Narrowboat Caught on the Cill: What to Do

A narrowboat caught on the cill is one of the most serious lock incidents a boater can face. It can happen quickly, especially when descending a lock, and it can put the boat, crew and nearby helpers at risk.

The good news is that cilling is usually preventable. With a clear understanding of how locks work, where the danger lies and what to do if something goes wrong, you can reduce the risk significantly.

This guide explains what the cill is, why boats get caught on it, the immediate actions to take in an emergency, and the everyday habits that help keep lock passages calm and safe.

What Is the Cill in a Canal Lock?

The cill, sometimes spelt sill, is the raised ledge at the upper gate end of a lock. It forms part of the lock structure and helps seal the lock gates when the chamber is full.

When a boat is going uphill, the cill is usually beneath the water and less likely to cause trouble. When a boat is going downhill, the danger increases because the water level drops and the cill becomes exposed inside the lock chamber.

If the stern of a narrowboat is positioned too far back over the cill while the lock is emptying, the boat can become caught. The bow continues to drop with the water, but the stern remains supported on the cill. This creates a dangerous angle and can lead to water entering the front of the boat.

Most locks have a cill marker, often a painted line or sign on the lock wall. This marks the approximate position of the cill. The boat must stay forward of this marker when descending.

What Does It Mean When a Narrowboat Is Caught on the Cill?

A narrowboat caught on the cill is usually stuck by the stern during a downhill lock passage. As the water level falls, part of the back of the boat rests on the cill instead of floating freely.

This is dangerous because the boat is no longer level. The bow drops, the stern is held up, and the boat can tip sharply. In a severe case, water can rush through the front doors, bow well deck drains, vents or other openings.

Once water starts entering the boat, the situation can worsen rapidly. A boat can sink in a lock if the paddles are not closed quickly and the water level is not restored.

Cilling is not simply a scrape or a bump. It is a genuine emergency that needs immediate action.

Common Causes of Cilling

Cilling usually comes down to position, communication or inattention. Locks are simple in principle, but they demand concentration every time.

Common causes include:

• The boat being too far back in the lock: This is the classic cause when descending. The stern drifts over the cill before or during emptying.
• Not watching the cill marker: The marker exists for a reason. If the stern is behind it, the boat is at risk.
• Leaving paddles open while distracted: A lock can empty quickly enough for a problem to develop while someone is chatting, walking away or looking elsewhere.
• Poor communication between crew: The steerer may see a problem before the lock wheeler does, or the lock wheeler may not realise the boat has drifted back.
• Using ropes incorrectly: A tight rope can hold a boat in the wrong place, while a loose or unattended rope may allow it to drift.
• Wind or water movement: Turbulence from paddles, by-washes or gusts can move the boat unexpectedly.
• Overconfidence: Familiar locks can still catch out experienced crews if attention slips.

Warning Signs to Watch For

Spotting a problem early makes all the difference. When descending a lock, keep checking the boat’s position and attitude as the water level falls.

Possible warning signs include:

• The stern is close to, on, or behind the cill marker.
• The boat stops dropping evenly with the water.
• The bow begins to dip more noticeably than the stern.
• The rudder or stern gear feels as though it has struck something.
• The boat appears to be hanging at an unusual angle.
• The steerer shouts or signals that something is wrong.
• Water starts approaching the front deck, well deck or door threshold.

If any of these signs appear, treat the situation seriously. Do not wait to see if it sorts itself out.

What to Do if Your Narrowboat Is Caught on the Cill

If a narrowboat is caught on the cill, the priority is to stop the lock emptying and refloat the stern. The exact actions depend on the layout of the lock and the crew available, but the core response is consistent.

1. Close the Bottom Paddles Immediately

If the lock is emptying, close the bottom paddles straight away. This stops more water leaving the chamber and prevents the bow from dropping further.

Do this quickly but safely. Use the windlass correctly, keep your face and body clear of the mechanism, and do not let the paddle gear run down uncontrolled.

2. Open the Top Paddles Carefully

Once the bottom paddles are shut, the lock needs to refill enough to lift the boat clear of the cill. Open the top paddles carefully and gradually.

A sudden rush of water can throw the boat about, so controlled refilling is important. If the boat is badly angled or taking on water, getting water back into the lock is urgent, but crew safety still comes first.

3. Keep People Off the Boat if It Is Unsafe

If the boat is at a steep angle or water is entering, do not send someone down into the cabin to rescue possessions. Boats can move suddenly when they refloat.

People are more important than property. If anyone is in danger, get them to a safe place as quickly as possible.

4. Communicate Clearly

Use short, clear instructions. Shouting can become confused around running water and lock machinery, so agree simple commands in advance where possible.

Useful phrases include:

• Close the bottom paddles
• Open the top paddles slowly
• Stop
• Hold the boat forward
• Everyone off

If there are bystanders nearby, give one person a specific job rather than accepting a crowd of conflicting advice.

5. Call for Help if Needed

If there is any risk to life, call 999. If the incident involves damage to the lock, a stuck boat, sinking risk or navigation obstruction, contact the navigation authority as soon as possible.

On Canal & River Trust waterways, use the emergency contact details displayed at locks where available, or contact CRT through their published emergency channels. On other waterways, follow the relevant authority’s guidance.

What Not to Do

In a stressful moment, it is easy to make things worse. Avoid the following:

• Do not keep emptying the lock: This increases the angle and the danger.
• Do not stand on a trapped boat: It may shift suddenly when the water level changes.
• Do not put yourself between the boat and lock wall: Crushing injuries are a serious risk.
• Do not use engine power aggressively: The propeller, rudder or hull may be in contact with the cill or lock structure.
• Do not ignore water entering the boat: Act immediately and get people to safety.
• Do not argue on the lock side: Clear action matters more than blame.

How to Prevent a Narrowboat Being Caught on the Cill

Prevention is much easier than recovery. Every downhill lock should be treated with the same care, even if it looks familiar or quiet.

Stay Forward of the Cill Marker

When descending, keep the stern of the boat well forward of the cill marker. The marker is usually painted on the lock wall, but it may be faded or partly obscured.

If in doubt, keep the boat further forward, while still allowing enough room for the bow to avoid the lower gates. On a shorter lock, positioning matters even more.

Keep a Crew Member Watching

Someone should always be watching the boat as the lock empties. The steerer has an important role, but a person on the lock side can often see the overall position more clearly.

Do not leave paddles open and walk away. Lock operation requires attention from start to finish.

Use Paddles Gradually

Opening paddles slowly gives you more time to spot movement and react. This is especially useful in unfamiliar locks or where the water flow is strong.

There is rarely any need to rush. A slower lock passage is far better than an emergency.

Manage Ropes Sensibly

Ropes can help control a boat, but they can also create hazards if used badly. Avoid tying a boat tightly while the water level changes, unless you are following a specific safe procedure under competent supervision.

A rope that cannot be released quickly can hold a boat down or pull it into the wrong position. If using centre lines or bow lines, keep them attended and ready to slacken or release.

Watch the Bow as Well as the Stern

The cill is at the stern end when descending, but the bow matters too. As the boat drops, check that the front is clear of the lower gates and not catching on gate projections, rubbing strakes or masonry.

Lock safety is about the whole boat, not just one hazard.

Lock Safety Habits for New and Returning Boaters

If you are new to narrowboating, or returning after a break, it is worth building a routine. Good habits reduce stress and make it easier for everyone on board to know what should happen next.

Before entering a lock, consider the following:

• Where is the cill marker?
• Which way is the lock working?
• Who is operating the paddles?
• Who is watching the boat?
• How will the steerer signal a problem?
• Are children, pets and non-essential crew in a safe place?
• Are windlasses, ropes and mooring lines ready?

During the lock passage, keep checking. A boat can drift even when it seemed well positioned at the start.

After leaving the lock, take a moment to reset. Close gates and paddles as required by local instructions, gather equipment, and make sure everyone is calm before moving on.

Single-Handed Boating and Cill Awareness

Single-handed locking needs extra care because one person must manage the boat, paddles and gates. There is less margin for distraction.

When descending alone, take your time. Position the boat carefully, check the cill marker, and open paddles gradually. Stay alert to the boat’s movement and be ready to stop the lock if the boat drifts back.

Many single-handed boaters use a centre line to steady the boat, but it should never be tied in a way that cannot be released as the water level changes. A trapped rope can become dangerous very quickly.

If a lock looks awkward, deep, turbulent or unfamiliar, it is sensible to wait for another boat or ask for help from competent nearby boaters. There is no shame in taking the slower, safer option.

After a Cill Incident

Once the boat is floating safely again, do not simply carry on as if nothing happened. Even a near miss deserves a careful check.

Look for signs of damage or water ingress, including:

• Water in the cabin bilge or under the floor
• Wet carpets, lockers or furnishings near the bow
• Damage to the rudder, skeg or stern gear
• Unusual engine vibration or steering problems
• Hull scrapes or dents
• Damage to lock gates, paddles or masonry

If the boat has taken on water, deal with that before continuing. Use bilge pumps if fitted and safe to operate, remove standing water where possible, and seek professional help if needed.

Report significant incidents to the navigation authority. This is especially important if the lock structure may have been damaged or the boat cannot move safely.

Why Calm Lock Working Matters

Locks are part of the charm of inland boating, but they deserve respect. Most passages are uneventful because boaters follow simple routines and stay alert.

A calm crew is a safer crew. Discuss the plan before entering the lock, especially if someone is inexperienced. Explain the cill marker, the direction of travel and what to do if a shouted instruction is heard.

Rushing, showing off or assuming someone else is watching can create unnecessary risk. Good boating is not about speed. It is about control, awareness and consideration for others.

Frequently Asked Questions

What is the cill on a narrowboat lock?

The cill is the raised ledge at the upper gate end of a lock. When descending, the stern of a narrowboat must stay clear of it. If the stern catches on the cill while the water level drops, the boat can tip dangerously.

Why is a narrowboat caught on the cill so dangerous?

It is dangerous because the stern can be held up while the bow drops with the falling water. This can cause the boat to sit at a steep angle and may allow water to enter through the front of the boat, potentially leading to sinking.

What should I do first if my boat is caught on the cill?

Close the bottom paddles immediately to stop the lock emptying. Then open the top paddles carefully to refill the lock and lift the boat clear. If anyone is in danger, prioritise people and call emergency services if needed.

Can using ropes prevent cilling?

Ropes can help control position, but they must be used carefully. Never tie a boat so tightly that it cannot rise or fall freely with the water. An attended line that can be released quickly is much safer than a fixed rope in a changing lock.

How can I tell where the cill is?

Look for the cill marker on the lock wall, usually a painted line or warning sign near the upper gate end. When descending, keep the stern forward of this marker. If the marking is unclear, stay well forward and operate the lock slowly.

Safe Locking Is About Awareness

A narrowboat caught on the cill is frightening, but it is also a risk that can usually be avoided with steady lock practice. Know where the cill is, keep the boat forward when descending, open paddles with care and make sure someone is always watching.

If something does go wrong, act quickly. Stop the lock emptying, refill carefully, keep people safe and ask for help when needed. With patience and good communication, lock passages can remain one of the most satisfying parts of narrowboat life.