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Old 08-09-2014
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Knotmeter Calibration

Hi All,

For those of us who only sail in tidal waters... I installed a new set of Signet Marine anolog (old school appearance) "sailing instruments". They were supposed to be already calibrated. I'm pretty sure my knotmemter isn't really accurate. Being as annal as I am sometimes I'd like to get that instrument to read as accurate as possible. How does one do that in a tidal area, especially one that changes direction 4 times a day?

I know... "why do you need that when you have GPS"? Short answer... because 30 years ago when I cut my teeth sailing on SF Bay... there was no GPS and all we had were the basic 3 sailing instruments, got used to them and I like looking at them. The boat came with 3 holes to fill, they look cool, yada-yada-yada.

Dave
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Old 08-09-2014
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Re: Knotmeter Calibration

Pick a slack time period and make runs in opposite directions comparing GPS speed to log speed.
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Old 08-09-2014
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Re: Knotmeter Calibration

Quote:
Originally Posted by sailak View Post
Pick a slack time period and make runs in opposite directions comparing GPS speed to log speed.
Theoretical idea is good... I've concluded this gauge will NEVER be accurate. I think I'll just watch the GPS.

SailAK.. looked at your pics... you guys are hard core! It's in my bucket list to sail from Puget Sound to AK, just don't think the wife will ever go for it.
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Last edited by Dave_E; 08-09-2014 at 05:40 PM.
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Old 08-09-2014
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Re: Knotmeter Calibration

Hey Dave I think that no matter how "calibrated" the crazy things are depending on where the little paddle wheel sits impacts the flow etc....but I don't know...here's what I do.

We got a mess of tide here in Vineyard and Nantucket Sounds...so yea, I look at the GPS, kinda figure how much tide there is, and then set it, adding or subtracting a bit. Then when I don't like what speed it says, I mess with it again. After doing this a few trips out, it gets "close."

With 2kts of tide here, speed over ground and speed over the water surface are only the same for about 20 mins every 6 hrs. After it's set, it's nice when you are somewhere unfamiliar to be able to discern if you have a favorable or adverse current...so I don't think it's too nerdy to mess with it until it's kinda right.
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Old 08-10-2014
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Re: Knotmeter Calibration

You could throw a measured line over the side with a chunk of wood on it. Time it until it stops running out. Retrive it . Do it again. Average it. Sit down with a pencil and paper, calculate speed thru water. Calibrate the fancy. May vary from tack to tack beacause of flow past hull. I think that difference between GPS cog and compass reading more informative .
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Re: Knotmeter Calibration

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Originally Posted by Capt Len View Post
You could throw a measured line over the side with a chunk of wood on it. Time it until it stops running out. Retrive it . Do it again. Average it. Sit down with a pencil and paper, calculate speed thru water. Calibrate the fancy. May vary from tack to tack beacause of flow past hull. I think that difference between GPS cog and compass reading more informative .
Capt Len...

I think you get the gold star on this one. Back on SF Bay with the bare boat Catalina 27 (and I do mean bare), I had bought myself a "knot stick". They are still available and I just ordered one. Like many others, I keep wanting to interject the GPS into the equation. If I'm thinking correctly here, I should be able to tweak the instrument to what the knotstick says at any time regardless of tidal flow... it is what it is... through the water speed, at that moment, with or against any current.

Thanks for the idea! Dave
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Old 08-10-2014
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Re: Knotmeter Calibration

Nice find Dave, that looks like a useful product for a couple of sailing buddies to share to calibrate their electronic knot meters.
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Re: Knotmeter Calibration

GREETINGS EARTHLINGS , Slack water and known transits (distance ) time your run over the distance do the maths and calibrate accordingly (On the River Mersey it's 34' range 7 1/2 knot run) Look up Liverpool UK things can go wrong so fast you have to do it right first time . It make a compitant seaman out of us. AS ALWAYS GO SAVE
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Old 08-10-2014
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Re: Knotmeter Calibration

You can also time how long some debris takes to go from bow to stern and apply the formula

L X .6 / T= S

L = boat length in feet

T = time in seconds

S = speed in knots
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Old 08-10-2014
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Re: Knotmeter Calibration

My old-school Sumlog knotmeter wavered too much to really be able to calibrate. I put it down to old-school goodness.

Also, it read 1.5 knots high - also a good thing on my old boat, it needed all the help it could get.
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