Measure speed through water without making hole through hull.

[Lars_L]

At the moment I have three holes through the hull under the waterline, propeller shaft, water intake, and water outlet. I do not want one more.

My home harbour is in the inlet to the Baltic (a little bit south of Copenhagen). The current in Öresund is depending on the wetter systems, so there aren’t any table to predict it.

Technically. I have the sender and receiver 0.5 m apart. The speed of the sound is much higher thru the hull than thru water. When I stop sending, I wait for 200 µs. During that time the sound thru the hull have past the receiver. Then I start listening and I will only here the sound that has gone thru the water, and it will last for 100 µs. If I send at 25 kHz, there will be five waves to measure on, and that’s enough.

[RaymondR]

You need a transmitter and two receivers, one forward and one aft of the transmitter, and measure the difference in arrival times or the phase angle variation between the two received signals then all the other factors cancel each other out. I'd be surprised if you could not buy something like that already.

[goboatingnow]

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.

The Devil is the detail. a flute after all is just a pipe with holes , played by putting you fingers over each one in a predefined sequence. That way a symphony can be created …...

[goboatingnow]

Originally Posted by RaymondR

You need a transmitter and two receivers, one forward and one aft of the transmitter, and measure the difference in arrival times or the phase angle variation between the two received signals then all the other factors cancel each other out. I'd be surprised if you could not buy something like that already.

there are many issues to deal with , including multi path distortion, errors due to aeration, hull laminar flow issues, noise floors, discrimination etc

In practice a little wheel is simpler !!

[Lars_L]

RaymondR I was did also have thoughts in that way. An other version of that is to have both a transmitter an receiver in both ends and first do a measure in one direction and then immediately after measure in the other direction. This way I can use transponders from a deep sounder or combined the instrument with the deep sounder.

I’d did also hope that it should already exit something similar, and that someone here on this thread could inform me about that. It’s really a simple instrument.

Many of the sources of errors that I have with a paddle wheel, for example hull laminar flow and aeration, I will still have whit a sound speedometer. But I don’t need another hole in the hull.

[keithasully]

Don't bother with the Knotstick. I bought one and returned it because it looks like a kids toy. Very fragile light spring has warning not to pull through water too fast or it will damage it. I returned it. Need something more substantial for the ocean and cruising. Not sure why guys are recommending it.

[Adelie]

Originally Posted by keithasully

Don't bother with the Knotstick. I bought one and returned it because it looks like a kids toy. Very fragile light spring has warning not to pull through water too fast or it will damage it. I returned it. Need something more substantial for the ocean and cruising. Not sure why guys are recommending it.

You thought k it looked like a kid's toy but you didn't actually try it in any conditions? That's your basis for knocking other' opinions?

To me as an engineer they look like an elegant almost bullet-proof solution.

Also they've been selling for almost 40yr so they must be doing something right.

I'm recommending it because it meets the OP's requirement of no holes. I inferred from the no holes requirement that he might also want something more robust than electronics. So I included it with several other possibilities.

And of course they're going to recommend against towing it too fast, there are morons out here that would use it on a ski boat then wonder why it's broke. At least it doesn't say "Not for use in navigation" like some chart plotters say. Does the documentation for your chart plotter say that?


Adelie

[keithasully]

I knew someone would probably take exception to my comments. I did not use the product. I was very disappointed when the product arrived. To me it appeared too fragile to hold up to the abuse of continual cruising. Also, the spring is stretched length ways over twelve inches (don't really remember exactly how far) to measure the speed and it did not appear it would be very accurate at the beginning of its use and much less hold it shape over a long period of time. This is not a tight spring, but a very light spring stretched almost completely out. It would not be consistently accurate. Just my opinion bro and sorry if you don't agree.
Do you have some vested interest in the product? Make money off of it? Maybe a family member or friend who makes it? I don't mean to be controversial, but I sent the product back because I was disappointed and had no confidence in the product.

[svHyLyte]

Originally Posted by keithasully

Don't bother with the Knotstick. I bought one and returned it because it looks like a kids toy. Very fragile light spring has warning not to pull through water too fast or it will damage it. I returned it. Need something more substantial for the ocean and cruising. Not sure why guys are recommending it.

The "Knotstick" is nothing but a spring scale in a graduated cylinder. Simple but effective. We have had one for several years and, when used as intended, they are quite robust. It would be very unusual for a sailing yacht to reach a speed in excess of what the device can endure and there are two different trailing log fittings included, a smaller one for relatively faster speeds. Of course, debris in the water, weed/kelp, or other, could become caught on the trailing log and over-load the device but that would be unusual.

FWIW...

[dpddj]

You know the length of your boat. Have someone toss a beer can (empty works best) into the water at the bow. Count the seconds until it reaches the aft end. Simple math - length beer can travelled divided by the time. Close enough.
If it's a Mikkeller, be sure to drain every last drop.

[RaymondR]

there are many issues to deal with , including multi path distortion, errors due to aeration, hull laminar flow issues, noise floors, discrimination etc

In practice a little wheel is simpler

I found that the tube worms loved nesting in mine and some of us are parenoid about thru hulls.

[Adelie]

Originally Posted by keithasully

Do you have some vested interest in the product? Make money off of it? Maybe a family member or friend who makes it? I don't mean to be controversial, but I sent the product back because I was disappointed and had no confidence in the product.

Is it normal for you to imply conflict of interest on the part of those that disagree with you or am I a special case?



Adelie

[David M]

You can flush mount it by building a recession into the hull, slightly larger than the size of the transducer. I have a total of 6 transducers on my hull including a Doppler log, all flush mounted.

[Island Time O25]

If the bottom portion of your underwater profile is solid fiberglass you can epoxy the transom transducer on the inside, just need to make sure there are no bubbles in the epoxy. After checking various potential spots I chose the one with the best signal (in the v-berth) and installed this set up on my old boat and it worked fine. I bought the whole set new for about $50 on c-list. At the time they were like $100 in stores

[rwidman]

Originally Posted by Island Time O25

If the bottom portion of your underwater profile is solid fiberglass you can epoxy the transom transducer on the inside, just need to make sure there are no bubbles in the epoxy. After checking various potential spots I chose the one with the best signal (in the v-berth) and installed this set up on my old boat and it worked fine. I bought the whole set new for about $50 on c-list. At the time they were like $100 in stores

How would that measure speed through the water?