Installing Logs and Depth Sounders

— Story and illustrations by Charles Moore

Cutting a hole for a thru-hull can be an unnerving proposition.

Many boatowners get cold feet when it comes to electronic instrument installations that involve cutting holes in the bottom of their pride and joy. But not to worry. It’s really not very difficult to achieve a professional caliber and leak-free installation.

The first task is to mount the hull unit, either a knotmeter impeller or sounder transducer. One aspect to consider when mounting the unit, is where crane or travelift slings will bear on the hull during haulouts. With some speed or logs, the impeller unit can be removed from inside the boat and replaced with a dummy plug to avoid haulout and launch damage or to clear fouling.

The simplest installation is on a runabout, where it mounts on the transom just below the waterline and at least 12" (30cm) from the outdrive. In larger cruisers and sailboats, the hull unit is best positioned in an area of low turbulence well below the waterline, approximately amidships for cruisers and forward of the keel in sailboats. This will not always be possible however, and a good alternative spot, especially on relatively hard-bilged, flat-bottomed boats (power or sail), is about 2/3 aft and off the centerline.

Figure 1 - An access plate under a bunk or settee.

Once the ideal spot is determined, it may still prove difficult to access, especially on boats with pan-type hull liners. Should you cut a hole in the liner or choose a less suitable but more accessible location? If you opt to cut, an access panel of wood or other material can be fabricated and screwed in place against a rubber gasket if watertight integrity is to be retained (Figure 1) .

Hole in One

Now you must cut the dreaded hole or perhaps not (see “Mounting Transducers Inside” on page 27). The usual method is to use a sharp, metal-cutting holesaw in an electric drill, but a technique described below in the section on cutting instrument head apertures may also be employed here. Be sure you are clear of bulkheads, tanks and other obstructions on the blind side. Measure twice and cut once. Drill a small pilot hole (easily repairable) to be sure, before committing yourself with the holesaw. Better yet, if the inside is accessible, drill from the inside out.

Log impellers don’t mind if they’re at an angle but sound transducers must be mounted on a perfectly flat part of the hull bottom or shimmed to a vertical, plumb attitude with angled fairing blocks inside and outside the hull (Figure 2) . Use a carpenter’s bevel and a bubble level (Figure 3) to transfer the hull angle to the block, then drill the transducer stem hole through the block (preferably with a drill press) before making the angled cut. With care you can cut the block with a handsaw, but it’s much easier to do on the tilt table of a band saw. The outer block should be streamlined to match or improve on the transducer’s shape.

Figure 2 - An angle block for mounting on an angled surface

Securely mount the transducer or impeller unit, bedding it with a generous amount of polysulfide or polyurethane (non-removable) sealant. Route the cable or wire to the instrument head location, carefully avoiding kinks and points of chafe. Keep depth sounder cables well away from other wires to prevent false readouts. Use nylon cable ties where necessary. Never attempt to shorten a sounder transducer cable. Coil any excess length and secure it with cable ties in a protected location.

Usually you will want to mount the instrument head on a forward bulkhead in the cockpit or the steering station dashboard. This will involve cutting another hole or two in the bulkhead or dash and sometimes in the liner behind it.

If your instrument head is circular and you have a holesaw available to fit, that’s the way to go, but if the aperture required is more than about 3" (7.6cm) in diameter, some alternative cutting technique must be employed. One way is to fabricate a simple jig that will allow you to cut an accurate circle with an electric drill.

Figure 3 - How to measure for the appropriate angle.

Obtain a piece of 1" (2.54cm) metal bar stock slightly longer than the radius of the proposed hole. Drill a shallow dimple centered close to one end of your bar stock, then use a compass to mark the desired radius on the bar. Now drill two 1/4" (6mm) holes separated by 1/64" (.4mm) or less, side by side inside the radius arc and drill out the original dimple to 1/4" (6mm) as well (Figure 4) . The two closely spaced holes must be at exactly the same radius from the center (dimple) hole.

Stick duct tape to the back of your jig to prevent scratches on gelcoat or paint. Mark the hole location on the bulkhead or dash with the compass and drill a 1/4" (6mm) hole dead center. Mount the jig snug but not tight with a 1/4" (6mm) bolt through the single hole end. Drill a 1/4" (6mm) hole in the panel through one of the closely spaced holes. Grind one side off the head of another 1/4" (6mm) bolt to use as a pilot anchor, and insert it in this hole. Now drill through the radius hole. Remove the bolt/pilot, rotate the jig slightly, and reinsert the bolt in the hole you just drilled. Continue “walking” the jig around the hole radius until you return to the starting point. Remove the jig and use a small file to remove the material from between the drilled holes. You should have a near perfect circular hole.

Figure 4 - A jig for cutting round holes to mount an instrument head.

A faster but less neat and precise method is to drill a pilot hole then cut around the marked circle with a sabersaw with a metal-cutting blade. Affix masking tape to the cut line on both sides to prevent chipping and to the saw’s bottom plate to prevent scratches. This mode is of course necessary if your instrument head is rectangular or square. In this case, you’ll need to radius the corners with a drill bit or holesaw to prevent stress cracks in the fiberglass.

Install the instrument head in the bulkhead or dash following the manufacturer’s instructions. Be sure to seal the cutout with sealant. For bulkhead mounts, you may have to cut a hole in the cabin liner for access to the back of the instrument.

If there is sufficient space between the cabin liner and outer bulkhead to accommodate the instrument head body, you can make an access panel similar to the one described in Figure 1 to cover the access aperture. Where instrument backs protrude through the liner or there is no liner, fabricate a box-like cover [see DIY 1998-#2 issue, “Good Boatkeeping” column — Ed] or use inexpensive plastic mixing bowls for individual covers (not pretty, but novel). Connect light and power supply leads to your breaker or fuse panel following manufacturer’s instructions, test for proper operation and the job is finished.