3 Submarines by L/D ratios. 2 Subs by I Ratios, eg. for Netherlands. India?

On October 12, 2019 Anonymous provided the following useful comments and calculations which compare: (A) Naval Group, SAAB and TKMS submarines against Length (L) / Diameter (aka Beam) (D) ratios, and (B) SAAB-DAMEN and TKMS submarines against Indiscretion Ratios (IRs). 
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(A)  Length (L) / Diameter (aka Beam) (D) ratios

Naval Group, SAAB and TKMS have proven submarines with similar dimension:

-  Scorpene AM-2000 (displacement 1,870 tons (t); length (L) 70m; Diameter (D) 6.2m, L/D = 11.2) 

-  A26 (1,930t, 63m, 6.4m, L/D = 9.8) and

-  Type 218SG (2,200t, 70m, 6.3m, L/D = 11.1), respectively. 

To achieve a larger submarine (say for the Netherlands) with a displacement of 3000t (if required?) with a simple hull extension (aka plug) the estimated L/D = 15-18 considerably deviates from the optimal L/D = 7-10 [1]. SAAB and TKMS proposed new submarines with bigger beam (to achieve an optimal L/D) presumably based on the existing (Collins-class) or mature paper designed submarine (Type 216). Unless Naval Group can offer submarine with optimal L/D, a successful bid will be a long shot.

(B) Indiscretion Ratios (IRs)

On the basis of rough estimates [2] the Indiscretion Ratios (IRs):

-  large SAAB-DAMEN A26 (3 x MTU diesels, LIBs, Stirling-AIP) = 3.8/% 

-  large 212CD (2 x MTUs, LIBs, FC-AIP) = 4.2%

So the IR of the SAAB-DAMEN A26 is slightly smaller than the 212CD's thanks to the A26's 3 x MTU diesels. These IRs are far superior to IRs of LAB-submarines (around 12%).

There are two strategies to achieve a low IR by using LIBs:

-  one is a combination of diesels and AIP, and 

-  another is the adoption of high power diesels. 

To achieve 4% of IR without AIP, SAAB-DAMEN needs 5 x MTU diesels [with LIBs] or 2 of the latest KAWASAKI diesels [with LIBs].

[1] “Optimum L/D for Submarine Shape”, by M. Moonesun and others. Indian Journal of Geo-Marine Sciences, vol.45(1), Jan/2016, pp. 38-43 at

https://pdfs.semanticscholar.org/0bb4/68b6b618d564c401285ecf31129312e677a2.pdf

(Conclusion is on page 42) The main achievement of this paper is the suggestion of an L/D = 7-10 as the optimal range for cylindrical middle body submarines.

[2] Assumption: AIP is used for 50-days submerged surveillance in a 70-day mission. Power consumed per hour is 0.2MW (c = hotel load 0.15MW + propulsion input 0.05MW). Then, total energy to be consumed for the surveillance is 240MW (e=c*24 hours*50days) consisting of an energy supply from AIP (d = c*24hours*submerge period by AIP (a) = 4.8*a) and that from diesel 

(j = 240MW-d). IR is related to daily energy supply (g = j/50days) from diesel with electrical output (b), namely, IR = 100*g/(24*b).

Where:

- SAAB-DAMEN: a=20days, b=3*MTU=3.12MW, d=96MW, j=144MW, g =2.88MW, IR=3.8/% and

- 212CD: a=28days, b=2*MTU =2.08MW, d=134.4MW, j=105.6W, g=2.11MW, IR=4.2%

Submerged period by AIP, hotel load and propulsion input are estimated in various articles.

Pete Comment - Naturally many other technical, cost and Dutch content variables will be part of the Netherlands' future submarine decision. Also, as Submarine Matters October 11, 2019 article records, Netherlands' domestic political factors and international relations with supplier countries will also influence the selection. India is also considering the selection of 6 Project-75I AIP submarines.  

Mainly Anonymous (Pete some editing, comments)