MovieChat Forums > Morning Departure (1950) Discussion > effect of the tides and air pressure

effect of the tides and air pressure

posted 9 years ago by kennethfrankel
2 replies | jump to latest

Several times during the rescue attempt they mentioned the tides. The sub is down in the open ocean. Except in certain odd places, like the Bay of Fundy, the tides will not seem to be much of a factor out at sea. Yes, the water will be a bit higher or lower, but so what. Once the sub is raised off the bottom, and held up by cables, why can't it be pulled up at a slow rate? Why would they have to wait for the tides to do that? Would they wait for a high tide or a low tide, or wait for the tide to rise or to start falling? Please comment if you know.
Sailing ships would leave a mooring "with the tide" because if you are in a river, or a protected port area, you may not have room to tack against the wind.
If the wind and the tides are against you, you can't go that way with sails. So the outgoing tide would carry the boat out into the open waters. None of which is relevant to the plot in this movie.
Separate topic -- several comments discuss leaving the sub when it is 30 feet down, at some point during the rescue after it has been raised up quite a bit.
You can stay down with scuba gear a long time at 30 feet without worrying about the bends or the nitrogen problems like rapture. A quick trip to the surface should not be too hard. What is the air pressure inside the sub? It might be close to surface air pressure. Quickly going from that to the surface means you would not fizz like a soda can. Nitrogen bubbles in the blood cause many problems. But there is something called a "bounce dive" - you can go down to 200 feet and back up quickly and not suffer much effects (not recommended). But if the sub has a higher pressure of air inside, then yes, you would have had time for nitrogen to build up inside of the people.
Several comments were made about other possible efforts. If the first group went out a flooded hatch, why couldn't a surface diver go to it, put supplies and rescue gear in it, and close the hatch. The men inside could drain the water out, into their cabin area. Yes, it would make a mess. Then they can go out of the hatch again like the first group. Maybe you can't close the hatch from the outside. They always seem to turn a big wheel to seal it shut.
I am not an expert on subs or salvage so I welcome explanations. I was a scuba diver. The second or third dives cause problems. Then the nitrogen builds up more and more, even if you rest on deck for a while. Airplane pilots found out the hard way that if you fly after diving, you are rolling the dice. Pressure in a big commercial plane may be similar to being at 3000 feet altitude. That is not figured into the diving tables. Again, this is not relevant to the movie plot.

[–] dj-68383 7 years ago

For a given latitude the tidal effect in terms of water depth are the same away from shore as near the shore, with estuaries like the bay of funds being exceptional due to,the geometry of the sea floor and the sides of the bay. So if there is an eight foot tidal rise at shore there will be a corresponding change in water depth offshore. I too was puzzled by the tidal references and found this https://youtu.be/-leA5EQVMKI that at about the 20 ish minute mark explains using two lift ships that ballast down at low tide, tighten up the cables under the sub and then at high tide dump the ballast and the tide plus ballast lift the sub (around great britan about 10 feet) and then the whole thing is towed to shallower water until the sub grounds again and they wait for low ide and repeat. The sub was in 15 fathoms when sunk or 90 feet so it would take about 9 lift cycles. Of course this method would be pretty useless closer to the equator where thevtidevis a foot or less!

As for why not just put some escape equipment in the escape areas and close the hatch, you can see that the hatch design on the Brit subs only allows closing from the inside and as this predates the existence of scuba, the divers were dependant on their air hoses so they could not go in and close the hatches behind them.

Good movie all the same!

[–] kennethfrankel 7 years ago

Thank you for your reply. I reviewed the later parts of the movie and I saw several scenes that matched the You-Tube video. The bottom line seems to be that they did not have winch/pulley systems that were strong enough to pull up a sub by themselves. If it was free of the bottom, and if you could raise it 3 feet, then why not 30 or 100 feet. They needed to rely on the force of the rising tide to lift 2 barges, with the cables taut, as you mentioned. Each tide would be about 6 hours later, so it might take a while. With lives at stake, that becomes really iffy at the end, even with good weather.