What do we mean by "reserve buoyancy"

[Deathblow]

quick technical question about sub dynamics.

I know that's its a measure of the ballast control of a sub, but what does the term "reserve buoyancy" specifically mean in terms of capabilities.

For example, how does a sub with 30% reserve buoyancy perform compared to a sub with 15% reserve buoyancy? Can it change depths significantly faster? Does it mean that the sub has better depth control to prevent broaching the sail in shallow waters?

And what where do the numbers "30%" and "15%" actually come from? Anyone that can shed some light on the subject?

[Terrax]

This question brings to mind a though I've had since reading Red Storm Rising. At one point, the skipper of the Chicago, I think, wanted to launch a torp form deep depth, and the XO protested, and said NO, they would use their emergency compressed air or something to that effect. It's been several years since I've read the book.

[Drebbel]

A sub with more reserve buoyancy has to take on more ballast (more water in dive tanks) before she can submerge.

But when submerged compared sub could have both neutral buoyancy. In that case there will be no difference, then the size of the sub matters more. Because 1 liter more ballast on the small sub has a much bigger impct than 1 lts ballast on a big sub

[Etienne]

Disclaimer : All my stabillity training is about surface ships.

Reserve buoyancy is the volume of the vessel that could be buoyant, but isn't, since it's above the waterline. (IE, accomodations, focstle store, etc). So a submarine with a lot or reserve buoyancy would be one ridding very high in the water when surfaced.

It'd take more ballast to submerge one such submarine, because of the whole Archimede thing (Essentially, to have neutral buoyancy, your submarine has to weight as much as its entire volume of water weight. Less than that, it's going up, more than that, it's going down)

[tycho102]

Reserve buoyancy is pretty much exactly as it's named. It's whatever extra buoyant force that a ship has above neutral buoyancy.

For subs these days, negative tanks have been discarded since dive times are pretty much....negligable. So a ship is made as neutrally buoyant as possible, and the ballast tanks as small as possible. Fill'em with water and the boat goes down, fill'em with air, and it provides 99.99% of the "reserve buoyancy" the boat has.

For surface ships, the reserve buoyancy is generally a couple times the ship's own dry weight. For subs, it's a bare fraction (~10%).



The measurement of reserve buoyancy is also directly linked to the amount of damage a boat can receive before going under...

[Etienne]

Originally Posted by tycho102

Reserve buoyancy is pretty much exactly as it's named. It's whatever extra buoyant force that a ship has above neutral buoyancy.

Ships can't have extra buoyant force. If that happened, the ship's volume would be reduced until the displacement is equal to the weight (IE, the ship would immerge)

Originally Posted by tycho102

So a ship is made as neutrally buoyant as possible, and the ballast tanks as small as possible. Fill'em with water and the boat goes down, fill'em with air, and it provides 99.99% of the "reserve buoyancy" the boat has.

Unless those empty tanks are fully immerged, they're not reserve buoyancy, they're just plain old buoyancy, since they form part of the ship's volume.

Is "Reserve buoyancy" in sub talk meant to be the extra buoyancy a submerged boat has that is compensated by the dive plane? (IE, a boat that's slightly buyoant being held down by the planes)

Originally Posted by tycho102

For surface ships, the reserve buoyancy is generally a couple times the ship's own dry weight. For subs, it's a bare fraction (~10%).

That's not reserve buoyancy per se, it's deadweight - althought if the ship's going around alight, it's pretty much reserve buoyancy, yes.

I don't think a sub's deadweight would be around 10% of the load displacement. I have here the Los Angeles' displacement as 6000T surfaced - That 6000T includes lightship (The weight of the ship, boiler water, some spares) AND deadweight (Crew, fuel, ammo, personnal effects, food, stores, etc)... 600T, total, for 127 some men, with their effects, the torpedos, missiles and whatnot? It's a bit close.

I don't know what's the reserve buoyancy of a submarine, but it's probably not much, especially if the sail is free-flooding.

[lesrae]

As I understand it, the reserve in % terms is a measure of how much of the ship's own weight the extra buoyancy can support - ie 10% reserve buoyancy on a 6000 ton sub could support 6600 tons.

Or something like that.

[Bill Nichols]

Originally Posted by tycho102

Reserve buoyancy is pretty much exactly as it's named. It's whatever extra buoyant force that a ship has above neutral buoyancy.

For subs these days, negative tanks have been discarded since dive times are pretty much....negligable. So a ship is made as neutrally buoyant as possible, and the ballast tanks as small as possible. Fill'em with water and the boat goes down, fill'em with air, and it provides 99.99% of the "reserve buoyancy" the boat has.

For surface ships, the reserve buoyancy is generally a couple times the ship's own dry weight. For subs, it's a bare fraction (~10%).

The measurement of reserve buoyancy is also directly linked to the amount of damage a boat can receive before going under...

Basically, for subs, reserve buoyancy is determined by the size of the main ballast tanks. The bigger the ballast tanks, the greater the reserve buoyancy (assuming the overall weight of the sub remains the same). The ballast tanks on US subs are typically sized to give about 10% reserve buoyancy when filled with air (thus, a 9000-ton sub that takes on 900-tons of water in a flooding casualty would sink, if it didn't use its propulsion to 'drive' the sub upwards).

Russian-design subs typically have about 30% reserve buoyancy, meaning they can withstand more flooding before sinking.

[Deamon]

logicaly, when blowing ballast tanks a sub with higher reserve buoyancy will get quicker to the surface.

Deamon