Offshoreonly.com - speedo accuracy?

This is the article from PowerBoat Magazine.

We have had several inquiries regarding the difference in the speed teadings in traditional
speedometers, a radar gun and a GPS. Many of our readers are experiencing different
readings themselves. Many have commented about the difference in the readings listed in
Powerboat's Performance Reports. Let me see if I can help clear this up.

One of the most common complaints is that a newly installed GPS digital or analog
speedometer is reading "slow." Most likely, this is not the case. It is more likely that the
conventional water-pressure-driven speedometer previously in the boat was reading high.

Depending on the manufacturer, some conventional water-pressure speedometers
read high. A few are pretty accurate. Pitot placement and style also play a major role in
speedometer accuracy. The most accurate pitots are transom-mounted and receive or
"see" clean water. Pitots integral to Bravo drives are pretty accurate depending on drive
height, but drive-trim angle can affect the pitot angle and the speedometer reading
accordingly. If you are testing a new or used boat for possible purchase, don't rely on the
speedometer for complete accuracy. Bring along a handheld GPS.

A GPS tells you how fast you are traveling around the circumference of the earth,
which is why the mph is really "speed over ground." It doesn't always accurately tell you
how fast you are traveling across the water. Currents, waves and navigational zigzags all
affect the GPS reading. But if you are on dead calm water, there is no current and you are
driving in a straight line, the GPS will give you an accurate overall reading.

Radar guns present another accurate option; Most modern radar guns can be used
from inside the boat and provide a pretty accurate reading, which is reflected from the
waterahead. Older radar guns could only record a reading from a land mass or had to be
held from a stationary point to radar a passing boat. The only way you can accurately
radar a passing boat is to have the boat coming directly at you or away from you. Coming
at you is usually more accurate because sometimes the radar gun picks up the rooster tail
from a boat from the rear, If you aim the gun at a boat that is passing by you, the reading
will be low because of the angle. Because of that, a radar gun used from inside the boat is
reflecting off the water, it tends to pick up wind waves. So if you are going against the
wind, it may read higher than if you are going with the wind.

During Performance Trials, we record the indicated speed on the speedometer
provided by the manufacturer. We install a separate Nordskog Performance Products
GPS with a memory function. And we use radar to record peak speed and to do
acceleration tests.

Looking at our data, you will note that certain tests show all the speeds
approximately the same. Most of the time the radar is a little higher than the GPS.
Generally speaking, this is because the radar records the absolute instant of peak speed
while the GPS is an average of the distance traveled each second. As far as the
speedometers go, we record what the readings are for comparison only. Many of the
boats tested last year had GPS speedometers or a GPS system in place of a conventional

speedometer When it comes to tournament ski boats, speedometer readings are
insignificant because the speedos are adjustable.

In other tests, you will note a fairly significant difference between the radar and
GPS reading. This is usually has to do with where the test was conducted. In other words,
if the tests were performed where the current was high, the wind was high, the water was
rough or a combination of all three, the readings will differ. The GPS top-speed readings
we report are the best achieved speed during the test. We also use the GPS to verily
efficiency readings, for the boat and for fUel economy. The radar readings are a
combination of a two-way pass, ideally one into the wind and one with the wind.

Certain types of boats react differently to the wind. Usually, cats run faster into
the wind because they get more lift, and V-bottoms run faster downwind because there is
less wind resistance. The radar usually will read higher into the wind because it is picking
up the wind waves coming at you, and, conversely, will read slower downwind because
the waves are going the same way as you are.

If the current is running the same way as the wind, the GPS will possibly read
higher than the radar gun and the speedometer but is not an accurate indicator of water
speed. And if the current is running upwind, the radar and the GPS could be
approximately the same. If you are running against the wind and up current in a V-
bottom, the radar will read high, and the GPS will read low. The point is there are many
variables.

Last year, our Performance Trials were performed on the Colorado River, which
has a 3- to 5-mph current, depending on the time of day. At that location, we average
speeds and find the water where the current is the slowest The florida tests were
performed at Captiva Island, which offers great offshore conditions on the Gulf of
Mexico, as well as some protected water inside the bather islands. This year, we had a lot
of wind and there were changing currents due to tides at this location. Long Beach, Calif
offers real Pacific Ocean offshore conditions and protected water behind the breakwater.
There is usually an afternoon wind and conditions deteriorate as the day goes on.

So what this all boils down to nothing is accurate for measuring speed on the water. I just use my speedo for reference. If it is reading a little higher after I try something, then I am happy.