My engineering background is rock solid, my artistic ability is what it is. I have built boats using wood strips, stitch and glue techniques, and molded fiberglass. Some have been better than others, but they all floated without breaking! I select a building process assuming an inexperienced builder, having few tools, who wants the process to be simple and progress to be fast. Visit my site @ http://www.johnsboatstuff.com
To quantify this low speed effect, I put my GPS on the dashboard of my car and recorded the velocity and heading every five seconds, while moving at a constant rate and heading. The results, Figure 1, show an error of 33% in velocity and 45% in heading. This is equivalent to a position error of 51 feet over a 100-second time. Using the 51-foot position error, I then calculated the expected error for higher velocities. Figure 2 shows the effect of velocity on error. Clearly sailboats can not expect much accuracy from the GPS for velocity or heading data. Using your GPS velocity data to evaluate sail trim or the GPS heading calculation to determine the amount of current or sideways slip is just not possible given the limitations of our present GPS system.
PREDICTED ERROR, 6 KNOTS =
15.7%
PREDICTED ERROR, 60 KNOTS =
1.7%
PREDICTED ERROR, 600 KNOTS =
0.2%
Figure 2
GPS is clearly the best navigation system available to boaters, but it does have limitations, especially the low speed effects caused by selective availability. When selective availability is "turned off" by DOD, the sooner the better, boaters can expect a dramatic increase in low speed accuracy.
My engineering background is rock solid, my artistic ability is what it is. I have built boats using wood strips, stitch and glue techniques, and molded fiberglass. Some have been better than others, but they all floated without breaking! I select a building process assuming an inexperienced builder, having few tools, who wants the process to be simple and progress to be fast. Visit my site @ http://www.johnsboatstuff.com