Earlier snorkel - a big missed opportunity for the U-boats?

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StanS
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Earlier snorkel - a big missed opportunity for the U-boats?

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I've always thought about the snorkel as a device hated by the crews turning the U-boats into a slow, deaf weapon. However, this assessment seems to have been completely wrong. The snorkel allowed the U-boats to penetrate the defenses around the British Isles. The shallow waters should have been a death trap, but the opposite was true - a U-boat laying motionless on the bottom was very difficult to locate by ASDIC or hydrophones. Aircraft had great difficulty finding them. Since the U-boats had problems with long-range radio communications, the value of Ulrta and radio direction finding were greatly diminished.

A very interesting book on the subject is Hamilton, Total Undersea War - The evolutionary role of the snorkel in Donitz's U-boat fleet in 1944-1945:

"When BdU surveyed the operations of the last few months they recognized that the sinking of enemy vessels was low, but this was a function of mathematics from their perspective. The average number of days in the operational area was significantly reduced compared with 1942 as a result of the much longer passage at a slow snorkeling speed, but also owing to a disproportionate time spent in port. In August 1942, out of 100 days the average U-boat spent forty in harbour and sixty at sea, of which forty were spent in the operational area. In December 1944 U-boats spent sixty-three days in harbour and thirty-seven at sea, only nine of which were in the operational area. The most surprising revelation of the survey was the fact that U-boat losses amounted to eighteen in four months – just over 10 per cent of those at sea. This was no higher than the losses recorded in the latter half of 1942. From BdU’s perspective, they could rightly believe they were now back on the offensive."

In mid 1940, the Germans captured the Dutch snorkel-eqipped O-26 and O-27. If they had decided to develop the device, they could have had some prototypes by mid 1941. By late 1941 they would have found that snorkels made operations around the British Isles possible. The historical German snorkel allowed a speed of only 6 knots. However, by the end of the war a more streamlined snorkel was developed by Oelfken. It contained a tube through which the periscope could be raised and it wouldn't suffer any significant vibrations due to higher speeds. This snorkel had a highly effective electro-pneumatic head-valve and anti-radar coating. It was calculated that it would enable speeds of 10-11 knots. This device could have been ready by mid 1942 and fitted on all U-boats by the end of the year. The timely adoption of the snorkel would have saved hundreds of boats lost to air power in 1943 and 1944. The transfer from mid Atlantic wolfpacks to coastal operations would have saved a lot of fuel and decreased the time wasted in travel. There is no point trying to find a convoy in the Atlantic when you can wait for it to pass overhead around the British Isles in relative safety. The escort carriers would have been a waste of money.

The Type VII U-boats could have been optimized for snorkel operations. A possible improvement would be to fit six torpedo tubes at the bow and two at the stern as historically suggested for the Type VIIC/43 [1]. Fitting a variable-pitch propellers and boosting the voltage of the motors was estimated to increase the underwater speed to 12 knots without any other changes [2]. Getting rid of the guns and fitting a streamlined tower would boost the underwater speed by another 5% and also greatly improve the depth-keeping which is very important for snorkeling [2]. The boats could get hull shape optimized for underwater travel. I would guess at least a knot of additional underwater speed for a total of about 13 - 13.5. Some additional batteries could be fitted instead of the ammunition room and parts of the internal fuel storage close to battery rooms A and B (probably about 42 cells, a 33% increase). The speed and additional battery capacity should allow some short-range maneuvering for convoy attacks. I would also add an air conditioner to de-humidify the boats as snorkeling led to a lot of condensation and short circuits.

I think the Germans missed a big opportunity here. As the Director of the Anti-U-boat Division Captain Clarence Howard-Johnson said in November 1944:

"The snorkel has had such far-reaching results that the whole character of the U-boat war has been altered in the enemy’s favour. Frequently he has managed to penetrate to and remain on our convoy routes in focal areas with impunity in spite of intensive air and surface patrols. With more experience in training and with the confidence engendered by his present immunity from air, and often from surface attack, he is likely, in the future, to do us more real harm than he has up to the present."

[1] [2] Rossler, The U-boat, 160, 161
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Re: Earlier snorkel - a big missed opportunity for the U-boats?

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StanS wrote: Thu Jun 20, 2024 5:43 am I've always thought about the snorkel as a device hated by the crews turning the U-boats into a slow, deaf weapon. However, this assessment seems to have been completely wrong. The snorkel allowed the U-boats to penetrate the defenses around the British Isles. The shallow waters should have been a death trap, but the opposite was true - a U-boat laying motionless on the bottom was very difficult to locate by ASDIC or hydrophones. Aircraft had great difficulty finding them. Since the U-boats had problems with long-range radio communications, the value of Ulrta and radio direction finding were greatly diminished.

A very interesting book on the subject is Hamilton, Total Undersea War - The evolutionary role of the snorkel in Donitz's U-boat fleet in 1944-1945:

"When BdU surveyed the operations of the last few months they recognized that the sinking of enemy vessels was low, but this was a function of mathematics from their perspective. The average number of days in the operational area was significantly reduced compared with 1942 as a result of the much longer passage at a slow snorkeling speed, but also owing to a disproportionate time spent in port. In August 1942, out of 100 days the average U-boat spent forty in harbour and sixty at sea, of which forty were spent in the operational area. In December 1944 U-boats spent sixty-three days in harbour and thirty-seven at sea, only nine of which were in the operational area. The most surprising revelation of the survey was the fact that U-boat losses amounted to eighteen in four months – just over 10 per cent of those at sea. This was no higher than the losses recorded in the latter half of 1942. From BdU’s perspective, they could rightly believe they were now back on the offensive."

In mid 1940, the Germans captured the Dutch snorkel-eqipped O-26 and O-27. If they had decided to develop the device, they could have had some prototypes by mid 1941. By late 1941 they would have found that snorkels made operations around the British Isles possible. The historical German snorkel allowed a speed of only 6 knots. However, by the end of the war a more streamlined snorkel was developed by Oelfken. It contained a tube through which the periscope could be raised and it wouldn't suffer any significant vibrations due to higher speeds. This snorkel had a highly effective electro-pneumatic head-valve and anti-radar coating. It was calculated that it would enable speeds of 10-11 knots. This device could have been ready by mid 1942 and fitted on all U-boats by the end of the year. The timely adoption of the snorkel would have saved hundreds of boats lost to air power in 1943 and 1944. The transfer from mid Atlantic wolfpacks to coastal operations would have saved a lot of fuel and decreased the time wasted in travel. There is no point trying to find a convoy in the Atlantic when you can wait for it to pass overhead around the British Isles in relative safety. The escort carriers would have been a waste of money.

The Type VII U-boats could have been optimized for snorkel operations. A possible improvement would be to fit six torpedo tubes at the bow and two at the stern as historically suggested for the Type VIIC/43 [1]. Fitting a variable-pitch propellers and boosting the voltage of the motors was estimated to increase the underwater speed to 12 knots without any other changes [2]. Getting rid of the guns and fitting a streamlined tower would boost the underwater speed by another 5% and also greatly improve the depth-keeping which is very important for snorkeling [2]. The boats could get hull shape optimized for underwater travel. I would guess at least a knot of additional underwater speed for a total of about 13 - 13.5. Some additional batteries could be fitted instead of the ammunition room and parts of the internal fuel storage close to battery rooms A and B (probably about 42 cells, a 33% increase). The speed and additional battery capacity should allow some short-range maneuvering for convoy attacks. I would also add an air conditioner to de-humidify the boats as snorkeling led to a lot of condensation and short circuits.

I think the Germans missed a big opportunity here. As the Director of the Anti-U-boat Division Captain Clarence Howard-Johnson said in November 1944:

"The snorkel has had such far-reaching results that the whole character of the U-boat war has been altered in the enemy’s favour. Frequently he has managed to penetrate to and remain on our convoy routes in focal areas with impunity in spite of intensive air and surface patrols. With more experience in training and with the confidence engendered by his present immunity from air, and often from surface attack, he is likely, in the future, to do us more real harm than he has up to the present."

[1] [2] Rossler, The U-boat, 160, 161
Hi! In my knowledge, snorkel was a mixed blessing. Yes, it allowed the sub to use diesels while submerged, but:

1-It disabled its primary detection sensor, the Eyeball Mk-1 of her crew
2-ASW radar could detect the snorkel, but the Uboat would get no warning of the oncoming air attack
3-The submerged hull would better propagate the diesel noise
4-The model used by the Type VII would allow (in my knowledge) a snorkeling speed of about 6 knots, far lower than the maximum surface speed of 21 knots, or even the cruising one of 10 knots

About what I high lighted, I have done some excels with voltages, amp, batteries and consumptions of some example subs I could find in the web, mainly German DKM boats. I doubt that a Type VII could have boosted her speed to such levels. It would have required doubling at least the electric powering, and there is no way you can accommodate double the weight of batteries with such limited modifications. And even if you can, that would give 12 knots for an hour, far less than what was achieved with the Type XXI design. I much respect Rossler, but this requires higher detail.
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Re: Earlier snorkel - a big missed opportunity for the U-boats?

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marcelo_malara wrote: Thu Jun 20, 2024 6:33 pm
Hi! In my knowledge, snorkel was a mixed blessing. Yes, it allowed the sub to use diesels while submerged, but:

1-It disabled its primary detection sensor, the Eyeball Mk-1 of her crew
2-ASW radar could detect the snorkel, but the Uboat would get no warning of the oncoming air attack
3-The submerged hull would better propagate the diesel noise
4-The model used by the Type VII would allow (in my knowledge) a snorkeling speed of about 6 knots, far lower than the maximum surface speed of 21 knots, or even the cruising one of 10 knots

About what I high lighted, I have done some excels with voltages, amp, batteries and consumptions of some example subs I could find in the web, mainly German DKM boats. I doubt that a Type VII could have boosted her speed to such levels. It would have required doubling at least the electric powering, and there is no way you can accommodate double the weight of batteries with such limited modifications. And even if you can, that would give 12 knots for an hour, far less than what was achieved with the Type XXI design. I much respect Rossler, but this requires higher detail.
Hello! :) Good points. Indeed, the snorkel had its flaws.

I would add to point 2 that according to The British Air Ministry ORS/CC Report Nr. 325 from 5 January 1945 titled Operational Experience Against U-Boats Fitted with Snorkel, the ASV Mark V's range of detection fell to about 4 miles from 13 for a surfaced boat. However, radar efficiency was very low in sea states more than 3 because of the sea returns. The snorkel could be operated in sea states up to 5 or 6. Indeed, the historical top speed was 6 knots. The Type XXI had a telescopic snorkel which should have allowed speeds of up to 13 knots but the vibrations of the periscope were so great that it was limited to 8.

The Oelfken design promised to solve this, but it never left the drawing board. In this design, the folding snorkel mast was raised when the periscope was down. When in the up position, the periscope was raised and lowered through a hole in a hydro-dynamically shaped hood covering connected to the snorkel. This reduced vibrations in the periscope shielding and provided significant stability when it was raised to the highest position. The Chief U-boat Engineer Admiral Thedsen believed that the Oelfken design was the best at the time and should be a standard fitting across all U-boats. Rossler mentions speeds of 10- 11 knots for it.

Some interesting statistics are mentioned in Hamilton's book. In the period July - Sep 44 26 ships were torpedoed with only 3 U-boats sunk after their attacks. Between mid-Sep to Dec, 3 ships were sunk with no U-boats lost (the U-boats were re-locating). From mid-Dec to Feb 45, 23 ships were torpedoed with 3 U-boats sunk after their attacks. From mid-Feb to May, 35 ships were torpedoes and 8 U-boats were sunk after their attacks. The Allied tactics were starting to get results, but the number of operational U-boats during this period was also greatly increased. During each of the first 3 periods one U-boat was sunk by aircraft and in the last - 6. These losses, however, are minor compared to the carnage in 1943 and 1944.

I also have doubts about the high speed. As far as I know, the Type XXIII had a motor from Type VII with boosted voltage which increased the power from 375 to 580 hp. Do you have any idea how much this could have increased the speed?
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Re: Earlier snorkel - a big missed opportunity for the U-boats?

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StanS wrote: Thu Jun 20, 2024 7:37 pm
I also have doubts about the high speed. As far as I know, the Type XXIII had a motor from Type VII with boosted voltage which increased the power from 375 to 580 hp. Do you have any idea how much this could have increased the speed?
You mean increasing the power of a Type VII from 2 x 375 to 2 x 580? In that case, a crude estimate, assuming a cubic variation of power to speed due to the hull being completely immersed in the fluid (ie no wave making resistance), would give a 15% increase of speed to a 54% variation in the power.

Anyway, I think that the Type XXI got its performance from two factors:

-streamlining. A quick comparison would give a clue to this. The Type VII needs 750 hp to propel a 800 t boat to 8.75 kt. The Type XXI 1800 t is propelled to 6 kt by her creeping 450 hp electric motor

-massive battery. May be not everyone is aware of this. The Type VII had 2 x 62 cells battery, total 124 cells, weight 61t. The Type XXI has 6 x 62 cells battery, total 372 cells, weight 235 t.

In my opinion not a single factor of this two would give a performance close to the Type XXI. You need both. And in turn this requires a big boat.
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Re: Earlier snorkel - a big missed opportunity for the U-boats?

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marcelo_malara wrote: Fri Jun 21, 2024 5:21 pm Anyway, I think that the Type XXI got its performance from two factors:

-streamlining. A quick comparison would give a clue to this. The Type VII needs 750 hp to propel a 800 t boat to 8.75 kt. The Type XXI 1800 t is propelled to 6 kt by her creeping 450 hp electric motor

-massive battery. May be not everyone is aware of this. The Type VII had 2 x 62 cells battery, total 124 cells, weight 61t. The Type XXI has 6 x 62 cells battery, total 372 cells, weight 235 t.

In my opinion not a single factor of this two would give a performance close to the Type XXI. You need both. And in turn this requires a big boat.
Wow! I didn't know streamlining could do that much. I think the only realistic way to radically increase battery capacity would be to fit in an additional section like on the Type VIID or F. I think 7 meters of extra hull would fit 82 additional batteries on the lower floor. With the 42 from the removal of ammunition and the decreasing of some internal fuel tanks, this would double the capacity. The space above this section could be used for a proper crew's quarters and an air conditioner. This would free up the bow for more torpedoes and maybe a reload mechanism. The resulting boat would be about 1000 - 1100 tons submerged. It would be almost as long as Type XXI but slimmer. Maybe it could have made 13 knots or so. We get a kind of interim boat between Type VII and XXI. If such a design was made in early 1942, the first could have been in service by mid 1943.
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Re: Earlier snorkel - a big missed opportunity for the U-boats?

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StanS wrote: Sat Jun 22, 2024 6:52 am

Wow! I didn't know streamlining could do that much. I think the only realistic way to radically increase battery capacity would be to fit in an additional section like on the Type VIID or F. I think 7 meters of extra hull would fit 82 additional batteries on the lower floor. With the 42 from the removal of ammunition and the decreasing of some internal fuel tanks, this would double the capacity. The space above this section could be used for a proper crew's quarters and an air conditioner. This would free up the bow for more torpedoes and maybe a reload mechanism. The resulting boat would be about 1000 - 1100 tons submerged. It would be almost as long as Type XXI but slimmer. Maybe it could have made 13 knots or so. We get a kind of interim boat between Type VII and XXI. If such a design was made in early 1942, the first could have been in service by mid 1943.
Hi. A 7m plug would float about 120 t, so yes, you could add the cells. Doubling the battery would double submerge range, for example she would make 40 hours at 4 kt in lieu of 20 hr @ 4kt. But I do not think you could increase speed, that would need a different engine that could couple with the extra voltage. The standard Type VII would connect the cells in series for max power, ie 124 cells * 2 volt = 248 volt, now you could have 248 cells * 2 volt = about 500 volts.
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Re: Earlier snorkel - a big missed opportunity for the U-boats?

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marcelo_malara wrote: Sat Jun 22, 2024 7:35 pm Hi. A 7m plug would float about 120 t, so yes, you could add the cells. Doubling the battery would double submerge range, for example she would make 40 hours at 4 kt in lieu of 20 hr @ 4kt. But I do not think you could increase speed, that would need a different engine that could couple with the extra voltage. The standard Type VII would connect the cells in series for max power, ie 124 cells * 2 volt = 248 volt, now you could have 248 cells * 2 volt = about 500 volts.
Hello! Thank you for that analysis! Rossler mentions that the voltage of the motors could be increased, but not by how much. If it could be doubled this would lead to a total of 1500 hp.

"A more difficult problem would have been the provision and installation of appropriate electric motors giving a higher performance. Almost certainly, the entire engine installation would have had to have been changed, and it would have been simpler to have incorporated variable pitch propellers, giving an increased number of revolutions from an increased voltage in the normal electric motors. (Without any change of shape of the outer skin, an increase in the submerged speed to approximately 12 knots was calculated.)" Rossler, The U-boat, 161

Also, improved hydrodynamics should increase cruising underwater speed with the same power, so range should increase more, i.e. 40 hours at 5 knots, 200 nm.

When the British modified HMS Seraph to experiment against high-speed U-boats, they boosted the motors from 1400 to 1600 hp., streamlined the hull and got 12.5 knots of speed. This boat is similar in size to a Type VII but about a meter wider. Maybe 12.5 - 13 knots could have been achieved.
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Re: Earlier snorkel - a big missed opportunity for the U-boats?

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StanS wrote: Sun Jun 23, 2024 6:56 am

Hello! Thank you for that analysis! Rossler mentions that the voltage of the motors could be increased, but not by how much. If it could be doubled this would lead to a total of 1500 hp.

"A more difficult problem would have been the provision and installation of appropriate electric motors giving a higher performance. Almost certainly, the entire engine installation would have had to have been changed, and it would have been simpler to have incorporated variable pitch propellers, giving an increased number of revolutions from an increased voltage in the normal electric motors. (Without any change of shape of the outer skin, an increase in the submerged speed to approximately 12 knots was calculated.)" Rossler, The U-boat, 161

Also, improved hydrodynamics should increase cruising underwater speed with the same power, so range should increase more, i.e. 40 hours at 5 knots, 200 nm.

When the British modified HMS Seraph to experiment against high-speed U-boats, they boosted the motors from 1400 to 1600 hp., streamlined the hull and got 12.5 knots of speed. This boat is similar in size to a Type VII but about a meter wider. Maybe 12.5 - 13 knots could have been achieved.
Hi! I do not think you can double the maximum voltage of the motors, but I am not expert in electric motors. The paragraph in confusing, may be there is a translation issue here. Anyway, If you put new motors or if you can double the voltage of the existing ones, the main problem to cope with double powering is making it useful for the propellers. You can do this in three forms:

-increasing rpm, but for a given propeller design this would lead to cavitation
-increasing diameter, but for sure this can not be done in the same hull
-variable pitch. At higher power and for increased speed you would put the blades at a coarse pitch, increasing the angle of attack of the blade to the oncoming flow. But you do not increase rpm in the same percentage as the increase in powering, because that would surely lead to cavitation
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Re: Earlier snorkel - a big missed opportunity for the U-boats?

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Increasing the angle of attack of the blade will increase the powering via torque and not via rpm. Remember that:

hp = torque * rpm/60
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Re: Earlier snorkel - a big missed opportunity for the U-boats?

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Interesting. I'll kindly ask if someone has Rossler's German edition to take a look and see if it has any additional information.
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Re: Earlier snorkel - a big missed opportunity for the U-boats?

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On the issue of streamlining, the Norwegians operated three Type VIIs after WW2 and cleaned them up. If I remember, the effect was not great in increasing the maximum submerged speed but the boats handled significantly better. If you intend to operate with a snorkel, you don't need any of the gun armament and can thus save some weight and drag.
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Re: Earlier snorkel - a big missed opportunity for the U-boats?

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Mostlyharmless wrote: Wed Jun 26, 2024 11:12 am On the issue of streamlining, the Norwegians operated three Type VIIs after WW2 and cleaned them up. If I remember, the effect was not great in increasing the maximum submerged speed but the boats handled significantly better. If you intend to operate with a snorkel, you don't need any of the gun armament and can thus save some weight and drag.
So the USN did have better luck streamlining theirs Balao in the GUPPY program.
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Re: Earlier snorkel - a big missed opportunity for the U-boats?

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Mostlyharmless wrote: Wed Jun 26, 2024 11:12 am On the issue of streamlining, the Norwegians operated three Type VIIs after WW2 and cleaned them up. If I remember, the effect was not great in increasing the maximum submerged speed but the boats handled significantly better. If you intend to operate with a snorkel, you don't need any of the gun armament and can thus save some weight and drag.
I think you might be referring to something Rossler also mentions. I might as well post the whole thing:

"Very possibly, even if inessential construction and appendages had been removed, and a more streamlined bridge shape adopted, no significant increase in submerged speed could have been achieved by the rather poor performance of existing electric motors. This opinion is supported by post-war trials carried out by the Norwegian Navy using the former U926 redesignated NKM Kya. Using the reshaped bridge style of the Walter U-boats, instead of the usual VIlC bridge (minus 'wintergarden', the AA platform behind the bridge, and armament, only an insignificant increase in speed (38 revolutions per knot as opposed to 40 revolutions per knot) resulted, but the depthkeeping properties were greatly improved.

A decisive improvement could only be brought about by changing the shape of the outer ship and enlarging the battery capacity and submerged power unit. As early as the summer of 1943, in an 'investigation into the increase in submerged speed of the VIlC/42 boat', suggestions were made concerning the trebling of the submerged power in Type VIlC. Additional battery capacity was to be obtained by removing the surface armament, by a reduction of the four reserve torpedoes and of the interior fuel-oil bunkers. A further proposal was the installation of larger battery cells even at the cost of accessibility. A more difficult problem would have been the provision and installation of appropriate electric motors giving a higher performance. Almost certainly, the entire engine installation would have had to have been changed, and it would have been simpler to have incorporated variable pitch propellers, giving an increased number of revolutions from an increased voltage in the normal electric motors. (Without any change of shape of the outer skin, an increase in the submerged speed to approximately 12 knots was calculated.) In any event, such conversions would have taken up a significant part of yard capacity and, inevitably, would have delayed the Type XXI programme, and this would have been totally unacceptable."

It would be great if we can somehow get the 'investigation into the increase in submerged speed of the VIlC/42 boat' study that he mentions.
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Re: Earlier snorkel - a big missed opportunity for the U-boats?

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It is possible that the Type VII was too narrow and too long, for surface riding, whereas submerge resistance depends mainly on surface friction, and this depends on wet surface. To illustrate the point:

-suppose a cube, 2m each side. Its volume would be 2 * 2* * 2 = 8 m^3. The surface of its 6 faces would be 2 * 2 * 6 = 24 m^2
-now suppose the same 8 m^3, in a prism 8 m long, 1 m wide and 1 m high. Its surface would be 8 * 4 + 2 * 1 = 34 m^2

No matter how much you remove appendages, the form is just not good for the job.
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Re: Earlier snorkel - a big missed opportunity for the U-boats?

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marcelo_malara wrote: Thu Jun 27, 2024 9:47 pm It is possible that the Type VII was too narrow and too long, for surface riding, whereas submerge resistance depends mainly on surface friction, and this depends on wet surface. To illustrate the point:

-suppose a cube, 2m each side. Its volume would be 2 * 2* * 2 = 8 m^3. The surface of its 6 faces would be 2 * 2 * 6 = 24 m^2
-now suppose the same 8 m^3, in a prism 8 m long, 1 m wide and 1 m high. Its surface would be 8 * 4 + 2 * 1 = 34 m^2

No matter how much you remove appendages, the form is just not good for the job.
Interesting. However, isn't a torpedo the optimal underwater shape? It is also long and narrow.
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