Figure 1 - Running into a sand bar

A typical boat travelling at 8 kph (just over 4 knots) is covering around 2 metres per second. Let’s assume the depth sounder is viewed every 10 seconds, it takes 2 seconds to react and 5 seconds to stop the boat. If the first indication of the river shallowing is just after you look, the boat can have travelled over 30 metres by the time you can stop the boat. This is more than enough time to go firmly aground. From this, it should be obvious that the depth sounder is not a substitute for a chart such as the River Pilot or the SA Waters Atlas.

Figure  2 - Outer Pontoon Running Aground 

Because houseboats are relatively wide, up to 8 metres across, the depth sounder could be a few metres from the outer edge of the pontoon. As a result, it is possible to measure a good depth and still have a pontoon go aground.

Depth sounders have both a maximum depth and a minimum depth that they can measure. For the river, the maximum depth isn’t an issue, but the minimum depth is important. The minimum depth may be deeper than the draught of the vessel. This can mean that as you inch your way across that shallow passage, the depth sounder may stop working just when you need it the most.

When considering whether to get a depth sounder, it is important to realise that it is not a magic cure, rather just another tool. It is still up to the Captain to consult the depth sounder, the charts and then make decisions.

Once you have decided to install a depth sounder on your houseboat, the question is where to mount it and how to mount it. Unlike a fishing boat or a speed boat, there is little point mounting the sensor on the transom – it may be 10-20 metres from the front of the vessel. The bow can be firmly wedged, while the depth sounder is still measuring many metres.

It is possible to mount the sensor directly to the pontoons, but this would need a bracket of some kind welded to the pontoon. Because it is a certainty that the houseboat will bump the bottom at some stage, it would be unwise to mount the depth sounder below the level of the pontoons.

Usually the most appropriate place for the sensor in along the centre line of the houseboat so that the distance to each side is the same. To provide some measure of protection from the riverbank and wharves, the sensor should be mounted behind the leading edge of the pontoons at the lowest point of the sensor. However, because the riverbank isn’t perfectly straight, it is possible for the sensor to hit the bank before the pontoons even if it is set back.

Because the sensor is likely to be struck at some time, either by the bank or floating debris, it is sensible to incorporate some form of shock absorber in the sensor mounting. The best form is probably some form of stiff spring, but it can be difficult to estimate the correct size of the string and fabricate it.

An alternate is to have some form of shear pin that breaks if the force on the sensor gets too great. The attached drawing shows the method used on the houseboat “My Lady”. This mounting system proved its worth at Cobdogla when wind forced the pontoons into the bank and the depth sounder sensor was also driven into the bank. The shearing bolt snapped and the sensor swung back out of the way. The shearing bolt was replaced at Kingston-On-Murray after it was noticed that the operation of the depth sounder was erratic as the sensor was pushed back and forth by the flow of the water.

There are few dimensions on the drawing because they will vary with each installation. If the installation instructions do not set a minimum depth to mount the sensor, the sensor should be mounted so that it is completely covered by the water at all speeds, but above the bottom of the pontoons.

There are often two modes for setting up a depth sounder. The first is for it to show the depth from the surface (above the sensor) to the bottom; the second is to show the depth from the bottom of the vessel (below the sensor) to the bottom. There are merits for either way.

If you just want to know how much water is under your vessel, then calibrate it so that the depth reading is zero when your vessel would be just touching the bottom. This way, you don’t need to remember the draft of your vessel and subtract it from the reading to see how much clearance you have. On the other hand, if you think that you may be sharing the depth reading with other vessels, it is better to calibrate the sounder so that depth is measured from the surface to the bottom. In this case, you don’t need to add the draught of your vessel to each reading. The attached drawing shows illustrates the measurements required for both calibration schemes.