Erratic speed log.

[Sneaky Pete]

I have an Airmar transducer connected to a ST60 tri data log. The problem I am having is the transducer does not show any recording on the instrument at the start of the day but once it has been removed for inspection paddle wheel turned then replaced everything works. The speed is a little erratic it does not maintain a steady(ish) speed fluctuating by as much as 1Kn sometimes when compared to the plotter SOG. What can be done to fix the problem?

 

[tillergirl]

I just think it's a little imprecise. Last week, I cleaned the impellor and put it back and was getting zero. I had it out again a day later and really gave it a v good clean including getting some very minor bits of weed from the spindle that didn't seem to be affecting it when I blew on it. Put it back and it worked. Yesterday, it seemed to be under reading by a knot while motoring. Wind came up and we had a good romp, barely touching 4 kts if you believed the log yet I got the usual 'whoomping' from the windward bilge keel which it does exactly at 5.5kts. At the end of the sail, dropped the sails and motored back to the mooring and it seemed to be reading at least a knot extra and back to what I would have expected. Maybe the spindle is getting worn. I believe you can buy a service kit for the spindle and paddle wheel. Perhaps it's time after 8 years.

But there is, of course, if I remember correctly a means of calibrating the instrument described in the manual.

 

[ga1135]

The tide effect will affect the speed log reading, running with tide will get lower reading & against tide will have higher reading. Have you check the GPS SOG data & the tide table ?

 

[Twister_Ken]

Taking out the impeller (from the hull) and giving it a really good clean should be the first step. I use and old, soft toothbrush to clean the impeller, but sometimes there's a bit of chalky growth on one or more of the cups on the paddle wheel which needs to be carefully scraped off. Once it's clean, put it back and see if you still have the problem.

As a general rule, I lift the impeller and replace it with the blank if I'm going to be away from the boat for more than two or three days. They are very vulnerable to fouling, and beasties like baby shrimps seem to like to hide in there.

 

[PCUK]

The tide effect will affect the speed log reading, running with tide will get lower reading & against tide will have higher reading. Have you check the GPS SOG data & the tide table ?

Sorry, that is total nonsense. The tide has no effect on the impeller speed whatsoever.

 

[nigelmercier]

Sorry, that is total nonsense. The tide has no effect on the impeller speed whatsoever.

Of course it does. Take it to the limit: you are anchored where there is a tidal flow, the impeller will read it.

My impeller has been disconnected for 4 years, it is so inaccurate that it is not worth having. When I get round to it I'm going to build a box to convert GPS speed into pulses, so at least the True Wind indicator will work.

 

[Sneaky Pete]

I agree the tide has an influence on the impellor speed +or- depending on the direction of travel. On a similar theme is there an alternative to the analogue measurement for speed (impellor), does anyone make a speed log instrument that takes readings from satellites similar to SOG.

 

[pvb]

As a general rule, I lift the impeller and replace it with the blank if I'm going to be away from the boat for more than two or three days. They are very vulnerable to fouling, and beasties like baby shrimps seem to like to hide in there.

Why not simply antifoul the impellor and its housing?

 

[V1701]

I've given up removing & cleaning the impellor & now just rely on the GPS...

 

[Mrnotming]

I've given up removing & cleaning the impellor & now just rely on the GPS...

You lack a source of information re current favourable or foul by not having the log to compare with the GPS speed indication.Useful always and even in the Med where there are sets or currents.There are French Hydrographic Office (SHOM) publications to show these.Tides I's sure are more predictable, but there is still the differences in springs and neaps.Seeing more information is always assuring in terms of ETA.
We gave our snorkler a wooden clothespin to clean ours last week, and accuracy returned! Sea temp 27C of course!
Raymarine transducer seems to tell this too!

 

[nigelmercier]

You lack a source of information re current favourable or foul by not having the log to compare with the GPS speed indication...

This is only true if you can get the two to agree in a slack tide. I have never managed to get within 1kn over the full range. If you want to know what the tide is doing, throw a small ball of loo roll over the side and time it to pass a known length.

 

[Twister_Ken]

Why not simply antifoul the impellor and its housing?

Now you're inviting a question as to which is the best a/f for impellors, and can you buy it in a 1cc pot! Antifoul away, but make sure you don't clag up the axle. Me, I'll carry on removing mine.

 

[pvb]

Now you're inviting a question as to which is the best a/f for impellors, and can you buy it in a 1cc pot! Antifoul away, but make sure you don't clag up the axle. Me, I'll carry on removing mine.

I do mine a couple of times a year. It only takes a few minutes, and I remove the spinning impellor to paint it separately, so no paint on the axle. It keeps unwanted growth at bay.

 

[Pye_End]

If you want to know what the tide is doing, throw a small ball of loo roll over the side and time it to pass a known length.

Whilst this is true whilst at anchor, how is this useful underway?

 

[Pye_End]

I agree the tide has an influence on the impellor speed +or- depending on the direction of travel. On a similar theme is there an alternative to the analogue measurement for speed (impellor), does anyone make a speed log instrument that takes readings from satellites similar to SOG.

The tide has no influence on the impeller whilst underway. It does however effect the difference between SOG and the log, the difference being the tide and any leeway made. The ST50 log (same impeller) I do not find very good at low speeds, but improves at maybe 2.5kn upwards.

Try cleaning the log by soaking in kettle de-scalent, or remove the impeller by punching out the shaft and cleaning the moving parts. If it is all running free then perhaps look to fouling on the hull around the log, though this seems unlikely in this case.

Remove log whilst not in use to stop build up fouling.

 

[nigelmercier]

Whilst this is true whilst at anchor, how is this useful underway?

Because the effect does not go way as you increase speed.

The tide has no influence on the impeller whilst underway...

Nonsense. The log is always affected by a tidal flow, except one dead on the beam.

 

[Pye_End]

Because the effect does not go way as you increase speed, the log is always affected by a tidal flow, except one on the beam.

Can you give example as I am struggling with this concept.

If you are doing say 4kn through the water with a 1kn tide in your direction, the loo paper will be moving at 4kn in comparison to you. Your log will be showing 4kn, and GPS SOG 5kn. If the tide goes to 2kn, the loo paper will still be moving away from you at 4kn, your log will still be showing 4kn, but your SOG will be 6kn.

Log shows speed through the water, even if that water is moving.

 

[nigelmercier]

Can you give example as I am struggling with this concept...

Take the example you gave and flip it so your SOG is constant.

 

[Pye_End]

Take the example you gave and flip it so your SOG is constant.

Nonsense. The log is always affected by a tidal flow, except one dead on the beam.

The log shows boat speed through the water pure and simple. The only way you would keep SOG constant in a changing tidal pattern would be to deliberately alter boat speed to keep SOG constant.

I am trying hard to understand the point you are making from the brief comments you make - is this the sort of behaviour that you are attempting to describe? I can understand that sailors with no log information might navigate in this way when considering tidal gates, but it is not very efficient.

Presumably you are not suggesting your boat speed through the water somehow increases as the tide picks up?

Of course it does. Take it to the limit: you are anchored where there is a tidal flow, the impeller will read it.

If you were in the Alderney race adrift with a tidal stream of 10kn, your SOG will be 10 but your log and boat speed will be zero. If you were anchored in the Alderney race your SOG will be zero, but your log will be 10kn - being underway is completely different from being anchored.

 

[GrahamM376]

Nonsense. The log is always affected by a tidal flow, except one dead on the beam.

As Pye_End says, the log is not affected by tidal flow when underway, it merely measures the flow of water you are passing through. What does change with tidal influence is speed over the ground as seen on the gps, which can be higher or lower depending on whether you're against, with or, cross tide.