Reliable numbers might be hard to find in this forum, but...
I can offer from experience with a 1980 Chris Craft 45' aft cabin with those engines, at 10 miles an hour you will get about one mile to the gallon. Up on plane the number will drop to about .4 miles per gallon.
The Bertram 46 has more dead rise than the Chris so the up on plane number might be slightly lower.
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