The Effect of Mud on Tanker Manoeuvres

In 1976 the Netherlands Ministry of Transport and Public Works ordered the Netherlands Ship Model Basin to investigate the effect of a layer of mud on the bottom of channels on the maneuvering characteristics of a deep draft tanker. After a chemical was found which represented on model scale the properties of the mud a model test program was carried out for a number of variations of mud layer thickness, mud density, keel clearance and ship speed. Planar motion mechanism tests resulted in the development of mathematical models and the calculation of standard maneuvers for the different conditions with mud and, as a reference, one condition without mud.

Fom the many results of this investigation, some general conclusions may be drawn:

• an appreciable increase of propeller rate of revolutions is necessary to maintain the ship speed when the
  channel is silted up
• ship squat and trim are less above the mud than above a hard bottom
• an internal wave develops at the interface between water and mud, diverging from the afterbody with a specific
  critical speed; the intensity depends on ship speed, mud thickness, keel-clearance and mud density
• inertia forces are nearly the same with mud or without, but damping forces increase appreciably with the
  presence of mud so that standard manoeuvres are affected as follows:

- course changing ability is hardly changed and is in some cases better,
- rate of turn is appreciably less and hence turning circles are larger,
- swept path and overshoot in zig-zag tests are much smaller,
- the manoeuvres depend greatly on mud thickness, keel-clearance and mud density.