Log calibration

PabloPicasso

PabloPicasso

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I finally connected up my paddle wheel log and it was over reading by roughly 2knots compared with the gps. I reached across tide in both directions and it was interesting to see the huge variance in the differences between both paddle wheel log and GPS.

I calibrated the log down a bit to be close to the GPS but to over read slightly as behooves good navigational practise. But the log is adversely affected by heel, and wave height/sea state, etc,.

Sometimes the log under reads compared to GPS, but mostly overreads a bit. How successful are the panel at calibrating these paddle wheel logs? How do you know?

NB I would use GPS for navigation mainly, but the log wheel is useful for racing through the water speeds. So not really essential that it reads 100% 1knot=1knot
 
Reaching across tide will give water speed higher than GPS SOG in both directions - draw the vectors. You need to measure against and with tide at to average it out. This has to be done at same water speed (throttle setting).
 
We recalibrate ours every so often, as it changes as the hull gets grubbier. If your unit will do it, two or three calibration points at varying speeds in still water will usually get us to within 5-10%. Given we're in Brighton Marina at the moment, which is huge, that gives plenty of time to set it up on the way out.
 
Paddle wheel logs rarely give linear read out compared to GPS. I set mine to read correctly at normal cruising speed. However at half speed it tends to under read, while at maximum speeds it tends to over-read. I triedsetting it at maximum engine speed, but it was even more adrift at low speed! AsOP points out the read out varies according to speed and angle of heel, as the flow over the paddle wheel varies according to the attitude of the boat.

But it has its uses both in sail trim (unaffected by changes in tidal flow) determining tidal flow rate and as a back up.
 
I have a mis-reading Raymarine log.

The manual advises doing the resets at various speeds, for maximum accuracy. At sea, etc in tide free conditions. A hassle.

My idea is, during winter ashore, to place a hose pointing at the paddle wheel and turn the tap on, to represent the various speeds. Having spoken to the Raymarine local dealer he says it won't work.

But I can't see why not? Can the experts here offer an opinion?
 
NPMR said:
I have a mis-reading Raymarine log.

The manual advises doing the resets at various speeds, for maximum accuracy. At sea, etc in tide free conditions. A hassle.

My idea is, during winter ashore, to place a hose pointing at the paddle wheel and turn the tap on, to represent the various speeds. Having spoken to the Raymarine local dealer he says it won't work.

But I can't see why not? Can the experts here offer an opinion?
Click to expand...

It won't work.

As others say, it is not possible to get a consistent reading over a range of speeds, so you just need to bear it in mind. I find that it is best not to expect to set it in one attempt. My practice is to judge what the rough error is and make some adjustment while motoring at around 6kn in still water. After that I will wait and see what the errors are over a range of speeds and make another adjustment so that it is reading +0.1kn at around 6.5 knots sailing.

There are so many factors influencing the result that it is hopeless to try for a perfect solution. My log reads slightly differently when motoring hard, maybe because the stern dips and the bow lifts, and shallow water <5m will slow the boat while making the log over-read. True still water without a current is hard to find - there appear to be currents even in canals such as the Kiel, so a flexible approach is needed.
 
NPMR said:
I have a mis-reading Raymarine log.

The manual advises doing the resets at various speeds, for maximum accuracy. At sea, etc in tide free conditions. A hassle.

My idea is, during winter ashore, to place a hose pointing at the paddle wheel and turn the tap on, to represent the various speeds. Having spoken to the Raymarine local dealer he says it won't work.

But I can't see why not? Can the experts here offer an opinion?
Click to expand...

Because it will not replicate the water flow across the hull which can vary according to hull speed.
 
NPMR said:
I have a mis-reading Raymarine log.

The manual advises doing the resets at various speeds, for maximum accuracy. At sea, etc in tide free conditions. A hassle.
Click to expand...

You don't need to do this in tide free conditions if you have a GPS and average over runs with and against tide (but you need to know tide direction, easiest in a river channel).

NPMR said:
My idea is, during winter ashore, to place a hose pointing at the paddle wheel and turn the tap on, to represent the various speeds. Having spoken to the Raymarine local dealer he says it won't work.

But I can't see why not? Can the experts here offer an opinion?
Click to expand...

In theory you could calculate the water velocity coming out the hose I suppose. You would need to measure how much water fills a bucket, say, in a given time, then calculate for the cross-sectional area of the hose. What was your proposed method?

But as noted above the velocity of water out the hose will still not equate to boat speed due to friction effects on the hull.

BTW, what log do you have? I am sure my Raymarine only allows a single correction.
 
While the speed readout from the log is not terribly important - other than for sail trimming and, as LittleSister says, for embellishment of achievements, is it safe to assume that the distance run is also likely to be in error? That could be more serious for navigation if you have to resort to old-school methods - though admittedly it would require the failure of all GPS-enabled devices on board before an EP would be your sole source of fix.
 
lampshuk said:
While the speed readout from the log is not terribly important - other than for sail trimming and, as LittleSister says, for embellishment of achievements, is it safe to assume that the distance run is also likely to be in error? That could be more serious for navigation if you have to resort to old-school methods - though admittedly it would require the failure of all GPS-enabled devices on board before an EP would be your sole source of fix.
Click to expand...

We calibrate our log so that it's reasonably accurate at 5.5knots.
This means it's not badly out at 4.5 to 7knots.
So if we play at old school nav and work out an EP, the distance run is generally not the biggest source of error.
If there's no wind and you can't hold 4.5 knots, then your course is generally all over the shop too, likewise if you're surfing at 8 knots half the time, so in these conditions you want big 'error bars' on your EP anyway.
Sea state tends to affect the accuracy of distance run, so even if the log is spot on at 6knots motoring, it won't be beating into a F4 at 6 knots.
In the English Channel, the tides will make a mockery of worrying about a few % of log error.
So each hour, you might just as well say 'we've been averaging about 5.5 knots, we've done 5.5NM.'
The old Walker towed logs were instrinsically much more accurate, but back in those days we still wanted other position information, dead reckoning was mostly about getting in sight of a series of lighthouses and aids like RDF were used a lot. Decca was nearly as good as GPS when it worked, but you had to do more 'proper nav' as a reality check than you do now.
 
Yes, those are good points.

In fact, I did get an old Walker log and streamed it on a cross-channel trip over the weekend, intending to compare the paddle wheel and the Walker. I forgot to bring the log book back with me, though, so can't comment at the moment.

And yes, it's probably going to be second-guessing what the autopilot thought he was steering that is going to be the main source of error.
 
NPMR said:
I have a mis-reading Raymarine log.

The manual advises doing the resets at various speeds, for maximum accuracy. At sea, etc in tide free conditions. A hassle.

My idea is, during winter ashore, to place a hose pointing at the paddle wheel and turn the tap on, to represent the various speeds. Having spoken to the Raymarine local dealer he says it won't work.

But I can't see why not? Can the experts here offer an opinion?
Click to expand...

The hose will spin the paddle wheel and it will read a speed on the log .... but I don't understand how this helps as you have no idea what the actual speed of the water coming out of the hose is and calculating it would require a Cray Supercomputer and, even then, it would be wrong as there are too many flow variables. :(

(If you don't have access to a Cray then a ZX81 might do the trick ;))

Richard
 
It feels like you are all overcomplicating my idea.

The Raymarine 'book' calls for calibration at various speeds.

I know that as an overall reading it is about 10% slow consistently.

So, I turn the tap until I get 1kt showing on the readout, adjust it to 1.1kts. Same for the other speeds. No need for heel, sideways slip or anything. Controlled simple speed. Adjusted as necessary!
 
I calibrate mine by sailing to Flushing, then going up the Walcheran canal, every year or so year. (rarely miss, except when doing the Cally canal) There is about .1kts of tide in the canal, so I do it both ways. I calibrate it to read correct at 5 kts & the log over reads .1 kts at 6kts & under reads .2 kts at 3 kts.
This is under power in flat water. Then I go & play in the Veersmeer to see what that does, but rarely adjust it anymore.
Once set it does not seem to need changing year to year by more than the tiniest amount.
 
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