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WHAT MAKES BOATS FLOAT ?!

WHAT MAKES BOATS FLOAT



BOATS FLOAT

You're at a boat ramp at the start of a beautiful day, and you're backing your trailer into the harbor. The boat—still resting in the trailer's cradle eases deeper and deeper into the water until the stern finally lifts and the boat begins to float. 

 How does this happen? What principles are at work that allows for a vessel constructed of non-buoyant materials such as aluminum, steel, or fiberglass to bob on the water's surface? 

 The Greek mathematician Archimedes figured it out while taking a bath. As legend has it, Archimedes noticed that when he got in the tub, the water level rose, and he determined that an object in water displaces a volume of water exactly equal to the volume of the object immersed in the water. 

 He further discovered that an object would sink if its weight was greater than the weight of the water it displaced. If the object floated, the weight of the displaced water was exactly the same as the weight of the object. The ratio of an object's weight to its volume is known as density. A chunk of steel, being more dense than water, sinks. A block of wood, being less dense than water, floats.

 Materials that are denser than water can still float, but you have to first change their relative densities. For instance, if you roll a 50,000-pound steel ball down a boat ramp, it's going to keep rolling until it hits the sea floor. 

 But, if you can melt, roll, form, and weld 50,000 pounds of steel into a shape that will displace 50,000 or more pounds of water, you've got yourself a floating object. By shaping the steel, you changed the relative density of the object its weight per volume. And depending on the shape you've created, you might just have a boat.

 As a boat is placed in water, it displaces water. If you were to collect and weigh the displaced water, you'd find that it is equal to the weight of the boat. 

STATIC TRIM


 The boat's posture at rest is known as its static trim. The line of contact between the water's surface and a boat's hull is called the waterline. If the boat was well-designed and well-built (and carries a normal load), the boat's waterline at rest will match the designed waterline (DVVL): the waterline the designer calculated while drawing the plans. Often, a boat's designed waterline is even painted on the hull.

 If an object is added to the boat, more water is displaced. The weight of the newly displaced water will equal the weight of the new object.

 A boat at rest should sit relatively level both fore to aft and port to starboard. If your boat does not, you may need to redistribute weight.

 If the boat's static trim is different from its designed waterline, you should try redistributing weight. In a smaller boat, this can be accomplished simply by moving passengers, spare fuel cans, anchors, or gear. On a larger boat, these actions may have little effect, and it may take some fundamental changes to the boat to restore balance. 

MOTION OF A BOAT


 Even while your boat is at rest, wind and waves can cause you to move in six different ways. 

 1. Roll describes the side-to-side rocking motion of a boat turning on its long axis—an imaginary line running from bow to stern through the boat. 

 2. Pitch describes the front-to-back rocking motion of a boat turning on its shorter horizontal axis the port-to-starboard axis.

 3. Yaw describes the pivoting motion of the boat turning around its vertical axis (i.e., when the bow is pushed to port and the stern goes to starboard, and vice-versa). Most boaters can immediately recognize a boat's roll and pitch, but it's also important to be aware of yaw, because it can push the boat off course. 

 4. Heave describes the up and down motion of a sea lifting and dropping the entire boat. 

 5. Slip describes a boat's sideways movement when pushed by wind or sea, or even its own momentum in a turn. 
6. Surge describes the motion of a boat being propelled suddenly forward by a passing wave, or the sudden deceleration of the boat slipping backward into the trough behind it.

STAYING UPRIGHT 


 Stability describes a boat's tendency to stay in, or to return to, its upright position. Transverse stability specifically refers to a boat's susceptibility to rolling motion. (Because a boat is longer than it is wide, transverse stability is usually of more concern than pitch.)
A boat is subject to six types of motion in the water. Three are pivoting motions: A boat rolls when it rocks side to side. Pitch is an up-and-down motion of the bow and stern; yaw is a port-to- starboard twisting motion. Three are linear motions: surge is a front- or backward motion caused by passing waves; slip occurs when wind or wave pushes a boat to the side; heave is the up-and-down motion caused by swells. All boats roll; some roll easier than others, and some can return from more severe rolls than others. As such, transverse stability can be broken into two components. Initial stability describes a boat's tendency to right itself from small angles of roll, while ultimate stability describes the maximum angle of heel a boat can endure before it capsizes. Both of these measures of transverse stability are governed by the locations of a boat's center of gravity and center of buoyancy.
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