Why the Dark Bits Deserve Your Full Attention

Our canal bridges and tunnels were built for horse-drawn boats, not for the glossy steel narrowboat you're standing at the tiller of. A bridge hole may be only 7 feet wide with a high, blind arch, and the old tunnel portals often curve inward so tightly that you have no view of what is coming the other way. Add a bit of rain, a rise in water level and a bend on the approach, and a moment's carelessness puts fibreglass and steel together.

The good news is that the skills involved are straightforward, and they are the same ones passed down from boat to boat on the towpath. Slow down, make yourself heard, know your headroom, and go in with everything stowed and lit. Get that right and the dark bits become the most atmospheric part of a day's cruising.

Approaching a Blind Bridge Hole

Treat every bridge hole as one-way traffic with no mirror. Begin by slowing to tickover a good boat length or two before the arch, then keep well to the right — historically the convention when meeting head-on under a bridge, as it echoes the rules of the road and gives the other boat the deep water in the middle.

If you cannot see through the arch, give one long blast on the horn: a steady, deliberate note of three or four seconds. Keep the horn to hand at the tiller so you are not scrabbling around in the cabin for it. Then listen. In summer, sound carries poorly along a tree-lined cutting; the reply may be muffled, so be ready to stop anyway.

A few points that catch people out:

• Shallow water near the edges slows your boat quickly, so your stopping distance changes as you move across the channel.
• Bridges on a bend often hide two boats, not one.
• Boats coming the other way may be silent because their crew is single-handed, or simply not paying attention. Assume nothing.
• After heavy rain, the river-fed sections can run fast through a bridge hole, shoving your bow sideways as you enter.

Checking Your Headroom Before You Commit

Air draught is the distance from the waterline to your highest point, and it is always bigger than owners think. Before a low bridge or a tunnel, run through the inventory: chimney, aerials, cratch board, pram cover, windlass left on the roof, boat pole, mop, bicycle, and that folding chair someone put up there an hour ago.

Remove the chimney entirely where there is any doubt — it saves several inches and stops diesel fumes drifting back into the cabin. Fold down anything taller than the cabin roof line and secure it, because a pram hood frame that swings loose inside a tunnel is a nuisance you do not want.

Some bridge arches carry a headroom gauge in feet and inches, marked at the centre of the span. Do read it as a guide rather than gospel: levels change with rainfall, and the gauge may be marked from a different waterline. If you are unsure, stop in a safe spot short of the bridge, walk forward with a boat pole and measure the clearance at the centre of the arch where your cabin roof will pass.

Tunnel Preparation: The Two-Minute Checklist

Tunnels run from a few dozen yards to well over a mile, and there is no turning round and no mooring inside. Take two minutes before you enter the portal:

• Lighting: switch on the tunnel lamp, plus cabin lights and a torch within reach. Oncoming boats need to see you early.
• Ventilation: keep engine room vents clear and watch the temperature gauge. Airflow is still, so engines and crews both get hot.
• Loose items: close windows and doors, stow the washing, and secure mugs and the kettle.
• Crew: everyone seated, inside or low on the counter, and briefed to stay still and quiet.
• Horn and signals: one long blast before entering, then listen for a reply before you commit.

If the tunnel is controlled by a keeper or traffic lights, wait for the green and follow the instructions given. Never follow another boat in against the signal.

Inside the Tunnel: Speed, Spacing and Sound

Once in, hold a steady slow speed — just enough to steer, no more. Wash bounces off the walls and can bounce your bow off them too, and shallow tunnels in particular make a boat wander. Keep to the centre where the water is deepest, and hold the tiller with a light, ready hand rather than wrestling it.

Leave a good gap behind the boat ahead. Their exhaust, noise and wash all concentrate in the confined space, and if they stop suddenly you want room to do the same. Do not overtake, do not stop, and do not attempt to turn. If you meet an oncoming boat where there is clearly no room to pass, slow right down, hug your side and edge past — the other helm will usually hold back where the bore is wider.

Sound the horn only when you need to. A long blast in a tunnel is astonishingly loud, and a needless one tells nobody anything useful. On the way out, ease gently back up to cruising speed as the light returns — the exit arch can hide a moored boat or a sharp bend just beyond it.