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SUBMARINES

Overview (10.1). SUBMARINES. 200+ Years Old (Turtle (1775) and Hunley (1864)) Navy mostly uses submarines (indefinite underwater endurance) Commercial industry uses submersibles (limited endurance) Expensive but stealthy!

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SUBMARINES

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  1. Overview (10.1) SUBMARINES • 200+ Years Old (Turtle (1775) and Hunley (1864)) • Navy mostly uses submarines (indefinite underwater endurance) • Commercial industry uses submersibles (limited endurance) • Expensive but stealthy! • Share characteristics of both surface ships and aircraft CSS Hunley

  2. Submarine Structural Design (10.2) SUBMARINES • Longitudinal Bending - Hogging & sagging causes large compressive and tensile stresses away from neutral axis. A cylinder is a poor bending element. • Hydrostatic Pressure = Major load for subs. Water pressure attempts to implode ship. Transverse frames required to combat loading. A cylinder is a good pressure vessel! • Recall: hydrostatic pressure =

  3. Submarine Inner Hull (10.2) SUBMARINES • Holds the pressure sensitive equipment (including the crew!) • Must withstand hydrostatic pressure at ops depth. • Transversely framed with thick plating. • Strength  = $ ,  , space  , but depth  . • Advanced materials needed due to high .

  4. Submarine Outer Hull (10.2) SUBMARINES • Smooth fairing over non-pressure sensitive equipment such as ballast and trim tanks and anchors to improve vessel hydrodynamics. • High strength not required so made of mild steels and fiberglass. • Anechoic (“free from echoes and reverberation”) material on outer hull to decrease sonar signature.

  5. Submarine General Arrangements (10.2) SUBMARINES • Main Ballast Tanks • Variable Ballast Tanks PRESSURE HULL

  6. Main Ballast Tanks (MBT) (10.2) SUBMARINES • Largest tanks. • Alter  from positive buoyancy on surface (empty) to near neutral buoyancy when submerged (full). • Main Ballast Tanks are “soft tanks” because they do not need to withstand submerged hydrostatic pressure. (Located between inner & outer hulls.)

  7. Variable Ballast Tanks (10.2) SUBMARINES • Depth Control Tank (DCT) • Alter buoyancy once submerged with little or no trim. Where is it located? • Compensates for environmental factors (water density changes). Rho*g*volume! • ‘Hard tank’ because it can be pressurized (has access to outside of pressure hull). • Trim Tanks (FTT/ATT) • ‘Soft tanks’ shift water to control trim (internal)

  8. U.S. Submarine Types (10.2) SUBMARINES • Ohio Class • Sub Launched Ballistic Missiles (SLBMs) aft of sail •  greater than many surface ships (i.e. BIG)

  9. U.S. Submarine Types (10.2) SUBMARINES • Los Angeles Class (SSN688)

  10. U.S. Submarine Types (10.2) SUBMARINES

  11. U.S. Submarine Types (10.2) SUBMARINES

  12. U.S. Submarine Types (10.2) SUBMARINES Virginia Class Displacement: 7,800 tons Length: 377 feet Draft: 32 feet Beam: 34 feet Depth: 800+ feet

  13. Submarine Hydrostatics (10.3) SUBMARINES USS Bremerton (SSN 698)

  14. Submarine Hydrostatics (10.3) SUBMARINES • Static equilibrium and Archimedes Principle apply to subs as well. • Unlike surface ships, subs must actively pursue equilibrium when submerged due to changes in density () and volume (). • Depth Control Tanks & trim tanks are used.

  15. Hydrostatic Challenges (10.3) SUBMARINES • MAINTAIN NEUTRAL BUOYANCY • Salinity Effects • Water Temperature Effects • Depth Effects • MAINTAIN NEUTRAL TRIM AND LIST • Transverse Weight Shifts • Longitudinal Weight Shifts

  16. Hydrostatics (Salinity Effects) (10.3) SUBMARINES Water density ()  as salinity level . • Decreased  = less FB •  sub weight > FB. • Must pump water out of DCT • Changes in salinity common near river estuaries or polar ice. • Mediterranean salinity is higher from evaporation.

  17. Hydrostatics (Temperature Effects) (10.3) SUBMARINES Water density ()  as temperature . • Decreased  = less FB •  sub weight > FB. • Must pump water out of DCT to compensate. • Changes in temperature near river estuaries or ocean currents (Gulf Stream, Kuroshio, etc.)

  18. Hydrostatics (Depth Effects) (10.3) SUBMARINES • As depth increases, sub is “squeezed” and volume () decreases. The string demonstration! • Decreased  = less FB •  sub weight > FB. • Must pump water out of DCT • Anechoic tiles cause additional volume loss as they compress more.

  19. Neutral Trim - General (10.3) SUBMARINES • When surfaced, geometric relationships similar except that “G” must be below “B” for sub stability. • Neutral trim on sub becomes extremely critical when submerged. Small changes to buoyancy can be mitigated with diving planes • Note the positions of “G”, “B”, “MT”, and “ML” in the following figures!

  20. Neutral Trim - General (10.3) SUBMARINES • Recall: these relationships can be used in transverse or longitudinal directions to find KMT or KML for a surface ship.

  21. Neutral Trim - General (10.3) SUBMARINES • Surfaced submarine similar to surface ship except G is below B. • For clarity, MT is shown above B although distance is very small in reality.

  22. Neutral Trim - General (10.3) SUBMARINES • When submerging, waterplane disappears, so no second moment of area (I), and therefore no metacentric radius (BML or BMT)! Equation? • “B”, “MT” and “ML” are coincident and located at the centroid of the underwater volume -the half diameter point (if a cylinder). • Very sensitive to trim since longitudinal and transverse initial stability are the same.

  23. Neutral Trim - General (10.3) SUBMARINES • When completely submerged, the positions of B, MT and ML are in the same place.

  24. Trim & Transverse Weight Shifts (10.3) SUBMARINES • Recall In Surface Ship Analysis: • GMT is found by equation (& Incline Experiment) to calculate the vertical center of gravity, KG. • Equation was only good for small angles () since the metacenter is not stationary at larger angles. • Large  only available from analysis of Curve of Statical Intact Stability.

  25. Recall for a Surface Vessel: Submarines • From the geometry, we got: M W f t G B

  26. Trim & Transverse Weight Shifts (10.3) SUBMARINES • In Submarine Analysis: • The calculation of heeling angle is simplified by the identical location of Center of Buoyancy (B) and Metacenter (M) (BM=0). • Since GM=KB+BM-KG, then GM=KB-KG=BG • This equation is good for all angles:

  27. Trim & Transverse Weight Shifts (10.3) SUBMARINES • Surface Ship analysis complicated because vessel trims about the center of floatation (F) (which is seldom at amidships). • Sub longitudinal analysis is exactly the same as transverse case since BM=0 for both longitudinal and transverse. For all angles of trim: • Moment arm l  t, so trim tanks to compensate.

  28. Submarine Stability (10.4) SUBMARINES USS Seawolf SSN-21

  29. Submarine Submerged Intact Stability (10.4) SUBMARINES

  30. Submarine Intact Stability (10.4) SUBMARINES • Initial stability simplified for subs. • The distance BG is constant (=GM) Righting Arm (GZ) is purely a function of heel angle. • EQUATION IS TRUE FOR ALL SUBMERGED SUBS IN ALL CONDITIONS!

  31. Submarine Intact Stability (10.4) SUBMARINES • Since righting arm equation good for all , curve of intact statical stability always a sine curve with a peak value equal to BG.

  32. Submerged Stability Characteristics (10.4) SUBMARINES • Range of Stability: 0-180° • Angle of Max Righting Arm: 90° • Max Righting Arm: Distance BG • Dynamic Stability: 2SBG • STABILITY CURVE HAS THE SAME CHARACTERISTICS FOR ALL SUBS!

  33. Submarine Resistance (10.5) SUBMARINES • Recall Coefficient of Total Hull Resistance • CV = viscous component, depends on Rn. • CW = wave making resistance, depends on Fn. • CA = correlation allowance, surface roughness and “fudge factor”.

  34. Submarine Resistance (10.5) SUBMARINES • On the surface (acts like a surface ship but with bigger wakes): • CV dominates at low speed, CW as speed increases (due to bigger bow and stern waves and wake turbulence). • Submerged (acts like an aircraft): • Skin friction (CF CV) dominates. (Rn is the important factor when no fluid (air/water) interface). • CW tends toward zero at depth. • Since CT is smaller when submerged, higher speeds are possible.

  35. Components of Total Hull Resistance • Total Resistance and Relative Magnitude of Components Air Resistance Hollow Wave-making Hump Resistance (lb) Viscous Speed (kts) • Low speed : Viscous R dominates • Higher speed : Wave-making R dominates • Hump (Hollow) : location is function of ship length and speed.

  36. Submarine Propellers - Odd # of Blades (10.5) SUBMARINES Stern planes could be rotated 45o and called “X” or dihedrals

  37. Skewed Propellers (10.5) SUBMARINES • Advantages: • Reduced Vibration (eases into flow). • Reduced Cavitation as tip vortex is smaller. • Disadvantages: • Inefficient backing. • Expensive & difficult to make. • Reduced strength. • Operational need outweighs disadvantages!

  38. Submarine Seakeeping (10.6) SUBMARINES • Subjected to same as surface ships • 3 translation (surge, sway, heave) and 3 rotational (roll, pitch, yaw). • Recall heave, pitch, and roll are simple harmonic motions because of linear restoring force. • If e = resonant freq, amplitudes maximized (particularly roll which is sharply tuned). • Roll motion accentuated by round shape. Why?

  39. Submarine Seakeeping - Suction Force (10.6) SUBMARINES • Water Surface Effect • Submarine near surface (e.g. periscope depth) has low pressure on top surface of hull causing net upward force. This is similar to squatting, but opposite! • Magnitude depends on speed, depth, and hull shape. • Minimize by reducing speed and having bow down trim. • Wave Action • Top of sub has faster velocity due to similar lower pressure effect as above. • Minimize by going deeper or beam on to waves.

  40. Submarine Maneuvering and Control (10.7) SUBMARINES • Lateral motion is controlled with rudder, engines, and props. Note that in a fast turn the sail may create lift, heeling the boat outward in to a “snap roll”, particularly if the sail is forward of Cp. • Depth control accomplished by: • Making the buoyant force equal the submarine displacement. • Finer and more positive control achieved by plane (control) surfaces.

  41. Fair-Water Planes (10.7) SUBMARINES • Primarily to maintain an ordered depth. • Positioning the planes to the "up" position causes an upward lift force to be generated. • Since forward of the center of gravity, a moment (M) is also produced which causes some slight pitch. • The dominant effect is the lift generated by the control surface.

  42. Fair-Water Planes (10.7) SUBMARINES • Primarily DEPTH CONTROL

  43. Stern and Bow Planes (10.7) SUBMARINES • Primarily to maintain pitch because of the distance from the center of gravity. • Positioning the planes to creates a lift force in the downward direction creates a moment (M) which causes the submarine to pitch up. • Once the submarine has an up angle, the hull produces an upward lift force. • The net effect is that the submarine rises at an upward angle.

  44. Stern and Bow Planes (10.7) SUBMARINES • Maintain Pitch • (better control than with fairwater planes)

  45. FINAL THOUGHT... SUBMARINES There are times when accurate control is nice!

  46. Principles of Ship Performance Good Luck and Good “Boating”!

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