SUBMARINES Overview (10.1) 200+ Years Old (Turtle (1775) and Hunley (1864)) Navy mostly uses...

SUBMARINESOverview (10.1)

• 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

SUBMARINESSubmarine Structural Design (10.2)

• 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 = gh

SUBMARINESSubmarine Inner Hull (10.2)

• 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 .

SUBMARINESSubmarine Outer Hull (10.2)

• 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.

SUBMARINESSubmarine General Arrangements (10.2)

• Main Ballast Tanks

• Variable Ballast Tanks

PRESSURE HULL

SUBMARINESMain Ballast Tanks (MBT) (10.2)

• 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.)

SUBMARINESVariable Ballast Tanks (10.2)

• 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)

SUBMARINESU.S. Submarine Types (10.2)

• Ohio Class • Sub Launched Ballistic Missiles (SLBMs) aft of sail

• greater than many surface ships (i.e. BIG)


• Los Angeles Class (SSN688)

Fairwater planes


Virginia Class

Displacement: 7,800 tons

Length: 377 feet

Draft: 32 feet

Beam: 34 feet

Depth: 800+ feet

SUBMARINESSubmarine Hydrostatics (10.3)

USS Bremerton (SSN 698)

SUBMARINESSubmarine Hydrostatics (10.3)

• 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.

0 0F M

g

SUBMARINESHydrostatic Challenges (10.3)

• MAINTAIN NEUTRAL BUOYANCY

– Salinity Effects– Water Temperature Effects– Depth Effects

• MAINTAIN NEUTRAL TRIM AND LIST

– Transverse Weight Shifts– Longitudinal Weight Shifts

SUBMARINESHydrostatics (Salinity Effects) (10.3)

• 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.

Water density () as salinity level .

SUBMARINESHydrostatics (Temperature Effects) (10.3)

• 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.)

Water density () as temperature .

SUBMARINESHydrostatics (Depth Effects) (10.3)

• 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.

SUBMARINESNeutral Trim - General (10.3)

• 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!


• Recall: these relationships can be used in transverse or longitudinal directions to find KMT or KML for a surface ship.


• 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.


• 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.


• When completely submerged, the positions of B, MT and ML are in the same place.

SUBMARINESTrim & Transverse Weight Shifts (10.3)

• 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.

SubmarinesRecall for a Surface Vessel:

• From the geometry, we got:

Zero pt.

tan

tanO F OG G G M

GM w t

W

t

G

M

B


• 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:

tanBG w t


• 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.

tanBG w l

SUBMARINESSubmarine Stability (10.4)

USS SeawolfSSN-21

SUBMARINESSubmarine Submerged Intact Stability (10.4)

SUBMARINESSubmarine Intact Stability (10.4)• 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!

sinRA GZ BG

SUBMARINESSubmarine Intact Stability (10.4)

• Since righting arm equation good for all , curve of intact statical stability always a sine curve with a peak value equal to BG.

SUBMARINESSubmerged Stability Characteristics (10.4)

• Range of Stability: 0-180°

• Angle of Max Righting Arm: 90°

• Max Righting Arm: Distance BG

• Dynamic Stability: 2SBG

• STABILITY CURVE HAS THE SAME CHARACTERISTICS FOR ALL SUBS!

SUBMARINESSubmarine Resistance (10.5)

• 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”.

T V W AC C C C

(1 )V FC K C

SUBMARINESSubmarine Resistance (10.5)

• 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 theimportant factor when no fluid (air/water) interface).– CW tends toward zero at depth.– Since CT is smaller when submerged, higher speeds are possible.

Components of Total Hull Resistance

• Total Resistance and Relative Magnitude of Components

Viscous

Air Resistance

Wave-making

Speed (kts)

Re

sis

tan

ce (

lb)

- Low speed : Viscous R dominates - Higher speed : Wave-making R dominates- Hump (Hollow) : location is function of ship length and speed.

Hump

Hollow

SUBMARINESSubmarine Propellers - Odd # of Blades (10.5)

Stern planes could be rotated 45o and called “X” or dihedrals

SUBMARINESSkewed Propellers (10.5)

• 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!

SUBMARINESSubmarine Seakeeping (10.6)

• 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?

SUBMARINESSubmarine Seakeeping - Suction Force (10.6)

• 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.

SUBMARINESSubmarine Maneuvering and Control (10.7)

• 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.

SUBMARINESFair-Water Planes (10.7)

• 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.

SUBMARINESFair-Water Planes (10.7)

• Primarily DEPTH CONTROL

SUBMARINESStern and Bow Planes (10.7)

• 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.

SUBMARINESStern and Bow Planes (10.7)

• Maintain Pitch•(better control than with fairwater planes)

SUBMARINESFINAL THOUGHT...

There are times when accurate control is nice!

Principles of Ship Performance

Good Luck and Good “Boating”!

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