Your safe cruising range is 2/3 your maximum cruising range.
For instance, my old 20' Seafari came with a factory 29 gallon tank, 27.5 gallons usable.
With my 90 Optimax, I used an average of 4.7 gph at cruise speed of 25 statute mph, or about 5.3 mpg. This gave me a maximum range of 145 miles on a single tank to fuel exhaustion.
My safe cruising range was therefore 97 miles. I used to just figure on 100 miles as my cruising range.
From time to time, I used to greatly exceed that, necessitating the carrying of extra fuel on the deck. I would go fishing and diving for a weekend out to the Florida Middle Grounds, to a spot that was 105 miles offshore, so I carried an extra 60 gallons of fuel in 2 30 gal. blue barrels on the deck, burning that fuel first, and then lashing the barrels on the foredeck/cabin roof. No problem.
No, I never made the trip alone as the only boat going. The smallest party was 3 boats, with a 41' Cruisers as our "motel" offshore. Once we had 7 boats cruising together out and back for a long weekend.
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Common Sense is learning from your mistakes. Wisdom is learning from the other guy's mistakes.
Fr. Frank says:
Jesus liked fishing, too. He even walked on water to get to the boat!
Currently without a SeaCraft 
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury
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