I finally got to sit down and play with realspeed. Thanks Mal. I haven't sailed much the last two months cos no wind. The last real sails I had I was really frustrated cos the max speed reading on the gps stayed the same no matter how fast I thought I was going. Well I downloaded the data and followed the foolproof instructions. I deleted invalid data as per instructions. Before I did this max speed was 27 something and after I deleted invalid data it was 30.27. Why? Which is correct?
Suzie, generally you do not need to remove spikes in RealSpeed. The software will automatically eliminate any runs that have spikes and provide results for all the good runs. If you had a spike in the middle of one of your best runs, then you remove the spike by deleting the invalid points, RealSpeed will then count that run, which is possibly what has happened.
If you want to be sure of the result, then send me the original file I'll take a look for you.
I'm using doppler for all measurements except alphas, no need to worry about invalids or deleting anything, but I just check the alpha track to make sure it's a "U" shape without any spikes or jumps in it.
If you're unsure about a particular speed it's usually a good idea to check the graph at the bottom of the screen. Click on your 2 sec speed and the relevant portion of the graph should become highlighted. If it's a valid speed you'll see the speed slowly rise up to the value and then drop off afterwards. If it's invalid you'll see it as a spike.
Take a little bit of practice but it will make you a better sailor if you analyse where you went well...
To be honest I'd be suspicious of your 30 result, but I can't tell without looking at the track. As others have said, if you use doppler you get much more accurate results.
but I knew it was wrong 
Doppler showed it as a 29.03 max which I knew was right
From Mal's RealSped site....
Trackpoint versus Doppler
Speeds calculated from Doppler speed data is significantly more accurate than speeds calculated from positional track point data. The reasons get a bit technical, but stem from how the two type of data are calculated. Track points are calculated by timing a signal from each of satellite being tracked. Doppler speed is calculated by the rate of doppler shifts that occur between the GPS frequency generator and the frequency of the carrier signal coming from the satellite. This doppler shift measurement is extremely accurate and not affected greatly by atmospheric conditions. The track point calculation using timing of a signal from the satellite to the GPS has significant error and can be greatly affected by atmospheric conditions.
If you use a Locosys GT11 or GT31 then you will have doppler data available. If you manage to capture NMEA data from any other GPS device then you will also have doppler data available and should choose the Doppler option. Otherwise use the track point calculation method.
Go here to download the Dopplespeed defaults http://www.intellimass.com/RealSpeed/Divisions.htm
TimeMachine is Mal the developer of RealSpeed, helpfull guy.
