On February 5, 2021 /Kjell provided an interesting comment on steel in upper structures of A26 submarines leading to a high center of gravity, with consequent stability problems.
The implication being Saab-Kockums is aiming to extensively use composites (for fins and elsewhere) in Sweden's Blekinge-class A26s and hopefully (for Sweden) in export A26s for the Netherlands.
/Kjell commented:
“It may happen that the Blekinge class (A26) will get rid of some steel and replace it with composite structure instead." [See Swedish language thesis (below) which has an abstract in English]:
“ABSTRACT
This report [of June 14, 2013] is a master thesis performed at The Royal [Swedish] Institute of Technology divisions of Lightweight Structures and Naval Architecture in collaboration with the Swedish defence material administration, FMV. There are several naval submarines used by the Swedish navy and more to be built.
The existing submarines have stability problems due to a high centre of gravity. Most parts on the existing vessels are made of steel, despite a position far above the vertical centre of gravity. By replacing the current steel casing covering the top part of the submarines pressure hull and the rudders placed on the tower, to a lighter composite structure it is possible to lower the centre of gravity.
In this report the composite structure of the rudder and the casing has been weight optimized to achieve a structure as light as possible, but still able to withstand the forces applied to the structure. An analysis of the applied loads shows that the dimensioning loads vary between 50 and 90 kPa depending on the specified part.
The results show that it is possible to decrease the weight of the rudder and casing [by] over 80 %. This would also give the submarine an increase in initial stability [by] over 40 % compared to the existing steel construction."
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This use of composites was earlier supported by this Swedish article of 2012:
"parts of the [A26] tower [aka fin or sail] and the molded [ie. outer] hull will be built with the same unique material that makes up the hull of the Visby corvettes, ie composite of carbon fiber and vinyl ester (thermosetting plastic). The purpose is to give the submarine better stealth properties."
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Pete Comment
As can be seen above the 3 versions of Saab Kockums A26s ("Oceanic "extended range" intended for the Netherlands?) have extensive fins/sails. Therefore the use of composites, rather than heavy steel, would be even more appropriate.
The sails have that unusual large, but blended, shape perhaps for hydrodynamic efficiency, acoustic stealthiness (reducing cavitation and other water flow noise), low magnetic signature and perhaps to house/mount a towed array sonar. (Artwork previously in this 2017 article).
Probably, internationally, most of the latest submarines designs would use composites, where possible, in those parts of submarines that are non-high load bearing. It is likely that steel (or part use of Titanium in some Russian and Chinese subs?) would particularly remain for pressure hulls.