That is called a wind maker Tim !

Wouter

Once on the lake the wind is pretty steady winds (both in speed and angle). Or we were very lucky that week... :-)

That's called a jib

"When you live in the alps its really funny watching the paragliders soaring one minute and then on the deck the next".
Wayne, it may look funny to you but it ain`t fun for the guys flying - Think of it as being similar to an instant 45degree header while you are two-up on trapeze, except that there is 3000ft of air below you, so you don`t just fall in the water!

Gill, looking at your tracks I see something I don`t understand :
your top 4 100 averages are all faster than your 1sec top speed. This makes no sense to me, as your top speed should show peak speed acchieved over 1sec, should theoretically be higher than any averages over a longer period.
Normally, the peak speed shows up any spikes in your tracks, which once deleted, brings your top speed (1s) down to a realistic level (my highest 1s speed was 65knots, after deleting spikes it was more realistic). Then your longer averages come down to also a more realistic average speed, but peak speeds should always be higher than averages.
I`m assuming you deleted spikes in GPSAR, but even so I can`t understand how 100m averages can be higher than peak speeds. In my head it is simply impossible to have 4 100m long average speeds higher than your all-out top end run. Am I missing something ?

Steve,
I uploaded the GPX in GPSAR and calculated the speed statistics, that's all I can do in the programm. Anyone who wants to check out the track, let me know.

Gill

Smiths Cat,
from "cruise-charter.net"
Let us take a sloop - its working sails are a mainsail and a working jib. The jib covers most of the fore-triangle (everything between forestay, deck and mast). The main sail plus the jib are the boat's primary source of speed. However, to get the most of it, to explore its limit to their extremes, you need one other sail called a Genoa.

The Genoa, or as they call it Genny, is a simple jib, whose clew reaches behind the mast (or also called abaft). Its size is directly related to the fore-triangle, and in fact it is calculated as a percentage of it. There are 4 main types of Genoa - the smallest is called Lapper (it is 110% of the fore-triangle, only 10% bigger than it), then we have Genoa 2 (135%), Genoa 3 (150%) and the Drifter (160% or even 165%). The name of the sail came from a regatta in 1927 for 6-m yacht; which was crushingly won by a crew which used a Genoa - extra sail attached to the jib; there were no rules banning this, so they won by this innovation and thus it took its name. The Genoa is not only for speed races in regattas, it has irreplaceable contribution when there is very light wind or still. It uses the space optimally, not even one cm remains open and the drive created is really considerable. The only negative side is that it blocks the forward view of the helmsman; and he/she better have a separate person from the crew to handle the Genoa. The sheets of the Genoa (ropes) are always run outside the shrouds.

Hi Gill,

Sure I want to have a look at your Track!

At Smiths_Cat,
I did use several gps already, and it seems they don't differ that much, as long as you remove spikes and take at least 10 seconds interval. See picture. For a test, I did use a Garmin 301 and a Qstarz 1300s gps during a regatta (almost no wind).
Scale is up to 10 knots and 500 meter. Black is Garmin, Blue is Qstarz. software is gpsar.

Interesting to see that the Garmin's output is more smooth. But in the end they are almost identical, if you would average the qstarz output.

This was typical for the whole track. Knowing that a GPS should get more accurate at higher speed I would think one should be able to compare pretty reliable different gps brands.

Geert

Hi Geert,

there are many ways to "remove spikes". That's why some report 20kts and other 17kts top speed for their boat. Actually a gps provides only a position signal not a speed signal and the devices software has to deduce the speed out of it, which requires to steps:
>translating position signals in WGS84 to distances. This is more tricky since the earth is not sphere (as asumed in many devices) nor an ellipsoid. But the error here is small.
>differentiating the distance over time. Sounds simple, but because the position signal is noisy, the error is amplified by the differentiating.
Finally you have to find an agreement what means best 100m?
Sailed track length (maybe with curves) or the fastest 100m straight distance.
At the end of the day you have measured ground speed, your preceived speed is through the water speed. A current can easily add 2kts to your measurement.

Cheers,

Klaus

Gill, if you download & then open your track in GPSAR, then compute speed statistics, you will see if you have any abnormally high speeds (my best speed today was 133knots over 100m, which I`m pretty proud of..)
Then, you select "show graphs" and select the right 3 boxes, ignore alt.
Under Trajectories, slect the ACTIVE LOG, then look at the bottom 3 graphs, if you see "spikes" in the graph, select your speed that makes no sense (if there is one), then hold shift down and select delete under trajectories. You then re-calculate speed statistics, and see if any spikes still exist, and go through the process again until you have a clean track. If you go to www.gpsspeedsurfing.com you might find a better description than I have given, but this is the process I followed to get rid of my 133knot top speeeeeed.

Gahh...

GPS velocity measurements are based on dopler shift NOT differentiation of the position data. As such they are a LOT more accurate at measuring velocity than they are at measuring position.

My first two google hits actually say velocity estimates are usually within 1cm/sec (0.02 knots!).

simple explanation (from an aircraft forum)
and
A technical peer reviewed scientific paper

You are right though in that there are other huge sources of error, like sailed track length (in 3D!) over which you are averaging.

Personally I think it's amazing that GPS works at all... they have to take into account the relativistic effects of gravity on time dilation to make it work!