Lifting a paddle wheel transducer in the water

[Rob_Webb]

Re: Sensitive adjustments...

I think I have a related problem - I have a NASA clipper paddelwheel log and when I first bought the boat a few months ago it worked fine.... then started playing up. I wnet swimming and cleaned the wheel with a sponge and dug out some grwoth with a small screw driver.... seemed to cure the problem but it need repeating every couple of weeks.

Now it reads 0.0 most of the time but occasionally jumps up to the 'correct speed e.g. 5 kts, then reads an obviously too low reading e.g. 2.2kts when we are scooting along... then it reads 0 again. I went swimming a couple of days ago and it seemed to spin fine - we were at anchor with a strong tide and I found the paddelwheel spinning when I went underneath - but 0.0 reading on the log.

The trip meter is also under-reading. I have seen the transducer in the bilges but have not attempted to open it up for fear of flooding - what should I do?

Thanks
Rob

 

[oldharry]

Re: Venturi effect

To get a venturi effect, the opening has to face away from the flow of the water. For optimum effect the hole should be at a right angle away from the direction of flow. To create a venturi effect the flow of gas or liquid has to be obtructed so as to create an area of low pressure downstream of the venturi orifice. Have a look at the suction bailers on a racing dinghy.

The idea is that at speed the low pressure area created in the water by the bailer sucks the water out. Most logs are mounted parallel to the flow, so that there is nothing to create an area of low pressure.

On a planing hull things might be different if the water flow is too fast to allow the water to flow in normally - but even so the water would try to enter at a rate far exceeding any venturi outflow from the static water inside. Of course if the leading edge of the hole were shaped to deflect the external water flow so that it did not enter the hull then a suction baler effect could be produced - but then that would adversely affect the operation of the log paddle wheel.

 

[vyv_cox]

Re: Venturi effect

Someone else suggested the venturi, not me.

I think that flowing water (or moving boat through water) will have a lower pressure than static water. So far as a log impeller is concerned I don't think there will be any discernable difference in the head whether the boat is moving or not.

 

[pvb]

Yes, silicone grease is best...

Get it from any plumber's merchants.

 

[roger]

The venturi effect reduces water hieght just outside the boat - so - look overside and check if the water level has gone down as far as the transducer hole. If it has, then you wont have a leak!

 

[charles_reed]

Venturi effect

Good theory, bad practice.

Only works well in small, enclosed systems.

 

[kdf]

Re: Que?

I didn't bother to do the research and find out why but believe me the inrush of water is way higher - The boat was doing about 8 knots and the jet hit the headliner over 6 feet above the opening.

Mind you we did get it working again.

 

[yachtcharisma]

Re: Que?

Partly forward facing hole maybe? Or the heeling of the boat putting it further under than at rest??

Patrick

==================
Sailing a Corribee
vzone.virgin.net/patrick.fox
==================

 

[Guest]

Re: Venturi effect

All box's containing the parts when bought include a blanking cap to screw onto the transducer tube when the paddle wheel is removed.
I have regularly lifted the paddle wjheel while afloat ... BUT normally when another is holding the blanking cap right next to the tube ready to fit immediately the paddle wheel assembly exits the hole. When I have done it alone ... to clean the paddle ... I have unscrewed the securing collar and held the assembly in the tube ... it has a interference fit rubber collar sealing inside the tube. Holding the blanking plug in your BEST hand, mine is the right, I withdraw the assmbly smartly with the left hand and screw on cap with right.

Averahge quantity let into boat is about 1-2 litres at most, and is more worrying before you ever do it ... once you've actually done it and see that its not so hard - - - -

I would suggest practice on a falling tide on a sand bank / mooring / BBQ beach etc. etc. before doing it for real !!!!!

 

[Guest]

Re: Easy....

Normally a small bullet magnet is embedded into the edge of one or two of the blades ... this gives the counter effect as it passes the detector mounted in main body of the transducer ..... when you cleaned the blades did you end up with a small hole in any blade edge ?

 

[Guest]

Re: Re missing magnets

Button magnets can be had from various outlets such as Maplins etc.
The thing only has to be a 'chip' epoxied back into the hole the original one came out of !
Maybe someone has a log that the head etc. has failed .... then if the paddle is same, 1 or 2 magnet type ... change the paddle .... not the transducer, as they are NOT all same.

 

[Guest]

If the magnets have gone on the paddle wheel and yours is a NASA unit you can get a replacement paddle wheel from NASA cheaply which is easy to fit. This saves messing about with non standard magnets and epoxy.



Steve Greenhough
"Frank" Liverpool Yacht Club

 

[Rob_Webb]

I (bravely/foolishly) removed my NASA paddlewheel unit whilst underway at the weekend with no drama. I found a four bladed wheel, with little lumps (about the size of match heads) on 2 of the 4 blades - are these the magnets? If so, they are covered in plastic and seem integral to the paddlewheel and not able to fall off.

Anyway, the wheel seemed to spin very freely but when put back I only get a reading occassionally - sometimes it reads the full boat speed, sometimes about 50%. Surely if it was an intermittent electrical problem I would either get the full correct reading or zero - not a range of readings in between?

I'm stumped on what else to try before more extensive/expensive investigation starts i.e. removing the unit from the boat and sending away etc.

Any ideas, please?

Thanks
Rob

 

[Artimes]

i TOO HAVE BEEN CURSED BY THE PADDLE WHEEL GREMLIJN!
HelpfullyI found I could test the operation of the wheel whilst under way by disconnecting from the instrument and connecting a good ohms meter. This showed a distinced flick as the paddle rotated (at low speed). Also the degree to which the paddle barrel is pushed into its tube seems to be sensitive (ie the projection below the hull) Finally, we found it must be withdrawn and cleaned monthly.

 

[Guest]

This may sound a silly question .... but is it mounted and projecting into 'clear - deepest' water from the boat.
Some people have been known to fit the paddle below waterline, but when in seaway / heeling etc. the paddle is prone to be in and out of good water ... another factor is if there are keels and other intakes etc. around / ahead of the paddle .... it should be in clear unobstructed flow of water. The act of heeling a boat will cause significant effects on water flow around a hull etc. particularly around keels etc. Even an engine intake strainer immediately behind a paddle can have an effect !

 

[jfkal]

Less water I suspect

 

[johnt]

Re: Venturi effect

at 90 deg to the flow a RAM effect is just as likely as a venturi, its all dependant on the external shape of the housing and the flow around it......an' I aint gonna try and find out! ... Ill pull my paddle wheel while stationary thank you !

 

[HaraldS]

With an intermittend electrical connection you may in fact get any reading. The instrument counts pulses coming from the sensor for a predetermined time window, say two seconds. If now pulses are arriving for the first second and none during the second, you would read half the speed.

I'd look for a broken wire, probably close to the transducer, or maybe a corroded wire close to the instrument, (asssuming the cable goes uninterrupted from transducer to instrument).

Don't know what transducer the NASA instrument uses, but most newer ones work with Hall-Sensors, which would mean there would be at least three wires, (or five if the is a water temperature sensor integrated).

The Hall Sensor needs three wires to work, one ground, one power (usually 12V provided from the instrument), and the pulse wire, (an open collecor fo the sensors output transistor). If any of the three get interrupted you would have the given problem.

I would assume that your instrument itself is o.k. If a wire is broken very close to the transducer it might be hard to fix. Try bending the cable at various places and have somebody watch the instrument while you are moving. Maybe you'll fing the right spot to cut the cable that way. And check the connections to the instrumnet.

Good luck.