Factbox-What are Sweden's new Saab-built A-26 submarines?

STOCKHOLM (Reuters) -Poland picked Sweden on Wednesday to supply it with three A-26 or Blekinge-class submarines being built by Saab, which are specially designed to operate in the Baltic Sea.

Following are details about the submarine, which is Sweden's largest to date:

CONVENTIONAL BUT QUIET SUBMARINE

The A-26 is a conventional submarine, meaning it is not nuclear-powered. It can remain submerged for weeks due to its three Stirling engines, which do not rely on air to run. Stirling engines, which are also fitted on Sweden's existing submarine models, are known for being very quiet.

At 66 metres (217 feet), the A-26 is much smaller than the biggest Russian or U.S. nuclear-powered submarines which can be around 170 metres long.

The smaller size makes it suitable for the shallow Baltic, which has an average depth of around 60 metres and is sometimes referred to as a "flooded meadow" by Swedish naval officers.

SEABED WARFARE

A unique feature of the A-26 is a 1.5-metre-diameter dive-lock, called a multi-mission portal, in the bow.

This will let remotely operated vehicles, autonomous vehicles or groups of divers in and out easily. The portal is designed for seabed warfare, including the protection or destruction of pipelines or other critical infrastructure on the seabed.

It can be equipped with torpedoes, mines and can carry groups of naval special forces, but cannot launch missiles, like some bigger submarines.

DELAYS AND COST OVERRUNS

Sweden originally ordered two A-26 submarines in 2015 at an expected cost of 8.6 billion Swedish crowns ($903 million) with delivery of the first one scheduled for 2023. Since then, the project has been hit by increased costs and substantial delays. The first delivery is now expected in 2031 with the cost for the two submarines now projected at 25 billion crowns.

($1 = 9.5190 Swedish crowns)

(Reporting by Johan Ahlander. Editing by Niklas Pollard and Mark Potter)