Is 20 knots fast for a cabin cruiser (38 ft)?

Yes, 20 knots is considered fast for a 38-foot cabin cruiser. Most cabin cruisers in this size range typically have cruising speeds between 15 to 20 knots, with 20 knots representing the upper end of that spectrum. Achieving this speed may require a more powerful engine and efficient hull design. Overall, while it's on the faster side, it's still within the realm of possibility for a cabin cruiser of that size.