You might want to re-visit 1.5 gph? Is this the small US gallons too?
I have logged most of my usage and with a 15 2 smoke Merc over all use as an average it's around 1.75nm a litre or 7.8 nm per UK gallon. However, some trips it has gone down to more like 1.4nm litre. There are a few topics on here about SIB fuel usage.
Our last trip was a pootle down the river at 5 kts for about 1.5 hrs followed by a blast along the coast about 10 miles out and then back. We used about 14 litres I think.
SIB + small 2 stroke + lots of WOT = a very un-economical way to travel.
Are you serious about going up to 40 miles offshore in a SIB and back??
Even on perfect flat water you won't average 15 mph. 30 mins in a SIB at WOT is enough to need a break to recover.
Your trip is like doing Land's End to John O'Groats on dirt roads in one day in a Renault Twizy...
We have a 32 litre bladder but fill it to around 25 litres.
For your trip you'd want to carry maybe 3 x 25 litre tanks.....