Measure speed through water without making hole through hull.

[Lars_L]

I want to measure the boots speed through water. But I don’t want to make any hole through the hull. Fore me a hole in the hull below waterline is a to great potential risk. Do there exists any such instrument on the market? I have an idée on how to built one, but do they exist then its unnecessary to invent it. Why, and the benefits of STW instead of SOG, are already subject to another ongoing discussion. A Doppler log can be mounted inside the hull but that measures a speed over ground.

[tcpip95]

GPS does it just fine for me.

[Wotname]

Well there is the older method say using a Walker Trailing Log. Measure the distance through water and time elapsed to determine speed but I'm thinking you want a more modern solution

[Adelie]

20 yr ago the club I am in had a Bernoulli tube that measured speed.

Currently you can buy a Knotstick for about $60, it drags a weighted line thru the water attached to a calibrated spring. You read speed by how much the spring is moved.

Or you can get transom mount paddle wheels for an electronic speedo.


Adelie

[44'cruisingcat]

You could transom mount a fishfinder + speed transducer on a bracket. That would save having a hole for a depth sounder too.

[Hydra]

Lars,
A Doppler log measures the speed over ground if the water is shallow enough to get a reflection on the sea bottom. This is probably the case in the Baltic sea where I suppose you are cruising.

In others area, a Doppler log measures the speed with respect to a water layer. Some systems even measure STW in shallow water: http://www.alphatronmarine.com/pdf/1_JLN-550.pdf

There are other, simpler and cheaper, solutions:
- a Dutch log (throw a piece of wood a the bow and measure how long it takes to arrive at the stern)
- a ship log (a piece of wood at the end of a line with regularly spaced knots)
- a patent log...

Alain

[rwidman]

There are plenty of transom mound speed sensors available. Normally these are mounted with screws into the transom so technically these would be holes in the hull although above the waterline.


I suppose you could use glue or double sided tape or a board clamped to the transom.

[svHyLyte]

Buy yourself a Knotstick. They work very well and are quite accurate. We use one to calibrate our thru-hull paddle-wheel knotmeter.

FWIW...

[roverhi]

Why are you so concerned with a through hull speed log?? The through hull is some form of nylon, very tough and not susceptible to corrosion. The speed transducer itself is pretty much solid so limited chance of leaking and the plug, if you use it, has virtual zero chance. These are not like metal through hulls with a seacock or other type of valve attached to a hose that have many points of possible failure especially with age.

Went with a Raymarine paddle wheel transducer through hull that has worked excellently.

GPS does give speed through the water but speed over ground. Does nothing to measure how efficiently you are actually sailing. Using a GPS, you could be doing 8 knots in some places and not have any means of propulsion because of current. For figuring out maximum efficiency sailing, a through hull knot meter is the best way to go.

[Lars_L]

Of course do I want an NMEA output, and I feel that with Knotstick I will have much work to make such interface.
I have mounted my deep sounder inside the hull. It looses some performance (max. 100 m deep), but it works good enough.
My idée is that the sound speed thru water is known, and if the water moves the speed of sound will change in the same way as the speed of the water. Since I can mount my deep sounder (that both sends and receives sound) inside the hull, it should be possible to mount a sender and receiver a bit apart (0.5 m) inside the hull. With these it should be possible to measure sound speed thru the water.

[svHyLyte]

Originally Posted by Lars_L

Of course do I want an NMEA output, and I feel that with Knotstick I will have much work to make such interface.
I have mounted my deep sounder inside the hull. It looses some performance (max. 100 m deep), but it works good enough.
My idée is that the sound speed thru water is known, and if the water moves the speed of sound will change in the same way as the speed of the water. Since I can mount my deep sounder (that both sends and receives sound) inside the hull, it should be possible to mount a sender and receiver a bit apart (0.5 m) inside the hull. With these it should be possible to measure sound speed thru the water.

B&G came out with a solid state combined speed/depth transducer such as you describe in 1987. The device mounted on the outside skin of the boat with small penetrations for the mounting bolts/wiring. (Google "Solid State Water Speed Transducer") Trying to reproduce that within a hull would be problematic/complicated by the speed of sound through the hull material itself although once could, in theory, apply a compensating algorithm. It certainly seems like a lot of effort for such a modest payoff. Why not simply mount a standard transducer in a fish-shaped block mounted on the end of a tube that could be extended into the water over the transom when needed/wanted?

[S/V Alchemy]

There are transom-mounted transducers for this purpose; there is GPS; there are Walker Knotmeters and similar semi-antique devices; there's the 60D/ST method of recording a transit between two fixed points on shore; and there's the venerable hourglass, chip board and knotted string method used on ships 300 years ago.

[boatman61]

If you are in a non-tidal situation your GPS SOG will be near enuf the same as SOW.. if currents enter the equation then you'll need a tide chart, tables and (maybe) a calculator.. then its SOG - current force and deduct for direction if required should give you SOW
I sailed from Gurnsey just after LW heading for the UK.. it was Springs and the GPS gave my SOG at 13kts.. however as the current up the Race runs at up to 7kts.. a quick glance at the chart and port info on tides I calculated SOW to be just under 7kts..
Knot accurate.. but nothing is 100%.

[ramblinrod]

Why be concerned about a thruhull?


Most 30+ foot boats have many.


A thru hull is not to be feared.


Transom mount speed sensors generally only work on power boats with a plumb transom to the waterline. Power boats have no choice, the transom is the only part of the boat in the water underway.


For a sailboat, especially with overhangs, go with a through hull speed transducer, and don’t worry about one of many thru hulls. Note that there may be accuracy variation, depending on speed and tack. Just like a compass deviation card, if you require high accuracy, you may wish to develop a knotmeter deviation card, based on tack, heel angle, and speed.


Rod Brandon





Certified Raymarine Installer and Service Provider

[ramblinrod]

Non contact Doppler flow sensor. There are such devices for measuring flow velocity (and calculated flowrate) through enclosed pipes. They work with varying success and are application dependant.




If I am reading between the lines correctly and you are considering "inventing" such a device for use as a sailboat knotmeter, be aware that the application variables will likely defeat you.







A pipe is a stationary object of fixed dimension and almost all other parameters but flow velocity.





In contrast a sailboat hull is subject to all kinds of variation: flexure, heel angle, roll, pitch, yaw, wave height, period, and angle, flotsam, entrained air, etc., etc., etc.