IXC U-Boats were predominately equipped with the Type I flange version that was installed on the starboard side of the U-Boat. Only very few received a Type II snorkel mast.

The exhaust trunking on both the Type IXC and Type VIIC was arranged differently. On the Type IXs it ran below deck, with the trunking entering the boot heel of the snorkel mast on the same starboard side as the intake trunking. On the Type VIIC the exhaust trunking exited the starboard side of the diesel engine room, then ran several meters forward below deck, before it came up above deck midway along the coning tower, before dropping back below deck next to the magnetic compass housing where it then entered the snorkel boot heel on the portside. A rather large shut off valve was integrated at the point the trunking returned below deck. The shutoff valve had an access hatch secured by nine bolts and was operated from within the control room by a turn wheel. The reason for this configuration remains unclear due to a lack of available primary sources. It should be noted that the Type IXs also had a shutoff valve installed that was operated by a turn-wheel in the control room.

The existing configuration diagrams show no specific reason why on snorkel-equipped Type VIICs the exhaust trunking was routed above deck. Once possible reason is that there was no space to run the trunking below deck. The investigation the author personally made during an archeological survey of the U-1105 wreck site in the Potomac River on numerous dives did not reveal any obvious space constraints that would cause the exhaust trunking to be routed above deck. What remains clear is that every Type VIIC equipped with a snorkel had this exhaust configuration without exception. The snorkel exhaust shutoff valve played a critical role in safe snorkel operations, especially on the Type VIICs as discussed below.

From l-r: U-1009 (VIIC/41) commissioned February 10, 1944 and received its snorkel retrofit in August that year, U-1058 (VIIC) commissioned June 10, 1944 and received its snorkel retrofit in November that year, U-1305 (VIIC/41) commissioned September 13, 1944 and is believed to have received its snorkel retrofit in January 1945, and U-1109 (VIIC/41) commissioned and received its snorkel retrofit in February 1945. Regardless if one of these U-Boats was a Type VIIC or VIIC/41, its date of commissioning or when a snorkel was added, all of these U-Boats show the starboard side snorkel exhaust trunking. This image was taken in the summer of 1945 after their surrender.

Initial scientific and medical testing occurred onboard U-235, U-236 and U-237 during a two month period of snorkel trials that lasted from October-December 1943. It was unfortunate for the crews of the snorkel-equipped Type VIICs that these studies occurred after the initial orders for the prefabricated snorkel kits were made due to the extensive changes that had to be introduced later in mid-1944.

Two post-war reports prepared by members of the Kriegsmarine medical staff provide unique detail on the impact of the snorkel on U-Boat crews. Dr. Hellmut Uffenorde was a lecturer at the University of Kiel and a former consulting otologist at the Sinnesmedizinischen Forschungsabteilung (SIMPA) des Marine Sanitätsamtes MOK Ostsee (Research Division for the Physiology of the Sensory Organs attached to the Medical Department of the Naval Station of the Baltic Sea). He authored the “Otological Experience with Snorkel-Equipped U-Boats”. Dr. Guenther Malorny served as the primary carbon-monoxide tester of the snorkel-equipped U-Boats from the fall of 1943 through May 1944. He authored “Carbon Monoxide on U-Boats”.

Dr. Malorny concluded that before the introduction of the snorkel there was no concern of any CO poisoning on board U-Boats. No one suspected what the impact was until the first testing began in the fall of 1943. It was determined that when operating a snorkel-equipped U-Boat of any Type at periscope depth a greater counter-pressure of the exhaust gasses had to be overcome for discharging the Diesel exhaust gasses than was required when operating the U-Boat on the surface. The negative pressure inside the U-Boat, which was permanently present during submerged operation, dropped rapidly when an operating snorkel head was submerged. Conversely, the pressure inside the exhaust tube increased significantly. The deeper the U-Boat submerged while the snorkel system was running the greater the pressure differential. The MAN diesel engines on the Type IXs generally proved capable of handling this differential, but the GW-engines on the Type VIIC could not.

Close up image of the starboard side exhaust trunking and shutoff valve on the U-1105. This image was take in the summer of 1945 after its surrender. This feature was never identified by the initial archeological survey team that documented U-1105 for the Maryland Historic Trust that oversees the U-Boat in the Potomac River. Divers that did see this piping on the wreck never knew what they were seeing or its important function, until now.

The GW-diesel engines on the Type VIIC did not produce enough power to counteract the pressure buildup in the snorkel exhaust trunking. This caused seawater to flood the exhaust system and the starboard diesel, followed by the port diesel. Toxic gas quickly filled into the diesel engine room. Simultaneously, the lack of oxygen flowing through the snorkel intake in those brief seconds caused an incomplete combustion of the oil. This served to increase the enrichment of CO in the already escaped exhaust gas.

This problem existed on the Type VIICs even when snorkeling was complete and the change over to the electric motors was underway. During those brief seconds that the GW-engines idled they were not powerful enough to counteract the pressure that built up in the exhaust lines. If the exhaust valve was not shut with the retraction of the snorkel mast, sweater rushed into the line and starboard engine forcing toxic gas back into the engine room. Special Experience with “Schnorchel”-Equipped U-Boat No. 4 was soon issued during the testing that directed that “if smoke is present in the Diesel engine room for a short period and if this lasts longer than 5 minutes, work has to be done there with rescue apparatus and protective goggles.” It went on to say that is prolonged exposure continued than the Diesel Engine Room had to be sealed off and the “…boat must be ventilated thoroughly with fresh air” after surfacing.ii This order made sense during training, but not during a war patrol as experience would prove. In the above context the outer exhaust valve formed the only barrier against outside water pressure. The tightness of the outer exhaust valve was a prerequisite for “perfect snorkeling.”iii

A composite of two wartime German technical diagrams of the Type VIIC snorkel installs captured by the Royal Navy from the U-Boat retrofit facility in Salamis Greece during the fall of 1944. These two schematics clearly depict the above deck starboard side exhaust trunking run and the shutoff valve in the Control Room.

Snorkel instruction and training was often just enough to make the crew familiar with the strange new mechanism. Each U-Boat had to master the apparatus on its own, often under the extremes of weather and combat. If it failed to do so, death and destruction of the U-Boat could occur as in the example of U-997 (VIIC/41). U-997’s snorkel was installed in February 1945 at Trondheim. One of the crew’s first training exercises with the new snorkel almost became its last.

“The boat was outfitted with a new snorkel and the crew was given brief training with it into Trondheim harbor before they set out for Harstad–in extremely rough weather. ‘About 5 o’clock in the morning we were ordered to go to snorkel drive, in spite of a very heavy sea astern. Sometimes we cut under and were submerged for minute. When the snorkel mast went below water, all the air was sucked out of the boat, and the crew had to quickly cut off the diesels.’ On one occasion the starboard diesel suddenly conked out, and 10 to 15 tons of water poured into the diesel room through open air intake and exhaust valves in the snorkel mast. ‘We tried to drain the water out of the diesel into the bilge, then we tried to clear the port diesel, but it wouldn’t start, either.’ Meanwhile, carbon dioxide from and exhaust smoke started to fill the boat, and almost every crewman, unable to grab his breathing device, started gasping and choking. Commander Lehman brought the sub back to the surface as fast as possible, trying to overcome the problems of sick crewman who could hardly function and 15 tons of water in the diesel room bilge.

Once more on the surface, the water was pumped out, engines start again, and U-997 began to recharge its batteries. The fuse box caught fire, the black smoke billowed out, and to top things off, the Gyro and magnetic compass is failed. During this bedlam, the bells screamed, and U-997 dived, pitching forward at a sharp angle, and Bootsmaat Sachse fell overboard. When Lehman surfaced again, he was unable to find his crewman.”iv

The frequent failure of the GW-engines to counteract the pressure in the snorkel exhaust trunking caused a high-rate of CO gas exposure on the Type VIICs that alarmed Malorny. Normal CO exposure to crews on non-snorkel equipped VIIC, IXC, and IXD2 U-Boats was measured at 0.013-0.038% of oxygen. On a VIIC snorkel-quipped U-Boat equipped with GW-engines this increased during normal snorkel operations to 0.08-0.12% of oxygen. But when the snorkel head dipped below the sea surface due to inclement weather or a tactical maneuver while the diesel engines continued to run this could increase to 0.4-0.7% or higher, which could be fatal.v It was determined that a high concentration of CO would induce acute poisoning within 15 minutes and a loss of consciousness or death in 45 However, according to Dr. Uffenorde “if air containing 0.2% volume of the CO is breathed for one hour, about one half of the hemoglobin is combined with CO. But, even the concentration of 0.07% volume CO is sufficient to saturate 50% of the hemoglobin if the period of its influence is prolonged accordingly.”vii

There was concern for the cumulative effects of frequent, but short duration exposure to CO gas. CO exposure has a cumulative effect and overtime has the same effect on the blood as if a long exposure occurred. This was not fully appreciate by the Kriegsmarine in 1943. Only after the completion of the CO testing in December of that year did the medical studies reveal the problem. As Dr. Uffenorde concluded in his monograph:

“Especially on the boats of the Type VIIC which were put to action in the beginning of the war, it happened repeatedly that men of the crew became definitely unfit after several months of “Schnorchel” cruising. The boats U235 and U236 were used for experiments. Therefore they had frequently and purposely to dive deeper with the result that dense smoke escaped to their Diesel engine rooms. After several months of this kind of duty the entire crews of both boats had to be relieved from duty as they showed symptoms of poisoning. They had to be sent on recreational furlough.”viii

When snorkel U-Boats went into action for the first time they only remained submerged for days at a time. By the late fall this increased to weeks, and by the spring of 1945 snorkel equipped U-Boats were remaining submerged constantly for 4-6 weeks. By the end of these patrols the crews were no longer functioning at peak capacity as the cumulative effects of CO took hold. This occurred in U-1231, a Type IXC/40 U-Boat, during its long submerged patrol into the St. Lawrence River. Here two E-motor machinists passed out from CO positioning due to frequent but long exposure to CO gas that caused the U-Boat commander to break off his patrol and find a quiet spot along the Canadian Coast to surface, check his snorkel exhaust line and vent his boat.ix This was an incomplete science at the time as no submariner from any anywhere in the world had stayed submerged for this length of time.

Dr. Malorny explained that “the significance of the carbon monoxide for the crews on U-Boats was not sufficiently estimated by the [snorkel] engineers. When I succeeded in demonstrating that a series of inexplicable accidents were caused by the effect of CO gas, expensive modification of construction had to be made later.”x The required modifications came in the form of new spring clutches for the snorkel exhaust trunking that would allow a mechanical blower to be attached when the diesel engines were in low revolution. This provided enough pressure to offset the buildup in the exhaust trunking.

Malorny retested CO measurements on frontline snorkel-equipped Type VIICs in May 1944. He was under the impression that the Kriegsmarine implemented the fixes he recommended almost six months earlier. He unfortunately found that the Type VIIC had high concentrations of CO and that no recommend technical fixes had been introduced. He recalled that due to the prefabricated nature of the snorkel systems and “due to difficulties of supply, only a few boats were fitted out with the new spring clutches.”xi The first order for 20 snorkels for Type VIICs was issued on August 12th, 1943. Just over a month later on September 24th, both the Naval Arsenal at Kiel and the Kriegsmarinewerft Wilhelmshaven (KMW) were ordered to urgently produce a further 100 snorkels for the Type VIICs in service and 40 for the Type IXC and Type IXD U-Boats. Records are not clear when the next order was placed, but likely as many as another 100 were ordered for the Type VIIC before the completion of the December trails. This meant that anywhere from 50-75% of snorkel-equipped Type VIIC did not have the new spring clutches upon the completion of their snorkel retrofit. This only serves to underscore the importance of the exhaust valve in maintaining effective operational snorkel-equipped Type VIIC U-Boats. However, U-Boat crews soon found work-arounds. For example, a fix was introduced by the crew of U-671 (VIIC), whereby a connection into the exhaust system was arranged from the high-pressure airline to the exhaust manifold. This allowed the U-Boat to economically blow all water out of the exhaust side of the diesels before starting up.xii There is no evidence that this fix was universally applied across all Type VIIC U-Boats. It is likely that a number of Type VIICs had the new spring clutches added later on in 1944 or early 1945, but there is no comprehensive primary documents that reveal how many.

U-995 in Laboe today. Original wartime photos of U-995 as well as postwar photos of this U-Boat in service of the Norwegian Navy clearly depict the starboard side exhaust trunking and shut off valve. The question remains, why was it removed from this snorkel equipped U-Boat when it was restored as a museum despite of its critical function?

Despite the importance of the above deck exhaust trunking and shutoff valve, it has gone unrecognized by historians and maritime archeologists who have documented U-Boat wrecks since the end of the war. As an example, the original maritime archeology survey of the wreck of U-1105 conducted in the mid-1990s never documented or acknowledged the purpose of this trunking despite it near perfect preservation on the wreck site.

It is unfortunate that no visitor to U-995 can see the starboard side exhaust trunking since it was removed during its restoration for some inexplicable reason. Photographs of U-995 during its renovation in 1970 published in Eckard Wetzel’s 2004 book U-995 clearly shows the exhaust trunking present on the starboard side. It might have been removed because of its condition when it was being restored. Perhaps in the future model companies and hobbyists will begin to include this feature on late-war Type VIIC and VIIC/41 U-Boat models as an acknowledgment of the important function it played during U-Boat operations.

About the author.

Aaron S. Hamilton is an avocational historian and amateur maritime archeologist.

i National Archives Records Administration U.S Naval Technical Mission Europe, Technical Report 517-45, “The German Schnorchel”, 27 October 1945, hereafter cited as NARA NTME/TR 517-45.

ii Dr. Guenther Malorny, “Carbon Monoxide on U-Boats” (1994).

iii Vorläufige Beschreibung und Betriebsvorschrift der U-Boot Schnorchel Anlage, Tiel 3, U-Boot Archiv, Cuxhaven, Germany.

iv Melanie Wiggins, U-Boat Adventures (Naval Institute Press: Annapolis, 1999) pp. 176-77.

v Malorny, and Dr. Hellmut Uffenorde “Otological Experience with Snorkel-Equipped U-Boats” (?).

vi Malorny.

vii Uffenorde.

viii Uffenorde.

ix U-1231 KTB.

x Ibid.

xi Ibid.

xii PRO/ADM 1-17549.