Log paddlewheels...why not copper?

[Woodlouse]

I have also been around for the last 40 years sadly ...

Quite a while ago Brookes & Gatehouse came out with an on-hull log system of 2 sensor plates a few inches apart, going by doppler effect; I don't know why it didn't catch on; maybe the high price, critical hull positioning ( I personally think the most likely snag ) or antifouling over the important bits !

There must have been issues with accuracy and calibrating. I've just been involved with installing the latest and greatest b&gs on a money no object program and they were still using paddle wheels.

 

[Seajet]

Woodlouse,

thanks; that amazes me, as I'd have thought one would want to avoid boundary layer effects ie towed trail log or on warships & subs towed array sonar.

I'd never use a paddlewheel log for navigation, but I suppose some punters just want a speed display; there's a story about the Concorde ' we're now doing Mach 2 ' cabin display - a crew member said " Actually we can wind it up to Mach 9.99 ! "

 

[vyv_cox]

There must have been issues with accuracy and calibrating. I've just been involved with installing the latest and greatest b&gs on a money no object program and they were still using paddle wheels.

I installed the latest Airmar transducer in a B&G system this year. The paddle wheel design is quite sophisticated with a kind of pelton wheel shape. A lot of the time we sail in no currents in the Aegean, no tides of course, and the log matches the GPS very well. We find it very helpful in letting us know when and where currents are flowing, a most useful instrument.

 

[Seajet]

Fresh & Salt water;

there must be a difference in calibration, if one is daft enough to rely on a paddlewheel log for anything more than a rough guide.

When I took my Caricraft 10 & Scorpion dinghies to Windermere I found they handled very differently compared to the sea, tending to bury the bow.

I'd guess a big clue is the fact ship plimsoll lines have very different levels for salt or fresh water...

 

[AngusMcDoon]

Fresh & Salt water;

there must be a difference in calibration, if one is daft enough to rely on a paddlewheel log for anything more than a rough guide.

Navigation (as in knowing where you are) in fresh water, certainly in Europe, is not normally much of a problem.

Every sailing syllabus teaches navigation using a log. Almost all logs manufactured for pleasure boats are paddlewheel now. Are you sure your reason for dismissing log based navigation isn't because you find it all too mathematically challenging? I do wonder sometimes. I know maths is not your strong point.

You may think it daft because of some bad experience you had with a particular model not made for 40 years, but that's not an opinion held by anyone else.

 

[AngusMcDoon]

Fresh & Salt water; there must be a difference in calibration,

Sea water is typically 3% more dense than freshwater, but it depends on temperature of course. So the difference is small. It also doesn't follow that because sea water is 3% more dense that a paddle wheel log will read 3% difference. It's likely to be much smaller. A more significant difference is likely to be caused by the higher waves generally found on the seas.

If you were really worried about it then all electronic logs can be calibrated. There are measured miles around the country in locations with little tide. I sailed past one off Arran a few weeks ago. Have you sailed past any this year?

 

[sailorman]

we launched in May, no a/f on the impeller ( i never have ) today i had to withdraw the log & clean it after 1000 mls, hardly fouled at all. The boat has been in her berth for 3 weeks whilst i painted the decks ect, this may have contributed to the slight crud. The impeller was cleaned , sprayed with silicone, inc O rings & replaced.

 

[Seajet]

Angus,

it's you I worry about as I could instantly tell the difference between salt & fresh water handling as an 8 year old, and I'd have thought your boat more susceptible to buoyancy changes !

I did manage to scrape ' O ' level maths - despite the pervert teachers who I'd like to meet around a dark alley some time - and I'd love to hear your explanation of how paddlewheel transducers are accurate despite boundary layer effect.

If in-hull is so spiffing, how come all decent warships and subs use towed array sonar ?

If GPS, the earlier SatNav and Decca hadn't been invented, we'd all have much better towed logs now, I still keep my WASP unit as a treasure for cross Channel trips when everything else might go ' Phut ! '

Stowe were moving in that direction, but still involved electrickery so probably best off with a Walker or Wasp - incidentally I have a lovely Walker log I bought in an antique shop in Petworth, boxed complete with spare spinners and oil can, they thought it was some sort of fishing gear !

 

[sailorman]

Angus,

it's you I worry about as I could instantly tell the difference between salt & fresh water handling as an 8 year old, and I'd have thought your boat more susceptible to buoyancy changes !

I did manage to scrape ' O ' level maths - despite the pervert teachers who I'd like to meet around a dark alley some time - and I'd love to hear your explanation of how paddlewheel transducers are accurate despite boundary layer effect.

If in-hull is so spiffing, how come all decent warships and subs use towed array sonar ?

If GPS, the earlier SatNav and Decca hadn't been invented, we'd all have much better towed logs now, I still keep my WASP unit as a treasure for cross Channel trips when everything else might go ' Phut ! '

Stowe were moving in that direction, but still involved electrickery so probably best off with a Walker or Wasp - incidentally I have a lovely Walker log I bought in an antique shop in Petworth, boxed complete with spare spinners and oil can, they thought it was some sort of fishing gear !

Shark spinner

 

[Seajet]

Saiorman,

precisely; just needs some sort of armoured bowden cable...

 

[Robin]

Yes, it was a rather brief post made from my phone

However, the first hit on google for the phrase happens to be the one I use: http://www.westmarine.com/buy/mdr--transducer-antifouling-paint--128225

It goes on very thin so it won't gum up the works, and it seems to do the job.

I bought it in the UK via eBay.

Pete

That is what we use, from West marine too. It seems to work but the transducer can always be pulled and cleaned for which I keep an old toothbrush handy in the bilge space by it. We have a $50 diver clean and anodecheck too every 6 weeks which helps as well but then we have been afloat for 2.5 years uninterrupted.

 

[Seajet]

Is a $50 dollar diver similar to the thing I used to play with alongside my sailing model Dragon, or akin to my ex, or both ?!

 

[Robin]

Is a $50 dollar diver similar to the thing I used to play with alongside my sailing model Dragon, or akin to my ex, or both ?!

$50 to have our bum sponged and zincs checked and changed when needed is not too shabby. Was Scuba Steve your ex?

 

[AngusMcDoon]

It's you I worry about as I could instantly tell the difference between salt & fresh water handling as an 8 year old, and I'd have thought your boat more susceptible to buoyancy changes!

I can instantly tell the difference between salt and fresh water by knowing roughly where I am. Why should my boat be more susceptible to a 3% change in buoyancy? Archimedes' law works in a roughly similar way for multihulls. Most multis are unsinkable - flood all spaces and they will still float. I think your concern of my foundering is unfounded, but I found it touching, so thanks anyway.

The density of seawater changes by 1% because of temperature variations. Could you detect that when you were 8 as well? I think when I was 8 the only change I could detect at sea was the scariness of the gert big waves.

I'd love to hear your explanation of how paddlewheel transducers are accurate despite boundary layer effect.

That's a big posh term you've heard your aviation chums talk about, isn't it? Why do you think a 3% change in density will have a big effect on it? Paddlewheel logs are accurate enough for the average leisure sailor's needs given what they cost, and that's why almost everyone has one and uses it. There is no such thing as certainty when measuring anything, only degrees of accuracy.

If in-hull is so spiffing, how come all decent warships and subs use towed array sonar?

They use those for detecting things under the sea, not for measuring how far they have sailed. Are you confusing a log with a depth sounder? It seems like it. Anyway, they have a budget of billions and have to spend it somehow. Most of us here balk at having to spend a few hundred £$€ on a log. That's a bit of a silly comparison of yours there. I'm sure even you can see that.

I still keep my WASP unit as a treasure for cross Channel trips when everything else might go ' Phut !

Have you done any of those this year?

 

[Seajet]

Angus,

if an 8 year old can feel the different density, I suggest you put down your tripewriter and go and try it.

Robin,

not as long as I have my breath, or you might see an A22 with depth charge racks !

 

[AngusMcDoon]

Angus,
if an 8 year old can feel the different density, I suggest you put down your tripewriter and go and try it.

Where do you suggest I take my boat to try it? Like almost all sea going sailors, my boat spends all of its time in salt water. I did go through the Crinan Canal a few years back, but Mhairi the lock operator got grumpy when I put my sails up and arrived at lock number 21 doing 18 knots (+/- 3%).

I'm a bit sailed out at the moment having just returned from a 3 month sailing trip. Maybe next week if you come up with a suitable suggestion.

 

[AngusMcDoon]

But as most living organisms don't like copper then why isn't it used moving parts such as these, or teflon or something modern i.s.o. old fashioned plastic ?

Back to the original question - I suspect it's because most marine electronic manufacturers buy in their transducers from Airmar, so they have to go with what's available. Why Airmar don't do it - cost maybe? You could contact them and ask...

http://www.airmartechnology.com/

 

[agurney]

Sureley the relatively small differences in salinity/density/buoyancy won't have any effect on how a paddle wheel log operates?
It shouldn't matter whether the liquid is treacle or alcohol, won't the paddle will still rotate at the same rate for the speed?
If that's not the case, then why isn't there a recommendation to calibrate cup anemometers, say for winter/summer, when the air density is different?

 

[Daydream believer]

? ' then I remembered how useless my Smiths paddlewheel job had been; one might as well tar and feather it, still just as accurate...

I seem to recall the smiths was not a paddle wheel but a kind of propellor on a mini saildrive with a cable connection
I was not aware of the smiths that had a paddle wheel.
But i can always stand corrected

 

[AngusMcDoon]

Sureley the relatively small differences in salinity/density/buoyancy won't have any effect on how a paddle wheel log operates?
It shouldn't matter whether the liquid is treacle or alcohol, won't the paddle will still rotate at the same rate for the speed?
If that's not the case, then why isn't there a recommendation to calibrate cup anemometers, say for winter/summer, when the air density is different?

There's always a slip ratio with this type of transducer - that's the speed of the tip of the paddlewheel/cups compared to the speed of the fluid. It will vary very slightly with density, but the density change for fresh/seawater is small, and paddlewheel transducers are not hugely accurate anyway for a whole host of other reasons. Therefore it can be ignored.

What wor Andy is worried about is the paddlewheel being in the boundary layer of the water flow and this changing with salt/fresh water. On a boat this layer is very thin (few mm) at the start of the flow (bow) and increases along the hull. That's why manufacturers say mount the transducer forward, because you don't want your transducer in it too much whatever the water type.

A bigger problem with log accuracy, whether paddlewheel or the older towed type, is non-linearity across the required measuring range. A log may be calibrated at a certain speed, but then be off at other speeds. That's the effect of the non-linearity. Better manufacturers can easily compensate for this non-linearity of the transducer response in software, but it's tricky to do mechanically.