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How to measure laser speed - polar graphs.

vteq

New Member
Hello,
I'm trying to find a method to measure laser speed as accurate as it is possible. The only thing that comes to my mind is car GPS. Of course, it works and shows speed but the problem is that there might a huge uncertainity in this measurements. It's car GPS so it's calibrated for high speed like >50 km/h. The highest value I get was only 12 km/h (wind speed: 3-4 m/s). Not to mention that average speed isn't higher than 9 km/h. I think that this device show speed more precisely with high speed like >50 rather than with those <10 km/h. What is more, the lag between moving from my real position to seeing it updated on a screen is something like 1s.

The other thing is that there are only 2 significant digits on a display so my uncertainity is at least like +/- 1km/h.

I was looking for something like electronic log. I thought that there might some kind of equipment with a propeller on the one end and display with another end but I realised that it doesn't exist and even if it exist it costs thousand of dollars.

I read some topics about measuring boat speed but in most cases there are only mentioned GPS systems.

I need this measurements to create real and very accurate polar graph of laser speed. I tried to find it also but the only things I got was "there is no official polar graphs for laser".

So finally I am asking you. Is there any possibility to measure boat speed accurate enough to create a polar graph? Are there any devices like windmeters that show speed of a boat? If no, how can I measure it more precisely than with this car GPS.

P.S Sorry for all language mistakes, I try my best but I'm not english native speaker :)
 

torrid

Just sailing
I don't think it's calibrated to any particular speed. The problems is it doesn't read speed instantaneously. The interval at which it updates speed is probably programmed into the unit.

If you are running a constant speed in a single direction in flat water, it will probably give you an accurate reading. If you are hitting a bunch of waves and having to steer through them, I would expect a greater uncertainty.

This is a GPS device made specially for sailboats:

http://store.velocitek.com/SpeedPuck

I've never used one, but I would be interest to know how it works. My thinking is since it doesn't have the overhead of maps and a display like a unit intended for a car, it may give better real-time information.
 

Rob Hair

Active Member
You may find one of the GPS based cycling computers to be useful. Garmin is the biggest maker of these. Using the included software you can show your course on a map (Google maps), which would give you heading information that was tied to speed. Good luck!
 

Wavedancer

Upside down?
Staff member
I was looking for a SpeedPuck as well, but it's quite expensive (over $300 and not discounted AFAIK).
The good news is that just recently, some GPS watches have become available at $100. They are not as multifunctional as the Garmin toys, but they appear to measure instantaneous speed just fine. I am tempted, but can't comment on how well they would work on a Laser; would like to know though...

Google 'Timex Marathon GPS Sport Watch'
 

cskudder

Active Member
There are many GPS units designed for boats that display speed and plot track on a map in real time, and they are obviously designed to perform well at slower speeds than cars, probably in the range of 10-30 knots, or 15-50km/hr. I use a Humminbird 161 plotting GPS-sonar fishfinder. It shows speed in 2 significant digits. I display it in knots, so I have tenth's up to 10 knots - for example 5.2 knots, up to 9.9, then 10, 11, 12, etc.
 

49208

Tentmaker
IF gps is accurate enough in Poland (and that's a big IF :)) you might want to look at getting an older used Garmin handheld from Ebay. The GPSMAP 76 will report speeds down to 10ths ie 4.5 and is quite accurate here in the USA (we use a couple of them on the big boats I sail from time to time and they are usually spot on with the boat's own instruments when there is no current involved) The device is waterproof as well. Should be able to find them for well under $100 I have compared this with the GPS on an IPhone 3S, which is not as accurate at low speeds
 

vteq

New Member
Actually, I don't need to have speed values instantaneously but the most important for me is accuracy of the reading like 5.21 km/h not 5 km/h. I might even read it after the measurement but it has to be as accurate as it is possible. I only think that this lag between my movement and device response is a sign of uncertainity of measurement.

I looked for this Garmin devices and it seems to be the best solution. The bad thing is that they're VERY expensive for me. If I only could I would buy the most expensive and accurate one but... "live is brutal" :D

Anyway, it's a good way of thinking. I'll try to look for this Garmin GPS and maybe I'll find it in some univeristy or institutes in my town and just rent it for a few days.

P.S. Does anyone know something about this laser polar graphs? :)
 

8817

Member
I made my own GPS data logger and you can set the frequency in the software. I put it in my kids boats while they race so we can review in Google earth later on. It shows speed, mark roundings penalty turns etc.
It isnt that difficult to do with an Arduino and I dont mind sharing the code with anyone who wants to try it out.
 

camino

New Member
8817, I actually ordered a gps module for the arduino to build the same thing a few days ago. I'd be interested in seeing your code. What kind of container do you use to keep it dry? I also picked up a tri axis accelerometer to log heel angle, which might be overkill but I think it will interesting data anyway.
 

8817

Member
I keep it in a Pelican case with pieces of foam to stop it from rattling around.
I have been running it for more than a year without issue.
Ill post the code as soon as I can find it, I think its on my other laptop.
 

8817

Member
This is a line of output from the device:
$GPRMC,185149.898,A,4341.2955,N,07919.7602,W,19.22,297.83,220711,,,A*41

The 19.22 is the speed in knots. *the logger was in my zodiac for testing

Here is a description of the data output.

$GPRMC

Recommended minimum specific GPS/Transit data
eg1. $GPRMC,081836,A,3751.65,S,14507.36,E,000.0,360.0,130998,011.3,E*62
eg2. $GPRMC,225446,A,4916.45,N,12311.12,W,000.5,054.7,191194,020.3,E*68


225446 Time of fix 22:54:46 UTC
A Navigation receiver warning A = OK, V = warning
4916.45,N Latitude 49 deg. 16.45 min North
12311.12,W Longitude 123 deg. 11.12 min West
000.5 Speed over ground, Knots
054.7 Course Made Good, True
191194 Date of fix 19 November 1994
020.3,E Magnetic variation 20.3 deg East
*68 mandatory checksum
 

cskudder

Active Member
Have you tried this fishfinder on a Laser? And if so, where do you put it?
Thanks.
I bought the Humminbird for the Laser, and have had it on there for like 5 years now. (I'm a recreational-only sailor.) The depth sounder and bottom profile display are the most important reasons why I got it. I sail mostly in reservoirs which are flooded river beds with bluffs and all, so there are huge variations in the lakebed profile that are hard to visualize or memorize, and the water level generally varies by 12-14 feet from high to low. I also plotted specific key points for which I know the elevation, so I have a decent idea of how much water there will be over those points.

Mounting- there was already an inspection port in the slanted part of the deck, next to the daggerboard. I screwed the display unit onto the top of the inspection port. I run it off 12 D cells in series, putting out an initial 18 volts when new. The unit will run on 10-18 volts. One set of batteries generally lasts like a season - 20 or 30 hours on the water. The battery pack is secured to the daggerboard trunk inside the hull. The battery rack isn't waterproof but it's not sitting on the bottom of the hull either. I've never had a problem with the power connections getting wet or shorting out. The transducer is epoxied into the hull just behind the mast tube. I'll take some pics over the weekend and post.
 

vteq

New Member
I made my own GPS data logger and you can set the frequency in the software. I put it in my kids boats while they race so we can review in Google earth later on. It shows speed, mark roundings penalty turns etc.
It isnt that difficult to do with an Arduino and I dont mind sharing the code with anyone who wants to try it out.
I completely forgot about the Arduino! I was going to buy one to build a slider controller for my camera but I post pone it. Now I see that I might use it for a few different things. Author of that slider says that I need:

Arduino Demilanove 328, this is the brain of the controller. Think this controller has been replaced by the Arduino Uno.


Can you describe what elements you used to build this logger and how it works? I mean is there any display, "start-stop button" and things like that. I saw that the measurements are quite accurate. I'll try to become an Arduino expert in a next few days but now I know only that I need Arduino plate and GPS receiver. I want to buy version that might be usefull in future for this slider.

Anyway, thanks for this message! :)

IF gps is accurate enough in Poland (and that's a big IF :)) you might want to look at getting an older used Garmin handheld from Ebay. The GPSMAP 76 will report speeds down to 10ths ie 4.5 and is quite accurate here in the USA (we use a couple of them on the big boats I sail from time to time and they are usually spot on with the boat's own instruments when there is no current involved) The device is waterproof as well. Should be able to find them for well under $100 I have compared this with the GPS on an IPhone 3S, which is not as accurate at low speeds
I finally found accuracy of my gps. It's somenthing about 1m. But still it doesn't show the speed as I want it. Buying Garmin GPS will be the last step when Arduino and other fail :)
 

8817

Member
I completely forgot about the Arduino! I was going to buy one to build a slider controller for my camera but I post pone it. Now I see that I might use it for a few different things. Author of that slider says that I need:



Can you describe what elements you used to build this logger and how it works? I mean is there any display, "start-stop button" and things like that. I saw that the measurements are quite accurate. I'll try to become an Arduino expert in a next few days but now I know only that I need Arduino plate and GPS receiver. I want to buy version that might be usefull in future for this slider.

Anyway, thanks for this message! :)



I finally found accuracy of my gps. It's somenthing about 1m. But still it doesn't show the speed as I want it. Buying Garmin GPS will be the last step when Arduino and other fail :)




Mine does not have a display, I want to use it during a race for logging data.
Its just an arduino with a prototype shield and GPS chip from Adafruit.
There is a small amount of soldering but I dont remember it taking very long.
You do not need to be an arduino expert.

I had completely forgotten about this post, I had meant to dig up the code as well.
Ill look for it this week and post some images/ code etc.

Mike
 

Beachcomber

Member
Frank Bethwaite's Higher Performance Sailing has a Laser Polar in fig 26.7. I presume he measured this as described elsewhere in his book. Namely, he would motor behind a fleet leader while racing with a GPS on board. After some time, they would stop and note wind speed and direction. I imagine his main purpose in doing this was to evaluate the performance of different designs. It's good enough for that purpose, but a fairly rough approximation of performance. Barring using much more technology, I think it would be hard for an individual to beat this accuracy.

If you really want to get more accurate, you'd want an electronic data aquisition system, a GPS/IMU (Inertial measurement unit - the accelerometers make the GPS more accurate, especially in accels and speeds), and an electronic wind strength and direction indicator.
 

Beachcomber

Member
Frank Bethwaite's Higher Performance Sailing has a Laser Polar in fig 26.7. I presume he measured this as described elsewhere in his book. Namely, he would motor behind a fleet leader while racing with a GPS on board. After some time, they would stop and note wind speed and direction. I imagine his main purpose in doing this was to evaluate the performance of different designs. It's good enough for that purpose, but a fairly rough approximation of performance. Barring using much more technology, I think it would be hard for an individual to beat this accuracy.

If you really want to get more accurate, you'd want an electronic data aquisition system, a GPS/IMU (Inertial measurement unit - the accelerometers make the GPS more accurate, especially in accels and speeds), and an electronic wind strength and direction indicator.
 
To create accurate polars I think you need a lot more than accurate boat speed.
For a given wave condition, I think you also need accurate true wind speed & direction, and boat speed & direction all simultaneous with ideal boat/sailing trim.
RRE
 
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