Are we overdesigning destroyers?

[P3D]

When I did extensive simming back then, you could pretty accurately simulate historical destroyers with the following rules:
a/ armament as mount and hoist for adequate weight
b/ Composite strength of ~0.90
c/ ~3-4t/torpedoes misc weight
d/ SHP/tonnage set to 60SHP/ton (iirc essentially unchanged for RN destroyers, from 1910 to 1940) - calculate SHP from speed, adjust speed to get engine weight

This is without any adjustment to destroyer speed - they'd need less SHP to reach the SS defined speed.

[Borys]

Ahoj!
Something from another board:
http://www.phpbbplanet.com/forum/viewtopic.php?t=3505&highlight=&mforum=warshipprojects

[Guinness]

Today I finally got my hands on DK Browns Nelson to Vanguard. FYI: that volume and the preceding Grand Fleet are now available in paperback from the USNI Press at quite reasonable prices.

On p. 212 to 214, he discusses seakeeping, asserting that the perception of whether a ship was a good seakeeper often hinged on the position of the bridge (the closer to the center of pitch the better, as that mean subjectively reduced motion in heavy seas), freeboard, and propensity to slam their bows, which was mostly attributable to a ship's draught. He also presents a rule of thumb introduced at the end of WW2: that any ship with more freeboard than the value of the square root of the ship's length multiplied x 1.1.

Judging from the values in a corresponding table, length in this case appears to be waterline length, and freeboard appears to be the freeboard at either the region of the hull break, or the at the region of the bridge position (which in British ships was almost always more or less the same place).

So, for instance, according to Brown's values, a V&W class DD, at 300 ft long, would have a required freeboard indicated by this equation of 16.4 feet. It's historical value was 17 feet, and thus it was a good seaboat. For reference, my sim of the V&W (which was actually for the last class of W's which were 9 feet longer at the waterline according to Friedman's dimensions) has a SS seakeeping of 0.62.

However, in his figures he attributes the M (WW1), and I classes as all being "good" seaboats, and my sims of these had SS seakeeping figures of 0.25 and 0.25, while he attributes the WW2 Tribals as not quite so good seaboats, and my sim of these have an SS springsharp rating of 0.85.

This to me is further evidence of the absolute inadequacy of SS's seakeeping value in evaluating the seagoing characteristics of DDs, and I submit that maybe we might adopt the equation Brown refers to for evaluating the seaboat characteristics of our boats. In fact given the RN's emphasis on seakeeping and operating theater in the North Sea and North Atlantic, I submit that any boat with seakeeping considered good by the Brits ought to be considered exceptional by us. To recap, the equation would be:

Calculating using the waterline length in feet to produce the desired freeboard in feet:

1.1 * sqrt(Lwl) <= freeboard at bridge = very good seaboat
1.1 * sqrt(Lwl) > freeboard at bridge = not very good seaboat

I'd like to further develop that into values for very good, average, and poor, but I suspect that may require more research using available sources, particularly american ships. For instance, using the Fletchers, the brits would expect a value around the bridge of 20.5 feet, but the Fletchers at load waterline would in fact have a freeboard there of around 12.5-14 feet. They weren't considered great seaboats, but they were certainly not poor, so maybe something like 70% of the equation recommended value is "fair"?

EDIT: fixed my representation of the equation, which was wrong.

[Sachmle]

Definitely interesting. I would wager that SS mostly uses freeboard and length to determine it's seakeeping values as well though. I wonder what 'formula' was instilled in it?

[Guinness]

I'm looking at the disassembled SS2 code for calculating seakeeping now, but I've either had too much wine already tonight, or not nearly enough, because it's giving me a braincramp. Displacement, freeboard, hull weight, weight of end belt armor, gun weight, length, beam, draught, frictional coefficient, wavemaking speed, and absolute speed all figure into it one way or another.

I can post the actual code, but it ropes in so many params that are non-obvious and calculated in other objects that I'm not sure it would make any sense to anyone. Needless to say, it's a very complicated (from the perspective of number of inputs) formula.

[Nobody]

Major difference between the formula and SpringSharp is that the latter tries to include the top speed in the equation. (faster ship --> lower seekeeping)

Question: Do you have data for the German destroyers, especially the Narvik-class? I would like to know how they perform under the rule of thumb compared to their bad seaboat characteristics.

[Guinness]

When I get a moment I'll hunt around for a decent scale drawing...

[P3D]

Dreadnoughtproject.org has some.

[Guinness]

I found a decent illustration on german-navy.de. Assuming it's accurate enough, here's a SS sim:

Type 1936a, Germany Destroyer laid down 1938 (Engine 1940)

Displacement:
   3,217 t light; 3,343 t standard; 3,605 t normal; 3,814 t full load

Dimensions: Length overall / water x beam x draught
   416.68 ft / 399.93 ft x 39.37 ft x 15.26 ft (normal load)
   127.00 m / 121.90 m x 12.00 m  x 4.65 m

Armament:
      5 - 5.87" / 149 mm guns (4 mounts), 99.87lbs / 45.30kg shells, 1938 Model
     Breech loading guns in deck mounts with hoists
     on centreline ends, evenly spread, 2 raised mounts - superfiring
      4 - 1.46" / 37.0 mm guns in single mounts, 1.63lbs / 0.74kg shells, 1938 Model
     Anti-aircraft guns in deck mounts with hoists
     on side, all amidships, all raised mounts - superfiring
      12 - 0.79" / 20.0 mm guns in single mounts, 0.29lbs / 0.13kg shells, 1938 Model
     Anti-aircraft guns in deck mounts
     on side, evenly spread, all raised mounts
   Weight of broadside 509 lbs / 231 kg
   Shells per gun, main battery: 190
   8 - 21.0" / 533 mm above water torpedoes

Machinery:
   Oil fired boilers, steam turbines,
   Geared drive, 2 shafts, 70,000 shp / 52,220 Kw = 33.92 kts
   Range 2,174nm at 19.00 kts
   Bunker at max displacement = 470 tons

Complement:
   232 - 302

Cost:
   £2.346 million / $9.385 million

Distribution of weights at normal displacement:
   Armament: 64 tons, 1.8 %
   Machinery: 1,783 tons, 49.5 %
   Hull, fittings & equipment: 1,339 tons, 37.2 %
   Fuel, ammunition & stores: 388 tons, 10.8 %
   Miscellaneous weights: 30 tons, 0.8 %

Overall survivability and seakeeping ability:
   Survivability (Non-critical penetrating hits needed to sink ship):
     1,312 lbs / 595 Kg = 13.0 x 5.9 " / 149 mm shells or 0.4 torpedoes
   Stability (Unstable if below 1.00): 1.09
   Metacentric height 1.5 ft / 0.4 m
   Roll period: 13.7 seconds
   Steadiness   - As gun platform (Average = 50 %): 50 %
         - Recoil effect (Restricted arc if above 1.00): 0.47
   Seaboat quality  (Average = 1.00): 0.60

Hull form characteristics:
   Hull has rise forward of midbreak
   Block coefficient: 0.525
   Length to Beam Ratio: 10.16 : 1
   'Natural speed' for length: 20.00 kts
   Power going to wave formation at top speed: 69 %
   Trim (Max stability = 0, Max steadiness = 100): 83
   Bow angle (Positive = bow angles forward): 27.25 degrees
   Stern overhang: 5.09 ft / 1.55 m
   Freeboard (% = measuring location as a percentage of overall length):
      - Stem:      22.64 ft / 6.90 m
      - Forecastle (15 %):   19.69 ft / 6.00 m
      - Mid (38 %):      18.04 ft / 5.50 m (10.76 ft / 3.28 m aft of break)
      - Quarterdeck (12 %):   10.76 ft / 3.28 m
      - Stern:      11.81 ft / 3.60 m
      - Average freeboard:   14.24 ft / 4.34 m

Ship space, strength and comments:
   Space   - Hull below water (magazines/engines, low = better): 174.7 %
      - Above water (accommodation/working, high = better): 76.6 %
   Waterplane Area: 10,735 Square feet or 997 Square metres
   Displacement factor (Displacement / loading): 84 %
   Structure weight / hull surface area: 72 lbs/sq ft or 351 Kg/sq metre
   Hull strength (Relative):
      - Cross-sectional: 0.94
      - Longitudinal: 1.73
      - Overall: 1.00
   Hull space for machinery, storage, compartmentation is cramped
   Room for accommodation and workspaces is cramped
   Caution: Lacks seaworthiness - very limited seakeeping ability

The 1.1 * sqrt(Lwl) calculation says 22 feet (6.7 meters). The freeboard in the way of the bridge is 5.7m (18.7 feet). So indeed the RN formula predicts she'd be a less than ideal seaboat, having only 85% of the desired freeboard.

The references cite a top speed in excess of 37 knots, but I simmed this to the SHP number, not the top speed. I expect that 37 knots is trial speed, and I have no idea what condition she trialed in, but I imagine it may have been at a bit lighter that normal displacement, and was almost surely before the twin bow mount was shipped. Note that navweaps says the 15cmL48 C/36 had an actual bore of 14.91cm, so I used that.

[Nobody]

Nicely done Guinnes.

Some interesting Notes:

• the SpringSharp bunker capacity is much lower than OTL (approximately half!)
  
• they look like they had a transom stern, how would that effect the SS-model?

[Sachmle]

Me thinks you may need more misc weight. She was supposed to ship 60 mines (probably the EMC type, 300kg charge unknown total weight), 4 DC launchers (each weighed 310kg, plus DCs, most likely the WBD type (180kg total weight)), as well as 8 21" G7a T1 torpedoes (1,528kg each). Even without the weight of the mine rail/dollies and the torpedo launchers themselves and the extra weight of the chain/casing for the mines it still comes out to ~38.65t.

Also NavWeaps says the outfit for the 15cm/L48 C/36 was only 120 rounds per gun, but it then says they also carried 80 Illuminating rounds. I presume this isn't included in the 120 rounds, as 40 rounds per gun is rather light. I guess 190 could be a good compromise, or you could outfit 120 and use misc weight to represent the 80 illum rounds, assuming they weight roughly the same as the HE rounds that would be ~4t.

Also wiki, and I know...wiki's wacky..says that the 1936A eventually carried 10 37mm and 20 20mm AA.

[Logi]

You guys do know when SS asks for the number of shells per main gun, they mean gun barrel, right? Calculations for magazine weight and shell weight show that.

Hence if a twin gun turret is given 200 shells in the value asked for by SS. In actually each gun barrel has 200 shells and the turret holds 400 shells total.

Considering that and the ROF of the guns involved, unless you are firing for over an hour, you will never deplete your magazine :-\ Not that's bad, but you won't be firing continuously that long so every shell (which robs the ship of some composite strength) could be removed for more useful things to be added to the ship.

Btw: Is the ship supposed to have 3 singles and a twin turret?

[Sachmle]

You guys do know when SS asks for the number of shells per main gun, they mean gun barrel, right? Calculations for magazine weight and shell weight show that.

Hence if a twin gun turret is given 200 shells in the value asked for by SS. In actually each gun barrel has 200 shells and the turret holds 400 shells total.

Considering that and the ROF of the guns involved, unless you are firing for over an hour, you will never deplete your magazine :-\ Not that's bad, but you won't be firing continuously that long so every shell (which robs the ship of some composite strength) could be removed for more useful things to be added to the ship.

Btw: Is the ship supposed to have 3 singles and a twin turret?

Ammunition stowage per gun 120 rounds

[Logi]

I know I was just noting that SS counts shells like that.

[Nobody]

Me thinks you may need more misc weight. She was supposed to ship 60 mines (probably the EMC type, 300kg charge unknown total weight), 4 DC launchers (each weighed 310kg, plus DCs, most likely the WBD type (180kg total weight)), as well as 8 21" G7a T1 torpedoes (1,528kg each). Even without the weight of the mine rail/dollies and the torpedo launchers themselves and the extra weight of the chain/casing for the mines it still comes out to ~38.65t.

Not to mention the sonar, radio, radar and fire-control systems. But since the ship currently is 1.0 composite this shouldn't be a problem.

Also NavWeaps says the outfit for the 15cm/L48 C/36 was only 120 rounds per gun, but it then says they also carried 80 Illuminating rounds. I presume this isn't included in the 120 rounds, as 40 rounds per gun is rather light. I guess 190 could be a good compromise, or you could outfit 120 and use misc weight to represent the 80 illum rounds, assuming they weight roughly the same as the HE rounds that would be ~4t.

Also wiki, and I know...wiki's wacky..says that the 1936A eventually carried 10 37mm and 20 20mm AA.

I think the number of illumination rounds is given per ship stored adjacent to two of the mounts, so 80/5=16 per gun on average in addition to the 120 normal rounds.


@Logi
Yes I know, but 120 shells / 12 rpm = 10 minute firefight (or 20 with a easier to keep 6 rpm).