Measuring speed without a gps

[two stroke mick]

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

[Pikey Dave]

Originally Posted by two stroke mick

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.

[willk]

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.

[Pikey Dave]

Originally Posted by willk

.....e.g. - I "know" that 3000rpm will give me 29-30kts.

Show off[emoji13]

[jonp]

Why not use your phone or borrow a car gps.

[Maximus]

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.

[willk]

Originally Posted by jonp

Why not use your phone or borrow a car gps.

Originally Posted by Maximus

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...

[willk]

Originally Posted by Pikey Dave

Show off[emoji13]

Soz - missed your post above - you must have thought there was an echo

[Davie]

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...

[Pikey Dave]

Originally Posted by Davie

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[emoji6]

[Maximus]

Originally Posted by willk

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

[Maximus]

Originally Posted by Pikey Dave

I usually throw the logs straight over the stern, saves getting streaks on the hull[emoji6]

It's all that Homemade Cake!

[two stroke mick]

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.

[beamishken]

Originally Posted by two stroke mick

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?

[Xk59D]

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?

[Tim M]

Assuming you're near the coast a 2 or 3 point fix would give you a very accurate position on the chart

[Poly]

Originally Posted by beamishken

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).

Originally Posted by Xk59D

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.

Originally Posted by Tim M

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).

[beamishken]

Originally Posted by Poly

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 [emoji106]