An outstanding fly reel with a 12% increase in line retrieval thanks to the super large Arbor. 
Fully sealed drag-clutch bearing
Narrow spool for less line stacking and increased backing capacity
Ergonomically designed asymmetric drag knob
3x stronger drag with zero start-up inertia
The Hydros SL has a narrow spool to eliminate line stacking, and offers increased backing capacity. To top it off, it has a fully sealed drag mechanism, which means the bigger reels are great reel for saltwater.

According to Sam Orvis (yes that's his real name), Orvis Rod and Reel designer, "This is a totally new reel, designed from square one and about the only thing similar to the original Hydros is the name. The original Hydros, while it had a long sealed drag, did not always retain its seal and the clutch bearing itself was not sealed at all. In the Hydros SL, the clutch bearing is fully sealed inside the drag mechanism and the inside edge of the spool is strengthened for better support. The Hydros SL has tighter tolerances and less spool wobble, and the tighter tolerances minimize the chance of backing or running line migration outside of the spool."

The spool edge is thicker and thus stronger than the original Hydros. Less spool wobble due to the increased shaft engagement will increase the life of the reel and reduced wear as the spool rotates only on the surface intended. The clutch bearing received an upgrade to a higher performance and more reliable bearing that won't free-spool. We also replaced the big, obtrusive drag knob on the original Hydros with a lower profile, ergonomic knob with thumb location indent and knurling for secure grip. The handle has been improved to an ergonomic machined Delrin handle.
 
Hydros I for line weights 1-3; 5.3 oz, 3¼" diameter
Hydros II for line weights 3-5; 5.6 oz, 3½" diameter
Hydros III for line weights 5-7; 5.9 oz, 3⅞" diameter
Hydros IV for line weights 7-9; 7.4 oz, 4¼" diameter
Hydros V for line weights 9-11; 8.4 oz, 4½" diameter