Sub physics - experts needed

[OneShot]

No kidding, I found one copy on Amazon.de but the company wanted a hefty 183€ for this ... Guess I pass on that one.

[Dr.Sid]

I guess I'll pass to. Anyway for the diving I now have all data needed. On the other hand I'm extra busy at work, I was working whole weekend and all evenings. So please be patient.

[Dr.Sid]

Just an idea how to determine real sub lift coeficients. Have you ever heard about diving sub without filling the ballast tanks ? It should be possible, at some speed and pitch angle. Or the opposite .. surfacing sub with tanks full (and staying surfaced).
I understand both maneuvers are nonsense, but maybe somebody ever this, even theoretically.

[Bubblehead Nuke]

Originally Posted by Dr.Sid

Just an idea how to determine real sub lift coeficients. Have you ever heard about diving sub without filling the ballast tanks ? It should be possible, at some speed and pitch angle. Or the opposite .. surfacing sub with tanks full (and staying surfaced).
I understand both maneuvers are nonsense, but maybe somebody ever this, even theoretically.

Nonsense maybe, but lets think this out.


Diving with the ballast tanks bown dry:

MAYBE, if you had bowplanes (they would be under the surface and thus effective).. and were going flat out on the surface, you MIGHT be able to push the bow down enough to get the water flowing over the hull, thus turning the upper hull into a massive plane surface. The old nuke boats of before the Albacore hulls (pre 637 hulls) had this problem at high speeds. They had to be careful at high speed putting angles on the hull less they lose depth control. I do believe that the rounded hull surface all but eliminated the issue.

They also have a LOT of reserve bouyancy designed into the hull. I do not think you could do this if you tried.

Surfacing the boat with the ballast tanks full:

Yes, easily. You just drive the ship up using the planes and the engines. Once on the surface you have the surface tension of the water to help support you (this is the same thing that allows a needle to 'float' on the suface of the water.

One term for surfacing without blowing the tanks is called 'broaching'. We hate doing it as when you have fairwater planes, you can make a rather LOUD noise and REALLY shake the boat when the planes come down back into the water.

[Dr.Sid]

Hm ! Then water tension !? I've seen something concerning waterline length in the book subguru sent me. Could be this. Must check it out.

As for the buoancy reserve, 688 at wikipedia shows 87% of hull volume submerged with standard surfacing procedures. That is 13% of weight which holds the sub 'up'. Really not much, if you consider what forces have effect on the sub at high speeds.

Well I guess I'll post the demo (this weekend quite probably) and I will let you judge if it feels right or not.

[Dr.Sid]

An idea ! I guess you all have seen those videos of 688 making emergency surface. If I knew the speed, I have everything ! Pitch can be seen in the video. From the speed, and amount of exposed hull, time it take to submerge again I can compute side lift & drag coefficients, and even forward-backward stabilization forces.
So the questions is .. what is the speed estimate of the sub in this video ?

If it is realtime, I could even capture the speed from the video. But is it realtime ?

[Chock]

I just made a rough estimate from that footage based on looking at a point on the ocean where it breaks the water and then timing how long it takes half the length of the boat to pass that point, which is about all you can do since the wave and spray obscures stuff soon after. Not very scientific, but it works out to around 21.5 knots by my estimation. Of course that is if the footage is not slowed down at all, which to be honest, it looks like it is a little.

Chock