Official Fleet submarine ranges are far from the truth?

[Rosencrantz]

Has anyone else here ever checked the official ranges given to the Fleet type submarines? I'm asking, because when I made some calculations for both Porpoise and Gato classes, I started to get weird results. A short, simplified example of my calculations:

Class: Late Porpoise (Perch and later)
Fuel oil load: 89 945 gals
Weight of fuel: 647 604 lbs (7.2 lbs/gal)
Assumed consumption: 0.46 lbs/BHP/hr
Overall lenght of the submarine: 301 feet
Waterline lenght of the submarine: 288 feet
Displacement, official: 1 350 - 1 370 tons surfaced
Estimated displacement, surfaced, diving trim: 1 634 tons
Range at 10 knots, official: 11 000 nm
Calculated SHP need for 10.25 knots, propeller: 665 shp
Calculated KW need for 10.25 knots, generator: 515 KW
Calculated BHP need for 10.25 knots, engine: 735 BHP

735 BHP x 0.46 lbs/BHP/hr = 338.1 lbs/hr = 47 gals/hr
89 945 gals/47 gals/hr = 1913 hr
1900 hr at 10 knots = 19 000 nm

0.46 lbs/hr shoud be more than enough for diesel-electric drive of those days. Geared systems used fuel even less and straight systems even less than geared. So 0.46 lbs/hr should be really enough. Steam turbines used typically 0.66 lbs/hr on those days. Now, if we take the official range of
11 000 nm at 10 knots, we get 1 100 hour endurance at 10 knots.

=> 89 945 gals/1 100 hrs = 81.77 gals/hr = 588.7 lbs/hr

588.7 lbs/735 BHP = 0.80 lbs/hr

Conclusion: either Wintons, GM's and FBM's used by the US Navy were very thirsty to the extreme levels (getting even steam turbines behind) or official ranges for fleet boats are either:

1.) Intentionally misleading (which I doubt - any qualified naval engineer is able to make pretty good estimates based on the previous details)
2.) Meant to be used as aid for operational planning like: "the boat is able to proceed about 5 000 nm to the area of interests, stay there about 30 days and make it back without refuelling".


Calculations for Gato gave exactly the very same directions. Official ranges seem to be about 40 - 45 % off. Any thoughts? Please, if you did find a mistake, report it.

Greetings,
Rosie

[CaptBones]

Your assumed specific fuel consumption is reasonable. IIRC, for the FM 38D8-1/8 engines of the day (in a diesel-electric plant) it was between 0.40 and 0.44 lbs/bhp/hr. NOTE: Fairbanks Morse was “FM”, not “FBM”. In my Navy that stood for Fleet Ballistic Missile.

But, one of the first things you need to know is that the required cruising radius established by the General Board had to take into account a 30% reserve for battery charging; in other words 30% of the fuel capacity was NOT included in the calculations for surface propulsion. You also have not accounted for the fuel required for the ship’s electrical service load, which typically uses 10-15% of the burnable fuel onboard.

“Burnable” fuel is also worth considering, since the fuel suction is not at the bottom of the storage tanks, but well above the bottom to allow for accumulation of water in the tanks, as well as other contaminates. When transferring fuel from a storage tank to a service tank, the first few minutes of pumping from the tank is “stripping” the tank to ensure water and contaminates are removed (pumped overboard) and not sent to the service tank. Because of this, “burnable” fuel is usually around 5% or so less than total fuel capacity, and that also is taken into account in calculating cruising radius.

Hope this helps.:salute: