Sources seem to vary about the Narwhals range. However, keep in mind that the Narwhal is built on the "U-cruiser" concept. The real Narwhal is MUCH bigger then the pseudo Narwhal you see in game.
As ranges go, sources vary.
According to Navsource.org:
The Narwhal has 182,778 gallons of fuel.
http://www.navsource.org/archives/08/08167a.htm
By way of comparision, a Gato class has 97,140 gallons of fuel
http://www.navsource.org/archives/08/08212a.htm
So it has nearly twice the fuel.
And then fleetsubmarine.com lists its range at 25,000
http://www.fleetsubmarine.com/narwhal-class.html
While i am skeptical of these figures to some degree, i figured for gameplay reasons, id just go ahead and use a figure of 23,000 since the missions for that boat will send it all over the damn theater, and you could make better time by running at a faster speed then 10 kts if you wanted to. You'll burn more fuel, but you also have more fuel to burn.