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Old 13 April 2017, 02:44   #1
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Measuring speed without a gps

Is there an easy way to measure speed in a rib without the use of a GPS? Is to allow for estimation of position on a chart
Tsm
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Old 13 April 2017, 03:06   #2
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Measuring speed without a gps

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Originally Posted by two stroke mick View Post
Is there an easy way to measure speed in a rib without the use of a GPS? Is to allow for estimation of position on a chart

Tsm


You can time yourself over a known distance, e.g between 2 buoys. You can compensate for tide if you have tide data to hand, it depends how accurate you want to be. After a while, you get to know your boat. I know that at 4000rpm with my usual load on calm sea I'm doing around 25kn
You could also use a pitot or paddle wheel speedo, but these can very inaccurate.
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Old 13 April 2017, 03:24   #3
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In calmish water, running at a "standard" load, you may find that a given rev will equate to a reasonably accurate speed. e.g. - I "know" that 3000rpm will give me 29-30kts.
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Old 13 April 2017, 03:26   #4
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.....e.g. - I "know" that 3000rpm will give me 29-30kts.


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Old 13 April 2017, 03:27   #5
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Why not use your phone or borrow a car gps.
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Old 13 April 2017, 03:31   #6
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Or for a Real world number....just go out with another Boat which Has!...and test at revs
If it's a real concern (I used a Zodiac SIB for years without one) you can pick up an older used unit,or hand held extremely cheaply.
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Old 13 April 2017, 03:47   #7
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Quote:
Originally Posted by jonp View Post
Why not use your phone or borrow a car gps.
Quote:
Originally Posted by Maximus View Post
Or for a Real world number....just go out with another Boat which Has!...and test at revs
If it's a real concern (I used a Zodiac SIB for years without one) you can pick up an older used unit,or hand held extremely cheaply.
I suspect that TSM isn't short of a GPS - but rather planning a bit of chartwork for educational purposes.

You know, Distance x Time = Fuel Burn or Position...
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Old 13 April 2017, 03:49   #8
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Soz - missed your post above - you must have thought there was an echo
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Old 13 April 2017, 04:22   #9
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Easy if you know the length of your boat, throw something that floats in the water and time how long it takes to get to the stern. You just have to be very good with a stopwatch...
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Old 13 April 2017, 05:14   #10
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Easy if you know the length of your boat, throw something that floats in the water and time how long it takes to get to the stern. You just have to be very good with a stopwatch...


I usually throw the logs straight over the stern, saves getting streaks on the hull
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Old 13 April 2017, 05:51   #11
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Quote:
Originally Posted by willk View Post
I suspect that TSM isn't short of a GPS - but rather planning a bit of chartwork for educational purposes.

You know, Distance x Time = Fuel Burn or Position...
..
I suspect all sorts....but I'm Polaroid
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Old 13 April 2017, 05:52   #12
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Quote:
Originally Posted by Pikey Dave View Post
I usually throw the logs straight over the stern, saves getting streaks on the hull
It's all that Homemade Cake!
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Old 13 April 2017, 07:20   #13
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I have a GPS but it has lost reception from time to time. The revs/ speeds ratio works well for smooth water but off the west coast in a small rib you tend to be on and off the revs a lot.
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Old 13 April 2017, 09:17   #14
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I have a GPS but it has lost reception from time to time. The revs/ speeds ratio works well for smooth water but off the west coast in a small rib you tend to be on and off the revs a lot.
If the speed isn't reasonably constant then there is little other than a GPS that is going to help with dead reckoning
Most dead reckoning theorys are aimed at displacement speed boats so won't work particularly well for high speed ribs
Maybe a second GPS as backup?
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Old 13 April 2017, 09:39   #15
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if gps is losing position then you probably have an issue, it is pretty hard for GPS to lose position on a boat unless severe weather.

i would replace the GPS antenna on your plotter if possible?
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Old 13 April 2017, 09:40   #16
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Assuming you're near the coast a 2 or 3 point fix would give you a very accurate position on the chart
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Old 13 April 2017, 10:13   #17
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Most dead reckoning theorys are aimed at displacement speed boats so won't work particularly well for high speed ribs
How does the theory change or fail when the boat is on the plane? The issue would seem to be that any other speed measurement device is likely to read instantaneous speed rather than an average over a short period. However pitot tube speedo's will give a rough idea, but are notoriously unreliable.

Many years ago before I had GPS on my boat I did some speed testing using a simple anemometer (type used by dinghy sailors - that you align to the wind). It actually worked pretty well (subsequently compared to GPS).

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if gps is losing position then you probably have an issue, it is pretty hard for GPS to lose position on a boat unless severe weather.

i would replace the GPS antenna on your plotter if possible?
It may well be water getting into a joint - mine drops its position if water gets into the external ant socket even although I am just using the internal ant.

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Assuming you're near the coast a 2 or 3 point fix would give you a very accurate position on the chart
It would but the whole reason for D.R. is that it is quicker/simpler than a 2 point fix, and can be done on the run. You can then back it up with a fix if you need it, but that is easier if you have a rough idea where you are (and thus what the things on shore you are looking at are).
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Old 13 April 2017, 10:21   #18
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How does the theory change or fail when the boat is on the plane? The issue would seem to be that any other speed measurement device is likely to read instantaneous speed rather than an average over a short period. However pitot tube speedo's will give a rough idea, but are notoriously unreliable.

Many years ago before I had GPS on my boat I did some speed testing using a simple anemometer (type used by dinghy sailors - that you align to the wind). It actually worked pretty well (subsequently compared to GPS).



It may well be water getting into a joint - mine drops its position if water gets into the external ant socket even although I am just using the internal ant.



It would but the whole reason for D.R. is that it is quicker/simpler than a 2 point fix, and can be done on the run. You can then back it up with a fix if you need it, but that is easier if you have a rough idea where you are (and thus what the things on shore you are looking at are).
The theory doesn't change but constantly changing speeds will make calculations complicated
And the use of logs or the boat length measurement will be quite difficult at planing speeds and if your picking your way through rough water the course is likely to change often too
Just trying to be practical
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