When a ship is drydocked or aground, there is a profound effect on stability. As the water level decreases, the keel will rest on the blocks or sea floor. A percentage of the ships displacement is now supported by these objects. Stability is affected as if removing weight from that point of contact. When weight is removed from the keel, there is a virtual rise in the ships center of gravity.
2. As buoyancy moves down, the ships metacenter moves up.
3. As the ships keel rests on the blocks (a weight removal low), gravity moves up.
Since the center of gravity always rises faster than the metacenter, the two stability points will eventually be in the same position. This results in neutral stability, where no righting arms are being produced. The draft where GM = 0 is called the ship's critical draft.
Calculating the ships critical draft is very important. When in drydock, dewatering of the dock stops just before critical draft is reached so it can be verified that the ship is properly supported by the side blocks. When aground, knowing the range of tide will determine if the ship might reach its critical draft.
CALCULATING CRITICAL DRAFT
When aground or in drydock, contact pressure is applied at the keel. This has the same effect as removing weight at the point of contact. Looking at the KG1 equation:Since weight is being removed at the keel, kg = 0 and (w x kg) = 0. The equation changes slightly:
Where, KGV = the new height of "G" after going aground
W0 = ship's original displacement (before grounding)
KG0 = ship's KG before grounding
3. Solve for KGV using the equation.
4. GM = KM - KGV
5. If GM > 0, choose a draft 3 inches below the last draft and complete steps 2 through 4 until GM < 0.
6. Plot GM values (top) for drafts (left) on graph. Curve will cross zero at critical draft.
EXAMPLE: The ship is aground at high tide and the range of tide is 2 feet. Will the ship reach it's critical draft, and if so, at what draft? Initial conditions: KG0 = 18.51 ft Draft = 15'3"
The Stranding Calculation Sheet will look something like this:
Using the graph to plot GVM values, critical draft is 13'5".GROUNDING / STRANDING
In most stranding cases, the following considerations will ordinarily constitute good procedure:
1. Attempts SHOULD NOT be made to refloat the ship under her own power if wind and sea conditions indicate the possibility of the ship working harder aground, pounding, or broaching to sea.
2. Anchors to seaward should be quickly laid if possible to prevent the ship from working further ashore.
3. The ship should be weighted down, not lightened, in an effort to help keep the ship from working harder on the beach, and secondly, to prevent damage caused by working and pounding of the ship on the bottom.
When a ship goes aground, the initial reaction on the bridge is to back down using the engines. Before attempts are made, consideration should be given to:
- Depth of Water
- Sea Floor Composition
- Possible Damage to Propellers and Hull
Surface Ship Survivability, NWP 3-20.31, paragraph 5.5.1 states, "If propellers are reversed and there is no tendency of the ship to back away, no further attempts to move the ship by means of the screws should be made."
The primary reasons we do not continue to use propulsion in a grounding situation:
1. The ship's screws become less effective in shallow water and the ship may squat. Propellers may also be damaged due to contact with the sea floor.
2. Propeller wash will drive silt and/or bottom aggregate in and around the hull, possibly causing a suction when the ship is pulled from its location.
3. This silt and aggregate can be sucked into sea chests, fouling necessary cooling equipment required to maintain the ship's propulsion systems.
WEIGHT DOWN THE SHIP
If attempts at backing down fail, the ship should be weighted down to firmly fix the hull in position. This is especially important if the tide is expected to rise and adverse sea conditions exist; tides or heavy surf may drive the ship further aground or cause it to broach. Weighting down is accomplished by ballasting tanks and if necessary, flooding low compartments.
INVESTIGATE FOR DAMAGE
After the ship has been weighed down, a careful investigation should be made to sound all voids, check fuel tanks for leakage and examine the interior of the hull for signs of structural damage.
DETERMINE TONS AGROUND
Determine displacement prior to grounding using daily draft report and Draft Diagram and Functions of Form. Read drafts after grounding and determine new displacement. The difference is the amount of tons aground.
CRITICAL DRAFT CALCULATIONS
Use the Stranding Calculation Sheet to calculate critical draft. If the tide is receding, determine whether or not stability will become critical. If so, lower the ships center of gravity by adding more weight low, jettisoning weight high, or shifting weight down.
Ground tackle should be rigged and kedge anchors laid seaward as quickly as possible. This will help to keep the ship from broaching. When a ship is broached, scouring occurs. Sand and gravel under the hull is washed away by the action of the surf. Currents produced by the swells breaking against the ship sweep around the bow and stern with great velocity. These currents remove sea floor material from under the ship and build them up in a sand spit amidships on the inboard side. As the material is cut away from under the ship, an extreme hogging condition results that will eventually cause failure of the hull.
The ship's boat should be launched to take soundings around the hull, determining the slope and nature of the bottom. These soundings should be continued in the direction toward which the ship is to be hauled off, in order to locate rock formations, coral ledges, or other under water obstructions. Currents which may effect the ship as she comes off should be noted.
CHECK FOR HOGGING/SAGGING STRESSES
The drafts are also checked to ensure that the ship is neither hogging nor sagging. If the ship is aground at one end, sagging stresses are increased, resulting in the need to remove weight amidships and relocate it at the bow and stern. If aground on a ledge or pinnacle amidships, hogging stresses are increased. Weight should be removed from the bow/stern and relocated amidships. Irregular rock or coral formations or sharp changes in gradient produce concentrated pressures that can crush hull plating and result in flooding. This damage can be intensified if the hull works or shifts position.
Hogging SituationMain deck: In Tension
Keel: In Compression
To relieve these stresses:
- Jettison FWD and AFT- Ballast Amidships
Main deck: In Compression
Keel: In Tension
To relieve these stresses:
- Ballast FWD and AFT
- Jettison Amidships
A request for salvage assistance should be made immediately, not delayed while refloating attempts are made. Early mobilization and dispatch of salvage assistance might mean the difference between success and failure of the salvage operation. When a request for salvage assistance is made, the following information should be provided:
- An accurate position of the grounding site, including latitude and longitude, applicable chart numbers, and means of fixing the position.
- Ships draft at last port and estimated time of stranding.
- Soundings along the ship from bow to stern, corrected to the datum of the chart area.
- Ship's heading after grounding with details of changes.
- Liveliness of the ship.
- Weather conditions, to include: wind direction and velocity, current weather at the grounding site, and any weather forecasts.- Sea and ocean current conditions, to include: direction and height of seas and swells.
- Location of grounding points and estimated ground reaction.
- Type of sea floor at the grounding site.
- Status of ship's machinery.
- Ship's cargo list or manifest.
- Amount and location of known hazardous materials.