Science Buddies: "Ask an Expert"

Hi,
My 5th grade son was attempting to do the project from Science's buddies project ideas called Making it Ship Shape https://www.sciencebuddies.org/science- ... p019.shtml. He did everything as described in the project guide. He had his grandpa help him carve four boats out of a 2x4 pinewood. Each boat was five inches long and had a different shaped hull. He put a hook in the top of each boat in the center and then weighed the boats. The heaviest boat was 200 grams so he put washers around the hooks on the other three boats so that each boat weighed 200 grams. I helped him construct a water way out of a piece of gutter, as described in the project, with a spill way at one end. The gutter was 10 feet long. When he went to do the experiment he stabilized the garden hose at one end of the gutter and attached the boat to the a 2.5 Newton spring scale to try to test its drag. No matter what he did he could not get the spring scale to move at all to get any measurement. He tried making the water force stronger (using the strongest hose nosel we had), he tried elevating one end of the gutter at a lot of different heights, he tried moving the boat and spring scale to all portions of the waterway, he tried adding more weight to the boats and he could never get any reading from any of the boats. We are wondering what went wrong since he seemed to do everything exactly as described? At the end of a very long (and cold) day he ended up just timing the boats moving a short distance (10 inches) with a stabilized hose and water pressure. Can he use the speed data he took to somehow show which boat had the least amount of drag? Would the fastest boat have the least drag? Or can somebody tell us what went wrong or another thing he could try. Unfortunately I don't think there will be another warm enough day to retry this outside before the science fair which is due this Friday the 28th.
Thanks for any insight!

jsmissey - So sorry you had difficulty with this experiment. I've never done this project, so will just offer my observation. The gutter is intended to get a flow of water established so that the hull drag could be measured with a simple scale. It would seem that too little flow would not produce enough hydrodynamic pressure to read very much on even a sensitive scale. A scale with about 2.5 Newton full scale seems to be not sensitive enough unless the water flow was in the neighborhood of several feet per second. My guess is that your water movement was far less than that. Too bad you don't have a more sensitive scale more in the range of 50 or 100 grams, then adjust your water flow to get readings within the scales capability. You say you measured the time it took a hull to move 10". Assuming the hull started at zero speed, what was that approximate time for the 10" travel? Near the end of the run down the gutter, the hull is probably moving at the water flow rate. I would say that your best chance to get a reading on this scale is to dramatically increase the flow of water. Don't use a nozzle on the hose, just let it flow pretty much full force out of the hose.

Another input, the different hull designs will have different transit times down the gutter if you let them sail free. From the moment the hull is inserted into the moving stream, the fastest hull down the gutter is the highest drag design and vice-versa. This takes a little thinking to understand that, but it is a fact.

Rick Marz