Vessel Stability How does Archimedes apply to Buoyancy? •When water is displaced, there is a force that pushes up on the object displacing the water. •As has been shown with displacement, an object will float when it displaces its weight in water. •The force of buoyancy pushing up on the object must be equal to the force of the mass pushing down. •The deeper they push the empty container, the more water is displaced and the greater the force of buoyancy, The key to floating is that the object must displace an amount of water which is less than its own weight. Buoyancy Materials Needed: 1. 1 Bucket 2. Smaller bucket to fit inside 5 gallon bucket 3. Water Empty Container Water Force Pushing Down (person) Water displaced Water displaced Buoyancy Force Hands-on Experiment : • Gently push the smaller bucket into the 5 gallon bucket of water. • The open container is displacing the water • Feel the force pushing back • Notice the force gets greater the deeper the smaller bucket goes, i.e. the more water that is displaced (Try a variety of diameter buckets that would displace different volumes of water and, therefore, have different buoyancy push-backs) 3 Simplified… Density = mass x volume Simplified, for an object to float the density of the object must be less than the density of the water it is in. Golden rule… A solid object will sink in a fluid if its density is greater than the fluid’s density, and will float if it’s density is smaller. What about boats? Boats are made of materials such as steel and aluminium that are much more dense than water so why do they float? Boats and Buoyancy Even though the materials of the boat are very dense, they displace a lot of water because the insides are full of air. The weight of the water they displace is far more than the weight of the boat itself, so the boat will float. Vessel Stability All vessels need to be designed to roll, pitch and yaw when pushed by the forces of wind and sea. Stability is the vessels ability to return to an upright position after being acted upon by such forces. Roll, pitch, yaw… Pitch is movement up and down between bow and stern Yaw is movement side to side on a ship. Roll is the ship rolling/rocking side to side. Roll pitch yaw https://www.youtube.com/watch?v=rlVw-SNU8cM Forces affecting stability The two main forces that affect stability of a vessel are: Buoyancy Gravity Metacentre and Centre of Gravity Metacentre (M) is the point which the vessel rolls around Centre of Gravity (G) is the point where gravity is pulling the vessel down. Its basically the centre point where the weight of the ship seems to be concentrated. Moving the centre of Gravity down makes the vessel more stable. For this reason heavy loads should be kept at the bottom of vessels. Relevance You want the centre of gravity to be well below the metacentre of a ship to make it stable. Overloading causes the centre of gravity to move up to the metacentre. Concentrating the load too high in the ship also raises the centre of gravity up to the metacentre. Correctly Loaded Ship The balance of a ship is called it’s TRIM. The ship on the left it in trim The ship on the right is not in trim A Canoe in water is like a see saw In Trim Out of Trim 100 Lbs 100 Lbs C G Out of Trim Back in Trim 100 Lbs 100 Lbs 100 Lbs C G 14 Canoe see saw… Upper, left image: Relate how the balance in this canoe is its TRIM Is the canoe in trim? (yes…it is balanced) The buoyancy force upward (blue arrow) is like the fulcrum of the see saw. The canoe itself is like the see saw board. Upper, right image: Relate how moving the weight to the end of the canoe puts the canoe “out of trim.” Center of Gravity has shifted. A Canoe in water is like a see saw In Trim Out of Trim 50 lbs C G C G Back in Trim Out of Trim 50 lbs C G 50 lbs C G 16 Canoe see saw… Upper, left image: • Is this canoe in trim? (Yes) • Why? (the weight is balanced) • How can the canoe be placed out of trim? (See next slide) Upper, right image: • Out of trim • Center of Gravity has shifted Free surface effect Water, cargo, fuel etc. moves from side to side with the rolling vessel. This can cause the vessel to topple over. Solution Fill one holding tank before the next one rather than half filling all tanks.
