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-   -   Boat speed measuring (http://www.rcuniverse.com/forum/speed-electric-109/6452989-boat-speed-measuring.html)

andreasturb 10-05-2007 11:41 AM

Boat speed measuring
 
Hi people!

I thought about starting a thread exclusively on speed measurements, mainly for two reasons:

- I am thinking of buying a GPS device to start recording the speed of my boats and so to be able to know what I gain with each new set-up
- to promote comparison of different methods (GPS,radar etc.) and different brands and models of speed measuring devices.

So, my first question would be of course:
Who can give me links to GPS device brands and/or radar guns, or anything that you or someone else could use to measure the speed of our boats? What are the advantages and disadvantages of each?

Any input is welcome!

thanks,
Andreas

SV27dayton 10-05-2007 12:14 PM

RE: Boat speed measuring
 
This should be a healthy discussion. Stinger was saying that he my be going faster than he thinks because he can't sustain top end long enough before loosing control and his GPS may not be registering his top speed b/c he wasn't there long enough. I did a road trip yesterday with my GPS (garmin 205 will register speeds over 100 if you were wondering, I was):). Anyway it seems to take 4-5 seconds to show the speed. (one drawback). I drawback of radar is you need to come straight at it to be most accurate (and the water conditions or lake shape may not be ideal in that direction. Also, you'll need another set of hands. Garmin is good for running and biking radar gun is not. Garmin adds weight to the boat, radar doesnot. If boat filps and breaches you can loose the GPS. Radar is safe on the shore. Garmin tells you total distance covered and average speed, which is kinda cool. Radar can't do that. That's all I got right now.

BJM-Maxx 10-05-2007 01:29 PM

RE: Boat speed measuring
 
Based on other scattered threads here I went recently with the Garmin Geko 201. It is nice and small, records max speed and total trip distance. I zip lock bag it and place it in my SV27. It has an update rate of 1 Hz and so you are getting a 1 second average which in a small boat might mean you have substantially beat the average. I hit 40.0 mph in my boat and I know I never hit a fixed top speed so my true peak would have been higher.

As far as accuracy goes I see a lot of inaccurate debate about GPS speed measurements. People make the argument that if a GPS is not better than ~30 m rms then how can speed measurements be accurate. This issue is that the errors in measurement are not truly random they are systemic. A number of years ago the US military invoked Selective Availabilty (SA) which intentionally dithered the satellites reporting of their time and position in order to cause an error. This was turned off during the Clinton administration as the US have now proven that they can selectively turn it on in local places around the globe as needed. So where the rms accuracy was approximately 75 m it is now around 20-30m. Speed measurements are different in that they require high resolution not accuracy. The errors that caused your position to be wrong by 50 m are the same errors when you recalculate about 1 second later. The difference in those positions is much more accurate than the absolute position themselves. Your GPS receiver has a resolution probably in the sub metre range. Atmospheric effects do not change rapidly. What will cause an error is dropping one satellite's signal for another, this causes your new errors to be stable for a while and then jump again when another satellite is lost.

Overall if you can repeat your top speed more than once you should be pretty confident that you have really reached that speed. Also you should not consider 40.0 mph to be a different value than something like 40.2 mph. I would not round off to better than 1 mph.

GregS 10-05-2007 01:42 PM

RE: Boat speed measuring
 
Although a GPS can effect speed measurements by adding extra weight to the boat it is very useful for comparing relative speed differences between different setups.

Greg

DUSTINNEWTON 10-05-2007 02:17 PM

RE: Boat speed measuring
 
I bought a Garmin Geko 101 for $50. Works very well and is waterproof. I doubt that it floats though. I can put it under the hatch on my SuperVee and it works perfectly through the fiberglass. Straps to the hood of mt T-MAXX, and can attach to a larger plan quite easily. I don't have my boats going fast enough to lose control. My thinking is if you can't control it for an extended period at top end, what's the use? I could easily make my SV go 50MPH every time, but I can guarantee that I would lose it every time. I want wicked fast, but repeatable and controllable speed. All that to say that I'm not terribly worried about losing the GPS. I believe the Geko weighs in at 3oz.

ryan_t888 10-05-2007 02:29 PM

RE: Boat speed measuring
 
A Garmin Geko - 3.1oz GPS in anything larger than a 25" boat will not slow it down.

I prefer the Geko line of GPS's. Extremely accurate and precise.

Ryan

Got RPM 10-05-2007 09:28 PM

RE: Boat speed measuring
 
The Geko 101 has been the standard of serious FE racers for several years. It is (was) cheap, small and very accurate. I ran many passes through official NAMBA and IMPBA traps and the GPS speeds were usually within 1 mph of the trap times. Differences were due to the way the GPS and the traps measure speed. The GPS gives maximum speed, while the traps give the average speed for 330 feet. If the boat is accelerating only a little, the GPS will indicate a higher max speed than athe average would be.

Radar is so limited that it is almost useless. Small FE boats cannot be picked up well by cheap radars until they are too close for safety. Anything but a direct head-on shot gives erroneous readings. Run straight away from the radar and the roostertail interferes. Radar is best used for larger boats to measure the speed as the boat enters a turn.


.

ryan_t888 10-05-2007 09:29 PM

RE: Boat speed measuring
 
Very well said, could not agree any more.

Ryan

SV27dayton 10-05-2007 10:24 PM

RE: Boat speed measuring
 
Ryan,
When you say "I prefer" a certain unit, I have to assume that you've used several. What others did you use and what did you find wrong. You say the gecko is precise. Were any others inaccurate. I have a 205 and I don't know if it's any better or worse then others. I know if I have my cruise set in my car and I'm on flat ground it will mainting the same number witin a few tenths for minutes at a time. If it were jumping around i'd be suspicious. But I don't have any hard data. I don't think it update that quick. Maybe accurate after 2 seconds

ryan_t888 10-05-2007 10:43 PM

RE: Boat speed measuring
 
I've only used the Garmin GPS units. Although all the Garmin GPS receivers that I have used have all performed the same. About 1 second refresh rate.

eTrex: Found it quite bulky but liked the amount of run time on AA cells. For some reason when I switched between screens it took a while for it to click in. May have had software issues.

Geko: Small enough, dislike the low amount of run time on small AAA cells

Forerunner: Found it ackward to use as it is made for your wrist. Band ended up ripping on meAlthough it was quite small and light. Once in a while I would get very strange speeds, like 14777km/h.

What I meant by Precise is it showed very similar data on a different run, different day, same setup specs.

Ryan

Quicksilver1 10-06-2007 12:56 AM

RE: Boat speed measuring
 
GPS has been talked about enough in this thread, but as for other methods, radar would probably be more accurate, well at least it should be..

Then there's the old fashion way by timing a given distance......speed over time. gonna be a long read here,you've been warned. I've found this to be really hard to accomplish and at least for me very inaccurate. I tried using video footage, with floating markers and even taking into account my camera's frame rate. I found you really need to do this on a big scale, which would require a real boat in order to set up properly. Something on the order of a half mile with a carefully measured course would be best. You could then lessen several variables that that screw with figures like these. When I was experimenting with video footage, I kept getting different results, from the same frame set. Unless you have a computer program that can dial in the exact times, disregarding frame rate, small distances are not accurate. I was doing this with my jet-villain(on my old 12cell sport cells) .. I was getting 12mph, 19, 25, 32......useless. I'm at a bit of a loss for what to do this late at night......I might go through some other newer higher quality footage.

If anyone knows of a free program that runs XP for video editing that could make things far more accurate..........that would be cool, I keep feeling it necessary to promote this freeware google: ICsoft.......very good video file converter

If I come up with a repeatable WOT speed, I'll mot only the number but how I did it.

See ya a few, If using somewhat simple speed over ground calcuatrions can make repeateable results, we must compare. Ive been up on the GPS stuff for decades now, shoot I know Loran-c(yeah not GPS) I've got access to all sorts of intergrated radar/GPS systems, only I dont have a mini GPS... :(

edit: p.s. if I can figure this out I could try to cross-reference your GPS figures with my own calculations from your videos.


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