Subject: SMML VOL 2740 Date: Sun, 30 Jan 2005 23:51:45 +1100 The Ship Modelling Mailing List (SMML) is proudly sponsored by SANDLE http//sandlehobbies.com For infomation on how to Post to SMML and Unsubscribe from SMML http//smmlonline.com/aboutsmml/rules.html ---------------------------------------------------------------------------- MODELLERS INDEX 1 Liberty Freeboard 2 WEM KGV reclined ladders and lifeboats question 3 Re Salinity & buoyancy -------------------------------------------------------------------------------- TRADERS, ANNOUNCEMENTS & NOTICEBOARD INDEX 1 Book on ebay U.S. Aircraft Carriers by Norman Friedman ---------------------------------------------------------------------------- MODELLERS ---------------------------------------------------------------------------- 1) From "Hank Lapa" Subject Liberty Freeboard I think the different load lines for different seas would only come into play if at or near (or over) maximum loading. I say this because the Liberty in Baltimore, in her empty wartime rig as a troop transport, rides very high in the Chesapeake (brackish water). Now, having seen a whole lot of red-painted hull up out of the water when this ship is tied up or underway, there's still a LOT of gray-painted hull up to the weather deck. However, there must be a storm factor cranked into the calculation of the load lines, and also for the frequency and height of storm seas to avoid breaking the back of a Liberty. Hank ---------------------------------------------------------------------------- 2) From Matthew Subject WEM KGV reclined ladders and lifeboats question Good day, Maybe a stupid question, but wanted to make sure I was going to do this right before attempting. The reclined ladders on the etch set seem to have rails, are these bent upwards, along with the steps pointing out? Just wanted to check. The lifeboats, are they coloured white? Same with the oars that WEM so kindly included? Yeah stupid questions, but I am more used to modeling airplanes and did not want to make too many mistakes. Many thanks, Matt ---------------------------------------------------------------------------- 3) From "Rick Nelson" Subject Re Salinity & buoyancy Tom stated that, "First, there is not a smooth gradient of salt water. There are definite changes in temperature and salinity that form layers." This is true as I had mentioned in my comments in 3) large salinity variations usually occur primarily around sources of fresh water, and they will have an affect on depth control depending on the extend of the density change. But they are not incurred very often out in the deep blue sea. The element that changes very significantly throughout the oceans is water temperature as Tom also mentions. As I had stated previously density (d) varies with Temperature and these temperature variations can occur over a very small depth change. These abrupt changes in temperature verses depth are referred to as "temperature gradients". Their affect on seawater density usually is not extreme enough to affect depth control very much but they due affect acoustics significantly by creating a "diffraction" layer. The abrupt change in density on one side of the thermal gradient to the density on the other side of the thermal gradient causes a change in the velocity of acoustic energy. Submarines have been outfitted with bathythermographs beginning in WWII which are used to measure water temperature in an attempt to detect "thermal gradients". This is what we have all heard referred to (and some of us have done) as "hiding under a thermal layer" from the enemy's sonar. To put Tom's comments about submarine hull compression in the context of the equations I presented in 3) one will see that if w = dgV remains constant (which it does for this discussion) and (V) decreases due to hydrostatic forces applied to the submarines' hull then (d)density increases causing the submarine to take on "negative" buoyancy. This can become an escalating event; the deep you go the greater the pressure acting on the hull and the deeper the boat wants to go! Couple this with having just run into a big fresh water patch and things can get exciting rather quickly. This occurs in northern latitudes. Hull compression is a very demonstrable phenomena and we would do this by stringing a line athwart-ship at periscope depth and make it taught. Then at test depth the line would take on a significant sag due to hull compression. This was a very graphic demonstration for the newbes. We also had Formica bulkhead panels in the middle level ops compartment passageway that were parallel to the centerline of the submarine which at periscope depth would be butted up against one another and at test depth would be overlapping by several inches demonstrating how much the hulls' length would be compressed. WWII era fleet submarines had buoyancy tanks, which are more correctly referred to as "Ballast tanks". There were four different types of Ballast tanks "Water" or "Main" (sometimes referred to as saddle tanks); "Variable"; "Special", and "Fuel". The term "saddle tanks" has been used for the buoyancy tanks because a large number of the water ballast tanks were "wrapped" around the cylindrical pressure hull and they were in pairs with a port side tank and a starboard side tank. However, there were other water ballast tanks like MBT #1 and MBT #7 that were not of that "style". All of the MBTs were free-flooding and not provided with flood valves. In other words, they were either full or empty. The MBTs had the greatest affect on the Volumetric change of the sub. The "Variable" Ballast tank group included; Forward Trim tank; the WRT tanks; Auxiliary #1 and #2; and After Trim tank. The WRT (Water 'Round Torpedo) tanks were as the name implies used to flood and drain the torpedo tubes. By "keeping" the water used for the torpedo tubes "on-board" minimized the changes in the subs' density because of torpedo operations. Forward Trim, the Aux tanks, and After Trim were primarily used to "fine-tune" the density of the submarine to match the density of the seawater it was displacing. The Aux tanks were located below the center of gravity of the sub and just affected the overall density of the boat. Forward and After Trim were use to maintain a "zero bubble" (no angle up or down). Water could be pumped between Forward and After Trim to adjust the angle of the boat and not change its' density. Now the "Special" tanks. There are three tanks in this group; Bow Buoyancy tank; Safety tank; and Negative tank. The Bow Buoyancy tank was used like a MBT in that it was fully flooded along with the MBTs when diving the boat. However, it is also used to correct excessive down angles and to give the ship an up angle during surfacing. The bow buoyancy tank is free flooding through ports in the superstructure plating along the outside boundary of the tank. "Safety" tank had a capacity of 23.23 tons of sea water located amidships and was always full. Safety could be blown to immediately give the sub a "positive" buoyancy in an emergency. It's interesting to note here that the difference between neutral and positive buoyancy on a fleet is only 23 tons. "Negative" tank would be dry when on the surface. When diving it was flooded along with the MBTs and was intended to give the submarine a significant "negative" buoyancy to facilitate getting below the surface as quickly as possible. Once submerged Negative tank would be blown "to the mark". This was a pre-computed quantity of water remaining that would bring the submarine back to "neutral" buoyancy. Modern nuclear submarine Main Ballast Tanks can still be considered "saddle tanks" in that they are still in pairs and surround the cylindrical pressure hull. (The San Francisco sustained major damage to her saddle tanks and the fact that her MBTs were still divided into pairs allowed her to make full use of the ones that were not damaged to help get to the surface.) Tom is correct in that with the advent of nuclear submarines the emphasis was no longer on quick dives so "Negative" tanks were removed. Also, the size of the "Safety" tank was no longer big enough to give the larger nuclear subs a "positive" buoyancy so it was removed from the design. The Auxiliary #1 and #2 remained up until recently as a means to control overall sub density. On fleetboats, the Aux tanks were external to the pressure hull and therefore were "Hard" tanks (could withstand test dept pressure). Nuclear submarine designs brought the Aux tanks inside the pressure hull making them "Soft" tanks. However, since the Aux tanks were inside the pressure hull, and internal hull space being such a premium, the Aux tanks have been removed to make either more room or a smaller design. Overall density is now maintained by more controlled use of Forward and After Trim tanks. A cubic foot of real estate inside a submarine is the most expensive real estate in the universe! Again, very long winded but I get carried away. Hope nobody minds! Rick Nelson "Damn the Pressure, Six-Zero feet!" "Boomers Hide With Pride" ---------------------------------------------------------------------------- TRADERS, ANNOUNCEMENTS & NOTICEBOARD ---------------------------------------------------------------------------- 1) From Mark McKellar Subject Book on ebay U.S. Aircraft Carriers by Norman Friedman Hi folks, I just listed this great book on ebay. I bought it for a project that never got off the ground. It's new, perfect. I'll ship it anywhere - you pay the freight. If you're a US carrier builder - you absolutely have to have this book. Mark Boston ---------------------------------------------------------------------------- Check out the SMML site for the List Rules, Reviews, Articles, Backissues, Member's models & Reference Pictures at http//smmlonline.com ---------------------------------------------------------------------------- End of Volume