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is there a non impeller speed-log?

8K views 24 replies 19 participants last post by  overbored 
#1 ·
Hi folks.

A few years ago I thought I heard tell of a speed log that used a sonar beam and reciever to measure speed, thus no moving parts and nothing to clog.
Have done some searching and cannot seem to find anything out there nowadays. Any ideas or recollections ofsuch a beast?


And when I said non-impeller...I did not mean the old VDO sumlog with the propellor-on-a-fin-driving-a-cable-type configuration. Have one of those...it came with the boat and gained us half a knot of speed when we retired it.

:)


Sasha
 
#3 ·
You're right - I recall reading about such a system a few years back - but can't remember who offered it. Haven't seen anything about it lately so maybe it never caught on.
The main drawback may have been the need to lead a wire down the keel to one of the transducers. (in one version that I remember)

I think it used the doppler effect or transit-time ultrasonic technique, a common industrial flow measurement method. As a flow measurement it is a bit unreliable and subject to errors that may be why it hasn't caught on here.

Makes sense, though, to eliminate the moving parts, doesn't it?
 
#6 ·
errr...no, it won't. GPS will give me (more or less) speed over ground, but that isn't always the info wanted.

Mind you, the GPS has also told me that I have sailed across the middle of a tall volcanic island in the pacific and that I was sailing at 78meters above sea level while parked at anchor....So it is not exactly perfect.

Sasha


Really want to track down this non-wheel transducer....Jeff H, any idea?


Sasha
 
#9 ·
THANK YOU!!!!!!!!!!!!!!!!!

It isn't the exact one I recall seeing...but it may just be better. I have sent them an enquiry and hope that the price quoted will not bring on a coronary....

(I figiure that if it beats five years of annoyance value at clogged and broken impellors then it is worth it)


Sasha
 
#14 ·
I wonder if this could be a function of a more advanced depth finder? I haven't done the research but I bet there is something like that out there in the fish finder/ depth finder market. Of course it would be limited by the depth of the water... once the bottom is past it's max range...
 
#15 · (Edited)
Check out Airmar at www.airmar.com They have some speed sensors that uses ultrasonic (I think). Search for CS4500. It looks like Airmar made transducers (sensors) for use on many brands of instruments eg. Raymarine, Navman, Simrad etc....
Interestingly, just early this week, I ordered a replacement ST40 Speed transducer and found Airmar transducer in the Raymarine packaging. I called the dealer to verify and was told its indeed correct.
 
#18 ·
Other than dual transducer units for use in shallow water the only non-wheel log I know of is the pitot tube type. I know of none suitable for boat mounting, and they are all subject to damage due to their protuding beneath the hull.
 
#19 ·
I have a very old (late '80s) cathode ray tube depthfinder called a Marinetek Video Sounder.

I have NO CLUE how it works, but it involves a squared-off hull projection with a glass window (I dislike this on general principles...). However, it did work last season and did give a decent and fairly vivid idea of the bottom, but it sucked at depths over 40 feet, and eventually grew a coat of algae that meant it recorded all depths as 3.2 feet...I have to clean it this winter.

Did I mention that the Video Sounder (when scraped clean) gives a pretty accurate speed under way? Over about 1.5 knots, that is. I don't have an impeller thru-hull, just this old thing and a 1990s Raymarine chartplotter that tells me when I'm doing over 2 knots.
 
#21 ·
f123olly said:
Speed=distance x time
A tennis ball on a measured string and a stopwatch will be about as good as anything you can buy...and its cheap!
Well, since our boat is faster, we found that shooting a Chihuahua dog over bord was more accurate...we tried several sizes. We found out that the ones whose eyes were sticking out more would be more accurate.

Good for foggy days, also, as we just shoot the Chuhuahua off the bow, and if he whines, then ther's something there!!!!

good thing is they swim back and are re-useable...

Just make sure not to throw your Chihuahua while in port, they don't have very good bounce back qualities!!
 
#22 ·
I note Sasha_V mention an old mechanical Sumlog VDO which had been "retired". What's the chance it might be hidding in a forgotten corner of a shed somewhere?
I'm keen to replace the white plastic rotor from my own Sumlog VDO unit. Keen to get in touch with anyone who might have one or know someone who has.

Thanks.
 
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