Manoeuvres & External Effects/Forces Flashcards
What is squat?
- Squat is the extra sinkage a vessel will have when moving through water.
How is squat generated?
- As the vessel moves through the water, the flow of water beneath the vessel is restricted by a smaller under-keel clearance, therefore the water flow increases velocity, causing it to decrease in pressure, leading the vessel to sink by a calculable amount.
What should be done to prevent squat when UKC is reduced?
- The vessels speed should also be reduced.
What are the symptoms which suggest squat is apparent?
Unusual noise or vibration.
Steering is slow.
The vessels speed slows.
A large stern or bow wave is evident.
Unusual trim.
What are the causes of shallow water effect?
Build-up of water ahead of the ship, with the longitudinal resistance, pushing the pivot point back reducing the steering lever.
Water being forced under the bow, at a higher speed, creating low pressure and loss of buoyancy.
(The vessel will now squat by the bow and can be as much as 2 metres).
How is turning a short round carried out?
Starting with the vessel on a set heading slowly reducing speed.
Put the wheel hard to starboard and the engine dead slow ahead.
When the vessel has swung 45 degrees, stop engines and put wheel amidships.
Put engines half astern.
Stop engines when approximately 180 degrees from the original heading.
Put helm to port and engines dead slow ahead.
Head on your new course using helm and engines as required.
What happens when vessels passing in close proximity?
- Due to the flow and pressure change between two vessels, many different scenarios may take place depending on the interaction of the two vessels. The different pressures at different parts of the vessel may cause them to repel from each other or suck together.
What is the bank effect?
- This interaction is very similar to the interaction of another ship, however this time it depends on which side of the vessel has the shallower water.
- The vessel becomes attracted to the shallower water.
How can the bank effect be beneficial for manoeuvrability?
- However, the bank effect can also be used to an advantage if used properly to assist vessels to turn around corners or bends.
What are the 6 degrees of freedom?
- Heave.
- Yaw.
- Sway.
- Surge.
- roll.
- Pitch.
As the OOW, what should be done before heavy weather?
Inform the master (If not done so already).
Notify the engine room and catering department.
Inform deck crew and clear decks/ secure deck items.
What should be considered when manoeuvring in heavy weather?
Before turning the speed should be reduced, and bold helm movements made.
When turning downwind and away from the head seas, you should wait for a wave crest to pass and turn immediately after when the vessel is in the trough.
When tuning up wind into heavy weather, the manoeuvre must be done as quickly as possible to prepare the vessel for oncoming seas.
What is synchronous and parametric rolling?
- Large rolling motions can be encountered in heavy weather due to wave periods.
- Parametric roll is a large rolling angle in the head or stern seas.
- This happens as a wave passes along the vessel’s hull, stability will change/ vary due to a change in buoyancy.
- A vessel with a large flare (outward curve of the hull) is more likely to experience parametric roll.
What is pooping?
Pooping occurs when the vessel falls into the trough of a wave and fails to rise, causing a wave to break on the stern poop deck area.
What is racing?
Racing is when the vessel gets pushed down the wave at speed (surfing the wave).
What is broaching?
Broaching is when the crest of the wave lifts the stern causing the vessel to steer off course this can cause a large heel and temporary loss of directional control.
What are sheering stresses?
Shearing stress is caused when two different cargo holds are loaded to different quantities causing each to try to float at its own draught, but this is not possible as it’s all one unit causing the issue of shearing stress.
What is pounding?
Pounding is when the forward part of the vessel is slamming down the wave due to bad weather or excessive pitching of the vessel.
What is panting?
Panting is the in and out movement of the ship’s hull or side plating due to the water pressure on either side, mainly because of swell.
What causes bending?
This happens when the vessel is unevenly loaded, causing the hull of the vessel to either.
What is sagging?
This is when the centre of the ship is much more heavily loaded than the forward and aft parts.
What is hogging?
This is when the forward and aft part of the vessel is much more heavily loaded than the centre part of the vessel.
