A SAWs trap is just a fancy chronograph, it times you across the measured distance of it and works out your speed from that. The problem with using a watch's chronograph (besides thae fact that you probably want to be looking at your boat not your watch on a speed run) is that of parallax, you can measure out a course on the banks, but unless you can run the same speed as your boat or can stand an infinite distance from the lake to watch it, you need to be in 2 places at once.
I have used an old fashioned Garmin Etrex handheld GPS, and a Hobbyking Quanum speed meter which is no longer sold but the SkyRC speed meter is an available equivalent. These all worked well to a point, being reasonably consistent in the vast majority of runs to the slower boats that I have run through the traps, and my cars speedo. They do wig out and produce nonsensical results on occasion, so they need a certain amount of filtering of the results, but if i get 2 similar results on the same boat I trust it to be within a couple of MPH of the results it gave. They do have VERY limited datalogging abilities though, with instantaneous maximum speed being the only useful information being recorded for your perusal on shore. Lots of people use these for bragging rights and taken for what it is, I think they are a pretty accurate method of measuring the highest speed a boat achieved during a run. It may have only achieved that speed for a tenth of a second however, which with faster boats is a very different thing to a two way average through the traps.
I have also used a friend's Garmin GPS running watch, which was by far the most expensive GPS I have used, and by far the worst, it and I believe all the watch type GPS units update at 1hz to extend battery life with their tiny batteries, and it gave readings all over the place. Whereas the handheld units, the RC specific units and I believe most phones, update at 10hz.
I have since switched to an RCM V2 GPS datalogger for my SAWs boats, which logs data at 10hz continuously during the run, it can also log battery, voltage, RPM and 3x temp sensors, the software will overlay a trace of the run onto a map, and you can get all that data at any point on the run, so when testing and tuning prior to a SAWs you can see your rate of acceleration, how fast you were going into the imaginary traps, how fast you came out, your average speed over the 100m/110yds, what your voltage dropped to under peak acceleration, etc.
If all you want is an answer for your mates down the pub who ask how fast it is get the SkyRC unit, it is a neat standalone box with an LCD screen, no wiring or other equipment needed.
If you want information that will help you tune your boat, go for the RCM. With power and all the sensors it is a bit of a mess of wiring, and you need a phone to display the data, so it is not for everyone, but there is a LOT more data available.
Eagletree Elogger and SM Unilog are dataloggers with 10hz GPS sensors available to them, and pitot sensors too, (which i don't believe are as accurate as GPS, but can be used to filter out erroneous GPS data (not that this is as much of a problem with a GPS logger as one of the speed readers as you can see the spike and the real data before and after it), I have an eagletree which records current, voltage, rpm, 3x temps, and capacity used, but its current shunt is only rated for 150A (it has been many years since I used it so could be misremembering it but I think it actually stopped reading at 200A), while you can add to the shunt and increase its capability to 300A but I don't have a way to measure the resistance well enough to calibrate it properly so it becomes an estimate used for comparison only, (being an EE I bet you have a great multimeter, and could calibrate it OK), the Unilog does the same things for the most part and has a 400A shunt available for it.
Prices for the SkyRC and RCM are similar, Eagletree is higher, and Unilog is higher still.
I also have a Castle Ice 50 ESC with datalogging built in, this is a smaller, neater and lighter solution for electrical datalogging. It offers 2 less obvious advantages too, the main one to me was the built in temp sensor is MUCH faster responding to the quick changes in temp (produced by very high currents of short duration, when running SAWs), than the external dataloggers can be with their external thermocouples. The other being that only the ESC loggers record ripple voltage, which allows you to tune your cap bank. On the down side I don't believe any of them have a GPS sensor so if you want that data you have to run a separate logger anyway, and ESCs with logging are often more expensive. IIRC my Ice 200s stopped reading at 320A.
Paul Upton-Taylor, Greased Weasel Racing.
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