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post #11 of 28 Old 1 Week Ago
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Re: GPS determined speed

Maybe on a straight road. but as others have said, GPS just knows where you were and where you are, not how you got there. Anyhow, for adjusting sails, the absolute speed doesn't matter; all you need to know is whether pulling in the sheet about a foot makes you go faster or slower; the absolute reading is irrelevant
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Re: GPS determined speed

I'm not following how one trims by boat speed. Seems SOG and STW are both after effects of proper trim. I find them both important to know, for different reasons. 0.1kt is close enough.


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Re: GPS determined speed

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Originally Posted by Minnewaska View Post
I'm not following how one trims by boat speed. Seems SOG and STW are both after effects of proper trim. I find them both important to know, for different reasons. 0.1kt is close enough.
no it's not. My B&G not only show changes in speed to .01 precision the show the TREND... speeding up, slowing down or steady. If you move the sheet an inch or two it MAY show as a speed trend... if often DOES. Same for steering.. change heading a degree and you can see a change in speed with .01 precision.

In the big picture it doesn't matter... but you CAN find the groove.

pay attention... someone's life depends on it
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Re: GPS determined speed

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Originally Posted by SanderO View Post
no it's not. My B&G not only show changes in speed to .01 precision the show the TREND... speeding up, slowing down or steady. If you move the sheet an inch or two it MAY show as a speed trend... if often DOES. Same for steering.. change heading a degree and you can see a change in speed with .01 precision.

In the big picture it doesn't matter... but you CAN find the groove.
Here are sailing in the grove close hauled in 9.6 knots true, just under 15 heel with .5 knots of fair current... STW is 6.72 and SOG is 7.2

Appears to be heading NW north of Shelter Island toward Greenport.
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pay attention... someone's life depends on it

Last edited by SanderO; 1 Week Ago at 01:25 PM.
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Re: GPS determined speed

Not sailing but relevant.

Recently decided to switch to an apple watch vs Garmin I had before. I'm a runner. Yea, gotta have the best gadgets? Apple as a running watch is horribly inaccurate compared to the Garmin which is incredibly accurate as compared to official race course lengths and official times. I'm about to abandon Apple and go back to Garmin.

I assume this is related to how often they sample GPS coordinates, how they interpolate between them, etc. This also probably is impacting power consumption that's particularly tricky in a watch.

So it would not surprise me if there is variation between vendors, software releases, etc. I don't think GPS implementations are all created equal.
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Re: GPS determined speed

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.....I don't think GPS implementations are all created equal.
Can be very different.
The older (5ish yrs) garmin plotter at my helm will bounce wildly in many conditions and so much so that i really dont rely on it for planning. ...3.5 to 7 = pretty useless. Not always so drastic, but not good
While the tablet running navionics is very stable.
I havent been using the newish garmin swingout plotter...and should check it.
Mx mariner app gives .3kn changes.
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Re: GPS determined speed

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Originally Posted by SanderO View Post
no it's not. My B&G not only show changes in speed to .01 precision the show the TREND... speeding up, slowing down or steady. If you move the sheet an inch or two it MAY show as a speed trend... if often DOES. Same for steering.. change heading a degree and you can see a change in speed with .01 precision.

In the big picture it doesn't matter... but you CAN find the groove.
You sure you're not a racer?

A one-hundredth of a knot change in boat speed could as easily be an oncoming wave or slight change in wind speed or direction. I certainly understand trimming for optimal speed, but one trims for wind and wind speed and boat speed is the result. Too many variables to dial in a speed that fine.


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Re: GPS determined speed

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Originally Posted by Minnewaska View Post
You sure you're not a racer?

A one-hundredth of a knot change in boat speed could as easily be an oncoming wave or slight change in wind speed or direction. I certainly understand trimming for optimal speed, but one trims for wind and wind speed and boat speed is the result. Too many variables to dial in a speed that fine.
No I don't race. I do at times in light breezes try to play with the "controls" to get the boat to go as fast as it can. By the time it's 10 knots true boat speed is not as sensitive to tweaking sail trim. At least that my experience. This is really fiddling around in the boat speed range of 4+ knots. And sure... wakes and waves do have an impact.... so the tweaking is mostly observable when those factors are not present.

think of all the controls available...
genny car position
halyard tension
head sail trim
main sheet
traveler position
vang
outhaul
leech lines
backstay tension

pay attention... someone's life depends on it

Last edited by SanderO; 1 Week Ago at 08:11 PM.
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Re: GPS determined speed

My newest Simrad GPS determines speed by satellite doppler shift, so it is all done in one measurement. It sometimes does not match my older Garmin GPS. I figure it is the more accurate measurement.

https://www.quora.com/How-does-GPS-c...d-while-moving

Bill

Last edited by wsmurdoch; 1 Week Ago at 09:55 PM.
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Re: GPS determined speed

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Originally Posted by SloopJonB View Post
I recently heard that automobile speedometers are changing over to GPS from physical sensors on the car.

I suspect that means GPS is pretty accurate.
The GPS is very accurate. But once we have current in play, as well as leeway, GPS is not terribly reflective of speed through the water. The computer on my bike now is just GPS instead of a physical speed sensor as well. It's fine unless there are a lot of tall buildings around in which case it can be off 20% easily.
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