WesWorld

The Belgian Navy is seeking tenders to fulfil the requirement for a small coastal submarine. The Ministry of Defence wishes to receive submissions from all interested parties.

The requirements laid down are:
Small size, must be able to fit into a Type 0 dock if possible
Diving Depth: given the mean depth of the North Sea is only 95m, the hull crush depth will be set at 200m.
Torpedo Armament: 8x 533mm bow tubes with sixteen torpedoes (eight in tubes and eight reloads). Stern tubes are not required. The provision to lay mines through the torpedo tubes is required.
Gun Armament: none required, 1x HMG/LAA cannon can be fitted if underwater performance is not affected
Speed: the emphasis on underwater speed requires a cruising speed of at least 14kts and an attack 'burst' speed of 22kts on electric power alone sufficient for one attack
Powerplant: diesel and battery/electric motor propulsion is mandatory, as is the fitting of a Snorkel to allow the operation of the diesels while submerged
Endurance: enough fuel and crew provisions for a continuous 14-day voyage must be provided (diesel fuel allowance for 14 days constant cruise at 10kts plus 5% time spent at 20kts, battery capacity to allow for at least 2 days constant cruise at 6kts plus 5% of that time spent at 22kts).
Fire-Control: attack and search periscopes, passive hydrophones with 360 degree coverage, retractable search-search radar, active ranging sonar, torpedo fire-control computer
Crew: the smallest allowing for efficient operation of the submarine

Costings: Costings should be made for development and construction of two boats, either by the designer or sub-contracted to a Belgian shipyard.

Edit: see revised endurance requirements and underwater speed requirements.

*looks at requirements* ... no wonder everyone makes jokes about the Belgians. You sure you do not want the additional requirement of masts with sails that can be raised should the submarine run out of fuel?

"fuel allowance for 14 days constant cruise at 14kts" means a minimum range of 14 x 24 x 14 = 4704 nm at 14 knots.

"plus 25% time spent at 25kts" means that a maximum speed of at least 25 knots is required as either surface of submerged speed and a minimum range of 14 x 0.25 x 24 x 25 = 2100 nm at 25 knots is required in addition to the above range.


... all that combined with a few other things, I doubt it would fit into anything that must be able to git into a Type 0 dock, unless you would settle for submarine that is 70 meters long and have a beam and depth in excess of 20 meters...

.. but ignoring the length requirement and putting the other requirements together, you would get something like this unsafe thing...


ABSOLUTELY-No-Smoking-Allowed-Inside-This-H2O2-Powered-Submarine, Belgian Submarine.
Date: 1948
Type: Coastal
Length: 170.0m
Beam: 10.0m
Draft: 10.5m
Crush depth: 225.0m
Light Displacement 2008t
Loaded Displacement 7943t
Full Displacement 8925t
wt fuel&batts: 5895t
Reserve buoyancy: 11%

Armament:
- 8 x 533mm (bow)
- 40 tons for mines or reload torpedoes
H2O2HP: 48100hp
DieselHP: 5900hp
Speed:
- Max Surf Speed: 14.0 knots
- Max Sub Speed: 25.0 knots (= minimum maximum speed required)
Range:
- Surfaced: 4,705nm@14 knots (minimum 4704 nm at 14 knots)
- Submerged: 2105nm@25 knots (minimum 2100 nm at 25 knots)
Tons Oil: 395.3t
Tons H2O2: 5500.0t
Miscellaneous Weight: 150 tons

.. or a safer one like this...

Alternative-Safer-Option-Using-Electric-Engines-But-Not-As-Good, Belgian Submarine
Date: 1948
Type: Coastal
Length: 170.0m
Beam: 10.0m
Draft: 10.5m
Crush depth: 225.0m
Light Displacement 6961t
Loaded Displacement 7961t
Full Displacement 8925t
wt fuel&batts: 3460t
Reserve buoyancy: 11%

Armament:
- 8 x 533mm (bow)
- 40 tons for mines or reload torpedoes
ElecHP: 32900hp
DieselHP: 33500hp
Speed:
- Max Surf Speed: 25.0 knots (= minimum maximum speed required)
- Max Sub Speed: 22.0 knots (= minimum electric powered maximum speed required)
Range:
- Surfaced: 3,584nm@25 knots (weight of fuel for range @14 knots added to weight of fuel for range @25 knots gives this range @ 25 knots
- Submerged: 69nm@16 knots (given underwater cruising speed requirement of 16 knots used)
Tons Oil: 960.0t
Tons Battery: 2500.0t
Miscellaneous Weight: 150 tons


... Japan thinks that the Belgians would be much better of with something based off [bragging mode on] the best submarine in Wesworld [bragging mode off]... the I-315.

I've edited the requirements slightly to:

Speed: the emphasis on underwater speed requires a cruising speed of at least 14kts and an attack 'burst' speed of 22kts on electric power alone sufficient for one attack
Endurance: enough fuel and crew provisions for a continuous 14-day voyage must be provided (diesel fuel allowance for 14 days constant cruise at 10kts plus 5% time spent at 20kts, battery capacity to allow for at least 2 days constant cruise at 6kts plus 5% of that time spent at 22kts).

I don't that should be impossible to achieve. What the Belgians have in mind is a small submarine destroyer, that is a sub that can loiter offshore and then attack any invading amphibious or raiding force making a series of quick attacks, engaging up to four targets (two torpedoes each) and then withdrawing to reload and hit again. Due to the small range of the North Sea littoral most support will be from the base so the two subs are only designed for two-week stints at the most. Thus overall range isn't as important as endurance. I think the lower speeds called for should allow for less fuel. The battery requirements are to ensure if the snort isn't operable for any reason or if enemy activity makes it unwise to use, it can still complete its mission without surfacing. 48hrs would be the absolute maximum due to breathable air etc. 2.4 hours of attack speed should be more than sufficient and is perhaps the more important of the two to achieve.
I will state no maximum surfaced speed, it doesn't matter too much as the operational patrol areas will be more or less on the doorstep and surface operation is mainly for transit or recharging on the surface, doing the laundry and suntan facilities (summertime only).

Also, Belgium has no submarine torpedoes so the winner of the contest will probably get that order too.

I'll take a stab at it, but the 22-knot 'submerged burst / attack speed' requirement makes it ambitious for most 1950s submarines, let alone a 1940s submarine. You really don't get that kind of performance without incorporating nuclear power.

Really, it sounds more like Belgium wants to bid on a coast-defense minefield, rather than a submarine...

Quoted

I'll take a stab at it, but the 22-knot 'submerged burst / attack speed' requirement makes it ambitious for most 1950s submarines, let alone a 1940s submarine. You really don't get that kind of performance without incorporating nuclear power.

Well, OTL there were the Japanese 1937 Submarine No.71 which was intended to go 25 knots (but only managed to get to just over 21 knots) and the German 1940 V-80 with its 28 knots thanks to its propulsion system so while ambitious, it does not look to be impossible for a late 1940s submarine...

... but I would think the better question you should ask is: why would Belgium, which (according to the encyclopedia) has no submarines, believe that it needs a maximum submerged speed of 22 knots for its submarine? Better question... How many submarines are there in Wesworld that can go that fast to make the Belgians believe that they should have a submarine that goes 22 knots under water?

Here's a list (hopefully got them all and got them all correct) of Wesworld submarines and their underwater speed. I only looked at the ones done with Subsim as it gives the underwater speed. I have a 23.2 knot sub planned for 1948 but have not posted it yet so I did not include that one in the list (guess I will post that one a bit later).

I-314 (Japan) ------------- 25.6 knots
I-315 (Japan) ------------- 22.8 knots
I-310 (Japan) ------------- 20.2 knots
I-402 (Japan) ------------- 19.9 knots
M401 (Russia) ------------- 18.6 knots
Durjeya (India) ----------- 17.9 knots
Emeraude (France) --------- 17.2 knots
Type XXI (Germany) -------- 17.2 knots
O Class (Great Britain) --- 17.1 knots
Type XXIII (Germany) ------ 17.0 knots
Gymnote (France) ---------- 16.6 knots
O-22 Class (Netherlands) -- 15.4 knots
Barrakuda (Russia) -------- 15.1 knots
S-class (Argentina) ------- 14.5 knots
SG-1 (Iberia) ------------- 14.4 knots
Tirador (Philippines) ----- 12.5 knots
Prakopa (India) ----------- 12.1 knots
Union (Chile) ------------- 11.4 knots
Gualcolda (Chile) --------- 11.0 knots
Type 42 (Atlantis) -------- 10.7 knots
Protée (France) ----------- 10.6 knots
Aluhaman (Philippines) ---- 10.3 knots
U Class (Italy) ----------- 10.2 knots
R-98 (Atlantis) ----------- 10.0 knots
Akula (Bulgaria) ---------- 10.0 knots
Roland Morillot (France) --- 9.9 knots
Adua (Italy) --------------- 9.9 knots
Mako (Philippines) --------- 9.9 knots
T Class (Italy) ------------ 9.5 knots
T Class (Great Britain) ---- 9.3 knots
Daphne (France) ------------ 9.2 knots
Thetis (France) ------------ 9.2 knots
Audaz (Philippines) -------- 9.2 knots
Type VII (Germany) --------- 9.1 knots
Héroe (Chile) -------------- 8.5 knots
Argonauta (Italy) ---------- 8.5 knots
Europa (Latvia) ------------ 8.5 knots
Bradán Feasa (Ireland) ----- 8.3 knots
Type IV (Germany) ---------- 8.2 knots
Perla (Italy) -------------- 8.2 knots
Lautaro (Chile) ------------ 8.0 knots
Pooka (Ireland) ------------ 8.0 knots
Antonio Sciesca (Italy) ---- 7.8 knots
Type IX (Germany) ---------- 7.4 knots

Quoted from "Rooijen10"

Quoted

I'll take a stab at it, but the 22-knot 'submerged burst / attack speed' requirement makes it ambitious for most 1950s submarines, let alone a 1940s submarine. You really don't get that kind of performance without incorporating nuclear power.

Well, OTL there were the Japanese 1937 Submarine No.71 which was intended to go 25 knots (but only managed to get to just over 21 knots) and the German 1940 V-80 with its 28 knots thanks to its propulsion system so while ambitious, it does not look to be impossible for a late 1940s submarine...

Both experimental submarines, though. I'd even classify the V-80 derivative Type XVIIs as an experimental submarine.

Most series-built diesel-electric submarines (mostly oceanic-sized subs) tend to have speeds of ~18 knots or lower, whether they're 1940s, 1950s, or even 1960s boats. For instance, the Oberon-class boats from the 1960s only make around seventeen knots submerged. (I forget the exact speed figure.) Coastal submarines through the 1960s usually didn't exceed 15 knots (for instance, the Russian Quebecs, the Japanese Ha-201, etc). I think anything exceeding those figures deserves a pretty careful and skeptical look.

Quoted

Both experimental submarines, though. I'd even classify the V-80 derivative Type XVIIs as an experimental submarine.

True, but with the Type XVII (which I actually had not looked at) you are not that far off if you do not care about the well-being of the crew of the vessel.

Quoted

Most series-built diesel-electric submarines (mostly oceanic-sized subs) tend to have speeds of ~18 knots or lower, whether they're 1940s, 1950s, or even 1960s boats. For instance, the Oberon-class boats from the 1960s only make around seventeen knots submerged. (I forget the exact speed figure.) Coastal submarines through the 1960s usually didn't exceed 15 knots (for instance, the Russian Quebecs, the Japanese Ha-201, etc).

Funny. 18 knots was what I was thinking of as a reasonable maximum speed based on the list I made. 17-18 knots should probably be the target to go for. As for the coastal submarine's 15 knots, would the size of those boats not play a bit of a factor in that?

Trying to make something work, a few more things with the tweaked requirements...

Quoted

I will state no maximum surfaced speed

Actually you did state a minimum maximum surfaced speed...

Quoted

diesel fuel allowance for 14 days constant cruise at 10kts plus 5% time spent at 20kts

That right there says "maximum speed using diesels is 20 knots minimum".


Throwing the values into a modified version of my planned 1948 sub as test...

2.4 hours at 22 knots is 52.8 nm. Entering that in subsim means that the amount of batteries required for the 52.8 nm @ 22 knots would be 1150 tons (which is something that boat can't handle as it is too small for that).

Now changing the submerged speed from 22 knots to 6 knots means that the 1150 tons of batteries is enough for a range of ~1300 nautical miles which is probably a lot more than any submarine in this time frame. Quickly making it bigger to handle the batteries gave me about the same 6 knot range result but with more battery weight.

... and here I thought that the 800 nm @ 5 knots of the I-315 was a little bit on the high side... At its top speed of 22.8 knots, the range is only 12 nm which is good enough for 31 minutes and 34 seconds and I think that 5-10 minutes max at attack speed would do. Once that is done, recharge the batteries when it is safe so 2.4 hours seems overkill to me...

Quoted from "Rooijen10"

Quoted

Both experimental submarines, though. I'd even classify the V-80 derivative Type XVIIs as an experimental submarine.

True, but with the Type XVII (which I actually had not looked at) you are not that far off if you do not care about the well-being of the crew of the vessel.

Precisely. HTP-powered boats are not really safe and barely functional prototypes. For instance, France's Gymnote works so poorly that it still hasn't, and never will, be commissioned as a warship in the MN...

Quoted from "Rooijen10"

Quoted

Most series-built diesel-electric submarines (mostly oceanic-sized subs) tend to have speeds of ~18 knots or lower, whether they're 1940s, 1950s, or even 1960s boats. For instance, the Oberon-class boats from the 1960s only make around seventeen knots submerged. (I forget the exact speed figure.) Coastal submarines through the 1960s usually didn't exceed 15 knots (for instance, the Russian Quebecs, the Japanese Ha-201, etc).

Funny. 18 knots was what I was thinking of as a reasonable maximum speed based on the list I made. 17-18 knots should probably be the target to go for. As for the coastal submarine's 15 knots, would the size of those boats not play a bit of a factor in that?

From what I can see, most of the coastal boats had lower speeds than the fleet boats, which I'd guess is due to a mix of the following design conditions:
-- First, a smaller coastal submarine is probably not going to have a lot of space for the larger engines and batteries required to achieve a particularly high speed. Some things, like the weapons and the crew don't scale down as the sub gets smaller.
-- Second, most coastal submarine designs tend to prioritize stealth as opposed to strategic or tactical maneuverability. If you're a coastal submarine, you're either planning to take a very stealthy, cautious approach to whack an unsuspecting target, or your pretending to be a hole in the ocean as the enemy steams ignorantly into the range of your weapons. Stealth is always going to be superior in every way to raw speed, since most of the current brand of antisubmarine hunters (namely aircraft and fast DDs) are always fast enough to overtake you; it's better to just avoid being seen.
-- Third, there seems to be a lot of comments on the historical high-speed designs regarding how handling of the boats was really... awkward. Issues with high drag, bad design or placement of the hydroplanes, etc., probably all of which are heightened by higher submerged speeds. If you're operating a submarine in a restricted, high-traffic, shallow environment like the Belgian coastline, "bad handling" is definitely not a desirable trait...

Here are two that I've got in my archive. I made a few very slight updates. France's DCNS therefore proposes this, mindful of the fact that neither actually meets the Belgian minimum performance requirements; but I don't feel the Belgian requirements are reasonable without using an experimental HTP powerplant. (I mean, not even Gymnote meets the speed spec, and the French *designed* Gymnote to test the limits of underwater operation...)

Quoted

Palinure-class Submarine
Date: 1948
Type: Oceanic
Length: 49.6m
Beam: 5.9m
Draft: 4.0m
Crush depth: 225m
Light Displacement 415t
Loaded Displacement 471t
Full Displacement 585t
wt fuel&batts: 115t
Reserve buoyancy: 20%

Armament:
- 4 x 550mm (bow)
- 16 tons for mines or reload torpedoes
ElecHP: 2100hp
DieselHP: 820hp
Speed:
- Max Surf Speed: 11.9 knots
- Max Sub Speed: 16.1 knots
Range:
- Surfaced: 4153nm@10 knots
- Submerged: 122nm@6 knots / 15nm@12 knots
Tons Oil: 40.0t
Tons Battery: 75.0t
Miscellaneous Weight: 24 tons

Crew: 22

Quoted

Grondin-class Submarine
Date: 1948
Type: Coastal
Length: 58m
Beam: 6.7m
Draft: 5.3m
Crush depth: 225m
Light Displacement 730t
Loaded Displacement 904t
Full Displacement 1030t
wt fuel&batts: 360t
Reserve buoyancy: 12%

Armament:
- 8 x 550mm (bow)
- 14 tons for mines or reload torpedoes
ElecHP: 2800hp
DieselHP: 2400hp
Speed:
- Max Surf Speed: 14.9 knots
- Max Sub Speed: 15.7 knots
Range:
- Surfaced: 7666nm@12 knots
- Submerged: 238nm@6 knots / 38nm@12 knots
Tons Oil: 160.0t
Tons Battery: 200.0t
Miscellaneous Weight: 53 tons

Crew: 32

I worked off my Alternative-Safer-Option-Using-Electric-Engines-But-Not-As-Good submarine and entered the current given requirements and got this...


Range requirements with diesel are 3360 nm @ 10 knots + 336 nm @ 20 knots and with electric they are 288 nm @ 6 knots + 53 nm @ 22 knots.


Requirements II, Belgian Submarine
Date: 1948
Type: Coastal
Length: 170.0m
Beam: 10.0m
Draft: 10.5m
Crush depth: 225.0m
Light Displacement 8673t
Loaded Displacement 8916t
Full Displacement 8925t
wt fuel&batts: 5453t
Reserve buoyancy: 0%

Armament:
- 8 x 533mm (bow)
- 40 tons for mines or reload torpedoes
ElecHP: 32900hp
DieselHP: 17200hp
Speed:
- Max Surf Speed: 20.0 knots
- Max Sub Speed: 22.0 knots
Range:
- Surfaced: 4,737nm@10 knots
- Submerged: 1591nm@6 knots
Tons Oil: 203.0t
Tons Battery: 5250.0t
Miscellaneous Weight: 150 tons

Minimum maximum speed using diesel is given as 20 knots and the minimum maximum using electric is 22 knots. With the range, I first entered the oil/battery values to get the high speed requirements and afterwards changed the cruising speeds to the lower requirements. I then calculated the range needed at the lower speeds and added to the converted high speed ranges and adjusted oil/battery weights to get the above values in the sim.

Note that the reserve buoyancy is 0% so I would have to make the submarine even bigger and then have to increase engine output and add more oil and batteries in order to get the reserve buoyancy up to the minimum of 10%... so if I were to tweak it even more (and I am a bit too lazy to do that right now), I think I would end up with a submarine that is in the 10,000-11,000 ton range... and the same weight in mines is probably going to be a lot more effective than one such submarine.

I hate to say it but I think that the current range requirements are even more demanding than the initial ones.

Well ok, at least we know where the bleeding edge is. In Belgium's defence they have no submarine arm and therefore one Captain reading a bit of Jules Verne probably hasn't worked out too well.

The third round would then revise further to:
Speed: the emphasis on underwater speed requires a cruising speed of at least 10kts and an attack 'burst' speed of 18kts on electric power alone sufficient for one attack
Endurance: enough fuel and crew provisions for a continuous 14-day voyage must be provided (diesel fuel allowance for 14 days continuous cruising at 10kts, battery capacity to allow for at least eight hours continuous cruise at 6kts plus sufficient charge for 1 hour at 18kts).

That should probably be more than enough electric propulsion time given sighting a target, closing in, making two attacks and then departing the danger zone. I guess at an emergency eight hours creeping would bring the sub back within friendly ship support or harbour if things got really sticky. Or the can just surface and rig those masts to pretend they are an innocent fishing smack!

Quoted

Well ok, at least we know where the bleeding edge is. In Belgium's defence they have no submarine arm and therefore one Captain reading a bit of Jules Verne probably hasn't worked out too well.

Sorry for being such a pain... uhm... I mean send that captain the the North Pole on a snowflake counting expedition!!!


Perhaps something like this...

IB-1 class, Belgian Submarine
Date: 1948
Type: Coastal
Length: 70.0m
Beam: 6.5m
Draft: 6.7m
Crush depth: 225.0m
Light Displacement 1275t
Loaded Displacement 1375t
Full Displacement 1524t
wt fuel&batts: 540t
Reserve buoyancy: 10%

Armament:
- 8 x 533 (bow)
- 30 tons for mines or reload torpedoes
ElecHP: 6000hp
DieselHP: 2000hp
Speed:
- Max Surf Speed: 13.1 knots
- Max Sub Speed: 18.5 knots
Range:
- Surfaced: 3,938nm@10 knots (minimum requirement 3360nm@10 knots)
- Submerged: 447nm@6 knots (minimum requirement 18nm@18 knots + 48nm@6 knots = ~390nm@6 knots + 48nm@6 knots = ~438nm@6 knots)
Tons Oil: 70.0t
Tons Battery: 470.0t
Miscellaneous Weight: 160 tons
Crew: 40

- Diesel-electric drive.
- Main electric engines output: 5,750 shp.
- Silent running engines output: 250 shp.
- Submerged speed: (silent running engines) 6.4 knots, (battery-electric) 18.2 knots.
- Range Surfaced (diesel-electric): 4,862nm@9 knots, 3,938nm@10 knots, 2,735nm@12 knots.
- Range Submerged (batteries, silent running engines): 1,865nm@3 knots, 657nm@5 knots, 447nm@6 knots.
- Range Submerged (batteries, main engines): 236nm@8 knots, 138nm@10 knots, 42nm@15 knots, 22nm@18 knots.
- Endurance: (surfaced, diesel) 16 days and 9.8 hours @ 10 knots, (submerged, batteries) 3 days and 2.5 hours @ 6 knots.

- 160 tons Miscellaneous weights breakdown:
--- 10 tons for Ninjatousaya System.
--- 60 tons for sonar, hydrophones, air/surface radar and radar warning receiver.
--- 34 tons for defense countermeasures and 'camouflage'.
--- 13 tons for air condition system.
--- 3 tons for 60 oxygen bottles (50 l, 150 atm, about 435 hours of oxygen total).
--- 5 tons for CO2 removal system + Soda lime.
--- 13 tons for damage control and fire suppression systems.
--- 7 tons for safety and rescue equipment.
--- 8 tons for demagnetization cables.
--- 2 tons for deep freeze unit.
--- 5 tons for desalinization gear.

Yeah, not bad.
Still has a lot of unnecessary misc weight though, could probably trim about 26 tons off the misc weight, that would boost the reserve buoyancy a little.

Just to throw the question out there... at what point ought the sub be simmed as 'oceanic' rather than 'coastal'? At least in terms of displacement, Walter's design is larger than avowed "ocean-going" designs like the Type IX Uboat, and approaching the size of a Type XXI.

Quoted

Still has a lot of unnecessary misc weight though, could probably trim about 26 tons off the misc weight, that would boost the reserve buoyancy a little.

The Japanese look at it differently but if the Belgians want that...

The thing that could be removed are the defense countermeasures and 'camouflage' but then they would probably not need the silent running engines either. I guess the Belgians can hold their breath for a long time so the air condition system, oxygen bottles and CO2 removal stuff will be removed as well as the safety and rescue equipment. Based on that, they probably do not care about a deteriorating hull or safety aboard the submarine so the demagnetization cables and damage control and fire suppression systems will be removed. They probably drink warm beer so we'll remove the deep freeze unit as well as the desalinization gear.

Quoted

Just to throw the question out there... at what point ought the sub be simmed as 'oceanic' rather than 'coastal'? At least in terms of displacement, Walter's design is larger than avowed "ocean-going" designs like the Type IX Uboat, and approaching the size of a Type XXI.

To be honest, in my opinion the limit should be at ~500 tons max (slightly higher than the Cleito Treaty max) so my vessel is well beyond the limit of that to be allowed to be a coastal submarine. To me, even the Type VII is an ocean-going submarine with the lightest version being ~600 tons. But having looked around, there are the French Daphne and Thetis and the Grondin you posted above and the the Filipino Aluhaman, Audaz and Tirador which are of similar size as or slightly bigger than the Type VIIA but with greater range and they are simmed as coastal submarines which to me is incorrect.




... so based on the above stuff, you would end up with this...


IB-1B class, Belgian Submarine
Date: 1948
Type: Ocean
Length: 70.0m
Beam: 6.5m
Draft: 6.7m
Crush depth: 317.5m
Light Displacement 1387t
Loaded Displacement 1477t
Full Displacement 1524t
wt fuel&batts: 522t
Reserve buoyancy: 3%

Armament:
- 8 x 533 (bow)
- 30 tons for mines or reload torpedoes
ElecHP: 5500hp
DieselHP: 1000hp
Speed:
- Max Surf Speed: 10.4 knots
- Max Sub Speed: 18.0 knots
Range:
- Surfaced: 3,364nm@10 knots (minimum requirement 3360nm@10 knots)
- Submerged: 439nm@6 knots (minimum requirement 18nm@18 knots + 48nm@6 knots = ~390nm@6 knots + 48nm@6 knots = ~438nm@6 knots)
Tons Oil: 59.8t
Tons Battery: 462.0t
Miscellaneous Weight: 70 tons
Crew: 39

- 70 tons Miscellaneous weights breakdown:
--- 10 tons for Ninjatousaya System.
--- 60 tons for sonar, hydrophones, air/surface radar and radar warning receiver.


Now trying to squeeze it to Coastal values...


IB-1C class, Belgian Submarine
Date: 1948
Type: Coastal
Length: 65.0m
Beam: 5.7m
Draft: 6.0m
Crush depth: 225.0m
Light Displacement 917t
Loaded Displacement 994t
Full Displacement 1112t
wt fuel&batts: 422t
Reserve buoyancy: 11%

Armament:
- 8 x 533 (bow)
- 30 tons for mines or reload torpedoes
ElecHP: 4500hp
DieselHP: 700hp
Speed:
- Max Surf Speed: 10.0 knots
- Max Sub Speed: 18.0 knots
Range:
- Surfaced: 3,367nm@10 knots (minimum requirement 3360nm@10 knots)
- Submerged: 440nm@6 knots (minimum requirement 18nm@18 knots + 48nm@6 knots = ~390nm@6 knots + 48nm@6 knots = ~438nm@6 knots)
Tons Oil: 47.0t
Tons Battery: 375.0t
Miscellaneous Weight: 70 tons
Crew: 29

- 70 tons Miscellaneous weights breakdown:
--- 10 tons for Ninjatousaya System.
--- 60 tons for sonar, hydrophones, air/surface radar and radar warning receiver.

... which to me is still 400 tons too heavy to be allowed to be simmed as a coastal submarine. But going for H2O2 can get you below 500 tons and will give you this...


IB-1D class, Belgian Submarine
Date: 1948
Type: Coastal
Length: 50.0m
Beam: 4.5m
Draft: 4.8m
Crush depth: 225.0m
Light Displacement 372t
Loaded Displacement 433t
Full Displacement 540t
wt fuel&batts: 41t
Reserve buoyancy: 18%

Armament:
- 8 x 533 (bow)
- 30 tons for mines or reload torpedoes
H2O2HP: 2800hp
DieselHP: 1200hp
Speed:
- Max Surf Speed: 14.0 knots
- Max Sub Speed: 18.1 knots
Range:
- Surfaced: 3,403nm@10 knots (minimum requirement 3360nm@10 knots)
- Submerged: 474nm@6 knots (minimum requirement 18nm@18 knots + 48nm@6 knots = ~390nm@6 knots + 48nm@6 knots = ~438nm@6 knots)
Tons Oil: 30.0t
Tons H2O2: 11.0t
Miscellaneous Weight: 70 tons
Crew: 22

- 70 tons Miscellaneous weights breakdown:
--- 10 tons for Ninjatousaya System.
--- 60 tons for sonar, hydrophones, air/surface radar and radar warning receiver.

Quoted from "Rooijen10"

Quoted

Just to throw the question out there... at what point ought the sub be simmed as 'oceanic' rather than 'coastal'? At least in terms of displacement, Walter's design is larger than avowed "ocean-going" designs like the Type IX Uboat, and approaching the size of a Type XXI.

To be honest, in my opinion the limit should be at ~500 tons max (slightly higher than the Cleito Treaty max) so my vessel is well beyond the limit of that to be allowed to be a coastal submarine. To me, even the Type VII is an ocean-going submarine with the lightest version being ~600 tons. But having looked around, there are the French Daphne and Thetis and the Grondin you posted above and the the Filipino Aluhaman, Audaz and Tirador which are of similar size as or slightly bigger than the Type VIIA but with greater range and they are simmed as coastal submarines which to me is incorrect.

Yes - the question's never really come up before. My own (very informal) rule-of-thumb was to keep coastal submarines under 1,000 t light or loaded displacement; but when I'm seeing coastal designs over 1,500 loaded displacement, I have to start wondering.

Quoted

My own (very informal) rule-of-thumb was to keep coastal submarines under 1,000 t light or loaded displacement; but when I'm seeing coastal designs over 1,500 loaded displacement, I have to start wondering.

Weight-wise, I would say that your 1000 ton max rule-of-thumb is way too high. Looking at the ranges of the above mentioned submarine classes, they really are ocean-going submarines and should be simmed as such. The Aluhaman with the shortest range of them has 58% more range than the Type VIIA. Compared to the Iberian SG-1 which is close to your upper limit but is listed as an ocean-going submarine, the Aluhaman has about 40% more range.


OTL Type VIIA ----- ocean ----- 626 tons ----- 6200nm@10 kts
Daphne ------------ coastal --- 600 tons ----- 10174nm@10 kts
Thetis ------------ coastal --- 675 tons ----- 10138nm@10 kts
Grondin ----------- coastal --- 730 tons ----- 11039nm@10 kts
Aluhaman ---------- coastal --- 768 tons ----- 9818nm@10 kts
Audaz ------------- coastal --- 675 tons ----- 10138nm@10 kts
Tirador ----------- coastal --- 849 tons ----- 11308nm@10 kts
SG-1 -------------- ocean ----- 952 tons ----- 7019nm@10 kts

What actual effect does the setting have in Subsim? Extra crew, heavier structure?

When I think coastal or ocean going I think more in terms of the role rather than size, which itself is usually dictated by the role and the specifications set by the nation building it. Atlantis for example labels subs with longer range fleet subs so naturally shorter range subs fall into the coastal category. Range and armament dictate the size of the ship but role dictates the range.

Atlantis sort of mirrored the British desire to have subs fast enough to screen the battle fleet and, as the British did, labeled them fleet subs. Designers quickly gave up on this idea given the rate at which battleship design speeds were increasing while submarine propulsion systems couldn't keep up (historically the British didn't give up easy on this concept) but the term fleet sub remained. As a result Atlantean subs don't place a premium on submerged speed as much as offensive capabilities and range, though the latter is still a bit short by foreign standards.