An examination of the Loch Ness sonar chart recorded on 8th August 1972


Creative Commons Licence
"An examination of the Loch Ness sonar chart recorded on 8th August 1972" by Dick Raynor is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. Permissions beyond the scope of this license may be available at

A New Examination of 08/08/1972 Loch Ness Sonar Chart Finds No Trace of a Monster


Dick Raynor, Inverness, Scotland.



ABSTRACT:       During underwater investigations at Loch Ness in August 1972 a chart-recording sonar machine recorded traces presented as indicating the presence of a large unknown animal, presumed to be the Loch Ness monster. This study correlates the recorded traces with the objects known to be within range of the equipment used and finds no evidence of anything remarkable on the paper record.



On the night of August 7th-8th 1972 a group of investigators from the Academy of Applied Science and the Loch Ness Investigation Bureau had deployed sonar and underwater lapse-time camera equipment below two boats, Narwhal and Nan, moored in Urquhart Bay, Loch Ness. Between approximately 0100hrs and 0210 hrs the sonar equipment recorded unexpected traces. When the underwater camera film was later processed it was found to contain three frames showing a disturbance, said to have been exposed during the same period as the unexpected traces. The sonar trace was shown to various experts who made comments supportive of the idea that the trace was caused by a large unknown object, presumably an animal. Those comments were then used to support the suggestion that the objects in the underwater photographs were the same as those on the sonar trace,  and represented part of a large unknown animal swimming in Loch Ness.

This paper correlates the sonar chart recordings with known objects in the water by means of contemporary accounts by the investigators themselves and an aerial photograph of the same area taken some seven months later. Once all known objects are accounted for in the sonar recording there are no residual traces requiring explanation.




A close examination of the sonar chart recording used in the  November 1972 media releases (Fig1), together with the more complete record published in the International Wildlife Magazine Volume 18 Number 3 March 1976, (Fig 2) show repeated similar patterns of four lines, the first two being equally thin and the second two being equally thick, over a period of one hour.  These are highlighted in Fig 3.


Fig 1. "AAS sonar chart 8th August 1972" as explained in contemporary press release.


Fig 2. Complete chart record between 0100 hrs and 0210 hrs with over-printed scale markings at centre.


Fig 3. Complete chart record showing repeated traces of two thin lines and two thick lines.

There is an important error in Fig 1. circulated by the Academy of Applied Science, as it shows both sonar transducer and camera / strobe unit deployed in a stationary mode on the bottom of the loch. Had the equipment been deployed in this way then all static objects would appear as horizontal lines on the chart paper. On the night in question, the equipment was suspended beneath two moored boats, free to move around independently under the influence of wind and current.  This layout is shown in Fig 4, below. The author asserts this to be the case as he was one of the crew on the sonar boat Narwhal on the evening in question, though not at the time of the sonar contact under discussion.

Fig 4. "Arrangement of boats on 8th August 1972"  - Raynor version.


This means that the camera equipment could  sometimes be within the area covered by the sonar beam, and sometimes outside it, and the distance between the sonar transducer and the camera unit could vary by a distance in the order of ten feet as the boats moved around. An example of the movement of a moored yacht in the same vicinity is shown in Video 1. below.

Fig 3 shows the way the camera unit (and perhaps its associated surface boat and mooring system) change their separation by about 5 feet over a period of about 5 minutes. The "down" direction on the chart in Fig 3 is "further away" from the sonar transducer, while the horizontal axis along the chart from left to right is Time, as shown by the hand written numbers along the bottom of the chart. The 4-line trace from the camera / boat / mooring system stays essentially the same in character, only changing in distance from the sonar transducer at the top of the chart.


How this particular sonar works

Sonar is a method of measuring the distance to something underwater by sending out a short pulse of sound and measuring the time it takes for an echo to return.  The chart recorder machine common until the mid-1980's used special paper which conducts electricity. A metal wire called a stylus on a belt or rotating disk  travels across the paper from top to bottom several times per second and every time it passes the top of the paper a pulse of sound is transmitted into the water from an underwater sound emitter called a transducer. This same transducer also receives echoes from objects underwater and these weak electrical signals are amplified and fed to the stylus which then burns a mark on the paper.

The further away the reflecting object is, the longer it takes for the echo to return and the further down the chart the mark is made. In the chart shown there is a "transmission mark" along the top of the paper at zero distance, and a number of other marks lower down marking echoes from other objects at varying distances.


In most chart recording sonars, also called fathometers, the user can change the "range" of the machine to suit the conditions by altering the speed of the stylus over the paper. The Raytheon DE-725C used in this operation had a choice of two speeds of rotation -  a high speed for short ranges up to 150 feet and a 3 times lower speed for longer ranges.

In commercial machines the paper chart is often 10 inches or 250 mm wide, allowing fine detail to be shown, while in machines aimed at the recreational market it was usually much smaller, around 4 inches or 100 mm. This smaller paper usually gave lower resolution of detail but in the case of the DE-725C an ingenious modification was introduced. Instead of one wire stylus carrying the signal to the paper there were three of them spaced 120 degrees apart on the rotating disc. The advantage of this is that the disc could spin at a high speed giving good resolution while the three styli effectively tripled the width of the chart paper. The drawback was, however, that the three simultaneous ranges being recorded were indistinguishable from each other. In normal use, pointing downwards, with a boat leaving the shore, the user would be aware of the increasing depth being shown and as the water depth passed the 50 foot mark the first stylus would stop recording on the bottom of the chart and the second stylus would start recording at the top. The paper has faint lines printed on it running along the roll, and every few inches there is a transverse line with numbers at the principal depths. One set  reads 10/30, 20/60, 30/90, 40/120 and 50/150, the next reads 60/180, 70/210, 80/240, 90/270 and 100/300, while the last reads 110, 120, 130, 140, 150. When the water depth exceeded 150 feet the user would change to the 300 foot range which effectively slowed down the rotating  stylus carrier to 1/3 speed. This is satisfactory when the equipment is used in the normal downwards-pointing mode to measure the depth of the water, but when the transducer is mounted horizontally as on this occasion there is no indication as to which of the three short range values or two long range ones is the correct one. They are simply recorded on top of each other.

As an example, look at the top image where a dense trace with about four separate components is labelled "about 30 feet long...with projections or humps". The explanatory diagram beneath the photo of the trace shows the object to be at about 120 feet range, but it could equally have been at 40 feet, or 80 feet, or 120 feet, or a combined trace of separate objects at all three distances. There is no way of knowing.

Let us now go back and look again at the trace labelled "about 30 feet long". It consists of, from the top, two weak lines, one well-defined thicker line and another softer but equally thick line.  This is what sonar operators call a "signature" - the kind of echo returned by an object of specific composition. From top to bottom on the chart this signature seems to be about 5 feet in extent, so the first weak line is an echo from something about 5 feet closer to the sonar transducer than the second soft thick line.  This "signature" can be found at  six points on the chart, spread over a period of one hour. It is always in the same general part of the chart, range about 35 to 40 feet, or 85 to 90 feet, or 135 to 140 feet, as discussed earlier no-one can be sure. I will accept for the present that it was really at 135 to 140 feet range, where the camera rig was suspended from the motor cruiser called "Nan".


The sonar beam.

A sonar pulse can be likened to a torch or flashlight beam in that it gets wider and weaker as it gets further away from its source. The transducer used on this occasion has a nominal beam width of 20 degrees, meaning that 10 degrees away from the centre line it has only half the power that it has in the centre. So at a distance of 140 feet this beam - a "cone of sound" - is 50 feet in diameter. What should appear on the trace if it is pointing towards an anchored boat with a camera rig suspended underneath its stern?

The equipment arrangement.

There is some historical confusion here from the start. The deployment  of the equipment according to the AAS explanatory diagram (Figs 1 and 4) shows the sonar standing on the bottom of the loch and the Edgerton Strobe unit also on the bottom of the loch some distance away and deeper down. With this arrangement all static objects in the beam will appear as constant parallel lines. On the night in question, the camera was on one boat and the sonar was on another, both of which were free to swing on their moorings as shown in Fig 3. A viewing of the 60 minute lapse-time video of an anchored yacht close to the old Narwhal mooring shows that boats swing around many tens of degrees and move back and forth several metres, so any fixed objects within range of the sonar could be intermittently in an out of the beam and at slightly different ranges. Echoes associated with another independently moving vessel within range would be even more erratic and the region near the camera unit would only be within the sonar beam intermittently.


Video 1."Lapse time video of anchored yacht close to Narwhal location" Copyright Dick Raynor 2012   Right-click on the image, then select "play"

Fig 3. "Arrangement of boats on 8th August 1972"  - Raynor version.


Fig 4. Arrangement of boats (i.e. no boats) on 8th August 1972 as presented by AAS, leading to erroneous conclusions by consultees.


Fig 5. Aerial view of moorings near Temple Pier taken early 1973. Photo Copyright (c) Holly Arnold


Fig 5a. Enlarged print shows Camera boat "Nan" mooring buoy 1, Sonar boat "Narwhal" mooring buoys 2, and two other red mooring buoys 3 and 4.

Measurements of red boat shed made in September 2013 to permit independent  analysis are: width of red frontage - 4.62m , 15'2"; total height to roof ridge - 4.97m, 16'4"; width under eaves 5.46m, 17'11"; total length of roof ridge  10.2m, 33'6". (See amended measurements below)


My provisional measurements indicate that the presumed Narwhal mooring buoys (2) are 39.3 metres or 129 feet away from the Nan mooring buoy (1), agreeing well with the sonar trace. Note also the way that the moored yachts are lying to the current, and not to the breeze which is on their port sides. The red buoy (3) is measured at 79 feet from the sonar boat buoys (2)

The sonar trace.

So what should be visible on a sonar trace when the beam is swinging around but pointing generally towards the camera unit?

Well, the camera and strobe units should be there unless obscured by a closer object, and being air-filled glass-fibre cylinders they will give a very strong echo. The submerged part of the boat hull could also appear, and the heavy chain from the mooring buoy will go down to the bottom only a few metres from the boat. These would appear as a series of echoes of a fairly constant nature. The existence of buoys 3 and 4 in early 1973 strongly suggests that they were present in August 1972. If so, they and their mooring chains would both be detectable by the sonar as it swung from side to side under with "Narwhal" rotation around buoys 2. Buoy 3 is of particular interest as it seems to be 79 feet from the sonar boat mooring and within 20 degrees of the camera boat "Nan" as viewed from "Narwhal" and so could have been detected just before or after the camera rig and the vessels swung around. Most significantly,  these marks do not appear on the chart at the same time as the "main" echoes, but shortly before or after them, suggesting that it may have been on a slightly different bearing.



Given the mobile nature of both sonar equipment and camera equipment there is no reason to expect consistent traces. The vessels employed responded differently to currents and changing breezes.  Such traces as were recorded, in particular the quadruple marks appearing about six times in a period of one hour, are what one would expect from highly reflective camera containers, mooring chains and boat hulls. Even the unusual trace at 0147 hrs, interpreted as an echo protuberance with a dimension of about 10 feet, has a precedent at 0115 hrs. The size of the camera tubes was approximately 8" / 20 cm diameter and 24" / 600 mm long and one could reasonably expect them to give very strong echoes. If an animal with 6 to 8 foot long flippers ( and presumably a body over 20 feet long) swam within a few feet of the camera tubes it would inevitably have presented a massive sonar trace for just two or three minutes before swimming away, and there is no evidence from the sonar of that having happened. My conclusion is therefore that the sonar chart shows intermittent echoes from the camera equipment suspended beneath the other vessel, and nearby mooring lines.


This version of 2013-10-04 includes measurements of the red boatshed to permit independent photogrammetry.

Update below on 2013-10-06 includes front view photograph of boatsheds and more accurate measurements made with Leica Disto 2.

Boat Houses in Urquhart Bay by Des Colhoun, from Looking north.

The boat house has wooden walls painted red which stand on an asymmetrical concrete base which is wider on the left (west) side. The eaves overhand the walls at both sides and each end. The left edge of the eaves is hidden in this photograph. The width of the red frontage is 4.87 m, the total length of the roof is 10.24 m and the total width of the roof is 5.48 m.





I can be contacted at dick (at) lochnessinvestigation dot org
"An examination of the Loch Ness sonar chart recorded on 8th August 1972" by Dick Raynor is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.
Permissions beyond the scope of this license may be available at permissions.html.

See also a related work on "The Flipper Photograph" at