R29 Performance Issues D4 320

[MidnightSun]

Racking my brain on a little problem and thought I would through it out to the community. My R29 is just about to turn 3 years old and has a D4 320. At 3250rpms I typically cruise at 16-17knts. But on my last trip, I was struggling to get 13knts at that rpm with significant fuel efficiency drop. Engine is running normally, no unusual knocks, vibrations or sounds. Bottom was cleaned before the trip and I was lighter than normal with fewer passengers and gear than I normally carry. My spring maintenance included fluid and filter changes for fuel, engine oil, gear box and new impeller. Stuffing box drips normally. Prop is in like new condition. Any thoughts?

Travis
Midnight Sun
Sitka, Ak