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magictime

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21 minutes ago, nicknorman said:

There are two ways to measure gps speed. One involves measuring the distance between consecutive fixes and dividing by the time. This doesn’t work well!

 

The better way is to use the satellite frequency offset. The satellites operate on a very high frequency and an extremely low bandwidth. So precise tuning is essential and Doppler shift caused by satellite velocity relative to the receiver has to be taken into account. The satellite’s known precise transmission frequency has to be tweaked / offset to allow for the Doppler shift. Satellite relative velocities are known from the ephemeris data and so the receiver velocity can be deduced with great accuracy, without needing successive fixes.

 

If you look at the spec for GPS receivers they often describe not only maximum velocity limits, but also maximum g (acceleration) limits. These relate to the maximum frequency offset and the maximum rate at which the offset can change (limited by filtering).

I just click the page up and it tells me how fast I am going, but on a slightly different tack (starboard)  when the car speedometer indicates I am doing 70 mph, Tomtom suggest I am doing 67 - 68 MPH

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6 minutes ago, ditchcrawler said:

I just click the page up and it tells me how fast I am going, but on a slightly different tack (starboard)  when the car speedometer indicates I am doing 70 mph, Tomtom suggest I am doing 67 - 68 MPH

Makes sense.  Most car speedos are deliberately miscalibrated by the manufacturers so you can't blame them if you get done for speeding.

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9 hours ago, magictime said:

I don't have any other guide other than my own fallible sense of how my speed relates to 'walking speed'.

 

If you know how long your bote is you can time how long it takes to pass a fixed point (ideally something casting a shadow, but not a bridge 'ole) and calculate the speed from that,

 

Eg: 48ft bote, takes 10 seconds, speed is 4.8 ft/sec or about 3.25mph. If you can't convert the units in your head, the speed limit is a smidge less than 6ft/sec. You could even do it in metric if you wanted. 

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32 minutes ago, Machpoint005 said:

 

If you know how long your bote is you can time how long it takes to pass a fixed point (ideally something casting a shadow, but not a bridge 'ole) and calculate the speed from that,

 

Eg: 48ft bote, takes 10 seconds, speed is 4.8 ft/sec or about 3.25mph. If you can't convert the units in your head, the speed limit is a smidge less than 6ft/sec. You could even do it in metric if you wanted. 

How do you measure the length of your boat, I think this may have been asked before

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12 hours ago, TheBiscuits said:

Makes sense.  Most car speedos are deliberately miscalibrated by the manufacturers so you can't blame them if you get done for speeding.

Is that still true? These days they use a hub sensor shared with other functions and seem far more accurate. Whenever I check my speed viewing the digital option on my 'Driver Display Module' aka speedo against roadside speed check cameras they agree to within 1mph.

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Just now, Slim said:

Is that still true? These days they use a hub sensor shared with other functions and seem far more accurate. Whenever I check my speed viewing the digital option on my 'Driver Display Module' aka speedo against roadside speed check cameras they agree to within 1mph.

Just depends on the manufacturer. In my experience Japanese cars tend to be nearly 10% optimistic. VW group cars about 4% and my old Saab was spot on. As you say, wheel speed sensors are accurate. But that is wheel rotational speed, not vehicle speed. The difference being tyre diameter and any deliberate error a manufacturer builds in..

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2 minutes ago, nicknorman said:

Just depends on the manufacturer. In my experience Japanese cars tend to be nearly 10% optimistic. VW group cars about 4% and my old Saab was spot on. As you say, wheel speed sensors are accurate. But that is wheel rotational speed, not vehicle speed. The difference being tyre diameter and any deliberate error a manufacturer builds in..

My Merc is getting on for 10% 'high' compared to both my GPS and those 'speed flashing' things in villages.

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We followed a boat down Farmer's Bridge and Aston Locks the other day, they were on the water point at the top when they saw us so set off as the locks were full. We kept pace with then but a gap opened up when I visited the weed hatch. As we approach Salford Junction we had closed up again. We followed for some time and the steerer kept checking his map, I kept a good few boat lengths away but as soon as he came to a straight section pulled over and waved me through.

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36 minutes ago, Alan de Enfield said:

My Merc is getting on for 10% 'high' compared to both my GPS and those 'speed flashing' things in villages.

 

My last Merc used to be about 10% high too, but this current one is almost spot on so when (if) it tells me I am speeding, I really am! There is only a tiny error margin built in.

 

That is assuming the GPS app in my phone is accurate, which I suspect it is, but have no way of knowing other than by all the speeding tickets not dropping through my door....

 

 

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1 hour ago, nicknorman said:

In my experience Japanese cars tend to be nearly 10% optimistic. VW group cars about 4% and my old Saab was spot on.

 

That tallies with my experience too - although the Japanese example was a British-built Nissan, some 30 years ago (probably the most boring car I've ever had).

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A few weeks back I was caught up by a boat along the straight at Bournville while heading toward Birmingham. We got separated a bit by having to take turns to pass a couple of boats going the other way either side of the bridge preceding the first of the railway bridge narrows. So rather than stopping to let the boat behind past I headed into that bridge and slowed to my customary crawl through confined channels meaning the boat behind closed right up on me. I accelerated away down the straight and when alongside the winding hole I waved the boat behind through only to get the response “There’s no point now”. Inevitably then, through the (old) A38 bridge and the second railway bridge he was having to knock out of gear and still almost ran up my rear fender.

 

Afer I emerged from the narrows I kicked back the prop to clear the dirt and of course he carried on down my inside (I wasn’t so bloody minded that I didn’t leave room) shouting “I don’t mean to be funny but I’ve been trying to pass you for an hour and you keep slowing down and then speeding up again when I catch up”. I tried to explain what happens when a deep-ish and square-ish boat goes through a confined channel and why he should have taken the opportunity to go past. I hope that got through for future reference because at his cruising speed he must catch up a lot of boats. His girlfriend was very apologetic and somewhat embarrassed. Maybe she had experienced similar before.

 

Canal Plan gives a cruising time of 10 minutes from where he first caught me up to where I waved him past but there’s no such thing as a moral victory so it would have been better for everyone’s peace of mind had I stopped ahead of the first railway bridge. Such episodes don’t do anything for your own enjoyment of the day no matter the rights and wrongs of those involved.

 

JP

 

 

Edited by Captain Pegg
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9 minutes ago, Machpoint005 said:

 

That tallies with my experience too - although the Japanese example was a British-built Nissan, some 30 years ago (probably the most boring car I've ever had).

Bluebird built in Sunderland?

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On 08/07/2019 at 09:28, Neil2 said:

Yes I think it's worth repeating that, because most folk do not have GPS or use an app, (I suspect because it conflicts with the "get away from it all" nature of canal boating) they think they are doing 4mph when it reality it's probably 3mph or less.  That's why when someone comes up doing nearer to 4mph they react with horror.  

 

Tunnels create the same illusion.  In my experience most folk go much too slowly through tunnels because it feels like you are going much faster than you are.   

It happens because there's a common assumption that walking pace is 4mph.  To maintain 4mph while walking is almost speed-walking.  A typical good brisk walking pace is about 3mph, maybe a touch over.  On the canals people typically stroll rather than walk and will be doing about 2-2.5mph.  So a boater who is overtaking walkers assumes they must be doing at least 4mph, when in fact that speed is virtually unobtainable in a narrowboat on many canals.

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15 minutes ago, Jerra said:

For a moment I was thinking no it wasn't then I realised you weren't thinking of the Bluebird.   Not that it would have been any use on a canal.

:) No, not the Coniston one.  I used to maintain a small fleet of courier vans and quite frequently the Nissan plant in Sunderland would run out of a certain colour of paint and production of a particular model would be halted. They would send an SOS to Bergers, now Ault Wieborg in Chadwell Heath East London. One of the vans useually  a 35cwt Transit would shoot round there and loat 3 or 4 40 gal drums of the paint and bomb up to Sunderland with it, sometimes to the plant and sometimes to their storage unit in Birtley just south of Newcastle.  The runs were always overnight, I did one once fror extra money.

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35 minutes ago, bizzard said:

Very reliable and popular with mini-cab drivers too.

My dad had such a Bluebird for donkey's years. After he died my brother in law had it for many more years. It ended up getting stolen and was found on it's arse in a ditch. Sad end to a superb (though boring) car. Had a new radiator and alternator in 30+ years and other than routine stuff that was it.

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1 hour ago, doratheexplorer said:

they must be doing at least 4mph, when in fact that speed is virtually unobtainable in a narrowboat on many canals.

I have been accused of speeding in an empty boat, towing another, on a canal.  "Hey, the limit is four mph".  That was just before Blisworth tunnel, going north.  True, I had been in the pub.

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3 hours ago, nicknorman said:

Just depends on the manufacturer. In my experience Japanese cars tend to be nearly 10% optimistic. VW group cars about 4% and my old Saab was spot on. As you say, wheel speed sensors are accurate. But that is wheel rotational speed, not vehicle speed. The difference being tyre diameter and any deliberate error a manufacturer builds in..

 

It also varies with tyre wear. On new tyres, the speedometer on my car reads about 3mph slow, and when worn to 2mm offered, the speedometer reads 5mph slow.

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Just now, cuthound said:

 

It also varies with tyre wear. On new tyres, the speedometer on my car reads about 3mph slow, and when worn to 2mm offered, the speedometer reads 5mph slow.

Umm... I think that should read ‘fast’. Speedos generally over-estimate your speed. It helps the mpg figures too. 

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Just now, WotEver said:

Umm... I think that should read ‘fast’. Speedos generally over-estimate your speed. It helps the mpg figures too. 

I think he's right. For a given road speed the bigger wheel will be turning slower. 

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