Practical projects
Peter Kersey has a plan to avoid drilling holes in his DrascombeÕs hull
Installing a portable echo sounder; a black ball fender ‘at anchor’ sign; safe mast storage and more reader tips
We have a Drascombe Lugger called that we have taken to the Isles of Scilly for two weeks each year for the last 15 years. She is ideally suited to exploring the archipelago with a shallow draught of about a foot, when rudder and centreboard are raised.
While visiting the islands, we often lay a couple of pots and do a bit of fishing. Both these occupations require knowledge of the depth of the water, which can change from a few metres to a few tens of metres in a very short distance. In the past we’ve used a lead line to measure the depth but that was cumbersome and with such rapid depth changes the result had often altered before the next swing of the lead was completed!
Another method needed to be found. While clearing out the garage I came across a NASA Clipper depth display and an old Seafarer transducer. To my surprise they proved compatible so I decided to make a portable echo-sounder.
I was not keen to drill holes or screw brackets onto Aunty Peg so hit on the idea of placing the transducer in a polythene pipe and suspend it over the side below the water level. As can be seen the transducer just fitted the pipe and the end was made waterproof with a jubilee clip. The pipe was also large enough to accommodate the redundant cable length.
This just left the mounting of the display head to sort out, the battery for power and the switch and recharging gear. A box was made from a
mock-up of the various parts and cut out and glued together. One important consideration is to allow clearance depth for terminals above the battery so that a short circuit doesn’t occur. To help prevent this I also covered the battery terminals with electrical insulation tape.
As well as turning the unit on and off, I wanted to be able to charge the battery without opening the box. The circuit therefore includes a double pole two-way (DPDT) switch, which alternates between ‘charge’, when the supply to the display unit is off, and ON when the charge circuit is disabled. Thus you cannot charge and use the unit at the same time.
A right-angled coaxial aerial extension was sealed in a drilled hole in the back of the box and glued in place. This enabled the transducer cable to be connected and disconnected without opening the box. Thus making the unit and transducer more easily portable.