speedometer vs. GPS
 

Has anyone noticed a big gap between your boat speedometer and a GPS? I've noticed as much as a 10mph difference and was wondering which was more accurate? I am assuming the GPS since it is accurate on the road. It is an auto GPS (Nuvi 350) with the Garmin winnepesaukee map on it. Looking forward to hearing if anyone else has noticed this.

GPS - as accurate as you will get
 

I'm amazed how accurate my boat speedometer is. It is usually within a MPH or two of the GPS reading. You can assume the GPS is very accurate, although it may have short glitches if the signal is weak.

Yes, boat speedometers are notorious for being off. Since most of them work on water pressure there are many variables which can affect an accurate reading. A GPS is extremely accurate at determining your speed.

As an fyi,... My boat speedometer get progressively more inaccurate as I go faster. Between 10 and 20 mph my speedo is relatively accurate, but when I speed up to say 50 mph on my GPS, my speedometer on the boat reads 55 mph.

Dan

What I noticed is when the boat is reading 35mph, the GPS is reading 45mph. Last thing I want to do is get pulled over for the "law that shall not be named" :laugh:

My Faria speedometer/Bravo 3 leg pitot is pretty darn accurate, according to my GPS. It's only off at speeds below 15 and above 45 or so, and then only by about 1.5 MPH at WOT.

A GPS calculates speed by change of position over the time between position readings. Theoretically if in the time between readings I went like a bat out of hell and turned around and returned to my starting position the GPS would report my speed as zero since I hadn't moved. The point is that the GPS is making some assumptions to calculate speed.

First is that the GPS has no way to know how you got from point to point. It assumes a straight line but you may have been traveling in a curve. The faster you are going the more distance you travel in a sampling period and the more it would effect the difference between a straight line and a curved path. If you are traveling in a straight line the accuracy is going to be higher. BTW, curved paths would result in lower speed calculated than reality.

Next, how accurate is your GPS positioning. Since speed is calculated by position and the sampling periods are relatively short, if the positions are accurate the speed calculation will be as well. Accuracy is a attribute of the design of the unit, the installation on your boat (antenna position, etc) and the ability to receive accurate satellite signals from minute to minute. There can be a lot of variability.

Not so fast there
 

You're partially correct, I believe a lot of GPS receivers use position differences over time as 1 input to the filtering used to calculate and smooth speed. But the most accurate input is averaged velocities calculated from the Doppler shift of the carrier signals from the satellites. Measurement of the frequency difference is very accurate and knowing about where you are on the Earth and where the satellite(s) are going you can calculate which portion of the velocity vector lies on the Earth's surface and hence your speed. Somewhere on one of my PCs I have a good paper that describes this understandable terms ... alas it isn't on this one. :(

Accuracy is not really that important, as long as the offset is consistent. It doesn't matter if the GPS knows exactly where you are, if it is off by 3 meters the calculations would be correct as long as the 3 meter offset error remained constant.

Most GPS units are doing several samples per minute, so even if you were traveling a non-linear route the speed calculation would still be accurate enough for anything other than a boat doing continuous donuts in a small area.

So what would happen if you were going 55 (which you thought was 45 due to your faulty speedo) and you got a ticket. How can you know your actual speed if your speedometer is that far off? I know one thing...I would be taking that one to court to let the judge decide. As far as I know, GPS is not required equipment...

Surprised
 

I was surprised how accurate my 23 year old VDO guages and pitot were when I took the GPS in the boat. 1-2 MPH difference all through the speed range max.

Speed...
 

Probably the same thing that would happen if you were actually doing 76 on Rte 93, but you THOUGHT you were doing 65. It is your responsibility to know how fast you are travelling. I doubt you would find a sympathetic judge.

keeping stock pitot is the key, on my old speedboat the oringinal pitot was within 2 mph of the GPS, the bracket broke and the new one that replaces it was off by 5-7 mph, not sure if the pcikup was longer on the old one or shorter. On my toon it's off by a consistent 5 mph at just about all speeds.

I somewhat disagree. Measuring speed accurately in a vehicle is much easier than on a boat, and speed limits have been a much more predominant item in automobile law vs. boating law.

It should be fairly clear by now that the reality is you're not likely to find yourself in this scenario. You would pretty much have to be traveling near the speed of light before you are going to get a speed limit ticket.

0.1 mph
 

My Garmin GPS steady state speed accuracy is 0.05 meters/sec or 0.1 mph. Position data is updated once per second. I might accept the GPS reading over the speedo.

While there are position errors in GPS, those errors change slow relative to the once per second update rate so distance traveled in one second is very accurate.

I have bought 2 "paddle-wheel" speedometers for 2 different boats. The packages included directions on how to accurise them. Basically you needed a measured distance so you could "clock" the boat and adjust the gear accordingly. There is probably a similiar requirement for "pitot tube" versions.

I doubt most factory boats have this procedure performed on them. Speed precision apparently was not considerred that critical since speedo/GPS discrepancy is occasionally discussed on other boat forums I visit.

If you want to accurise your instrument it is simpler to use a GPS than a measured course. During a calm period (no wind or chop) operate your boat at various speeds on a lake or large pond. Compare the readings. Adjust your speedo according to the directions. Repeat the procedure until you are comfortable with the results.

If you don't have, and cannot get, directions for your version examine the back of it for calibration access. Make a note of the current settings if possible. Then expiriment to see what makes it show higher or lower.

If you are not comfortable working on these systems then make a mental note or written table of the discrepancy at various speeds.

That's all I think I know about that.

Good luck!

LEO article
 

I was reading an article posted in 'Heloise' the other day. A LEO from Texas says to never use the GPS as a spedometer as it is inaccurate????

He claims to be pulling people over who says: 'Officer, My GPS says I am going the speed limit!' They were traveling over the speed limit. He hinted on Heloise that people should not use their GPS as a speedometer as they are inaccurate.

As stated in this thread I find the boat's speedometer readings off from my GPS reading. I am planning on using GPS to keep within the speed limit on the lake. I have observed my GPS as 'dead on' with the speedometer readings in my truck. I also heard that stored data in GPS is being used as evidence in court cases.

Should I be confused????? :confused:

When going in a straight line your GPS is extremely accurate. The problem becomes if you take readings while taking turns. Since your GPS only works on forward momentum, accuracy diminishes while turning. The sharper the turn the less accurate it becomes. I would trust my GPS whlile going straight way more than any speedometer.

As far as GPS being used as evidence in court cases, I have not heard of this, but who knows, in this day and age of "big brother" looking at everything we do I wouldn't doubt it. I have heard of On Star being used in court cases however.

Dan

ishoot308, great explaination of the gps speed. I have seen in many situations that in a slight turn it will actually show a greater speed then the true speed. This is why when racers are trying to break speed records they have to complete two passes to ensure accuracy.

My gps speedo needs a new battery, but livorsi wants $200 to do the replacment. However, there is no law that I have to have a speedo on the boat. So frankly I don't know how fast or slow I am going. Would be very hard to make that stick in court. ;)

On Saturday, I tested mine out. The speedo is accurate up to around 45 to almost 50, then it goes my way :laugh:

I will be testing the Garmin against the Tom Tom next weekend. As of now, the best GPS I have in my boat is the iPhone, with a $9.95 Navionics East app in it. I can see everything better, including the marker buoys, than with the Garmin 440. Interestingly, I have the optional $200 card in the Garmin.

$200? Depending on the model, that is like 70-80% of the price when new. You should bring it by, my guess is the battery could be replaced for about $10.

I have been online and apparently if you send it back to livorsi they give you a new model for $200 as an exchange. I have been told I can run a line to my new GPS once I get the network up and running which may negate the need to replace the battery or unit at all.

But frankly, I don't care what speed I am at. I am driving safe and prudent. Until there is a law that says I have to have a speedo it really doesn't matter.

I put a portable GPS in my boat for a weekend and wrote down the tachometer revolutions for particular speeds.
My pitot tube kicks up over 60 on Lake Winnisquam.. :rolleye2:

My speedometer sucks. It doesn't even read until about 25 MPH and then when it does, it's off by 10MPH when compared to my GPS.

However, my boat tops out at 48.5MPH according to my GPS, and that's on flat water with only two people. I don't see a ticket in my future anytime soon:yawn:

There's that "Responsibility" word again...
 

The statement ignorance of the law is no excuse is centuries old:

http://www.rbs2.com/cc.htm

The same source continues:

Please explain how someone on a boat is supposed to know exactly how fast they are traveling if their boat does not have a speedometer? There is no law that requires a boat be equipped with a speedometer. That would be my challenge to a judge if I were to get a ticket.

I am not being ignorant of the law, as I am perfectly aware that there is a speed limit. Without a speedometer, I might be ignorant of my exact speed however.

Yeah tell it to the judge!
 

Yeah, I suggest you don't try tellin that to the judge. My pickup doesn't have a speedo or doors for that matter. I got a speedin ticket that night when that college boy Hooper just kinda rolled out'a the truck on the salt pond road, and it didn't do no good when I told judgy I couldn't tell how fast I was going cause I aint got no speedo. No, give up on that excuse mister.

Absolutely would bring it up to a judge. It is a lot easier to gauge ones speed in a vehicle on a road than in a boat. For one, in a car you can compare your speed to other vehicles. You can judge your speed based on time/distance to fixed objects. In a boat, there are way to many variables to even think about guessing speed.

Are speedometers required equipment in cars?

Boat Speed
 

Seems like if you don't have a clue how fast you are going....maybe you shouldn't be behind the helm.
If you want to captain a vessel, you should be ready to accept responsibility for all aspects of the operation of said vessel. Short of that, keep your boat tied up to the dock. That will help make the lake safer for the rest of us.

I have been driving boats my entire life. If you don't have a speedo it is difficult to know exactly what your speed is. If you know your boat you know that based on the RPM level where around your speed is but no one knows exactly without a speedo gps or otherwise.

As an example going at cruising speed on a flat windless day at 3000 rpms feels slow (in my boat at least)

You change that into a strong head wind with 2 - 3 foot chop it feels like warp speed. It is all relative. However, you may actually be going slower.

As long as you are traveling at a safe and prudent speed for the conditions the # is arbitrary.

Be careful what you complain about!
 

Of course if enough folks complain that is is wrong to have a speed limit but not require speedometers on boats, some intrepid legislator will listen and file legislation to require speed measurement devices on boats travelling on waters with limits.

And then some of those same folks will complain about "nanny state" and "ridiculous legislation".

And this, my friends, is a perfect example of how we get all these rules and regulations that we are all (most of us anyway, myself included) so fond of complaining about! :)

agreed Skip... However that would be extremely tough to regulate considering boats may not or may not have been manufactored with them. The logistics would be crazy.

Aye!
 

Aye, Not like going down the pond chasin' bluegills and tommycods...we're talkin' 'bout up around 40 knots or so- or maybe 22 in the wee hours. You certainly know if you're approachin' the max legal speed- 'specially if there's a small 3-4 foot chop on the lake. If you have the urge to go max legal speed and not a prudent one- then get yerself a good working speedo or one o' them new GPS things. Any keep yer eyes open an' not on tha' damn electronics stuff!

what about bumps
 

We use the GPS on our sleds. My wife has noticed many a time when traveling across a lake at 90-100mph that if she hits a bump the gps will jump up to as much as 150mph.
I’m not sure if this is due to the higher rate of speed or what but she has never commented on it happening when using it on her boat which reaches speeds of 50, or on my baja which gps’s at 66.

Required equipment?
 

I do not see where a speedometer is required in a car, nor do I see it as required in a boat.

If it was required in a car, I would expect that it would be part of our vehicle safety inspection. While not required in a car, I would not really want to stand before a judge and say my speedometer cable (I have a really old car) broke three months ago, so I really didn't know I was doing 70 in a 55 (my really old car is a big old beast that rides like a dream). I think that once the Judge stops laughing at me(like this :laugh: ), I would be reaching for my checkbook to pay the fine. Judge would then thank me for making his day.

I think that if any of us get a speeding ticket on the lake, and our defense is that we have no means of telling our speed, the Judge may again thank us for making his day (that being right after letting us know just what the fine would be).

Bottom line is we have a speed limit, like it or not, and some of us will find ourselves in a position of being caught exceeding the limit (just as we may on our roadways), and when we do, we may very well have to pony up the bucks. We may be able to challenge "radar" qualifications, but the lack of a speedometer would not be a defense, at least in my humble opinion.

Regardless, go out and enjoy the lake, and if you see the Upthesaukee as you fly by me, please do give me a wave...I do wave back! Enjoy your summer.

Nothing to worry about
 

the laws have been passed, no doubt about that. But the State gutted the MP budget, which had already been gutted. Let them feel good about what they've done, and don't worry so much about some silly measuring device.

I don;t really care though. Heck, I joined this forum to talk about sailing :emb:

I'd expect there's a limit
 

Yes speedos are required equipment on cars. One of the times this topic came up I even listed the Fed regulation to that effect. There's also an accuracy requirement, I think it was +/- 2%. In any case you might plead to the judge "I didn't think I was going that fast" and if you were only 5-10 mph over, he might believe you. On the other hand if you doing 80+ mph in a decked out 300K$ gofast with twin supercharged engines ... my guess is he's not going to buy the story.

whats next?
 

I know of 4 boaters who have gone up to between 38 and 48 foot cruisers after selling their performance boats.
We are a 2 boat family and always have been. I'm selling my 25 Baja and we are looking to upgrade to a cruiser. We don't need one but if we are going to boat on this lake we are going to be comfortable doing so.
Even in my wife’s 26 foot Chaparral you get banged around on any weekend. My Baja is great but I'm not going to waste my money if I cannot enjoy it to its full potential. So we are looking to buy a boat we probably would not have but because of the new speed limit we will.
So all the boaters complaining about all the big waves, well it is only gonna get worse.
Blame the speed limit. Oh and what about shore line erosion?

Watch out. They'll go after the size of the boats next.......

don't do anything rash.. news to follow. ;)

does not matter
 

It's just much more economical to become a one boat family. The lake isn't so bad now but when it picks up to its original level prior to this recession even in the 25 outlaw there are days when I just wish I was in a wake maker instead of on the receiving end.
I know that at one time those proposing the speed limit were also boasting they were going to go after the big cruisers next but I just do not see that getting any support. Just look at how many big boats there are and many do not even leave their slip.
Guess we will have to wait and see.
Someone said to me recently "don't worry eventually these people pushing the unnecessary laws just die off" Wish Pilliod would.