kbus888 wrote:Hi folks (and Mr Weber)
Displacement ?? Total weight while empty ??
Bunkerage ?? Maximum cargo weight ??
Freeboard ?? Ocean level to top deck while full of cargo ??
I am not an expert on nautical terms
?? Can anyone either explain these terms or give me a URL where I can find the definitions myself ??
I need a lot of help here !!!
R
Sorry.
"Displacement" is the amount of seawater (expressed in tons) which a ship's hull displaces at a given loading. It gives a more accurate value for the ship's actual weight than the older "burden" tonnage calculation did. "Burden" tonnage (also sometimes shown as "burthen," especially for ships from earlier centuries) was a calculation of the cargo capacity of a merchant hull of the same dimensions, thus USS
Constitution had a tonnage of 1,570 or so during her active career (when it was calculated as "burden") but has a
displacement tonnage of around 2,100-2,200 tons.
Even "displacement" values for ships can be slippery when it comes to comparing them, however. When the Washington Disarmament Conference wrote the Naval Treaties which limited the size of battleships, they had to specifically
define "standard displacement" in terms of a vessel's displacement as intended for sea duty, with one half designed fuel capacity on board, machinery fully lubricated, steam plant with designed water levels and reserve feedwater, designed ammunition load, etc. Even so, the treaty participants discovered that they could "save" hundreds of tons through purely paper maneuvers. For example, the US Navy reduced its "normal" ammunition allotment from 100 rounds per battleship gun to only 80 . . . but provided sufficient magazine space for an "emergency war load" which actually pushed the ammunition allotment back to slightly above the previous level. Deciding how many boats the ship would carry as part of its "normal" allotment was another way to cheat. Or, then, you could always take the Japanese approach and simply lie, telling the world that your eight-inch only displaced the 10,000 tons the treaties allowed when they actually displaced in excess of 13,000 tons. But I digress.
"Bunkerage" refers to the fuel capacity of the ship. Coal, the initial seagoing fuel, was carried in "bunkers," in essence storage holds with external access from deck level to allow coal to be put into them and doors ("scuttles") to allow the stokers to remove coal from them in or near the fire rooms. When sealed fire rooms were adopted to allow for "forced draft," the coal scuttles had to be outside the fire rooms' airlocks, which were required in order to maintain the overpressure in the fire rooms which in effect "fanned" the fires under the boilers to produce higher temperatures. Today, with liquid fuels, "bunkers" are fuel tanks which are normally located in the bottoms of the ships, which is very useful for stability purposes. Using coal-fired boilers, naval designers used the coal bunkers for what amounted to additional side protection, using the coal itself to stop projectiles and the bunkers as a sort of explosion chamber which was supposed to at least partially contain and channel the explosive force of detonating shells. What was not realized for many years — until World War One's experience with battle damage — was that it was impossible to incorporate watertight bulkheads into coal bunkers, because the shock effect of explosions sprung or warped the scuttles, which left big, water-gushing holes in the bulkheads in the event of a torpedo hit.
Steamships are designed to float at a specific level — "design waterline" — with the loads aboard that the designer expects them to carry. When you see the bunkerage data tabulated for a vessel, it will usually (especially for coal-fired ships) be listed as two values with a "/" between them. Thus the River-class at "480/1,172," which indicates that the ships will float at their "design waterline" with 480 tons of coal on board but that they have sufficient additional space in their bunkers for another 692 tons. That higher total — the 1,172-ton figure — represents the ships' maximum fuel load, and will cause them to float deeper than their design waterline.
A properly designed vessel should still be perfectly safe under normal sea conditions at the maximum load, however the draft will be deeper and what may be safe for "normal sea conditions" may be distinctly
unsafe in combat. For example, armored ships normally have "belt armor," which is designed to protect the side of the ship against horizontal shell hits. Its primary function, however, is to protect machinery spaces (normally located below the waterline whenever possible) and the waterline itself against penetrating hits which would flood the ship, so it normally doesn't extend all the way to the top of the hull. That is, there are usually levels within the hull above the belt but below the "weather deck" (the lowest deck exposed to the weather [i.e., not enclosed within the hull]). This means that if a ship is so heavily loaded that all or a significant portion of its armored belt is immersed, rather than extending above the waterline as intended, it loses almost all of its protective value and hits which would not normally have flooded the ship (because they would have been well above the waterline) instead allow flooding.
It was not uncommon during the period of coal-fired boilers, especially when steam pressures were relatively low and hence less efficient, for ships being deployed over great distances to leave port with deck loads of coal, in addition to that which was in their bunkers. In conditions like that, freeboard (see below [G]) could be dangerously reduced, although generally not to a degree which would have threatened the ship's safety under normal sea conditions. When the Russian fleet steamed from the Baltic to the Battle of Tsushima, its ships left port with heavy deck loads, which considerably increased the distance they could steam before being required to refuel ("coal," used as a verb). With the advent of oil-fueling, that became impractical, but the increased caloric efficiency of oil, coupled with the substantially higher steam pressures available, also enormously increased a ship's cruising radius for the same tonnage of fuel. It was . . . unusual in the extreme to see a European coal-fired battleship with a designed endurance of much over 3,000 miles prior to
Dreadnought (1905), and even though that had been increased to somewhere between 5,000 and 6,000 miles for the majority of British dreadnoughts between 1905 and 1916 (the Battle of Jutland), it was still substantially less than that for many of their ships (
Canada = 4,400 miles;
Agincourt = 4,500 miles
Revenge [oil-fired] = 4,200 miles, etc.). That relatively short cruising endurance was acceptable for European waters but explains why the Royal Navy required a worldwide system of coaling stations. It also explains why American battleships were designed with stupendous endurances compared to their European counterparts; it's roughly 3,500 miles from New York to the English Channel, and the US didn't
have coaling stations. American ships had 5,000-mile radii while European vessels were being designed for 3,000 miles, and by 1910 that had been pushed up to 8,000 for the US, while the standard became
10,000 with the
Nevada-class designed in 1911. Many European naval historians have failed to understand the logic behind such long endurances, and even many who have understood that American naval architects had to design ships that could sail all the way to European waters, fight a battle, and then sail home again, have failed to appreciate that one of the reasons "backward" American battleship designers stuck with reciprocating machinery as long as they did, rather than following the more "advanced" Europeans into using turbines, was that reciprocating machinery delivered lower sustained speeds but were far more fuel-efficient at lower
cruising speeds than current-generation turbines.
But I digress . . . again. [G]
"Freeboard" is normally defined as the distance between water level and the edge of a vessel's weather deck. That is, the distance (height) between the waterline and the edge of the "top deck" of the
hull but not the top of the
superstructure.
In this case, however, I'm using the term somewhat differently. These ships are "casemate ironclads," which means that rather than having their guns mounted in rotating turrets, like the
Monitor, they carry them on normal gundecks inside armored superstructures ("casemates") with gun ports. The "freeboard" in this case is the
armored freeboard — i.e., the distance from the design waterline to the top of the casemate. They have short foredecks and afterdecks, mainly as someplace for the line-handlers to stand, but the freeboard to deck-edge is only about 5 or 4 feet, and the lower sills of the gun ports are only about 6 feet above the design waterline, which is only about half the height required to efficiently work the guns in a normal seaway. That is, the height required to keep waves from washing in through the gun ports in the aforesaid "normal seaway." This is why frigates, with only a single gundeck, normally have between 10 and 14 feet of freeboard to their port sills. Ships-of-the-line, with the tonnage constraints imposed by the great mass of guns crammed into their hulls, seldom had that kind of freeboard to their lowest gundecks. Until after about 1820 or 1830, construction techniques restricted the length of a wooden-hulled ship to a maximum of only about 200 feet, which imposed a hard limit on displacement, which in turn meant that cramming 74-100 and guns aboard a ship automatically meant that it was going to sit a lot deeper in the water. What it
also meant, however, was that you were limited to no more than about 30 gun ports per gundeck, because of the restriction on length, so getting those extra guns on board also meant that you needed additional armed decks. That meant the lowest of those decks had to be closer to the water and that you simply had to accept that under adverse sea conditions (i.e.,
normal conditions outside harbors and similarly sheltered reaches), you weren't going to be able to use the lowest gundeck's guns — which were always the heaviest ones on board.
As long as the gun ports could be properly sealed, however, they didn't normally pose any threat to the ship's survival even under highly adverse conditions, since the water couldn't get in through them, anyway. So while the River-class ironclads couldn't work their guns under typical mid-Atlantic conditions, they have plenty of freeboard for river defense or even for coast defense under normal sea state conditions, and their lack of freeboard in itself doesn't pose any significant threat to their survival under
most blue water sailing conditions.
Hope this helps and wasn't
too digressive.
Oh, one other thing I should mention while I'm at it, although I've mentioned it several times in the books. The term "knot" is actually very specific — it means "1 nautical mile per hour" (which is why some of us tend to cringe when we hear someone say "the ship was moving at ten knots per hour"). Now, a nautical mile and a statute mile are not the same animal, which means that when someone talks about an Old Earth ship moving at ten knots (ten
nautical miles per hour), the speed for landlubbers is 11.5078 (approximately [G])
statute miles per hour (or, for our metric friends, 18.52 km/h). On Safehold, courtesy of Eric Langhorne, there are
only statute miles, so when these ships' maximum speed is given, it is approximately 13% lower than the same speed — in knots — from our own maritime experience. In other words, these ships are slower than they would seem from the listed speeds to someone accustomed to thinking in terrestrial maritime terms.
