Ultrasonic Logs

rwoofer

rwoofer

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I'm get sick and tired of constantly cleaning and recalibrating my Raymarine paddlewheel log. I had to up the correction factor by .24 this weekend alone.

I know ultrasonic logs were introduced in the past and disappeared again. I've also heard rumours of their re-introduction but not seen anything. Does anyone know if any are going to come out?

Soon as Raymarine do, I will be first in the queue.
 
I remember seeing a speed transducer in the Echopilot catalogue that did not use a paddlewheel. I think that was an ultrasonic one.
 
The idea has been around since 1977 at least when a doppler log was sold. I bought one at a boat jumble a few years back and finally replaced it with a paddlewheel, because this has one moving part and only fouling as an evil attribute.

What was wrong with the 1977 version (which was a short-range echosounder using an angled beam)

Low sensitivity.
Needed air bubbles or mud in the water to bounce the sound off
Picked up radio stations and went off at 10 knots , despite having a tuning selector to avoid.
Was jammed by radar on nearby boats and would race away.
Was plagued by corroded connectors.
Worked only with a level hull. Read low at high speeds and low speeds, but on average was about right if you looked at the counter counting distance.
Needed a solid hull . Had two transducers in an effort to fix the need to have a transducer a couple of inches below waterline (ok on displacement powerboat , no good on anything else)

Another idea is the electromagnetic log where a magnetic field induces a voltage in the moving water. Apart from fouling this one looks promising .....
 
A non paddle wheel transducer was featured in YM Feb 09, the Airmar (for Raymarine) DST900-EM electro magnetic transducer will be available later this year, price estimated at £300. Can be installed in existing hull fittings.
The thought of not having to pull the transducer every time I go on the boat, and clean the paddle wheel with a toothbrush, then mop up the water which comes in even with a built in valve is worth £300 to me.
I hope it works better than the ultrasonic (which is much more expensive anyway)

Check the airmar website.
contact jg tech.

Regards,

Michael.
 
Go through same routine myself and would consider 300 pounds worth it. Especially since I need to up all the bedding to do the clean.
 
Fouling here is far worse (faster) than UK and long ago I remoived the padle wheel system. I now either do without the speed info or rely on GPS.

A friend has an old Brookes and Gatehouse system from 1989 which is ultrasonic doppler I think and seems to be faultless. olewill
 
Agree with your sentiments and I too am waiting for the new airmar electromagnetic version but have been unable to get any information on their release. I did try to get one in the US earlier in the year but couldnt source one there either. Seems we must wait....

Out of interest how do you manually adjust the calibration factor? I went to the trouble of going into Port Solent to calibrate my log a couple of weeks ago and it's still wrong :-(

Now I need to adjust the calibration and I do have a rough idea of how much I need to adjust by....but I don't know how you do it. Oh, it's and ST60 btw
 
I calibrate at a typical cruising speed, where you want most accuracy say 5-6 knots.

I try to pick an area that is either slack or has a consistent tidal stream. I then motor for a couple hundred yards with a fixed throttle and then take a GPS speed reading, then without touching the throttle do a 180 turn and motor on the reciprocal course for a couple hundred yards before taking a second GPS speed reading. Still leaving the throttle alone, I add the two GPS speeds and divide by two to give the actual speed. I then adjust my ST60 until I get to the actual speed.

I may repeat the reciprocal runs a few times to get it spot on.
 
If any of these devices were any good and reasonably priced, they'd be selling like hot cakes. However, we do live in hope...

/forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif
 
Thanks. I think my problem was I calibrated in Port Solent & was therefore restricted to 3knots or so. When out at the weekend sailing at higher speed the correction factor must have multiplied my speed somewhat as I was doing about 7.5knots upwind.......not bad for a twin keeler :-) The real speed was probably 6 something. I guess any correction factor error gets exagerated the further you deviate from the calibration speed
 
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