What is more accurate for speed, navionics or nasa log?

[Crinan12]

My navionics speed registers roughly a knot more than nasa speedo. Say 4 knots on log would be 5 or even 5.5 with navionics.
What would be more likely to be correct?
Navionics fluctuates a bit more whereas the log is consistent
Thanks

 

[Deleted member 478]

Are you taking any currents into account?

 

[Crinan12]

No but it is pretty much always like that, even when say just motoring somewhere with no tide.

 

[Deleted member 478]

Okay, where is each instrument getting its data from? A log or gps or ..... ?

 

[Crinan12]

Okay, where is each instrument getting its data from? A log or gps or ..... ?

Sorry yeah
Nasa is a paddle wheel, navionics a tablet with gps/glonass


Thanks

 

[Deleted member 478]

IMHO, the Nasa needs calibration. There should be instructions in the manual

 

[ash2020]

NASA logs are notoriously unreliable. Mine works for 2 weeks after going in the water and then doesn't do anything for the rest of the season.

 

[greeny]

GPS speed over ground. Paddle wheel speed through the water. In no current scenario they should be the same as previously quoted. Re calibrate the NASA either to the GPS or do a run and return over a shore based measured distance. If there is one nearby to use. I usually set mine up to the GPS which is pretty accurate.

 

[Tranona]

They are measuring different things. They will only be the same if there is no current. Using measured distance only works if no current or 2 runs in opposite direction and averaged to take out any effect of current or wind.

Speed through water is now of limited value except to measure change in boat speed, for example after changing sail trim or engine RPM.

 

[Crinan12]

Yeah i get the speed over ground /speed through water thing
But even say in the canal navionics shows faster
Wondered if one was more likely to be correct
I can try my phone as well and see what it says

Thanks

 

[Buck Turgidson]

The gps speed is more accurate, your log needs calibration. Even after that it Will be different on either tack and possibly on different points of sail. It’s only measuring the local speed of water normal to the paddle wheel.

 

[alahol2]

The GPS derived speed will be the more accurate for speed over the ground. Calibrate your NASA log to match the GPS when you are in the canal.

 

[Pete54]

It depends on where the paddle log is. Inevitably water has to take a somewhat longer route around a hull, than the hull straight through the water. Hence the need to calibrate the paddle log to compensate for the slightly higher speed of the water around the hull.

 

[vyv_cox]

Speed through water is now of limited value except to measure change in boat speed, for example after changing sail trim or engine RPM.

With 'clever' navigation systems the log speed is used to calculate and display optimum sailing tack, effect of current, etc. Our B&G Triton displays all sorts of information that uses log speed, in conjunction with true and apparent wind speed and direction.

 

[MontyMariner]

Apart from trying to compare the equivalent of chips with mash, what state is your paddle wheel and surrounding hull in? Any growth, paddle wheel misalignment, spindle wear will change it's output.

 

[Dellquay13]

Speed through water is now of limited value except to measure change in boat speed, for example after changing sail trim or engine RPM.

For me, no matter how accurate my very basic nasa target log is, as long as it is consistent, my STW is my most important guide to how well I am sailing and handling sail trim etc, my gps SOG is so dependent on external factors like Pembrokeshire tides that are beyond my control that I hardly ever bother about it, until I am about to miss a crucial access lock to the marina.

 

[johnalison]

They are measuring different things. They will only be the same if there is no current. Using measured distance only works if no current or 2 runs in opposite direction and averaged to take out any effect of current or wind.

Speed through water is now of limited value except to measure change in boat speed, for example after changing sail trim or engine RPM.

Speed through the water has immense value, at least to me. In terms of navigation over distances, naturally the ground speed takes priority and frees one from the need for dead reckoning. I use water speed for:
Getting True wind speed and angle
As my most important reference when close-hauled, almost more so than the wind angle
As a guide to how well my engine/prop are performing. There have been several occasions in open water when I have noticed my speed drop about a third of a knot due to weed fouling. This would not have been noticeable from the GPS and no vibration was felt through the saildrive, and a short burst astern cleared the fouling, allowing me to continue at full speed, important over a leg of perhaps 100 miles.

 

[Chiara’s slave]

For me, no matter how accurate my very basic nasa target log is, as long as it is consistent, my STW is my most important guide to how well I am sailing and handling sail trim etc, my gps SOG is so dependent on external factors like Pembrokeshire tides that are beyond my control that I hardly ever bother about it, until I am about to miss a crucial access lock to the marina.

We’re very much seat of the pants, rather than log. The log is something to raise a glass to when it shows a good number, but it seems far too flaky to trim a sail by. Telltales, jib, main leech, and a set of lightweight ones on the shrouds, are our biggest sailing asset. Nav wise, obviously COG and SOG rule the roost, plus a bit of human and instrument assisted judgement on how to maximise them. I’d barely miss the log. Do not miss the log if it has ingested something from the river, though I do get in the water and give it a clearout when it needs it, for forms sake.

 

[Dellquay13]

Maybe I described it wrong, my log is my most important of my 3 instruments, log, basic plotter and depth. The sails, telltales and the feel of the tiller are more immediate references but the log tells me when I am doing well, or more usually not well, and to look to the sail trim some more.
Being relatively new to sailing from motorboats, the log is more apparent that something isn’t quite right and needs addressing, until sail trim hopefully becomes more intuitive.

TBH it all feels a bit academic these days, my boat was only afloat for 2 days this season with brand new sails and standing rigging before it was lifted back into the yard 10 weeks ago to have a leaky stern tube sorted, with no action from the repairers in ages…
My apologies to all for moaning

 

[Bilgediver]

Navionics gives very accurate speed over the ground. Your log gives speed through the water and less accurate, Depends what you want.