Boat Design Terminology & Physics Principles

Note: Forward and aft are directional terms. Directions on a Ship Forward/ Bow Port A game may be played similar to “Simon Says” that will make it fun to learn these ship directions Make 4 large cards labeled Forward, Aft, Starboard and Port (large enough to be seen across the room) Place one of the cards on a wall of the classroom For example: A part of the room is designated with a sign as the forward part of a ship. Students stand up. The other directions are called out one at a time and the students must instantly turn to face that direction. If a student does not turn to face that direction right away, or if they change their mind, they must sit down. Last one standing is the winner (or, don’t wait for one to be last, go to next bullet) Change the part of the room to be one of the other directions for variety in the game and to practice all directions. Aft/ Stern Starboard Note: Forward and aft are directional terms.

Beam: The width of the boat from the port side to the starboard side at the widest point. Centerline: the measure of the overall length from one end (stern or bow) of the hull to the other end along the center line of the hull.

Setting up your design in Autodesk Inventor Set origin on centerline, intersecting baseline, at the forward perpendicular.

Types of Hulls Flat bottom boat - These boats are generally less expensive to build and have a shallow draft (the part of the boat that's under the water). They can get up on plane easily but unless the water is very calm they tend to give a rough ride because of the flat bottom pounding on each wave. They also tend to be less stable and require careful balancing of cargo and crew. Examples of flat bottom boats might be Jon boats, small utility boats, and some high speed runabouts. Round bottom boat - These move easily through the water, especially at slow speeds. They do, however, tend to roll unless they are outfitted with a deep keel or stabilizers. Many trawlers, canoes and sailboats have round bottoms. Vee bottom boat - The vee bottom tends to have a sharper entry into the water which provides for a smoother ride in rough water. They do, however, require more power to achieve the same speed. Many runabouts use the vee-bottom design. Multi-hull boat - Catamarans, trimarans, pontoon boats and some house boats use a  multi-hull design. The wide stance provides greater stability.

Buoyancy What is Buoyancy? Buoyancy is the upward force that a fluid exerts on an object less dense then itself. http://dsc.discovery.com/tv-shows/mythbusters/videos/lets-talk-buoyancy.htm .

Archimedes's principle. An object immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the object. Put picture of floating boy here G:\PHS 2013-14\Adv Draw\SNAME\Videos\Archimedes' Principle - YouTube.htm

Center of Gravity & Trim Center of Gravity - the point on an object where it is balanced. Trim – difference between the forward and aft draft. Draft – vertical distance from the waterline to the baseline.

A See-Saw in Trim + CG = Center of Gravity 380 lbs 20 lbs 400 lbs CG Board = 20 Lbs CG 400 Lbs Support (fulcrum) can hold 400 Lbs Relate the upward force from the container pushed into water with a see saw analogy Explain Center of Gravity What does the fulcrum force upward represent? (buoyancy) The fulcrum can support 400 lbs of force Are the upward and downward forces equal? (yes…each = 400 lbs including the see saw board) What happens if the weight is increased more? (Weight and board will fall down…relate this to what would happen to a boat) What would happen if the weight was moved to either end of the see saw board? (the board will tilt of balance) This balance on a boat is called its TRIM CG = Center of Gravity

A Canoe in water is like a see saw In Trim 100 Lbs Out of Trim CG 100 Lbs CG 100 Lbs Out of Trim CG 100 Lbs CG Back in Trim 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 Lower, left image: Also “out of trim” if the weight is moved to the other side of the canoe Center of Gravity has shifted the other way Lower, right image: Why is the canoe “back in trim”? (because the weights on it are balanced)

A Canoe in water is like a see saw In Trim Out of Trim 50 lbs CG 50 lbs CG Out of Trim 50 lbs CG Back in Trim 50 lbs CG 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 Lower, left image: Also “out of trim” if the weight is moved to the other side of the canoe Center of Gravity has shifted the other way Lower, right image: Back in trim (weights are balanced)

Center of Gravity What is the Center of Gravity? The center of gravity is the geometric property that represents the average location of the weight of an object. We can balance the scale by moving the fulcrum to the center of gravity. Refer back to the balancing can Read definition of center of gravity to students. Demonstrate how to use a string and a weight to find the center of gravity of an rectangle. Demonstrate how to do the same calculation on an irregular shape

Center of Buoyancy Is the center of gravity of the displaced water The center of buoyancy changes as the ship rolls and pitches The balance between center of buoyancy and center of gravity affects stability of the ship

Metacenter A fixed point in space above a boat about which it rotates. Ship Water A fixed point in space above a boat about which it rotates.

Center of Gravity Center of Buoyancy

Ship rolled at a small angle

Line of force for new center of buoyancy perpendicular to water line New center of buoyancy

The intersection of the two lines of force form the metacenter

The distance from the metacenter (M) to the center of gravity (G) is called the metacentric height (GM). M G

Metacenter A positive GM is needed to make a stable ship The higher the GM the more stable the ship The higher the GM the faster the ship will right itself. If this number becomes too high the ride will become rough. GM = metacentric height