Loss of speed


I've lost 2 kts of speed from last year to this with my 26 Fortier with a 43 KAMD Volvo 220 hp engine while running at 3400 rpm.  For years I got 15-16 kts at slack at 3400 and this year it has been 13-14 kts and often closer to 13 kts.  The loss is about the same throughout the typical cruising range of 3100 to 3500.  I had the prop serviced two years ago with no damage since.  I had the belts changed this year and the valves checked.  It ran normal right through last year and had slower speeds right away this year.  The boat is a 1988 and the engine (a 1998 replacement) has about 1500 hours.   
Seems to be a load problem since there is no limit on typical rpm range and no smoke or vibration.  I got in the water and looked at the running gear and brushed the hull.  I worry about through-hull intrusion but have no evidence of this.  Any ideas or related experiences?    Thank you.  

Fortier Boats Inc.  •  34 Riverside Ave.  •  Somerset, MA 02725-2869  •  508-673-5253