Greater Underwater Propulsion Power Program

The Greater Underwater Propulsion Power Program (GUPPY) was initiated by the United States Navy after World War II to improve the submerged speed, maneuverability, and endurance of its submarines.

The navy began the program by testing and reverse engineering two German Type XXI U-boats—U-2513 and U-3008—obtained as war reparation.

That analysis led[1] to four goals—increasing the submarines' battery capacity, streamlining the boats' structures, adding snorkels, and improving fire control systems.

A similar program for the Royal Navy involved modifications to 24 wartime and post-war British T- and A-class submarines, which were provided with streamlined hulls, fin-type conning towers, and increased underwater performance during 1948–60.

Externally, they featured improved streamlining of the bridge and shears structures, and periscope and radar mast supports.

An SV radar aerial was added to the top of the sail, creating a distinctive side bulge.

All capstans, cleats, and rail stanchion supports were redesigned so they could be retracted or removed when rigged for dive.

The Sargo II was developed to be lower cost than the Guppy battery while providing most of the performance.

Unlike the GUPPY conversions, these boats retained their original deck structure, bow, and storage batteries.

Submerged performance of the Fleet Snorkel boats was therefore significantly inferior to any GUPPY conversion.

İnönü, were converted to a Fleet Snorkel Submarine in 1953, the work being done first in Turkey's Gölcük Navy Yard and completed in the United States.

Some boats had the high-pressure air compressors relocated to the lower level of the forward engine room.

GUPPY IB was an informal designation for a limited upgrade and modernization given to four boats for transfer to foreign navies.

These boats had snorkels and were generally similar to the GUPPY IA, except that they were not equipped with the modern sonar, fire control systems, or ESM.

[6] The GUPPY II conversions suffered from very cramped internal conditions due to the four-battery configuration.

The removal of the sonar room from the forward torpedo spaces allowed an increase in the number of reloads.

[7] The GUPPY III conversion was part of the Fleet Rehabilitation and Modernization (FRAM) program.