SailNet Community banner
1 - 14 of 14 Posts

·
Registered
Joined
·
712 Posts
Discussion Starter · #1 ·
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. :laugher I'm pretty sure my knotmemter isn't really accurate. Being as annal as I am sometimes :eek: 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
 

·
Registered
Joined
·
712 Posts
Discussion Starter · #3 · (Edited)
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.
 

·
Registered
Joined
·
1,865 Posts
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.
 

·
Registered
Joined
·
4,726 Posts
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 .
 

·
Registered
Joined
·
712 Posts
Discussion Starter · #6 ·
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
 

·
Registered
Joined
·
360 Posts
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
 

·
Registered
Joined
·
9,376 Posts
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
 

·
Doesn't sail enough
Joined
·
623 Posts
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.
 

·
Old as Dirt!
Joined
·
3,491 Posts
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. :laugher I'm pretty sure my knotmemter isn't really accurate. Being as annal as I am sometimes :eek: 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
David--I realize you have by now already received a number of answers but for the sake of the exercise...

There is no such thing as a "pre-calibrated" knot meter. The dial may have been "calibrated" to the paddle wheel speed at the factory, but the speed of the paddle wheel relative to the speed of the yacht through the water will depend upon the position of the paddle wheel in the hull and the flow of the water across the wheel at that point in service.

On a calm day-i.e. no wind so that there is no wind effect on one's speed-one can use one's GPS to calibrate a knotmeter, despite the presence of a tidal stream. Firstly, one sets one's engine speed at a convenient number-say 2500 RPM-and thereafter leaves the throttle untouched. With that one heads "down stream" for a fixed period of time, e.g. 5 minutes. (To ensure one is actually motoring "down stream", the heading on one's steering compass should match the "COG" read out on one's GPS.) During one's first timed run, one records the GPS Speed over the ground ("SOG") reported at convenient intervals, say every minute in a 5 minute speed run. The "average" of the recorded speeds is then computed (e.g. say 7 knots). One then reverses course, motoring on the reciprocal heading over the same path "up-sream", again for (in this case) 5 minutes, and again recording reported SOG at the same interval and then computes the average, for example, say 3 knots. The average of the up- and down-stream speeds is, of course, the speed of the yacht through the water (in this case 5 knots) and the difference between that and the GPS reported SOG on either course is the "set", or speed, of the current, in this case, 2 knots. With this, one again turns down-stream, motoring on the original heading for a similar time period, recoding the speeds reported on one's knot meter at the same intervals. The average of the reported speeds on the knotmeter (say 4 knots) is then compared with the average of the recorded GPS speeds (in this case 5 knots) and the difference (1 knot) is the amount by which the knotmeter must be, in this example, dialed up to properly calibrate the knotmeter.

The foregoing actually takes longer to describe than it does to perform but the process does work. The alternative, of course, is to use a "chip log", or its modern equivalent, a "Knot Stick" to do the calibration (as suggested by others).

FWIW...
 
  • Like
Reactions: trantor12020

·
Registered
Joined
·
2,256 Posts
In the outer channel where I sail are 2 towers 1 NM apart, called a "measured mile" it's a little known feature of many harbors. It can be a little like "Where's Waldo" because there's no real standard for marking a measured mile and they can be difficult to see on a chart.
 

·
Registered
Joined
·
9,376 Posts
In the outer channel where I sail are 2 towers 1 NM apart, called a "measured mile" it's a little known feature of many harbors. It can be a little like "Where's Waldo" because there's no real standard for marking a measured mile and they can be difficult to see on a chart.
Usually the start and finish beacons are paired similar to range marks. That way you get an exact start and finish to the measured mile.
 
1 - 14 of 14 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top