Question

What general effects might continued running at service speed in heavy weather have on marine machinery? What abnormal stresses may be imposed on to the following:-

(a) Intermediate shafting,

(b) Propeller Shaft,

(d) Engine supports &

(e) Thrust Pads.

Answer:

A ship which runs for a considerable period at service speed, in heavy weather would be experiencing severe rolling, pitching and vibration. In addition there will be hogging and sagging of vessel in heavy weather.

In main propulsion engine, there will be racing due to propeller coming out of the water, which may cause turbo charger surging and its damage. Whirling of propeller shaft, offset axial thrust as well as bending stresses may cause excessive loading on crankshaft and its bearings. Frictional wear in liners and piston rings will be aggravated. The short length stiff shafting in comparatively flexible ship structure causes damage to bearing material and cracks in bearing housing due to hull distortion by hog or sag with the severe sea condition.

Failure of lube oil pump suction or cooling water pump suction due to rolling can cause operational problem to all other running machineries.

(A) Stresses on Intermediate shafting:-

Intermediate shafting will be subjected to severe torsional shear stress due to propeller racing in heavy weather and can be quite alarming at "service speed due to the foil torque development. This can cause surface cracks on the shaft, damage to shaft bearing material due to heavy pounding and cracks on bearing housing due to excessive load and vibration.

End thrust from the propeller which induces a compressive stress will be small compared to other stresses. Bending stresses could arise due hogging, sagging and yawing of vessel which may damage the shaft bearings.

(B) Stresses on Propeller shafting:-

Propeller shaft is subjected to torque, end thrust and bending.. Torsional stress is due to the transmission of shaft torque into propeller, compressive stress is due to the end thrust which is the reaction of water thrown aft and bending stress is due-to the propeller weight at the end of a cantilever beam.

The bending moment due to the full weight of the propeller when it comes out of water in heavy seas is quite appreciable and combining with it the fluctuating torsional stress and end thrust can considerably reduce the fatigue strength in a corrosive weather. Damage to stern tube bearing, oil seal leakage can be envisaged if run at service speed in heavy weather for a prolonged period.

Coupling bolts are tightened to a specific stress to bring the flange faces together such that friction between flange faces provides some proportion of the drive and they are perfect interference fit in the flange holes for maximum transfer of load and designed to carry the shaft torque in shear.

Whirling of the shaft, racing of the engine combined with bending due to hogging and sagging in heavy seas cause excessive shear stress on the coupling bolts and bending stresses. These abnormal stresses, specially if run at service" speed with full torque, may cause surface crack, fatigue and failure due to shear. Failure of one bolt aggravates the stresses on the rest and further damage results

(D) Stresses on Engine supports:-

Engine supports are subjected to transverse vibration due to cylinder pressure and inertia forces giving a resdnn^ou^about the engine centerline which may be aggravated due to propeller torque variations in heavy sea conditions. Such vibrations can cause failure of engine stays and cracking of welding joints at bedplates.

Axial and torsional vibration generated by whirling of propeller shaft and offset thrust can also cause substantial damage to engine supports.

(E) Thrust Pads:-

Relative movement of the hull, misalignment due to droop from propeller weight, slackening of propeller blade load and whirling of tail shaft can create axial vibration problem which cause thrust block to rock, panting of tank top and structural damage.