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The green line was likely fine.This was a construction artifact for whalers of that vintage, and unless there was actual delamination it is normal. Here is an article referencing it:
http://www.whalercentral.com/articles...icle_id=75

100hp is not too much power, but the weight of a 100 hp 4stroke may have introduced problems. I think you are describing cracks originating from the sump and heading towards the RPS. This might be troubling.

Good luck with your search.
Dennis