Episode 29 Depth Gauge

Bolero's depth gauge decided to not gauge the depth after getting back into the water from her January slip.

To be fair, the depth gauge unit was Navman Multi 3100 with an LCD screen, about 15 years' old. It did everything else nicely, including a read out of the battery voltage, but as it had given up the ghost in its chief purpose of gauging depth, it had to go.

Being a cheapskate, I looked for the cheapest working alternative. I wanted an in-hull fitting, because I don't like the idea of putting holes into the hull (or bulkheads) and because I did not want to slip the boat again, at $350 a pop at least, to fit a new instrument. For me, the most important thing was a reliable reading of depth in water shallower than about 2m so that I would not run aground, and so that I would know how much anchor chain to let out when needed.

Having succeeded in fitting a Lowrance Hook 2 5x to the inside of the hull of my Austral 20, I decided to downgrade to a Lowrance Hook 2 4x with GPS for my Cavalier 28, which had a similar solid GRP hull, although much thicker. The unit cost about $160. With my Austral 20, I simply glued the bullet to a puddle of epoxy resin in the bilge, and it worked nicely to give depth readings. 

One feature I like is that you can mount the head unit on a bracket screwed onto a bulkhead, so that it can be easily removed and stowed inside when the boat is unattended. This also saves having to cut a large new hole into the bulkhead, which not only weakens the structure but also presents another opportunity for water to get into the boat.

Initially, the 4x would not give a reliable reading of depth. To be fair on the Lowrance Hook 2 range, the transducers or bullets are designed to be mounted in the water on a bracket on the back of the transom as fish-finders. I was mounting mine inside the hull, and I was not interested in fish as much as the depth to the bottom, especially in shallow water. 

I had glued my bullet with FixTech 180 to the bilge a little forward of the previous and now-redundant through-hull fittings. The 4x gave some readings, but proved to be unreliable. The shallow-water alarm went off in the middle of the channel near Peel Island, which is maybe 8m to 10m deep. Thinking I might have mounted the bullet over some obstruction in the hull construction, I prised it off, damaging it in the process, and tried blue-tacking it into several alternative locations. With the blue-tack, the depth readings disappeared entirely and I thought I might have bought a dud.

I phoned the shop I bought it from, Boating and RV in Tingalpa. Richard suggested I make a wet well inside the boat into which I should place the bullet. He suggested PVC pipe with a screw-on end, but I opted for cutting the bottom off an old Sistema clip-top food container and fixing it to the hull with FixTech 180. The clip-on top was needed to keep some water in the wet well when the boat heeled, and there was precious little room in the bilge for a larger screw-top arrangement.

I am happy to report that the wet well approach works well, and I got a nice, steady reading of a depth of 2.2m at my berth, which is what I expected (allowing for the bullet being about 0.3m to 0.5m below the waterline - I'll have to measure it more accurately). It also showed some fish, which were in no danger of being hunted by me at the time. 

So, to recap, you can use a Hook2 4x Fishfinder as an in-hull depth gauge subject to two important conditions;

1. The hull you are shooting through is solid GRP construction, not a core-filled sandwich
2. The bullet is placed inside a wet well inside the hull, which you will have to make, including a lid to keep the water in. A food container or sandwich box with a sealable lid will do nicely

The end result is ugly and agricultural. The plastic food container was more difficult to cut than I had anticipated. However, as we engineers say, "If it is stupid and it works, its not stupid".

Sistema food container with bottom cut off, glued to hull with FixTech 180

Wet well filled with water, bullet inserted and lid on (the lid needs to be sealed properly)

Depth gauge reading on head. Not easy to read in direct sunlight, but the head can be tilted down on its bracket.