Calibrating the knotmeter Unsure of the accuracy of your speedo? Use this tow-line method for constructing a speed curve
All of us have, had or will have the experience of sailing along at hull speed when the reading on the knotmeter drops to zero. After a tack, the knotmeter reading is normal but again soon drops to zero.
Sooner or later, the owner goes below, pulls up a floor board and pulls out the transducer (through-hull plug at the ready, of course), gives the paddle wheel a spin with a flick of the finger, and
someone topside shouts, “It’s working again.”
As slimy as the transducer gets, I’ve always wondered how it could work and how accurate it was. It seems a method of calibrating the knotmeter would let me know it was working accurately.
One way of measuring speed through the water is the chip log. The chip log method consists of tossing a small wooden chip over the bow and measuring the time it takes the chip to pass down the side to the stern. For example, for a boat 30 feet long, moving at 6 knots, the chip will be astern in three seconds after being tossed over at the bow. However, the chip log is not suitable for small craft because of the requirement to measure short time periods. Besides, the Environmental Protection Agency may have something to say about leaving many wooden chips in your wake.
Another method—using a line with knots tied in it—seems to be a popular way to measure speed through the water. Consider a line with a knot every 100 yards. The boat would be towing the line with the bitter end in the water with a small drogue. To determine speed in knots the drogue is allowed to pull the line off the boat for three minutes. The number of knots, during three minutes, equals the boat speed. Measuring the length of line remaining before the next knot will help determine fractions of a knot. This method looks good until you consider that a boat moving at 10 knots would have 1,000 yards of line out at the end of a three-minute run.
Other methods also have their limitations. The geographically fixed “measured mile” suffers from unknown and variable currents, and a GPS measures speed over the ground, not speed through
the water.
Towing line There is one method that I have found to be very reliable, allowing me to measure with accuracy periods between 10 and 30 seconds with a stopwatch. This method uses a special calibrating line I made for use on a heavy-duty trolling rig.
It consists of a drogue, made of a 36-inch length of half-inch wooden dowel with a lead weight sufficiently heavy to make the dowel float upright. I fastened the towing line to the dowel at the lead weight. The towing line can be of any type of fishing line; 20-pound test mono-filament works fine. Following this towing line are two measured lengths of a different type line. The first measured length is 100 feet. The second measured length is 200 feet.
The purpose of the two lengths is to allow a choice in measuring distance and hence the measuring time. The two measured lengths are separated by something that can be seen or heard as the line runs out. For example, a fishing line swivel or piece of red yarn that will pass through the guides on the fishing rod will work fine. When sketched out, the rig will look similar to that in Figure 1.
To use the speed-measuring rig, tow the drogue with the towing line, putting the reel in “free spool” and stripping off about 10 to 20 feet of the first 100 feet of measured line. Start a stopwatch and let the stripped line run free. The drogue will sink to the vertical position and help strip off the remaining measured line. Use a stopwatch to measure either the time to run out 100 feet or 300 feet of line. This measured line must be of a different size and texture from the remaining line on the reel. For example, if the towing line is 20-pound test mono-filament, then the measured line could be something like 60-pound white squidding line.
Use the table in Figure 2 to determine your speed. The first column shows full seconds of running time for various measured distances, with the adjacent columns showing the corresponding speeds in knots, miles per hour and kilometers per hour.
Say, for example, you let 300 feet of line run out and this takes 26.5 seconds. When calibrating the knotmeter, which is done under power, I make several measurements at the same rpm and use an average of the times measured. For argument’s sake, say that 26.5 seconds is the average of four measurements. For partial seconds you can interpolate the speed in the table. The table shows 6.8 knots for 26 seconds and 6.6 knots for 27 seconds, so I would figure 6.7 knots as being the most likely speed for 26.5 seconds.
Don’t use measured times that vary greatly from the rest. If you measured times of 25.5, 26.6, 27.4 and 35 seconds, I wouldn’t use the last one to figure the average. Most likely, the noticeably longer time is the result of inattention in operating the stopwatch or some other variable. If you want your boat calibrated in miles per hour, use Columns 4 or 5. Likewise, if you want to calibrate your boat in kilometers per hour, use Columns 6 or 7.
Running calibrations
So now that you can accurately measure boat speed through the water, it’s possible to calibrate your knotmeter by doing a few test runs under power. I would start out at idle, say 1,000 rpm, and then increase engine speed by 250-rpm increments until the engine tops out. Record and compare rpm vs. speed vs. knotmeter reading, using the resulting data to construct a speed curve. Some people find it useful to construct a graph—rpm vs. speed—while others prefer to use the data in a table format. I like to have both aboard. With the speed curve plotted, you now have a simple reference for finding the boat’s speed at a given rpm.
When making your calibration runs, stay clear of other boats, stay in calm water, allow the boat speed to stabilize when changing engine rpm or course, and do the runs with no more than 10 knots of wind. The data you get here should be the “best case” information. That is, do it with a clean bottom and nothing dragging. If you use this data, it will be easy to see when you need a hull scrubbing or a propeller cleaning.
If the measured distances in the table above are not right for your purposes, a couple of minutes with a desk calculator will allow you to make a table for any distance or time. The formula is: Distance = Speed x Time. If your measured distance is in feet and you want the speed in knots, divide your measured distance by 6,076. Should you want miles per hour, divide by 5,280. If you want kilometers per hour, divide your measured distance by 3,281. If you want to use seconds to express the time period, divide the number of seconds by 3,600, being the number of seconds in an hour.
However, the chip log is not suitable for small craft because of the requirement to measure short time periods. Besides, the Environmental Protection Agency may have something to say about leaving many wooden chips in your wake.
There is one method that I have found to be very reliable, allowing me to measure with accuracy periods between 10 and 30 seconds with a stopwatch. This method uses a special calibrating line I made for use on a heavy-duty trolling rig.