Tech Directions - August 2007 - (Page 19) used in making the hull. Each team must list all the materials it uses in a Materials section of a final report. Teams must also decide on hull design, with flat bottoms, tunnel hulls, and V-bottoms being among the options. Three studentbuilt boats, each with a different hull design Testing Testing concerns include the total mass of the boat, the density of the hull, and total amount of water displaced by the hull. The following is an example of data obtained regarding a watercraft: Total mass of boat—The team got this information by placing the boat on a balance and reading the data: 2,371 grams. Density of hull— To obtain this information, we cut a small piece of coroplast and submerged it in water in a graduated cylinder. We then took the mass of the sample and used the formula Density (D) = Mass (M) ÷ Volume (V), or D = M/V For our boat, D = 0.27/0.5 = 0.54 g/ml Left side: 973.5 cm2 Right side: 1,000.62 cm2 Thus, to determine the total area of the hull: 973.5 cm2 + 1,000.62 cm2 + 917.85 cm2 + 1,784.76 cm2 + 486.75 cm2 = 5,163.48 cm2 Then: = 5,163.48 cm 4 cm 0.27 g x 4x = 1,394.1396 x = 348.5349 2 2 equals 2,600 g of water. According to Archimedes’ principle, the boat must displace an amount of water equal to the boat’s total weight. (In my team’s testing, the boat actually displaced slightly more water than its weight. But throughout our experiments, we were working with nonstandard conditions. We did come very close to the expected results, but we did not have a scien- Mass of hull—This step of the testing process is time consuming because you cannot simply determine the mass of the hull by placing it on a balance, since the hull has other materials epoxied to it. To find the mass, you must first find the area of the hull, then set the area of the hull equal to a sample piece of coroplast with a known mass. For example, we used a sample that was 4cm2 with a mass of 0.27 grams. You can then calculate the area with the following formulas: Rectangle: l × w Trapezoid: h × (b1 + b2)/2 Triangle: bh/2 We calculated Front: 917.85 cm2 Bottom: 1,784.76 cm2 Back: 486.75 cm2 tific laboratory in which we could keep the amount of constants true.) Additional Options If bodies of water are not readily available for testing, students could attach wheels to the bottom of the craft to produce land vehicles. Alternatively, in cold climates, students could attach skis to the craft for testing on the snow in winter. Several schools might take on this project and conduct inter-school competitions. To find the amount of water displaced by the boat, the teams first fill a utility tub with tap water. Then, place the boat in the tub and draw a line on the tub at the top of the water. Remove the boat and add specific measured amounts of water until the top of the water reaches the line. In our case, for example, we added 2,600 mL, which roughly To see these boats in action, visit www.techdirections.com/ rcboats.html www.techdirections.com TRANSPORTATION 19 http://www.techdirections.com/rcboats.html http://www.techdirections.com
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