Some of you guys have emailed me about trimming and ballasting submarines. First of all I’m no expert and there's probably a lot of ways to do this. I am not saying that’s the right way, and by no means the only way, its just one that works for me.
This is the simplified version
Ok there are 2 important variables, Centre of Gravity, and Centre of Buoyancy. the vertical distance between the two is the metacentric height. The first thing to work out is what type of boat you want to end up with. One that handles like a speed boat, a supertanker or somewhere in between. I normally go for somewhere in between but if you a beginner or you sail in unfamiliar or murky water, you may want it like a tanker. The greater the metacentric height the more stable the sub will be about the pitch (bow/stern up/down) and roll axis. With a single prop sub having a greater metacentric height will make the boat less susceptible to torque roll generated buy the prop. Subs with contra rotating props don't suffer with torque roll if both motors are running at the same speed. Unless you are building a racing sub you want the metacentric height to be as great as possible, but too much and your sub will handle like a supertanker. So to increase metacentric height you want the CoG low down and the centre of Buoyancy high up. You achieve this by placing lead weight low down in the hull and foam (the blue closed cell type- not white polystyrene foam) high up (but below the surface running water line (foam above the water line has no effect- if you have set up your sub right in the first place). If you have a dry hull sub, there is no foam to add. Ok CoG first (you work out where this is by balancing your sub on the edge of a ruler) The further forward the CoG, the more stable the boat is in the yaw axis (less responsive to the rudder), and the further back it is the less stable it is (more responsive to the rudder). So once again you need to decide if you want a sub that turn like a speed boat or a super tanker. I keep my CoG just forward of the middle point of the boat. For most nuclear subs this is just behind or below the sail (conning tower). This works out in line with the centre of pressure exerted by water on the external surface of the sub. (Nuclear subs usually have more surface area upfront and taper toward the rear – hence in front of the midline if the boat) If you have a central ballast tank you want to place it directly above the CoG. If you have a twin tank, you want them to straddle the CoG equally. If the position of your ballast tank is fixed and you cant move it, then you want to set you CoG to be below your ballast tank (obviously you have now lost some flexibility as your cant chose your CoG depending on how you want your sub to perform). If you don't set the CoG below your ballast tank then your sub won’t rise and submerge level.
Ok now fill you ballast tank fully. You now want to add/remove foam below the desired surfaced waterline (set by an empty ballast tank, or a partially filled one if you want some reserve buoyancy for emergencies), so that the that the sub achieves a state of neutral buoyancy with the sub sitting level suspended in the water. (Don’t remove any lead weight that will upset your CoG, just add or remove foam. If you have a dry hull sub (no foam obviously) not only will you need to add/ remove lead you may need to redistribute it to maintain the CoG. No if it surfaces in a level attitude, it should submerge again in a level attitude. If it doesn’t then you have trapped air under the deck. You need to drill holes at strategic locations of sufficient minimum size (there is probably some formula to work out how big they need to be to overcome surface tension but I don’t know what it is). Voila! You have just trimmed a sub. Ok, now when may you want to add foam above the waterline? (Remember this is best avoided). If you have a wet hull sub (one with a WTC) and when surfaced some part of your WTC is (if your WTC is not symmetrical to the sub - most aren't) above the waterline, your trim is will be upset when you move from submerged to surfaced. Similarly if you have an odd shaped sub with say a disproportionately sized sail that’s stuck on one end of the sub rather than closer to the middle, the same thing will happen. Now you need to move some of that foam from below to above the water line (don't add extra, move what you got already). But you will now find that your sub now sits lower in the water when surfaced as your ballast tank is no longer large enough to maintain your desired waterline you set when you calculated the size of the ballast tank you needed (unless you built in some reserve buoyancy – some of which you will now loose). So if you have the chance to set the position of your WTC, it is advantages to set it so that the top of the WTC is level or just below the surface waterline. If you have a dry hull sub, you don’t have much choice as you surface a significant proportion of the buoyant part of the hull be broach the surface. Once again if its not symmetrically shaped you will find that the surface attitude is not level. The only way to correct this (if it is bad enough to bother you) is with some form of onboard trimming system (moving weight/ water from one part of the sub to another). You can of course fit such a system to a wet hull boat to intentionally alter its trim (make the bow heavy for a crash dive?)
Some of you may have seen the setup video of my HMS Trenchant posted elsewhere.
http://media.putfile.com/Trenchant-TimmingI have set this boat up as my ‘sprinter’, so I have set a smaller metacentric height. You may notice some pitching of the boat during static diving and surfacing. When underway however the boat is very responsive to the dive planes, leans over nicely in sharp turns, and yet its easy keep level with an auto leveler with the gain on high setting.
I'm sure you guys have your own way of doing it so lets have some tips!
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Topic: How to Ballast a Submarine (Read 4067 times)