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Usually normal engine 'harmonic imbalance'. Check the vibration with and without the boat in gear (forward AND reverse).
The usual cause of such harmonics are 'shaft whip' due to poor engine alignment, poorly set/aligned or worn-out cutless bearing, and improper stuffing box packing (unequal circumferential loading of the packing by the packing gland).
Suggest that if you're concerned that you deduct the cost of an engine, etc. realignment in the negotiated price. What's the 'clearance' status of the cutless bearing?
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