Matata
Well-Known Member
Can I simply calibrate the log using a gps, say over 5 miles, with no tide running or am I missing a trick? Nik
Calibrating ships log.....
- Thread starter Matata
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VicS
Well-Known Member
Yes if there really is no tide or current.
Unless you are doing it because you want an accurate indication of speed through the water the why bother ?
Unless you are doing it because you want an accurate indication of speed through the water the why bother ?
Last edited:
Norman_E
Well-Known Member
If there is genuinely no tide and no current I really don't see why not, though the instructions with my own log say no. You would have to keep speed constant. At very least it would give you a rough starting point to calibrate the log.
It might be better, and not much more difficult, to make timed runs between waypoints in both directions as instructed in most log installation instructions.
One important issue is to do your calibration at your normal cruising speed, because paddle wheel logs are prone to read a bit slow at low speeds, and read higher at the top end.
It might be better, and not much more difficult, to make timed runs between waypoints in both directions as instructed in most log installation instructions.
One important issue is to do your calibration at your normal cruising speed, because paddle wheel logs are prone to read a bit slow at low speeds, and read higher at the top end.
jwilson
Well-Known Member
I see nothing wrong with picking near slack water and light wind, doing a steady motoring 6 knot GPS run in one direction for a quarter of a mile, then reverse course and go back to start point, then repeat the first leg to see if it's still near-as-dammit 6 knots. If so the mean of 6 and your GPS reading from the other direction run is what to set your log to show whilst turned back and doing leg one again, all with the engine revs untouched.
john_morris_uk
Well-Known Member
The short answer is yes you can, but I would be interested to know how you are so confident that there is no tidal stream or current?
lw395
Well-Known Member
I think a good way is to make a variety of estimates of the best cal factor over the season, based on various GPS measurements.
Don't just correct it one week to the latest best guess and throw away the old information.
You may find it generally reads a little different on one tack or at fast or slow speeds.
It will vary according to how clean the hull is, even if you clean the impeller.
It may vary with sea state.
Or angle of heel.
If you want to get into serious dead reckoning, it is handy not only to have a well calibrated log, but also an idea of how far out it might be under different conditions.
As an exercise, you could try working out the cal factor from any timed run for which you have decent tidal data. do you get a consistent answer? *
Alternatively, just set it to max sensitivity, so that small changes in boat speed are most easily seen for tuning purposes and use the GPS
(* On my boat the answer was usually 'no' or +/- 5% on a good day)
Don't just correct it one week to the latest best guess and throw away the old information.
You may find it generally reads a little different on one tack or at fast or slow speeds.
It will vary according to how clean the hull is, even if you clean the impeller.
It may vary with sea state.
Or angle of heel.
If you want to get into serious dead reckoning, it is handy not only to have a well calibrated log, but also an idea of how far out it might be under different conditions.
As an exercise, you could try working out the cal factor from any timed run for which you have decent tidal data. do you get a consistent answer? *
Alternatively, just set it to max sensitivity, so that small changes in boat speed are most easily seen for tuning purposes and use the GPS
(* On my boat the answer was usually 'no' or +/- 5% on a good day)
gerry99
Well-Known Member
If you really wanted to do it then there is a measured mile in southampton water (if you are near there) and I am sure others around the UK. I have done it for the fun of it more than anything else
nigelmercier
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I've tried to calibrate my Raymarine log, but it is still rubbish: if I get it right at 6kn, it is way out at 2kn etc. As a result, the transducer hasn't been fitted in two years.
The only reason why I would want it is to get the true wind direction, tides and currents one can deduce from the track. I've therefore been considering making a device that will get the speed from the GPS and generate a SeaTalk signal. Any thoughts on this?
The only reason why I would want it is to get the true wind direction, tides and currents one can deduce from the track. I've therefore been considering making a device that will get the speed from the GPS and generate a SeaTalk signal. Any thoughts on this?
Blue5
Well-Known Member
Glad I am not the only one. I find it useful for tides etc but get frustrated with kit when it does work properly.
Seajet
...
There is a measured mile in the Solent West from Gilgicker Point.
however I agree that in-hull paddlewheel logs are useless, I can never understand why so many people tolerate the things with the required hole in the hull; when I had a boat with one we were forever withdrawing the log to remove weed or wee beasties and it made no contribution whatsoever towards navigation.
Conversely I can't understand why decent economically priced trail logs aren't generally available, I still have my Wasp which has always done well, and it's a good feeling to have a Plan B rather than be totally reliant on someone elses' satellites.
however I agree that in-hull paddlewheel logs are useless, I can never understand why so many people tolerate the things with the required hole in the hull; when I had a boat with one we were forever withdrawing the log to remove weed or wee beasties and it made no contribution whatsoever towards navigation.
Conversely I can't understand why decent economically priced trail logs aren't generally available, I still have my Wasp which has always done well, and it's a good feeling to have a Plan B rather than be totally reliant on someone elses' satellites.
Matata
Well-Known Member
I go everywhere at 5 knts 6 on a good day so i'd calibrate it at that speed. Just curious really ! I do like ep and dr and adds to the interest on the way. Also I think its good to know as it is another tool. I find GPS very unrewarding. Just sailed down SE the Irish sea 54 kmiles and by doing tidal flows course to steer lee way etc ended up 5kmiles from where we intended 13 hrs. Do you think thats good going ? Just wondering? Should it have been more accutate? Nik
david_bagshaw
Well-Known Member
Also have sufficient depth while doing calibration runs. Until 3 x draft is reached our log is inacurate. likewise canal calibration is difficult from the side shallows effect
lw395
Well-Known Member
I don't think they are useless.
They give a useful speed indication.
When racing, it is very useful, the log will show you a 0.1 knot or less change in speed long before you notice it.
You can get ultrasonic in-hull logs. They still need calibrating to some extent, because the water next to the boat moves with it to some extent.
BAtoo
Well-Known Member
Top of the tide & run in opposite directions for 5 minutes should be good enough; no need to do 5 miles.
If its Raymarine & linked to the GPS, IIRR, you can "tell" the log to accept the GPS speed as the true speed by being in the right part of the calibration program.
Always used to do mine in the Veerse Meere when there as definitely no tide.
If its Raymarine & linked to the GPS, IIRR, you can "tell" the log to accept the GPS speed as the true speed by being in the right part of the calibration program.
Always used to do mine in the Veerse Meere when there as definitely no tide.
Porthandbuoy
Well-Known Member
I'm with SeaJet on this. Through hull paddlewheel logs are a complete waste of time as far as an aid to navigation is concerned. Forever stopping, or to be a little pedantic, failing to start. Reads differerent from one tack to the other despite tweaking the angle of attack and electronic calibration.
Okay, if you're a racer it might just tell you if your going an nth of a knot faster than 10 minutes ago.
I'm seriously considering plugging my hole up this winter.
as I've got a trailing log for navigation purposes.
Okay, if you're a racer it might just tell you if your going an nth of a knot faster than 10 minutes ago.
I'm seriously considering plugging my hole up this winter.
as I've got a trailing log for navigation purposes.
ghostlymoron
Well-Known Member
I think the paddle wheel log is useful but, like any device, should not be relied on 100%. I calibrated mine by doing back and forth between two GPS waypoints at slack tide. 1 - 1.5 nm should be fine.
Seajet
...
The ultrasonic logs of the 1970's died out long ago, but I think there is a firm - forget which - with a twin sensor system, presumably working on doppler rate.
It will still be in the boundary layer of the flow next to the hull, and still subject to position errors and tack to tack differences unless one has twin sensors each side of the boat / keel.
Hull mounted logs may give a clue as to tuning in racing for short distances - if they work at the desired moment - but have no place on the vast majority of boats they're fitted to, it seems to me just a desire to 'have a speedo like in a car' !
lw395
Well-Known Member
Airmar do an ultrasonic sensor for about £650!
http://www.jgtech.com/pdf/CS4500Data.pdf
Many top end race boats will have two, one for port tack, one for starboard.
You have to have accurate boat speed info if it is to be worthwhile calculating true wind or VMG.
On my last boat we gave up trying to calibrate the true wind system and decided the G in B&G stood for 'guesswork'.
http://www.jgtech.com/pdf/CS4500Data.pdf
Many top end race boats will have two, one for port tack, one for starboard.
You have to have accurate boat speed info if it is to be worthwhile calculating true wind or VMG.
On my last boat we gave up trying to calibrate the true wind system and decided the G in B&G stood for 'guesswork'.
onesea
Well-Known Member
OK New boat has through hull speed log, I calibrated it by:
Looking at a passing buoy realising there was no current (there was little wind or I would of been sailing and not caring).
Steamed at constant speed (passage speed), then calibrating to GPS speed (smoothed over short period for accuracy LOL).
You could calibrate over 5 miles 1 mile etc but then is the log that accurate?
If it was trailing I would expect more accurate but through hull... If using it for DR how accurate where your tide calc's and the tables themselves? Not worth wasting time on.
Then again when working a compass course on small boat I round it to nearest 5 degrees. I do not feel many can steer more accurate on small boat at sea, if your compass is that accurate? etc
Although we should know HOW to be accurate, I think its more important to know how in-precise this can be...
Which reminds me you have GPS? do not bother with finding a measured mile, your GPS will be accurate enough over a longer distance.
Steam at decided speed with no current and straight run. Hit MOB on GPS and start your timer, maintain straight course for required time/ distance.
Use Formula:
Speed = Distance/Time
With the accuracy of GPS 10m over 1000m that's only 1 or 2% good enough for me.
Also if you decide to swing your compass use known position light house? and GPS over 1000m what is 20m in position error going to make? less than 1 degree?
Looking at a passing buoy realising there was no current (there was little wind or I would of been sailing and not caring).
Steamed at constant speed (passage speed), then calibrating to GPS speed (smoothed over short period for accuracy LOL).
You could calibrate over 5 miles 1 mile etc but then is the log that accurate?
If it was trailing I would expect more accurate but through hull... If using it for DR how accurate where your tide calc's and the tables themselves? Not worth wasting time on.
Then again when working a compass course on small boat I round it to nearest 5 degrees. I do not feel many can steer more accurate on small boat at sea, if your compass is that accurate? etc
Although we should know HOW to be accurate, I think its more important to know how in-precise this can be...
Which reminds me you have GPS? do not bother with finding a measured mile, your GPS will be accurate enough over a longer distance.
Steam at decided speed with no current and straight run. Hit MOB on GPS and start your timer, maintain straight course for required time/ distance.
Use Formula:
Speed = Distance/Time
With the accuracy of GPS 10m over 1000m that's only 1 or 2% good enough for me.
Also if you decide to swing your compass use known position light house? and GPS over 1000m what is 20m in position error going to make? less than 1 degree?