Seakeeper 35

The Seakeeper 35 is designed to eliminate up to 95% of boat roll on boats up to ~100 tons, vessels ~85' and larger. Multiple units can be fitted for larger boats. Unlike traditional external fins, the Seakeeper 35 requires only modest electrical power, has no external appendages—
meaning no drag or risk of snags—and can be installed virtually anywhere on board.

$309,300
(installation cost varies)