Transducers Conundrum

[Dockhead]

My boat was built with 3 transducer holes. Filled with:


1. Depth
2. Speed
3. Forward looking sonar.


I've been struggling hard with speed, that bane of anyone who wants valid true wind data, and I have a whole box of speed transducers. Since none of the ones I have any hope in, will fit the installed Airmar housing, I think I'm going to acquire yet one more and obtain a Signet Blue Top one like the racers use.


So that's one hole.


I acquired a B150M CHIRP sonar transducer, which plugs into the sonar port of one of the Zeuses, which is made for a 2" hole, but without a housing. It's all bronze and looks like it would need to be Sikaflexed in permanently. I think about putting that into the second hole in place of the regular depth sensor. It would give sonar viewable in the plotters, and the plotters will translate that data to simple depth for the other instruments. Maybe.


What about the FLS? It doesn't work all that well. And the transducer projects down causing drag, and, I'm afraid, creates turbulence which interferes with the speed log (could that be why I have such bad luck with STW data?).



So maybe better to get rid of the whole FLS system, and put the sonar there? I'll have to sleeve the 2 1/2" hole, but in the grand scheme maybe that's ok.


Then I'll have a spare hole for a normal depth transducer.


Or I could go radical and drill a fourth hole and put the new speed log there. Then I'd have a spare Airmar-sized hole for whatever might come up later.


Thoughts?

[Shrew]

I wouldn't bother. Most if not all speed transducers are giving you Speed Through Water (STW). Run with a current or tide (let's say 2 knots) and your STW is 2 knots slower than Speed over Ground (SoG). Run against a current or tide (same 2 knots) and now your SoG is 2 knots slower than your STW. Either way it makes arrival time estimates increasingly inaccurate over lengthening periods of time.

I just use the SoG from my GPS which is more accurate, overall. Just my .02.

[rslifkin]

Originally Posted by Shrew

I wouldn't bother. Most if not all speed transducers are giving you Speed Through Water (STW). Run with a current or tide (let's say 2 knots) and your STW is 2 knots slower than Speed over Ground (SoG). Run against a current or tide (same 2 knots) and now your SoG is 2 knots slower than your STW. Either way it makes arrival time estimates increasingly inaccurate over lengthening periods of time.

I just use the SoG from my GPS which is more accurate, overall. Just my .02.

On a powerboat, I agree. But for sailing, speed through water is useful.

[Shrew]

Originally Posted by rslifkin

On a powerboat, I agree. But for sailing, speed through water is useful.

It's been my understanding that STW is useful for collision avoidance, rather than navigation. I can see how this might be useful in racing.

I'd like to take this as a learning opportunity. What am I overlooking?

[rslifkin]

Originally Posted by Shrew

It's been my understanding that STW is useful for collision avoidance, rather than navigation. I can see how this might be useful in racing.

I'd like to take this as a learning opportunity. What am I overlooking?

It's for sailing performance. SOG is for getting where you want to go, but STW will tell you if you're actually sailing slower for some reason or if you just have a foul current. Basically STW tells you how fast you're sailing, SOG tells you how fast you're getting where you're going.

[dlj]

Originally Posted by Dockhead

My boat was built with 3 transducer holes. Filled with:


1. Depth
2. Speed
3. Forward looking sonar.


I've been struggling hard with speed, that bane of anyone who wants valid true wind data, and I have a whole box of speed transducers. Since none of the ones I have any hope in, will fit the installed Airmar housing, I think I'm going to acquire yet one more and obtain a Signet Blue Top one like the racers use.


So that's one hole.


I acquired a B150M CHIRP sonar transducer, which plugs into the sonar port of one of the Zeuses, which is made for a 2" hole, but without a housing. It's all bronze and looks like it would need to be Sikaflexed in permanently. I think about putting that into the second hole in place of the regular depth sensor. It would give sonar viewable in the plotters, and the plotters will translate that data to simple depth for the other instruments. Maybe.


What about the FLS? It doesn't work all that well. And the transducer projects down causing drag, and, I'm afraid, creates turbulence which interferes with the speed log (could that be why I have such bad luck with STW data?).



So maybe better to get rid of the whole FLS system, and put the sonar there? I'll have to sleeve the 2 1/2" hole, but in the grand scheme maybe that's ok.


Then I'll have a spare hole for a normal depth transducer.


Or I could go radical and drill a fourth hole and put the new speed log there. Then I'd have a spare Airmar-sized hole for whatever might come up later.


Thoughts?

I took the forward looking sonar off my boat as it didn't work worth a darn. The sensor on the hull looked like a tumorous growth and I didn't want the drag for what it didn't provide... I fiberglassed the hole as I'm a fan of having the fewest holes in my boat.

Depending where it was, could be it interfered.

It's easy enough to drill a hole in the future if you decide you want something else. Then you can make it the right size also.

dj

[motion30]

Originally Posted by rslifkin

It's for sailing performance. SOG is for getting where you want to go, but STW will tell you if you're actually sailing slower for some reason or if you just have a foul current. Basically STW tells you how fast you're sailing, SOG tells you how fast you're getting where you're going.

So I'm not a sailor but I am curious, What can you do with that information?

[osprey877]

Originally Posted by motion30

So I'm not a sailor but I am curious, What can you do with that information?

The difference between your SOG and your STW will tell you the strength and direction of any current you are sailing in, which is very useful when plotting a course. And when combined with AWS and AWD, will tell you the true wind speed and direction, which is useful for comparison with forecasts and grib charts etc, all of which use TWS and TWD.

[motion30]

OK thank you
I know sailors have different requirements than us dumb power boaters, Helps me to understand a little more

[wholybee]

Originally Posted by motion30

So I'm not a sailor but I am curious, What can you do with that information?

It is performance data. If you know the apparent wind speed and direction, you can calculate or know from experience what your speed through water should be. If you are sailing slower than that, your sails aren't trimmed properly. This only works with STW and AWS. If you rely on Speed over ground or Wind over ground, the performance metric doesn't work. So, if you are involved in racing, STW is a very important measurement, and it is notoriously difficult to measure accurately.

On my boat for example, the paddle wheel reads 10%-20% different on starboard and port tack at the same actual speed. An ultrasonic sensor fixed that, but was darn expensive and kept failing. I can't afford to keep replacing it so had to reinstall the paddlewheel.

[Dockhead]

Originally Posted by Shrew

I wouldn't bother. Most if not all speed transducers are giving you Speed Through Water (STW). Run with a current or tide (let's say 2 knots) and your STW is 2 knots slower than Speed over Ground (SoG). Run against a current or tide (same 2 knots) and now your SoG is 2 knots slower than your STW. Either way it makes arrival time estimates increasingly inaccurate over lengthening periods of time.

I just use the SoG from my GPS which is more accurate, overall. Just my .02.

Originally Posted by rslifkin

On a powerboat, I agree. But for sailing, speed through water is useful.

Originally Posted by Shrew

It's been my understanding that STW is useful for collision avoidance, rather than navigation. I can see how this might be useful in racing.

I'd like to take this as a learning opportunity. What am I overlooking?

For sailing, you need STW. If you use instruments at all. Of course we have all sailed with nothing but a windex and a compass -- it can be done. But if you want to measure sailing performance -- are you going as fast as you are supposed to? -- and if you want to know true wind, so that you have valid laylines, so that your pilot works properly downwind, you must have STW.


Because we sail in the interface between air and water, not between air and ground. The water is always moving in relation to the ground, so SOG is not a valid proxy for STW.

[Dockhead]

Originally Posted by wholybee

It is performance data. If you know the apparent wind speed and direction, you can calculate or know from experience what your speed through water should be. If you are sailing slower than that, your sails aren't trimmed properly. This only works with STW and AWS. If you rely on Speed over ground or Wind over ground, the performance metric doesn't work. So, if you are involved in racing, STW is a very important measurement, and it is notoriously difficult to measure accurately.. .

Indeed, and it's worth emphasizing that you cannot calculate a wind triangle, you cannot calculate true wind speed OR true wind angle, without STW.


And yes, it is indeed "notoriously difficult to measure accurately". The main function of expensive sailing computers like the B&G H5000 is to provide elaborate calibration of the speed log, correcting the data for linearity, heel angle, etc.


Racers spend a huge amount of time on this.


But this is all relevant to cruising, too, if you care about sailing well.

[Tupaia]

Have you thought about a magnetic speed transducer, no moving parts.
https://www.nasamarine.com/product/electromagnetic-log/

[Dockhead]

Originally Posted by Tupaia

Have you thought about a magnetic speed transducer, no moving parts.
https://www.nasamarine.com/product/electromagnetic-log/

I've used mostly ultrasonic speed logs for the last 15 years.


First an Airmar CS4500, which was the best speed log I've ever used, but far from perfect.


Then an Airmar DX900i which is electromagnetic, and directly measures leeway, which i thought would be very useful, but it was dead on arrival.


Now using an Airmar UST850 ultrasonic one, which is not very good and which I have been unable to calibrate.


In my box of new speed logs I haven't tried yet is a NKE ultrasonic.


And a Garmin/Nexus GST 43, highly recommended by racers. This is a paddlewheel, pulse one which is supposed to work well with the H5000.


I had not heard of the NASA electromagnetic one; thanks for the tip. I'll look into it. NASA are budget sensors not normally known for high accuracy, but you never know -- the Garmin GST 43 is also cheap as chips and yet is supposed to be very good.

[Tupaia]

We are expecting a full and detailed review of all these transducers. With so many on hand you have to be in a position to a world expert tester.