5052 is highly resistant in an marine environment. It is commonly used today to build high end yachts for example.
Steel is still the best for volume production. Although aluminium has been used for large ocean going OPVs such as Ocea’s OPV 270 class, very few shipyards are qualified to build large aluminium hulls.
1,300 to 2,500 tonnes displacement (full load) is the minimum size for decent seakeeping, 6,000nm+ range, and payload capacity (this is a rough range based on existing OPV designs). In Europe alone there must be 20+ smaller shipyards that can build large patrol vessels in that size range.
Ultimately many existing OPVs could probably be used as a starting point for a mass produced Flower class. All you really need is space in the stern for a towed sonar, diesel electric propulsion with ~12kt speed in electric mode, 22-24 knot top speed, and enough fuel for a 2,000nm transatlantic crossing at an average speed of advance of 18-20 knots (plus say 30% fuel reserves). Effectors can be a mix of LWTs welded to the deck, UAVs dropping sonobuoys, ideally also a few ramp launched ASROC that fit in the footprint of a 20ft container. Perhaps 1-2 small davit or ramp launched USVs can also help (though I’m not convinced they can maintain the speed of advance, so probably best used against already identified sub contacts).
In France alone there are 4 shipyards building multiple OPV designs that might work, from the small 1,300t Patrouilleur Outre-mer (Felix Eboue class), to the larger 1,500t Gowind 87 (Adroit/Bouchard class), and the new 2,400t Patrouilleur Hauturier (Auguste Techer class). All use very robust commercial systems including identical diesel electric propulsion (ABC diesels from fishing trawlers and tugboats and Scania truck diesels as gensets), Flume passive stabilization tanks, simple box coolers to dissipate engine heat etc.
Production costs start at ~€45M / $50M for a 1,300t OPV. Probably a lot less if you build in lower cost yards in Romania, Bulgaria, Poland and the Baltics.
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