The Submarine Reserve Buoyancy Problem
A bothersome factor when addressing the underhull configuration of Parche, and even those of Halibut, Seawolf, R.B. Russell, and, to a lesser extent, the Jimmy Carter, is the concern for maintaining sufficient reserve buoyancy during extensive hull modifications. For a submarine, reserve buoyancy is the difference in the maximum upward force provided by its empty ballast tanks and the dead weight of the ship's hull and all its contents. The reserve buoyancy must be able to provide sufficient freeboard when the ship is surfaced at its design waterline.
Naval architects build sufficient reserve buoyancy into a ship design so that it can accommodate future modifications and additional equipment—but only up to a point. I recall seeing some hull plans for a 637 identifying those vertical "stabilizers" on the stern planes as "space and weight" for a future passive, wide-aperture-like sonar that never materialized. Reserve weight is designed in by including many hundreds of tons of fixed lead ballast into the design trim and waterline. These ballast blocks are attached to the inside of the hull, usually low in the frame bays of the pressure hull or elsewhere. I've been through a new construction, a decommissioning, and too many overhauls and extensive yard periods. When a boat has had some or all of its fixed ballast removed, it floats much higher than under normal circumstances—and it's "tippy."
I"m not a naval architect and don"t have access to actual numbers, but I"d estimate that under normal circumstances, a modern SSN probably has between 15–20% reserve buoyancy, or possibly even less. If anyone knows the actual numbers, please correct me. For an Archerfish-class boat of about 4300 tons, that means the addition of about 750 to 850 tons of extra weight would sink the ship. In any case, it"s not the actual numbers that matter but the idea of the sensitivity a submarine has to adding extra dead weight to the hull.
When Parche went through her initial conversion, the key visible alteration was the addition of quite a massive object called the DSRV Simulator, well aft of her center of gravity. To compensate for this weight, a lot of fixed ballast would have had to have been removed from the engineering spaces to maintain an acceptable draft and fore-aft trim on the surface. When one considers that there may have also been the mysterious "gondola" attached to the underside of the hull, and the additional dead weight that it entailed, the reballasting of the ship must have been a big headache for the naval architects.
To illustrate my point, here is an undated photo of Parche that appears to have been taken sometime shortly after her initial conversion overhaul. Notice the amount of trim aft compared to the average 637 or 678 boat. Obviously, when submerged, shifting around variable ballast could give her a zero bubble, but the photo illustrates the issues of maintaining sufficient reserve buoyancy and satisfactory surface trim.
The "DSRV Simulator" wasn't the only thing visibly added the vessel during her career. Early on, based on available photos from the Web, other structures appeared:
And, of course, later on was the big extension. This is what was visible from topside:
So, preserving sufficient reserve buoyancy was obviously a critical task. Considering the ship's substantial freeboard in her final configuration, there must not have been too much uncompensated weight added to the ship out of sight.
A bothersome factor when addressing the underhull configuration of Parche, and even those of Halibut, Seawolf, R.B. Russell, and, to a lesser extent, the Jimmy Carter, is the concern for maintaining sufficient reserve buoyancy during extensive hull modifications. For a submarine, reserve buoyancy is the difference in the maximum upward force provided by its empty ballast tanks and the dead weight of the ship's hull and all its contents. The reserve buoyancy must be able to provide sufficient freeboard when the ship is surfaced at its design waterline.
Naval architects build sufficient reserve buoyancy into a ship design so that it can accommodate future modifications and additional equipment—but only up to a point. I recall seeing some hull plans for a 637 identifying those vertical "stabilizers" on the stern planes as "space and weight" for a future passive, wide-aperture-like sonar that never materialized. Reserve weight is designed in by including many hundreds of tons of fixed lead ballast into the design trim and waterline. These ballast blocks are attached to the inside of the hull, usually low in the frame bays of the pressure hull or elsewhere. I've been through a new construction, a decommissioning, and too many overhauls and extensive yard periods. When a boat has had some or all of its fixed ballast removed, it floats much higher than under normal circumstances—and it's "tippy."
I"m not a naval architect and don"t have access to actual numbers, but I"d estimate that under normal circumstances, a modern SSN probably has between 15–20% reserve buoyancy, or possibly even less. If anyone knows the actual numbers, please correct me. For an Archerfish-class boat of about 4300 tons, that means the addition of about 750 to 850 tons of extra weight would sink the ship. In any case, it"s not the actual numbers that matter but the idea of the sensitivity a submarine has to adding extra dead weight to the hull.
When Parche went through her initial conversion, the key visible alteration was the addition of quite a massive object called the DSRV Simulator, well aft of her center of gravity. To compensate for this weight, a lot of fixed ballast would have had to have been removed from the engineering spaces to maintain an acceptable draft and fore-aft trim on the surface. When one considers that there may have also been the mysterious "gondola" attached to the underside of the hull, and the additional dead weight that it entailed, the reballasting of the ship must have been a big headache for the naval architects.
To illustrate my point, here is an undated photo of Parche that appears to have been taken sometime shortly after her initial conversion overhaul. Notice the amount of trim aft compared to the average 637 or 678 boat. Obviously, when submerged, shifting around variable ballast could give her a zero bubble, but the photo illustrates the issues of maintaining sufficient reserve buoyancy and satisfactory surface trim.
The "DSRV Simulator" wasn't the only thing visibly added the vessel during her career. Early on, based on available photos from the Web, other structures appeared:
- The "Bustle"
- The aft fairing
- The bow blister
And, of course, later on was the big extension. This is what was visible from topside:
So, preserving sufficient reserve buoyancy was obviously a critical task. Considering the ship's substantial freeboard in her final configuration, there must not have been too much uncompensated weight added to the ship out of sight.
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