Current speed and log

Roberto

Roberto

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Lorient/Paris
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The speedometers of quite a few boats I sailed, including mine, showed a puzzling behaviour.

With a moving boat, it indicates a generally correct speed from say 0.5 to 7 knots or above.

OTOH when moored to a buoy, no wind, the boat perfectly aligned with currents of >2 knots, all I get is a meagre 0.1kt at most /forums/images/graemlins/confused.gif

For example last week at springs in the Odet river there must have been around 4knots of current, the speedometer just read 0.0 or 0.1, so I reckon it is not only related to slow speeds.


(Of course, talking speed through water, transducer correctly located, not fouled, boat lying exactly bow against current, etc etc)

any explanations ?

/forums/images/graemlins/confused.gif
 
Strange! How about an experiment: next time you're on a buoy or anchored and in a strongish current check you're getting this low reading then start the engine and just take up the slack on the mooring or anchor and see if the speed is any different.

I can't see why there should be any difference.

You could also check the current with a Dutchman's log to make sure it's really as strong as you think it is.
 
I wonder if the movement / vibration in the hull induced by the act of sailing or motoring, could reduce the friction ("stiction") in the sensor. In calm flow, with nothing to help the little prop go round, it might therefore underread.

Perhaps test it by applying something like an electric drill body to the area near the sensor ?
 
If the transducer is forwards, it might be in the turbulence from the mooring-chain and not reading fully. Putting a shear on the helm might move the boat across enough to change the reading.
 
That's why I never bothered to get mine working again. SOG is more useful if you are trying to hit a tidal gate. Logs are just old technology with all that that entails in terms of unreliability.
 
I think Sarabande has it...
At speed the energy in the flow would turn the impeller, in slow currents it will catch, and have to overcome static friction (stiction) to get the impeller to turn. Sometimes the flow will be too weak to do anything more than occasionally move it, hence the substantilally reduced reading. A perfect transducer wouldn't do that, but we probably wouldn't pay the price.
 
I don't think I agree. My experience is that the first thing a paddle-wheel does when sticky is fail to read low speeds, and usually kicks in at about 0.3. If Roberto's log is reading 0.1 or 0.2 it sounds as if it is in good working order.
 
How can it be good reading 1/10th of the flow he believes to be true ? Something is preventing it getting up to 1 or 2 knots... either the design the maintenance, or the installation.
 
It is not constant, but I too have noted your observation.

On the pontoon at waterford we were travelling at 2kts, at anchor after going wide and hitting the sandbank at Shell Island in North Wales I had 3kts speed as the tide flooded and we waited to get off and follow the correct route into the harbour.

Yet on my mooring, we have enough current that needs often full throttle on the dinghy to get alongside, nothing, nada, the log does not read at all.

Even if I clean and spray ptfe, so it is not down to clogging.

My thoughts are the log must need to be exact with the current, at anchor we swung dead on, but my mooring is a trot, so might be just off. Though, we always seem to get all or nothing, I would have thought, just off current we would have a low reading.. also, the boat swings back on forth on the trot and the log never (so as I have seen) springs to life, I can't believe we are never in-line with the flow.
 
Yes that is what I thought, either 0.00 or full speed, but maybe sarabande explanation goes in the right direction: I compared SOG and log readings while approaching a pontoon exactly parallel to the current, perfect ferrygliding /forums/images/graemlins/smile.gif

I had SOG going little by little to zero, say 0.2-0.3kt while approaching the pontoon, while speed on water was at 2-2.5kt.
This until I was a few metres away from the pontoon.

Then I had to moor the boat and could not read the instruments any more... you guess it, once mooring ropes had been made fast, I went back to the displays and they both showed 0.0 (sog and over water) /forums/images/graemlins/shocked.gif

Incidentally, I tried to measure things a bit more carefully with my boat, but with at least 3-4 other boats I chartered in places like the Sound of Chausey, Little Russel channel, or gulf of Morbihan no one of them actually ever indicated any current while stopped /forums/images/graemlins/frown.gif
 
Maybe the vibration of sailing/motoring along continually breaks the stiction so that the full speed is recorded, but the static calm nature of the flow while moored fails to do that?
Does it get worse with older units where trhe bearings are knackered?
 
Hmmm. I have seen this behaviour with speedboat type transom mounted speedometers which work on water pressure but my experience with a paddlewheel log is that it either works from 1/2 Knot or less or doesn't work at all 'cos its weeded up.

Have you got a speedometer or a log?
 
Maybe difference is turbulence under boat. When motoring / sailing the boat has motion that causes some turbulence. When sitting at mooring the boat moves less and water flow is more smooth ?
We have noted that when testing engine alongside the log can show speed due to the water flow under the hull feeding / exiting the prop.
 
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