EGlideRider
Lieutenant Junior Grade
- Joined
- Dec 14, 2008
- Messages
- 1,000
Many on here seem to be obsessed with speed. Must be our testerone...
But I read of lots of speed comparisons that originated from inaccurate data.
Thus this post:
While it is true that a GPS is the most accurate instrument we have to measure speed, the speed must be interpreted accurately if a true comparison is to be made.
A GPS measures speed across land (or water bottom) not speed through the water. Our boat speedos measure what we really want, the speed through the water. But of course everyone knows that the speedo's are unreliable.
Let's assume we do have an accurate speedo and look at a couple of scenereos. Let's also assume no wind and we are in a river flowing due south with a 10 knot current.
Going north, we glance at the GPS and see a speed of 10 knots at WOT. The speedo shows a speed of 20 knots. If we reverse course, the GPS will indicate a speed of 30 knots while the speedo continues to indicate 20 knots.
So, to get the speed through the water with a GPS, you must average the speed in two opposite directions. To increase accuracy, you should do so several times to minimize the effects of wind and current. In the example above, we would average the 10 knots northerly speed with the 30 knots southerly speed to get the true speed through the water of 20 knots.
So the next time you read here of a GPS speed such as 20 knots or 20 mph, you must remember that the speed must be averaged along the same course if you are to use the data for comparison purposes.
Hope this helps.
But I read of lots of speed comparisons that originated from inaccurate data.
Thus this post:
While it is true that a GPS is the most accurate instrument we have to measure speed, the speed must be interpreted accurately if a true comparison is to be made.
A GPS measures speed across land (or water bottom) not speed through the water. Our boat speedos measure what we really want, the speed through the water. But of course everyone knows that the speedo's are unreliable.
Let's assume we do have an accurate speedo and look at a couple of scenereos. Let's also assume no wind and we are in a river flowing due south with a 10 knot current.
Going north, we glance at the GPS and see a speed of 10 knots at WOT. The speedo shows a speed of 20 knots. If we reverse course, the GPS will indicate a speed of 30 knots while the speedo continues to indicate 20 knots.
So, to get the speed through the water with a GPS, you must average the speed in two opposite directions. To increase accuracy, you should do so several times to minimize the effects of wind and current. In the example above, we would average the 10 knots northerly speed with the 30 knots southerly speed to get the true speed through the water of 20 knots.
So the next time you read here of a GPS speed such as 20 knots or 20 mph, you must remember that the speed must be averaged along the same course if you are to use the data for comparison purposes.
Hope this helps.
