Trailing log question

[Kyle2]

I plan to fit a NASA Clipper Duet Log and Echo Sounder, to replace my old Seafarer equipment. It occurred to me that a trailing log (such as the Seafarer) has at least 2 distinct advantages over the paddlewheel type (such as the NASA).
1) no need to cut a big hole in the hull
2) impeller won't get clogged with barnacles etc. as it is only in the water when in use
I would guess that speed logs work by either making or breaking the circuit with every rotation of the impeller, and so different makes and types of impeller might be interchangeable. Does anyone know if it is possible to use a trailing type impeller with a NASA log? If so, what type?

 

[William_H]

Your right about a log my own choice is to use the GPS for speed and nav and do without a log. However to comment on your question. I don't know but it may be possible to find out the number of pulses per Nmile fo NASA log. If you knew you may be able to find out or deduce the number of turns of the trailing logper Nm. You would need to build or adapt a transducer for the trailing log. Typically one or more magnets rotated with the log whose poles rotate near a magnetic reed switch or coil. More magnets give more pulses ie 2 magnets twice as many pulses as 1 magnet. If you use a coil pickup you will need some electronics to convert the induced AC to pulses. It is highly unlikely that the number of pulses (or a multiple) per Nm would be comparable. The NASA log may have some adjustment /calibration provision. Electronics could be used to divide or multiply the number of pulses (whole numbers) or you could acheive the desired result by eg multiplying by 100 and dividing by a number from 2 to 99 electronically. An interesting hobby electronics type project but ultimately more unreliable and trouble than it is worth. PM me if you really want more ideas on the electronics but there is a long way from idea to practical good luck will

 

[john_morris_uk]

There are also disadvantages to using a trailing log:

Then fact that you have to trail it to use it and remembering to stow it before you start doing any parking manoevers at the end of your passage.

Many the log that has ended up round a prop shaft....

I don't like the idea of just using the GPS for speed - after all it only gives you part of the story 'Speed over Ground'. If you have an accurate log as well, you can see what the tide is doing to you.

Regarding through hull logs:

I guess the vast majority of boats have through hull fittings for thei logs. Most boats don't sink.

Some manufacturers have 'flap valves' on the transducer housing that stops some of the water coming in when you withdraw the log for cleaning. They are not perfect - you still tend to get half an armful, but I put up with log impellor cleaning as one of the hazards of boating.

If you are interested, we have a through hull log and carry a Walker mechanical trailing log for when/if everything else were to fail.

 

[Evadne]

I have used a Stowe trailing log since acquiring the boat but unfortunately the Stowe is no longer made. I would not want to change it for a through-hull log, I find it accurate, easy to clear of seaweed and dead easy to maintain. I would find another trailing log if this one packed in. This question popped up a while back and I found Google on "trailing log" found at leasty one new instrument for sale. The disadvantages would be:
No NMEA output so can't interface to any integrated nav. package (no problem for a luddite like me).
You do have to remember to take it in at the end of each voyage, which takes all of 30 seconds.
You do get people shouting at you that you have a line trailing in the water at regular intervals.
That's it.