Saab Offering 2 Types of Sub to the Dutch: L/D Ratios.

Following this article Locum on February 9 has provided some interesting comments which have prompted Pete to look into L/D ratios: a pretty complex, controversial topic.

Length of a submarine / Diameter of a sub = (L/D) ratio, a key measure. A L/D ratio of 7.0 may be good for a coastal (littoral brown water) submarine. A submarine with good oceanic (blue water) qualities ideally has a ratio around 10 up to 11.5. 

The Netherlands Navy and Defense Ministry want, a largish, around 2,900 tonne, tailor-made sub. But this is expensive. Such a sub would have range longer than 10,000nm miles at 10kn cruising speed for the shortest possible transit time. This would be good for missions from the Netherlands to the Dutch Caribbean (monitoring and/or special forces) or Netherlands through Mediterranean to the Middle East (electronic and ASW monitoring).

A lot of people, even some Dutch politicians, think that a Saab-Kockums A26 Oceanic Extended Range concept is the best contender for the Walrus replacement.

A. Saab-Kockums may be offering the Netherlands two designs. That is:

-  A26 Oceanic of 65m/ (maybe) 6.75m 2,000 tonnes (surfaced), 6,500nm at 10kn sub yielding a L/D ratio of 9.63. Its very close to standard Blekinge A26 dimensions. This makes it cheaper, hence of the type most of the (non-Navy/non-Defense Ministry) economic ministries want.

OR

- A26 Oceanic (Extended Range) An 80m long (diameter unknown lets say 7.8m) L/D of 10.26 3,000 tonne (surfaced) 10,000nm at 10kn. Around 3,300 tonnes (submerged). Greater weight alone makes it a more expensive choice, however its dimensions are similar to Kockums operational Collins sub. Its around the dimensions the Netherlands Navy wants. But that navy can then shoot lower, to 2,700 tonnes, and thus appear conciliatory.

B. Naval Group is offering versions of the Barracuda and Scorpene [Dimensions, hence L/D unknown, being kept Secret.]

C. TKMS is offering the Type 212CD E. [Dimensions, hence L/D unknown, being kept Secret?]

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Navantia was offering the S-80 Plus. [Though note “Navantia was not accepted as a bidder.” by the Netherlands’ future submarine selection panel.]

Drawing illustrates a submarine's L/D ratio, wherein Diameter (D) of Hull is a proportion of Length (L) of Hull. In this example L/D ratio is quite good 8.75 (where L = 8.75 D). See upper right corner of Drawing. (Drawing courtesy Google Images which Google located from this source).
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L/D ratio imposes hydrodynamic efficiency, but this ratio is also governed by the type of missions a particular nation's submarines performs.

Low L/D allows rapid turns (to clear the baffles) and rapid tactical changes in direction (good for dodging other subs, ship’s hulls and even torpedos). Low L/D also allows for easier steering around rocks and narrows and a more compact shape to fit in seafloor "holes" (eg. in the Mediterranean or Baltic Seas).

A higher ratio makes for more efficient oceanic-blue water travel (surfaced, snorting of fully submerged).