Log underreading ? - suggestions

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westernsailor

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Had a fabulous days sailing yesterday.

15kts plus of favourable wind, sun shining - most of the time, low seas.

Sailed from Kilrush Marina to Loop Head and back in 4.5 hours. Raymarine series 600 tridata log indicated we travelled 25 miles. Paper charts over 32 miles. This suggests a problem.

I had log out of its casing two weeks ago when boat was being boat weighed, it looked fine. It also appears to have reseated properly. I had a suspicion that it was under reading all along.

Garmin hand held GPS indicated we were regularly doing 7/8 knots and even up to 10 for short periods. Log on boat never got above 6 kts. I may not be the a great sailor, bit a 41 footer under full sail I know I can more than six knots.

32 miles in 4.5 hours suggests an average speed of 7.12 kts.


Suggestions.
I had log out of its casing two weeks ago to get the boat weighed, it looked fine. it also appears to have reseated properly. I had a suppission that it was underreading all along.

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Sometimes limescale buildup from the water can provide extra friction to a log paddlewheel. I've found kettle de-scaler helpful.

Might the paddlewheel have been damaged during the liftout? (bent the paddlewheel axel)

Also, (don't know your area), but were you travelling with the tide for a goodly part of your trip. Out with the tide and back with it? This will boost your ground distance/speed compared to paddlewheel based measurements

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adjust it. dont nkow your particuklar setup, but most modern logs allow you to adjust them.

but take care about tides. tide tables are point measurements and not that accurate over a distance, but the direction of flow is a bit better. so do some decent cross tide runs and record the distance run. ignore moment to moment speed readings.

then simply adjust by a percentage up or down according to how up or down the distances are.

PS make sure your paddle wheel isnt all clagged up first. tiny little barnacles can wreak havoc. and make sure the paddle wheel is correctly aligned fore and aft.

<hr width=100% size=1>this post is a personal opinion, and you should not base your actions on it.
 
Find two fixed points an easily measurable distance apart. Transits are good for this, or pierheads or similar; buoys at a pinch and if you use buoys measure the distance between them by GPS because they may not be exactly where charted. Ideally the two points should be uptide/downtide and about a mile or so apart. On a quiet day and at a time when you can expect the tide to be pretty well constant for a bit (middle two hours of tide cycle), motor between them in one direction steering as accurately as poss, turn very tightly and motor back the same way. Set the log to zero when you pass point A and read it when you arrive back at point A. If the log is correct it will have recorded 2x the charted (or GPS) distance, because tidal flow will have cancelled itself out. If it hasn't adjust the log up or down as required and repeat the exercise until hunkydoryosity is achieved.

BTW, A mate had a similar sort of problem using Raymarine ST60 kit. It transpired that the log/speedo would work accurately at low speeds but not at higher speeds because of the way waterflow changed as speed built up. He soldiered on until the end of the season then resited the impellors during the winter.

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Calibrating the log.

I agree it is very useful to do this, but be aware that this will only enable you to calibrate the log exactly at one speed. You cannot assume the log will then be accurate at other speeds: most will not be.

It's worth drawing up the equivalent of a deviation curve for your log.

However, calibration at various speeds is easier said than done. The comparison can be made against measured distances or with the GPS. But you have to find somewhere you can do a good straight run at steady speed, and where the effects of tide, current, or even wind-driven surface drift can be avoided.

(i) If you visit Holland some of the the larger slow-running canals that have distance markers along the bank are very convenient for this.

(ii) There are measured miles along the coast here and there which can be used, but only near to slack tide on a quiet day. The average of a there-and-back run is required, it is absolutely essential that you maintain the exact course which is prescribed for the measured mile.

(iii) A straight-line course between two buoys or similar at approximately slack tide, comparing log speed with GPS. Again go there-and-back and take the average.

P.S. If you cannot hold exact steady speed then it will be better to use distance as the basis of the calibration.

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In water logs vary in their readings for all sorts of reasons :-
alignment of impeller - very easy to alter when you pull it out and put it back.
protusion from hull - maybe this can alter with each insertion
weed fouling
bearing friction as indicated.
angle of heel
fore and aft trim
fouling forward of the impeller
Frankly its a wonder they work at all or for a reasonable period even.
Just like the unlamented RDF they are not very good.
By all means adjust the calibration - but threat it with suspicion and keep checking - remember even the GPS goes on the blink sometimes.

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Doubt your log was that innacurate.We did the same trip with a very similar result
and have our log calibrated accurately in flat water /non tidal conditions.
It is all down to a strong ebb tide running yesterday afternoon up to approx 15.15.You turned around for the return trip pretty much at low water and had flood tide home,difference of log to chart is all tide.
PS your Raymarine log can be calibrated .
Samphire.

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Had the same problem end of last season. One of the paddles was broken off. Dried out at Herne Bay. Swindlery said "Oh no cannot sell you just the paddle wheel" So off with it, drilled hole in remaining stub and fitted hardwood plug. Then rigged up a mold and filled with epoxy. When dry filed it to shape. Bingo, cost ziltz
Thinks if I file them all smaller will I sail faster?
Briani

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I had this problem and it took me a long time to discover that it was a build up of slim and fine growth on the hull around the transducer which was causing it to under read. I removed the log and cleaned the hull in this area with a scrubber tied to a rope up to the deck on each side. Had a person on each side deck pulling it back and forth. This corrected the problem.

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I never believe logs. I have an old mechanical log, a 1 year old Raymarine tridata and a Garmin 128 fixed GPS. They never agree on anything and when checking against distanced travelled on chart ( making all necessary corrections for tidal flows etc) I do not believe, IMHO, that logs are accurate to much better than + or - 20%.

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My log consistently underreads - the difference is usually about 1 knot inside a locked marina (not very much tide!). The other giveaway is that either the North Sea is slowly emptying into ipswich or the log is wrong, as the difference between the log and GPS on the River Orwell is about 3 knots on the flood and only about 1 knot on the Ebb, when heading up the river.

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There will almost certainly be a calibration parameter in the log instrument - you'll probably need the manual to figure out how to use it.

Find a measured mile - they're marked on the chart. The two I know of in my sailing area are on the west side of Southampton Water north of Fawley oil terminal and south of Swanage between St Alban's Head and Anvil Point.

Sail and/or Motor each way along the measured (Nautical Mile) making a note of the start and end trip meter reading on your log instrument for each transit. Take the average of the two readings to remove the effect of any tide and you will see how much over/under your log is reading. You can then (using the directions in your log manual) calculate the new calibration factor and enter it into the instrument. TIP (from bitter experience) - once you have entered the new calibration factor turn off the power to your log and turn it on again - we couldn't understand why the change appeared to have no effect to begin with. My unit (Silva Sea Data) doesn't appear to update the in use calibration factor unless the power is cycled. Once you've entered the new calibration factor, do a further two runs to validate it is now reading accurately. Mine was 5% out and is now spot on.

If you want to get really serious, you can do a series of runs at different speeds and set up a log deviation table...


Good luck!

Ed

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I sail out of Kilrush also and without checking yesterday's tides and assuming your log is reasonably OK, my guess is that you went out on the ebb and back on the flood. In my experience, a typical tide in the estuary can give you 3 knts+ roughly from outside Scattery to west of Kilcredaun Lt. Our speed record so far is about 10.5 knts (on a 29' boat)!

BTW - last year I thought we found an eddy running counter to the ebbing tide, when we moved close under the Clare shore to avoid the nasty race outside. It seemed we picked this up from maybe 2-3 miles W of Kilcredaun, right up to the Lt. Will look into this some more this year.

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