From: somewhere in Massachusetts
This is a two page, rather long read, so you might to go to the fridge and pop a cold one prior to diving in!
Endurance is a measure of how far (in nautical miles) a ship of a given displacement and optimal fuel load, can travel, at a given speed (in knots). Simply put, its a vessel's operating range for a fixed set of parameters. This definition is valid for real navies, as well as WitP AE virtual counterparts.
The amount of fuel oil consumed to maintain a given shaft rpm is directly related to Endurance. This is the Fuel Rate and is defined as fuel consumtion per unit of time. During WW2, the (US) fuel rate was measured in gallons (gals.), barrels (bbls.), or tons (t. or l.t.). A standaed barrel = 42 gals. (This is a traditional measure dating from early in the century; not to be confused with our standard 50 gal. drum.) A ton, in marine usage, is a long ton, or 2200 lbs. (I know, damm sailor's can't call anything what it really is!)
Here are some more definitions to confuse and baffle you.
A propeller is basically a screw machine. It is defined by its diameter, number of blades, and pitch. Pitch is the ideal distance a prop will longitudinally travel if rotated one revolution. If you own a small boat (or big yacht), you already know from your fuel bill, that ain't what really happens! A propeller's efficiency, through water is a function of Slip. All props have slip, which is the distance it didn't move, expressed as a percentage of the distance it was supposed to move. Example: a prop with a pitch of 12 ft. is observed to move only 9 ft.; 12 - 9 = 3. It didn't move 3 ft.; 3 / 12 = .25; the slip is 25% This is called negative slip. 15% to 20% is an average range of slip for a large vessel on a calm day, with little or no current. But such days aren't always the case. A fully loaded ship, off cape Hatteras, heading south to Miami, nosing into a small gale fights not only the wind and seas, but is fighting the Gulf Stream current as well! If the ship hasn't seen a dry dock in several years, hull fouling (sea grasses and crusaceans, whch climb aboard for the ride) will also slow her pace. If the gale is strong enough, the ship may actually go backward for the day (slip in excess of 100% is not uncommon). Of course, the reverse is also true, a lightly loaded ship, being pushed by following winds, currents, and seas could warp speed you from Hatteras to Miami in far less than a day!
Trying to measure a consistant fuel rate over an observed distance, under varying weather conditions, is a useless task. That's why we sailor's invented another term, Engine Miles. Engine miles is the total number of turns of the propeller, over a given length of time, say a day, times the propeller pitch. Engine Miles less Observed Miles divided by Engine miles is Slip. Thus, using Engine Miles and shaft revolutions, at steady steaming conditions, allows us a method of determining Fuel Rates for a given class of vessel. The US Navy, of course, figured this stuff out a long time ago, and developed real time performance curves for their vessels. For most US ships engaged in WW2. this performance data, entitled War Service Fuel Consumptions of US Naval Vessels, FTP218, (from which I gleaned all relevent data) can be found at www.ibiblio.org/hperwar/USN/ref/Fuel, or you can simply Google 'USN fuel consumption' and follow your nose to the site.
In WitP AE, a ship has three speed states: stopped, which is 0; Cruising Speed (which is 15 knots for most combatants)and Maximum Speed (which varies by vessel class). In game terms, maximum speed is limited to an effective 30 knots, since no ship can move more than 9 hexes (360 n.mi.) per phase, 720 nautical miles total. In reality, most larger ships cruised at 17 to 18 knots. Due to fuel, machinery and hull considerations, maximum speed was used only to meet specific mission goals or in defensive manuevering. Maximum speed was in the 30-ish knots range for most latter built (DD through CV) ships. Of all US BB's, only the Iowa class could hope to reach this speed.
Merchant ships and Naval auxiliaries rarely could or would exceed their cruising speed for any length of time.
In game terms, WitP implementation of ships speed is adequate. Comparing the variuos Ships Information screen speed ratings, in all cases, was consistant with published data for Naval and Merchant vessels. The displacement tonnages were all fairly accurate and reflect the 'light' (unloaded) tonnage, in long tons. The listed fuel capacity is in long tons. The fuel capacities, as listed, often varied somewhat from FTP218 listings, and appeared to reflect the Maximum, or 95% capacity. For DD's and smaller ships this is not an issue, since space limitations clearly dictated dedicated fuel tanks. However, on larger ships, there are tanks which can carry fuel/salt water ballast and/or remain void for reserve bouyancy. Each class of ship has recommendations from BUSHIPS as to how these tanks should be maintained in combat areas. The implementation of these recommendations was at the discretion of the vessel's commander. This brings about a conflict, not only in the tonnage of fuel carried which can vary from 700 to 1300 tons (depending on class) less than 95% capacity; but also brings Endurance Ratings into question.
Also, each ship's rated Endurance, as it appears in FTP218, refernced above, is an Endurance Rating for Engine Miles In many cases the the Endurance listed on the Ships Information screen parallels (though exceeding) the FTP218 rating. Unless there is another reason, which I cannot easily ascertain with the available infomation; the data suggest rather inflated Endurance values present in the games code, for each vessel class. Propeller slip, weather, currents and hull fouling are not factored in.
To model fuel consumtion the developers use only one fuel rate for each ship class. That is the rate at Cruising Speed; which for most combatants, is 15 knots. Thus, a combat ship travering one hex will lose 40 (miles) Endurance; and the fuel capacity will decrese 40 times the tons/mile fuel consumption rate (or 1 times the tons/hex fuel consumption rate). If Endurance at cruising speed closely parallels FTP218 values, then it follow that the fuel consumption rates coded into the game, closely follow FTP218.
Wisely deciding not to encumber the game data base with a gaggle of class specific engineering data, and allowing one other ship's speed to be represented, that of Maximum Speed, the developers cleverly defined Maximum speed Endurance (i.e.fuel consumption) as a function of Cruising Speed Endurance. Maximum speed Endurance is is defined as Crising speed Endurance plus 240 Endurance. Thus a US combat ship at Maximum Speed consumes Endurance at 280 Endurance per hex traveled; which is a ratio of 7 times the cruising rate. We can safely assume that the fuel consumption rate is seven times that of fuel consumption at cruising speed. A close examination of the tables in FPT218 shows that from 15 knots to Max speed (30 - 32 knots) the fuel consumption rate of the various classes increases by a facto of 6 to 8; It seems that a factor of 7 times Enurance was a very valid choice.
Although the relevant data for each ship is pretty much in line with published Naval and Marad data; it seems the developers failed to account for propeller slip, weather, currents and hull fouling in their Endrance calculations. Also, fuel capacities for larger ships may be inflated. A reduction of enudrance in the range of 15% to 25% for each ship class is required to bring endurance in line with realistic operations.
Your comments, suggestions and insights on the above commentary is welcome.