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sunbear1 · 19-05-2015, 13:10

Years ago I read an article about how to install/make a permanently mounted tube knotmeter (It has a bead that moves up and down as the speed changes) and thought I had kept a copy--but can't find. Does anyone have such a beast or know dimensions?? Davis used to make a hand held one. Wanted to try. Thanks.

PDA1 · 25-09-2015, 18:18

Quote:

Originally Posted by sunbear1

Years ago I read an article about how to install/make a permanently mounted tube knotmeter (It has a bead that moves up and down as the speed changes) and thought I had kept a copy--but can't find. Does anyone have such a beast or know dimensions?? Davis used to make a hand held one. Wanted to try. Thanks.

I have a wind speed meter like you mentioned made by Dwyer-

But, I'm searching for the same thing- an analog device to measure boat speed. Since the Dwyer wind meter works by a difference in air pressure (Bernoulli's principle) I assume the tube knot meter you're referring to would work the same way.

There's the "knotstick" but is too expensive- something around $60.

I made a chip log and have used it quite a bit. It's reliable but since I have to "heave the log" and don't have the line on a spool it gets tangled upon retrieval.

Counting the seconds it takes for, say, a "bubble" in the water to pass from the bow to the stern is easiest. Here's how to do it-

Boat Speed

...for a 30 foot boat.

Determine boat length in nautical miles

30 feet divided by6,076 feet (nm in feet)= .00493' boat length in NM

Multiply Boat length x seconds in 1 hour= .00493 x 3600= 17.748 Boat Value

Speed= Boat Value divided by seconds for stuff to pass from bow to stern (figures below are rounded).

1sec 17.8 kn

2sec 8.9 kn

3sec 5.9 kn

4sec 4.4 kn

5 sec 3.6 kn

6 sec 3.0 kn

7 sec 2.5 kn

8 sec 2.2 kn

The results are easy to memorize or just write them down.

Let me know if you find anything....and I'll do the same.

mousenik · 25-09-2015, 19:25

wow-thats actually some good info, thxs--hopefully someone will find some old school drawing of the knotmeter--I seem to remember seeing a design that had a tube thru the hull and some way to read pressure generated by the different speeds.

PDA1 · 26-09-2015, 04:27

A Pitometer, which is what I think we have in mind is mentioned in Dutton's book; Navigation and nautical astronomy.

Here's more about it- https://en.wikipedia.org/wiki/Pitometer_log

Except for the mercury I think it would work just as well with oil or glycerin (stuff that wont evaporate easily).

A Pitot tube is probably the correct device to use;

The Photo-Pitometer

rognvald · 05-10-2015, 09:11

Sunbear,
Why do you need a knotmeter if you have a GPS?

Jman · 05-10-2015, 09:21

Quote:

Originally Posted by rognvald

Sunbear,

Why do you need a knotmeter if you have a GPS?

Because you care about the difference speed over the ground and speed through the water?

Sent from my iPhone using Cruisers Sailing Forum

**newhaul** · 05-10-2015, 09:42

Just spend the $50 bucks and be done with it will cost more to make your own unit.
Knotstick Sailboat Speedometers

rognvald · 05-10-2015, 10:10

Quote:

Originally Posted by Jman

Because you care about the difference speed over the ground and speed through the water?

Sent from my iPhone using Cruisers Sailing Forum

Jman,
You can determine your exact speed over water from your hourly fixes by your GPS. Speed devices are notoriously innacurate and can be influenced by current, tides and leeway. When my speed log crashes, it will not be replaced. It's only a rough guess of your actual speed. And, it has a reputation as an excellent weed and barnacle catcher. However, speed logs aside, do you race that beautiful yawl in your avatar?

TeddyDiver · 05-10-2015, 10:18

Quote:

Originally Posted by rognvald

Jman,
You can determine your exact speed over water from your hourly fixes by your GPS. Speed devices are notoriously innacurate and can be influenced by current, tides and leeway. When my speed log crashes, it will not be replaced. It's only a rough guess of your actual speed. And, it has a reputation as an excellent weed and barnacle catcher. However, speed logs aside, do you race that beautiful yawl in your avatar?

It's your GPS frelled by current, tides and leeway. Thou sailing in lakes (great or small) there's not tides..

Jman · 05-10-2015, 10:21

Quote:

Originally Posted by rognvald

Jman,

You can determine your exact speed over water from your hourly fixes by your GPS. Speed devices are notoriously innacurate and can be influenced by current, tides and leeway. When my speed log crashes, it will not be replaced. It's only a rough guess of your actual speed. And, it has a reputation as an excellent weed and barnacle catcher. However, speed logs aside, do you race that beautiful yawl in your avatar?

It is correct you gps gives speed over the water (or over ground depending on how you want refer to is). It does not give speed through the water. As you say it is affected by current as one example. But knowing the difference between the two, along with heading vs. course over ground allows you to calculate that current.

If that is important to the OP I don't know.

To your question, yes, although only one or two regattas a year.

Sent from my iPhone using Cruisers Sailing Forum

rognvald · 05-10-2015, 11:10

Quote:

Originally Posted by TeddyDiver

It's your GPS frelled by current, tides and leeway. Thou sailing in lakes (great or small) there's not tides..

Teddy,
Not quite true. The Great Lakes do have wind-driven currents, tidal differences along the coasts due to direction and strength of the wind and we certainly have induced leeway when sailing abeam on lumpy days in a shoal draft vessel. However, we lived aboard for ten years in South Florida and the Caribbean and are very familiar with these important factors although not, perhaps, as large as a factor as is in Alaska. Many interesting sites are offered by NOAA for Great Lakes sailors that record wind, current, temperature, etc. One of them is:Great Lakes Currents Map Good luck and good sailing. P.S. We'd love to sail your beautiful waters one day.

PDA1 · 05-10-2015, 12:04

I need an analog speed device because;

1- I want to use a device that doesn't require batteries.

2- The sailing I do now, with a small boat, doesn't need or require a GPS.

3- It's easier, and more enjoyable, for me to use a chart and plot positions using a compass or by measuring horizontal sextant angles.

The easiest speed determination method I've round so far is timing an object floating from bow to stern.

rognvald · 05-10-2015, 12:28

Quote:

Originally Posted by PDA1

I need an analog speed device because;

1- I want to use a device that doesn't require batteries.

2- The sailing I do now, with a small boat, doesn't need or require a GPS.

3- It's easier, and more enjoyable, for me to use a chart and plot positions using a compass or by measuring horizontal sextant angles.

The easiest speed determination method I've round so far is timing an object floating from bow to stern.

PDA1
The knotlog Newhaul recommended is an excellent choice. We sailed with that exact device for several thousand miles on our first boat before we bought a GPS. The only problem with that device is that the trailing line can become snagged with debris and may break. However, it never happened to us. Good luck and good sailing.

PDA1 · 05-10-2015, 14:15

Quote:

Originally Posted by rognvald

PDA1
The knotlog Newhaul recommended is an excellent choice. We sailed with that exact device for several thousand miles on our first boat before we bought a GPS. The only problem with that device is that the trailing line can become snagged with debris and may break. However, it never happened to us. Good luck and good sailing.

I'd be easy to make that gadget.