I am interested in measuring the velocity of a competitive swimmer who is moving linearly across the surface of the pool (and at the beginning of the length to be swum the swimmer would be around 3ft underwater). The swimmer's velocity is typically between 0.8m/s at the lowest end and up to 2.5 or possibly even 3.0m/s at the top end. The idea I have so far is using some sort of fishing line and attaching it to the swimmer via a waist belt. I would have the line rolled onto a spool on the deck at the edge of the pool. This seems like it could work especially well if the spool has a large diameter such that the thickness of the fishing line is negligible as it spools and I can just assume a constant spool diameter for calculating velocity. (I hope that makes sense?) It seems like the easiest way to measure the velocity using this system would be to get a reading of the angular velocity of the spool or a larger disk attached to the spool (such that both have the same angular velocity). I have come up with two methods of measuring the angular velocity so far. The challenge is I need at least 15readings per second so I need to sample at a rate of at least 15hz when the swimmer is moving slowest (around 0.8m/s linear velocity). What seems like the best method is placing evenly spaced markers/reflective tape on the outer edge of the spool or disk and counting them with an optical sensor. The second idea my friend suggested is placing evenly spaced magnets on the outside edge of the disk/spool and utilizing a hall-effect sensor to count and measure angular velocity of the spool. Looking at a spreadsheet I put together a 300mm diameter spool would give me a 51rpm ang. vel. at the minimum swim velocity 0.8m/s and a 25-50mm diameter spool gives 611-306rpm respectively. So I suppose I am wondering if anyone has any suggestions as to if I am on the right path or where I could purchase any of the equipment to make this?