How to Run New Wires Through a Boat (Chase Tubes, Pull Lines and Deck Penetrations)

Turn off the battery switches and any charger, inverter or solar controller. Make sure the wire you're replacing, and any wire you'll be working beside, is de-energized.

Look under helm consoles, at the transom and in the bilge for the ends of the chase tube. Peek inside with a flashlight or inspection camera to see how crowded it is and whether there's an old pull string already in it. If there is, it's your best friend: tie a new string to it and pull both through so you always keep one in place.

Feed the fish tape in from whichever end gives you the most room to move your hands. BoatUS suggests going slowly and trying different angles until you find the path of least resistance, since a crowded chase can take several attempts. Use a tape with no sharp edges. Steel tapes work but can damage existing wiring; nylon or fiberglass is gentler. A clever free option BoatUS describes is the stainless inner wire from an old control cable, with the ends cut off, a small loop bent into one end and the cut tip filed smooth.

Small wires can sometimes be taped to the fish tape and pushed straight through. For anything complex, BoatUS recommends pushing the tape through, tying on a strong pull string, and using the string to pull the wire. Make the string long enough to leave a spare behind for next time, or pull a second string through with the wire.

How you attach the wire decides whether the pull works:

• Skip bulky knots. Twist the conductors through the loop of the tape or string.
• Wrap electrical tape over the whole joint, tapering it smoothly well above and below so it can't catch, building a bullet shape over any plug.
• Make it strong enough for a hard pull. Breaking halfway through is the worst outcome.
• Pulling several wires at once is fine, but lay them side by side; don't twist them together, or you'll never get one out again later.

Use a wire-pulling gel or silicone spray to ease friction; BoatUS says body powder also works if you want less mess. Avoid soap and petroleum-based products, which can break down insulation over time. On heavier cables, have a helper push while you pull and gently work the cable back and forth at tight spots.

Fish tapes tend to wander in open pipework like T-tops and rails. BoatUS describes two tricks:

• Vacuum: put a shop vac on the exit hole, feed yarn (a cotton ball or bit of tape on the end helps) into the entry, and let the suction pull it through. Then use the yarn to pull a stronger string, and the string to pull the wire.
• Magnet: tie a steel washer to the string and drag it along inside the tube with a strong magnet from outside.

If there's no chase tube, or it's full, look for another path: up inside the gunwale between hull and liner, through lockers, or through cabin structure on bigger boats. A construction diagram or an inspection camera helps. BoatUS says to keep the route away from moving parts, heat sources, fuel tanks and sharp edges. Adding a new length of chase tube where you can reach is often worth it for future projects.

Vibration slowly saws through insulation wherever a wire rubs. BoatUS cites an ABYC standard that calls for chafe protection where wires pass through or against a bulkhead, stringer or other sharp edge. Use a grommet in every hole you drill, and split loom or chafe sleeve where wires cross rough fiberglass edges or rub against each other.

Route wires away from exhaust, manifolds and other hot surfaces. BoatUS insurance data show that engine-room wiring problems start many boat fires, and recommends keeping all wiring supported away from heat. Keep wires up out of bilge water, and make connections in dry, accessible places.

Watch where the new fuse or switch ends up. A new circuit often brings an inline fuse holder, a switch or a small fuse block. Blue Sea points out that the Coast Guard (33 CFR 183.410) requires ignition-protected devices in spaces where gasoline vapor can collect, such as a gas engine room or a fuel-tank compartment, and that the same risk exists anywhere a gas can, propane bottle or leaky fuel fitting shares a space with a spark-producing part, even on a diesel boat. Ignition-protected parts are marked as such. If your route passes through one of those spaces, put the fuse, switch and splices outside it, or use parts marked ignition-protected; if you find an unmarked device already in there, have a marine electrician check it.

Secure the new wire at least every 18 inches along its run. BoatUS notes nonmetallic supports are fine in most places, but in the engine room use metal (cushioned) clamps, which won't melt and drop the wiring onto hot parts. Inside a long chase tube the tube does the supporting, but secure the wire where it leaves each end so it can't work back and forth.

BoatUS advises treating a hole in the deck like a hole in your house's roof. Two fittings are common:

• Deck gland (cable seal): a rubber gland squeezes around the cable as you tighten the fitting. The hole in the gland must match the cable diameter. Split glands let you fit the cable without removing a factory plug.
• Gooseneck (swan neck): an inverted J-shaped tube that sheds water and takes several wires of different sizes. BoatUS suggests sealing the entry with silicone or a little expanding foam, especially where seas can wash over it, and leaving extra wire on the outside so you can pull the plug out later.

For either type, seal any exposed deck core with epoxy before mounting so water can't rot the core, and bed the fitting with marine sealant. Put a drip loop in the wire just below the deck so any water that gets past drips off instead of running into the connection. Place the cable where crew won't grab it as a handhold. See our sealants and rebedding guides for the bedding steps.

Label every new wire at both ends with what it feeds and its circuit number; BoatUS recommends labeling both ends and keeping an updated wiring plan aboard. BoatUS tells of an owner who disconnected a pile of identical black wires for winter, reconnected them wrong in spring, and damaged the whole electrical system. Then make your terminations with proper crimps and heat shrink, power up, and check the new device works and the voltage at the device is close to the battery voltage.

Make the labels last. In BoatUS's guide to drawing your own wiring diagram, Tom Neale found that office-printer labels lose their glue in the oily, damp air of a boat and end up in the bilge. Use quality marine wire labels, or good white or yellow electrical tape wrapped all the way around the wire so it sticks to itself, written on with an indelible marker. Then add the new circuit to your wiring diagram, with its color, wire size and breaker.

If you ever need to find where an existing wire comes out of a chase, have a helper watch the far end while you tug the wire back and forth. A continuity test with a meter can mislead, because ground connections can show "ghost" continuity; seeing the wire move is the surest check.