Thanks guys - the shafts, cutlass bearings, props, rudder post bearings are all "as new" with only 20 hours on the engines since launch. I have been under to take a look and there is no play anywhere, also no movement or vibration in the shaft log or anything else forward of the shaft, including gearboxes, engines etc when the 1500 plus RPM fwd vibration is going on at the prop ends.
There is approx 80 mm of shaft overhang between the aft cutlass bearings (we have cutlass bearings at each end of the 1500 mm shafts) and the prop hubs that provides room for shaft anodes. Prop tip clearance is approx 90 mm for the 480 mm(19") diameter.
Bill may be close to the answer (though we cannot lift
a rudder as they are fixed) as I have talked with Ben Hawke at Seahawk who has suggested possible wash from the blades against the rudders and/or the bottoms of the hulls which are flat right above the props. It is very unlikely to be prop balance or shafts as there is absolutely no vibration at full bore in reverse. The skegs may be suspect as there is a relatively large grounding bulb at the bottom of each skeg which may be causing a vortex as water
passes it at higher speeds which is then picked up by each blade tip as it comes around, the blades on these 19" props extending almost the the bottom of the skeg.
It is a bit of a head
scratcher but we are taking her out of the water at month end to have a better look at any effects of prop wash and, most likely, to fair the skegs. I'll let you know how we go.