Subject: SMML VOL 2742 Date: Wed, 02 Feb 2005 01:11:16 +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 Re Recommended Airbrushes? 2 Malta Class Carriers 3 Compressed Air on Subs, Bouyancy, etc. 4 Re Salinity and buoyancy 5 Re Salinity and buoyancy 6 Plimsoll marks on ships ---------------------------------------------------------------------------- Model club & SMMLcon Information 1 Escort 49 ---------------------------------------------------------------------------- MODELLERS ---------------------------------------------------------------------------- 1) From "Devin J. Poore" Subject Re Recommended Airbrushes? >> Does anyone have recommendations on airbrushes primarily suitable for spraying fine detail? I use a Tamiya Sprayworks purely for large area coverage and manual brushing for detail/camo schemes. << I have and love my Badger Sotar 20/20. Works fantastic for modeling purposes. With the medium tip and needle setup, you can free hand splotch camo on 1/350th scale warships. It will also "open up" wide enough to do overall same color spraying on the same sized models. The best thing, though, is how easy it is to clean. I hardly every need to strip it, as all of the "internal" workings are visible in the color cup; I just swab it out with a Q-Tip and run thinner through it and it's clean. I will break it down once a month or so and do a thorough check and clean, but the break down of the entire thing takes less than a minute. One of the best modeling tools I ever bought. Devin ---------------------------------------------------------------------------- 2) From Mike Bartel Subject Malta Class Carriers Hi Shane, A question for the group I need information (drawings, hull plans, etc. on the cancelled Malta class carriers. If you can help, please contact me at ihphobby@hotmail.com. Thanks! Mike Bartel ---------------------------------------------------------------------------- 3) From "Hank Lapa" Subject Compressed Air on Subs, Bouyancy, etc. It is purely a matter of weight -- compression is irrelevant. The total weight of all the air molecules in the tank does not change whether compressed or not, whether diving or rising (presuming tanks aren't blown or refilled). Consider a full tank of air. Now assume that, by magic, the volume of air, originally the same volume as the tank containing it, is compressed until it fills only one fourth of the tank, the weight of the air, and the sub, stays the same. Again, because the weight of all the air molecules is fixed when not blowing or flooding. The other three fourths of the tank now holds a vacuum, which of course has no weight at all. And so, the weight of the tank doesn't change, and that's the important point. The whole concept of bouyancy is about differential weight, whether a sub floods its tanks to dive or blows them enough to surface. These actions cause differential weight. Thus, these actions cause a change in bouyancy. In the surface ship world (or for a surfaced sub), it's a matter of displacement. Just like before, differential weight is the answer, only this time the two main considerations are the total weight of the ship and the weight of the water displaced by the part of the hull of this same ship which is under the surface. Bouyancy constantly reacts to changes in weight. At it's destination, the ship should be more bouyant because of fuel being burned, food being eaten and eliminated, passengers offloading, lines heaved, etc. So assuming an equal density of the sea, the ship will surely ride a little higher, the displacement self-adjusting to the ship's loss of weight. The effects of increased pressure outside or inside the sub are minute and arise only when sea pressure causes deformation of the containment, slightly decreasing it's original volume, and raising the air pressure inside the hull. Actual deformation may seem physically small, but change of internal pressure can be quite painfully felt in those with ear problems when diving deep at anything more than a dead-slow crawl so that human equalization can take place, just the opposite of pressure chambers used for deep diving. Nothing above is meant to confirm or deny details of construction or operation of any real submarine of any nation. Hank ---------------------------------------------------------------------------- 4) From "Rick Nelson" Subject Re Salinity and buoyancy Hello again, Kevin asks about "air in the dive tanks". As I had mentioned in my previous email that discussed the tank arrangement on a fleetsubmarine the Main Ballast tanks which are external to the pressure hull and free flooding can only be in one of two conditions either completely empty, or completely full of seawater at pressure. So there is no air to be compressed in the Main Ballast tanks (sometimes called the saddle tanks). The Variable and Special Ballast tanks can be partially full of seawater and air. However, they are not "free-flooding" or, you could say they are "sealed", i.e. they have valves that control the entry/exit of both water and air. If the tanks are external to the pressure hull they are "Hard" tanks in that they can withstand test depth and will not collapse if only partially filled. If they are internal to the pressure hull they are "Soft" tanks and are not subjected to external sea pressure and do not need to be able to withstand test depth, just normal internal air pressure. So, any tank that can hold a mix of seawater and air does NOT allow sea pressure into it or affect its' contents. The amount of water in each of these tanks are controlled using a system referred to as the "Trim and Drain System", (a topic for another discussion) or are emptied (Negative and Safety tanks) by blowing the water out through a valve using high pressure ships air. Therefore, there is no need to be concerned about any air being pressurized and un-pressurized in the buoyancy tanks. George in his 2) mentions the ebb and flow currents going through the Straits of Gibraltar. This condition does exist and is still taken advantage of by submarines. However, the condition only produces a current flow of about 2-3 knots. Enough but you're not going to get a speeding ticket riding it. Gibraltar is probably one of the very few places where this occurs and it takes an entire body of water the size of the Med to create it. I can personally attest to the fact the Med is much warmer than the Atlantic at depth. I speculate that these ingress and egress currents have indirectly reduced the chances of submarine head-on collisions in the Straits (which are very narrow, especially the deeper you go) because of the fact that the currents run at different depths and subs, by taking advantage of them, create an inherent vertical separation. But if someone doesn't play by the rules then it can get hairy. George also discusses the THERMOCLINE which according to Websters' is defined as "the region in a thermally stratified body of water which separates warmer oxygen-rich surface water from cold oxygen-poor deep water and in which temperature decreases rapidly with depth." Thermocline is an application of the term "temperature gradient" which is the change in temperature compared to the change in distance, hence "thermally stratified". A thermocline does not act like an acoustic reflector, it has no coefficient of reflectivity. Now how does temperature affect acoustic energy (sound waves)? The basic equation for sound Intensity (I) is I = (P*P)/2dv where (P)Pressure-amplitude squared divided by 2 times (d)density times (v)velocity of the sound wave. Acoustic energy is affected by density (d). For example; you can hear sound in air but there is no sound transmission in a vacuum. Or, sound travels faster in water than air. Both of these generally accepted statements are based on the differences in density (d) of the medium the sound is traveling through. As I had described in my previous email temperature affects density(d) and this abrupt change in density at the boundaries of the thermocline a diffraction layer on both sides of which the density is different. Sound waves are bent not reflected due to the change in density of the seawater. However, it is the bending phenomena that has saved many a submarine from detection. The direction that the sonar operators' equipment tells him the source of the sound is at is not correct due to this bending. The sea/air boundary creates a very significant diffraction layer also. George is right when he says that thermoclines work in both directions. The direction of travel of a sound wave will be bent regardless of which side of the temperature gradient the source is. Dipping sonars from helos and sonabouys are not necessarily solutions to the thermocline problem. Usually they cannot get below the thermal stratification. You can't believe how easy it is to hear a helo with its' blades beating on the ocean surface. Sonabouys must remain partially on the surface in order to transmit their sonar data. In WWII the objective was to remain undetected from the surface ship enemy so if a boat found a thermocline it attempted to stay below it. Now the game is primarily sub vs. sub and both have equivalent capabilities. How you use a thermocline depends on what your objective is. If you're a missile boat you use it to hide. If you're an attack sub and want to evade detection you use it to hide. If, however, you are on the hunt then you get on the same side of it as your adversary and bet on your superior quieting and tactics to win. Rick ---------------------------------------------------------------------------- 5) From Reynold.Oh@defence.gov.au Subject Re Salinity and buoyancy G'day Kelvin Mok. George Oh from Sydney, Australia here. 1. Yes - air is highly compressable and a given volume is much smaller at depth than at the surface. If taken underwater in a childs balloon (say) the water pressure would make the balloon smaller by compressing the air. On the other side, if a scuba diver inflates a balloon underwater with the air from his lungs (lungs refilled from breaths taken from his bottle of compressed air) and takes it to the surface, the balloon will expand (and possibly burst) as the water's pressure on air inside reduces with the reducing depth. This is why submariners (and scuba divers) are trained to breath out as they assend from the depths (to NOT hold their breath) - because their lungs are exactly like the balloon. However, the air inside a submarine (assume it is equal to the air-pressure at the sea's surface - the hatch was open when they were on the surface) will not be compressed because the sub's pressure hull protects it from the water pressure (unlike the balloon). 2. To surface, a submarine must get rid of the seawater in its diving tanks and replace it with something that will make the submarine more bouyant. It does this in one operation - by blowing compressed air (from the compressed air bottles it carries) into the diving tanks - hence the order "Blow tanks". The air forces the seawater out of the bottom (through opened valves) of the diving tanks. At a shallow depth, the submarine will need the air from (say) two bottles to fill the diving tanks. At a greater depth, the submarine will need air from more bottles (say seven) to fill the same diving tanks. Why more? Because the submarine is releasing the compressed air to the outside of the pressure hull. The air becomes one big bubble (inside the diving tanks) around the submarine. In the diving tanks, the air is kept compressed (to a much lesser degree that it was inside the sub's compressed air bottles) by the pressure of the water. As the submarine rises, the air in the diving tanks expands (like the balloon above) and the extra volume thus created, overflows out of the diving tanks from the still-open bottom valves. Once back on the surface, the submarine starts its air compressors to refill its compressed air bottles 'cos it'll need them next time it wants to surface. It will probably also start its engines to recharge its batteries. 3. On the surface, the diving tanks remain filled with air, and continues to be a bubble of air around the submarine, so it floats. To submerge, the dive tank are flooded (with seawater - hence the order "Flood tanks") and the submarine's supporting bubble is 'destroyed'. So, the submarine 'sinks'. How are the dive tanks flooded? The diving tank valves - both top and bottom - are all opened. The valves at the bottom let seawater in, and the ones at the top let the air out, all at the same time. Using propellers, diving planes (= horizontal rudders) and ordering "All (spare) hands, Forward" hastens the diving procss (= a crash-dive). Once underwater, the diving tanks are full of water and all of the valves on the diving tanks are closed. Just before the required depth is reached, some compressed air is pumped into the diving tanks, till the amount of bouyancy they provide makes the submarine neutrally bouyant, and the submarine hangs at that depth - the dive has been arrested or checked. Air is added to or releasd from the diving tanks to alter the submarine's depth. 4. There are also trim tanks that can be flooded or drained to make the submarine float bow up, or bow down, or horizontally in the water. Of course, operating a submarine is much more complex than that quick theory lesson. And there is the tactical situation to consider. George Oh, out. TX ends........................ ---------------------------------------------------------------------------- 6) From "Paul Giltz" Subject Plimsoll marks on ships There is a two page description of the history and appearance of the Plimsoll Mark in the 1936 edition of Felix Riesenberg's book " Standard Seamanship for the Merchant Service" ( published by D. Van Nostrand, NY ) This is a wonderful book for anyone interested in merchant shipping of that era, Well illustrated with photos and drawings. 942 pages and 627 illustrations. (winches, how to rig cranes and kingposts, special gear for loading coal, how to stow a locomotive in a lower hold using bales of compressed hay, using "tally Sticks" when loading cargo in China with Coolies, which officer is responsible for stowing passenger luggage marked 'not wanted on voyage',formulae for paint one gallon of "Hospital Ship Green"-(Chrome green in oil 7lbs.-9 1/2 oz.,Raw linseed oil 5pts-2gills,Petroleum Spirits 3&3/10ths gills,drier 2 gills,spar varnish 1/2 gill) wonderfull stuff..... From the book - "The subject of loading of vessels is closely connected with the name of Samuel Plimsoll, who , as a member of Parliament, conducted in the early 1870's, a strong campaign for the fixing of load lines on vessels under the British flag. In Plimsoll's opinion the unusual number of disasters at sea that had been occurring for years were due chiefly to the overloading of vessels, and he contended that Parliament should establish a load line for every vessel.... the merchant shipping act of 1876 provided that a circular disc, with a line drawn ( horizontally ) through the center, should be painted on both sides of every British vessel... this mark, which became known as the Plimsoll mark, was to indicate the greatest draft to which vessels should be loaded." The book describes how ( by 1936) the American Bureau of Shipping had incorporated a slight variation of the system. There is a drawing of the" current" marking which shows the circle bisected by the horizontal line. Next to it is a vertical line with five horizontal lines branching from it. These lines are listed ( in descending order) as "FW" (fresh water),"IS" ( Indian Ocean-Summer), "S" ( summer) , "W" ( Winter in waters other than the North Atlantic),and "WNA" ( North Atlantic in Winter, October to March inclusive) Some previous owner of the book very thoughtfully pencilled in most of the dimensions of the drawing. In addition he pencilled in changes that must have taken place. At the top there is now a "TF " marking ( Tropical fresh )above "FW" ,and "IS" is now "T" ( tropical). The book also discusses a little bit about how the placement is made in relation to deck-line and draft.Plenty of information for even the most compulsive of modelers.... Should you be one of those interested in the actual dimensions/geometry of the Plimsoll mark send me an e-mail off site. ---------------------------------------------------------------------------- Model club & SMMLcon Information ---------------------------------------------------------------------------- 1) From brownatfareham@surefish.co.uk Subject Escort 49 The website (www.smallwarshipgroup.org.uk) was updated earlier today. Those of you who have asked for pdf copis should have already received them (I hope!). Regards Les ---------------------------------------------------------------------------- Check out the SMML site for the List Rules, Reviews, Articles, Backissues, Member's models & Reference Pictures at http//smmlonline.com ---------------------------------------------------------------------------- End of Volume