Following this article Locum on February 9 has provided some excellent comments which Pete has rearranged a bit for clarity, given its a pretty complex subject.
Length of a submarine / Diameter of a sub = (L/D) ratio, a key measure. A L/D ratio of 7.0 is the ideal. A submarine with good hydrodynamic qualities needs to be short and 'fat' [like the current Walrus - see below]. That is good for an expeditionary submarine.
[This is what the Netherlands Navy and Defense Ministry want, but its expensive. A short fat sub for the Netherlands is ideal as it would yield a long range of > 8,000nm miles and a high cruising speed for the shortest possible transit time. Good for missions from the Netherlands to the Dutch Caribbean or Netherlands through Mediterranean to the Middle East (for electronic monitoring, etc)].
A lot of people, even some Dutch politicians, think that a Saab-Kockums A26 Oceanic Extended Range concept is the contender for the Walrus replacement.
Incorrect, Saab-Kockums and Damen Schelde are offering a concept sub, that's 73m long and has a diameter of (approx) 8m.
But, Saab-Kockums marketing wants us to believe that a stretched A26 version will do. However if we simply stretch the A26 to from its Swedish Navy standard of 63m to eg. 70m [while retaining its Swedish Navy diameter of 6.4m] we will end up with an unfavourable L/D ratio of 10.94 .
A. Instead Saab-Kockums is really offering two designs. That is:
- 73m x 8m sub yielding a L/D ratio of 9.13. [An expensive choice, of the dimensions the Netherlands Navy wants] This is a new design, with some recycled and/or evolved elements of the Walrus, Collins class and A26. Displacement is approx 2,900 - 3,000 tonnes [submerged]. [This compares with the current Walrus’s 67.73 x 8.4 = L/D ratio of 8.06 at 2,650 tonnes (submerged)].
- and (probably) the standard A26 [Its standard dimensions make it cheaper and of the type most of the (non-Navy/non-Defense Ministry) economic Ministries want]. which is approx 63m x 6.4m yielding a L/D ratio of 9.84.
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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PETE COMMENT
An ideal L/D ratio not only has hydrodynamic efficiency (for low fuel use movement) through the water, but it also adds structural strength, eg: a perfect sphere L/D = 1, or even an egg, are strong shapes to crush with water pressure.
Low L/D also 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 Singapore Strait or Baltic Sea).