Diving and the MBT Gauge

[Dredd52]

Hi fellas,

I've been trying to understand how the ballast tanks in an American submarine work, using the NAVPERS 16160 Fleet Type Submarine manual. The impression I've received is that in order to dive, the main ballast tanks (MBTs) and negative tank fill, giving the submarine negative buoyancy - in other words, it displaces less water than it weighs, and sinks. While underwater, the MBTs remain completely filled, and the variable tanks are used to achieve negative, neutral, or positive buoyancy. I've been struggling a little to understand exactly how buoyancy works, but I've read that it does not alter with depth, and that a "dive" setting of the submarine will mean continuous descent, no matter the depth, and a "surface" setting will mean continuous ascent.

Upon executing a dive in SH4, however, instead of immediately filling, the MBT gauges fill up steadily as the submarine sinks, and level off when the submarine does. This suggests to me that a submarine has to take on so-and-so amount of weight to descend to a specific depth, and must take on even more weight to descend further - which contracts my present understanding of buoyancy. I've added a couple images demonstrating this - one at 150 feet, the other at 200.





Am I reading the wrong gauge, and is it something to do with outside water pressure? Or am I misunderstanding how submarines work? I've added RFB and the RSRD campaign mod, if that makes a difference.

[les green01]

dont go by sh4 things dont work like they should and some not even been finish

[fireftr18]

A lot of it is just eye candy and doesn't really mean anything. The depth gauges, the telegraph, the rpm meter, and the rudder angle indicator all work correctly. I'm not sure what else works correctly.

[Sniper297]

In my opinion the game designers understood submarine operation about as well as the average Aroma Therapist does.

The way it worked in reality is they would make a daily trim dive, leveling at 100 feet and slowing down, pumping fore-aft in-out until they had the boat balanced and as close to NEUTRAL buoyancy as they could get. Then they would set a pointer on the negative tank gauge for the amount of water it held at neutral buoyancy.

When diving all the tanks were flooded for negative buoyancy to get under in a hurry, then before getting too deep the order came "BLOW NEGATIVE TO THE MARK". The negative tank blown to the previous mark gave the sub approximately the same neutral buoyancy from the most recent trim dive, the depth was then adjusted DYNAMICALLY with the diving planes to go up or down or hold depth.

In real life unlike the game you couldn't stop motors and go dead in the water (zero knots) and be able to hold a steady depth or even stay level fore and aft, since perfect neutral buoyancy is nearly impossible. So they maintained a minimum speed of 2 knots at all times while submerged, changed or held depth only with the diving planes and never fiddled with ballast at all unless something happened to change the balance or neutral buoyancy. When they surfaced they blew air into the tanks to get positive buoyancy, and also used the diving planes to go up. But for going from 100 feet down to 300 feet or rising from 300 feet to periscope depth it was diving planes only, never touching the ballast controls.

[razark]

Wait, the gauges in the game aren't completely random?!?


I've owned this game since it came out and never noticed?

[Sniper297]

I have SH4 1.4 and have never seen the MBT gauge shown in the OP's pics, so I assume those are from 1.5 or the Uboat expansion. The other gauges aren't random, they accurately show depth heading and speed, but the physics model for buoyancy and balance is so oversimplified it shows either a complete lack of knowledge for how it actually works or complete apathy. So if they added a ballast tank flood gauge I would expect it to reflect that same ignorance or apathy. When underwater all the main (external) ballast tanks are usually completely full, but if the safety, bow buoyancy, and negative tanks are empty the sub will have positive buoyancy and rise to the surface.

Many "experts" will tell you that you can never have any air in the external ballast tanks when submerged because the external pressure will collapse the tanks. Completely forgetting that for US subs the flooding ports at the bottom are open to sea and air is compressible, so automatic equalization of internal and external pressure. "Put a bubble in number 7" to compensate for compartment flooding aft was commonly done, but in this game you either have full or empty ballast tanks, no in betweens.

"must take on even more weight to descend further - which contracts my present understanding of buoyancy"

I think you mean "contradicts" rather than "contracts", but trust me, your understanding is better than the designer of that gauge.

[TorpX]

Originally Posted by razark

Wait, the gauges in the game aren't completely random?!?


I've owned this game since it came out and never noticed?

I didn't know the MBT gauges ever changed.

At least the dive plane indicators work sensibly. Too bad we can't have manual control of these.

One of the things I miss from SHCE, is having a working odometer.

[Dredd52]

Thanks for the explanations, guys - fantastic game nonetheless.

I think you mean "contradicts" rather than "contracts"

Whoopsy daisy, didn't catch that.

[ETR3(SS)]

Sniper hit it pretty much right on the nose. (Not bad for a brown shoe ) The salinity of the water you're in along with depth can also affect the ships buoyancy. There's a lot of math that goes into calculating ships buoyancy before you even leave the pier. And then when you're out at sea more calculations are done as consumables are used and sanitary tanks pumped overboard in order to maintain that state of near neutral buoyancy. The amount of calculations involved would overwhelm the game and it is much easier to just have submarines that hover like UFOs.