Towing and Being Towed

Prerequisites

This lesson is part of the RYA/MCA Yachtmaster Offshore syllabus. You should already:

• Be confident handling a yacht under power in confined spaces and at sea, including the effects of prop walk, windage and stopping distances (Day Skipper and Coastal Skipper boat handling).
• Know the knots and line handling used aboard: bowline, round turn and two half hitches, rolling hitch, cleating and surging a line under load.
• Understand VHF procedure and the Pan-Pan urgency message (SRC level).
• Know COLREGs Rule 24 and the lights and shapes in Annex I, and the sound signals of Rule 35, as covered in the COLREGs lesson.

The Yachtmaster Offshore syllabus lists towing and being towed under emergency situations. Candidates may be asked to rig and explain a tow or come alongside another vessel for an alongside tow during the practical exam.

Learning Objectives

By the end of this lesson you should be able to:

• Assess whether a tow is safe and appropriate, and when to accept, offer or decline one.
• Agree communications between the two vessels before the line is passed.
• Rig a bridle and towline to strong points with chafe protection.
• Explain catenary, towline length and why a bar-taut tow is dangerous.
• Pass the line safely and take up the tow, then control speed and course.
• Carry out an alongside tow for close-quarters manoeuvring.
• Release a tow in an emergency, and avoid the common mistakes.
• State the lights and shapes and legal and salvage aspects of towing.
• Map your knowledge to the RYA Yachtmaster Offshore syllabus: the emergency situations section (G158 syllabus and logbook), covering towing and being towed, preparing the casualty, rigging a towline, controlling the tow and releasing it, together with the related COLREGs lights and shapes (Rule 24) and sound signals (Rule 35) and the examiner's expectation that you can explain the physics (catenary, snatch loading, yaw) as well as the seamanship.
• Estimate the loads in a tow and choose line and fittings with a sensible safety margin.
• Prepare and manage your own vessel when you are the one being towed, including accepting a tow from a lifeboat.

Loads in a Tow: Why Numbers Matter

A tow looks gentle: two boats moving at three knots. The loads are not. There are three:

1. Steady load. The force needed to drag the tow through the water at the chosen speed, plus the force of the wind on the tow's hull and rig and the push of the waves.
2. Dynamic or snatch load. When the boats surge out of step and the line comes up bar-taut, the stored kinetic energy of a heavy yacht is dumped into the line and fittings in a fraction of a second. Peak loads of three to five times the steady load are normal in a seaway, and more with a stiff (low-stretch) line.
3. Side loads. When the tow sheers or yaws, the load on the bridle and the fairleads is applied at an angle, which concentrates it on one cleat.

Estimating the steady load

The wind force on an object is approximately:

Force (newtons) = 0.5 x 1.2 x (wind speed in m/s) squared x drag coefficient x frontal area in square metres

To convert knots to metres per second, divide by two (roughly: 1 knot = 0.51 m/s). To convert newtons to kilograms-force, divide by ten.

Worked example. A dismasted 12 m yacht lies head to a Force 6 wind of 25 knots (12.9 m/s). The hull side and cabin top, together with the stump of the mast and rigging trailing, present a frontal area of about 10 square metres with a drag coefficient near 1.0.

• Wind force = 0.5 x 1.2 x 166 x 1.0 x 10 = about 1,000 N, or 100 kg-force.
• Hull resistance towing a 9 tonne displacement yacht at 3 knots (well below her hull speed of about 4.2 knots) is perhaps 200 to 300 N on flat water; steep seas can multiply this several times because the yacht is repeatedly stopped by wave faces.
• Steady load in these conditions is therefore of the order of 1.5 to 2 kN, say 150 to 200 kg-force.
• A snatch factor of 4 gives peaks of 6 to 8 kN, or 600 to 800 kg-force.

Choosing the line

Typical breaking loads for new three-strand nylon rope are roughly: 12 mm about 2,200 kg; 16 mm about 3,600 kg; 20 mm about 5,500 kg. Treat these as approximate and check the manufacturer's data. Then apply honest reductions:

• wet nylon loses about 10 to 15 per cent of its strength;
• every knot or tight bend can cost 30 to 50 per cent at that point, which is why splices and well-laid hitches matter;
• an old, UV-damaged or chafed line may have lost half its strength without looking bad;
• a sensible working load for a tow in a seaway is no more than about one fifth of the new breaking load.

For the 16 mm line in the example: 3,600 kg, less 15 per cent for wet, is about 3,000 kg; a working load of one fifth is 600 kg. The predicted peak of 600 to 800 kg is right at the limit, which is exactly why you would tow slowly, use a long line and ensure a good catenary. If the peaks were likely to be higher, two lines in parallel, or a doubled line, would be sensible.

The same logic applies to the fittings. A typical 8 mm bolted stainless cleat on a 10 m yacht may be good for only 500 to 1,000 kg if it is bolted through a thin deck without a backing plate. The mast foot of a keel-stepped mast, the windlass base (if through-bolted to the deck and bulkhead) and a samson post going down to the keel are far stronger. Look at what is underneath the deck before you choose.

Taking a Tow: Preparing Your Own Yacht

Sooner or later most skippers are the casualty. Being towed safely is a skill of its own, and the examiner may ask "your engine has failed and a fishing boat offers a tow: what do you do?"

1. Call for help first if there is danger. If you are being set towards a lee shore or are in a busy shipping lane, Pan-Pan the coastguard before anything else. Tell them you have been offered a tow and who by.
2. Agree terms. Ask the towing vessel's skipper for the tow destination and the arrangement (see the salvage section below). Never sign anything you have not read under pressure; agree a fixed price or "assistance only" on the radio and log the time.
3. Pick the strongest attachment on the bow. Often this is the anchor windlass base, the samson post, or a bridle led from both bow cleats and a tape or strop round the mast foot. Do not tow from the pulpit, a stanchion base or a single cleat.
4. Lead the line through a fairlead, not over the pulpit rail. Pad everything with hose, rag or leather: nylon will saw through gelcoat and stainless steel alike.
5. Rig the release. Make the line fast with a round turn and several figure-of-eight turns round a cleat, or with a round turn and two half hitches. Keep the free end flaked clear and keep a knife where you can reach it.
6. Crew in lifejackets, clipped on if on the foredeck, and then keep everyone off the foredeck once the line is tight. Only one crew, with a knife, should be near the line.
7. Steer. Keep someone on the helm. A yacht that steers to follow the towing boat's wake is safer and easier to tow than one that wanders. If your rudder has been lost, trail a bight of warp or a drogue astern. If the helm is free, a small amount of weather helm can keep the yacht tracking, but avoid fighting the line.
8. Be ready to anchor. If the tow parts near land, you need an anchor ready to let go. Keep the engine on standby if it can be started safely.
9. Keep communicating. Use the agreed channel and the agreed hand signals. Report any chafe, noise or rising water immediately.
10. Prepare for the arrival. Agree beforehand whether you will be towed alongside into the harbour, who will take the lines at the berth, and how the tow will be released: the tow-er will usually stop, you will slack the line and cast off at their command.

Taking a tow from a lifeboat or coastguard vessel

RNLI lifeboats and coastguard craft use their own towing gear and are practised at it. The lifeboat coxswain is in command. Expect:

• a crewman to come aboard for a short time to assess damage and to fit the lifeboat's own line; follow his or her instructions;
• the lifeboat's line (usually a heavy, braided or three-strand rope) to be made fast to your strongest forward fitting, sometimes around the mast;
• to be asked to stay clear of the foredeck and let the lifeboat crew do the work;
• the lifeboat to take you at a speed it considers safe, often 4 to 6 knots, which may be faster than you expect: be sure your forward fittings and rudder can take it, and say so if not.

Do not argue about how the tow is rigged: lifeboat crews do this many times a year. But do speak up if you can see a problem (a loose deck fitting, a hatch that will go under, water coming in forward).

Special Cases

Towing a dinghy, RIB or small craft

A dinghy towed astern is a normal sight, but it is also a regular cause of lost dinghies and fouled propellers. Tow from the bow eye of the dinghy, with a bridle if there is a tendency to sheer. Keep the painter long enough so that the dinghy rides on the back of the second wave behind you rather than surfing into your stern. Take the outboard off and stow it aboard. Never tow a dinghy with crew in it unless they are wearing lifejackets, and never at more than displacement speed. In rising wind and sea, lift the dinghy on deck or on davits (check your stability calculation: davits add weight high, see the stability lesson) or cut it loose rather than let it endanger the yacht.

Towing under sail and short tows

A sailing yacht can tow a small casualty under sail, and a casualty with a lost engine in a confined anchorage can be towed alongside by a dinghy under outboard power. The principles are the same, but there is less margin for error. Sail balance is changed by the load on the stern, and the helm may be stiff. Reef early and keep the main sheeted hard to maintain steerage.

Towing a vessel with no rudder, no engine or no crew control

A vessel without steering will yaw in a way that can overpower a small towing boat, and will be dangerous to bring alongside in any swell. Consider streaming warps or a drogue astern, towing at lower speed with a longer line, and (if conditions allow) taking the crew off first. A casualty with a fouled propeller should shut off the engine and put it in neutral, because the rotating shaft with a line on it can overheat the gearbox or damage the stern gland.

Aground

Do not attempt to tow a grounded yacht off with brute force. Check first whether the tide is rising (it may float off by itself), whether the hull is damaged (a keel strike and a hull breach should be dealt with in line with the damage control lesson), and whether pulling will move her into deeper water or only cause more damage. Pull from the direction she came from, at an angle to the keel so that the keel does not dig in further, and avoid snatch loads.

When Things Go Wrong: Fouled Propellers and Parting Lines

• Line round your propeller. Stop the engine at once and put it in neutral. Cut the line free if you can reach it safely (a diver or a person on the bathing platform with a serrated knife, in a flat calm), or have the casualty's crew slip the tow and anchor if you are near the shore. Never start the engine again until you have checked the propeller is clear. An engine started with a rope round the shaft can break the shaft coupling or stern gland.
• Parted towline. Recover the broken ends carefully, because the snap-back of a nylon line can injure. Inspect the line and the fittings. Look for the cause: chafe, a poorly made-up bridle or a surge in a following sea? Do not re-use a line that has parted without checking its strength; a replacement or a stronger arrangement may be needed.
• Tow overrunning the towing vessel in a following sea. Slow down gently, add length to the line, and trail a drogue from the casualty.
• Casualty taking water. Slow to minimum speed to reduce water pressure at the damage, keep pumping, and consider whether to take the crew off. The casualty skipper's immediate responsibility is the crew, not the boat.
• Crew overboard from the tow. The towing vessel must stop but beware the tow running down on her. Both skippers should have a plan for this: slip the line, then recover the person as in the man overboard drill.

Before You Tow: Risk Assessment

The first question is whether to tow at all. Towing puts big, unpredictable loads on the gear and hulls of both boats, and places people near lines under tension. A 10 m yacht towing a heavier vessel in a rising sea can be pulled off course, have its cleats torn out or a line wrapped round its propeller, turning one casualty into two.

Consider:

• Power and displacement. Can your engine control the combined displacement in the expected wind and sea? A rough rule is that a yacht can tow a vessel of similar size in moderate conditions. A much heavier tow, or one with a lot of windage, may be beyond you.
• Weather and sea. Wind against tide or a steep sea makes towing much harder and increases shock loads.
• Distance to safety, and hazards. How far to the nearest suitable port? Is there a lee shore? A tow at 3 knots covers only 3 miles in an hour.
• Crew. Enough experienced crew on both vessels to steer, handle lines and watch the tow?
• The alternatives. Would the casualty be better anchoring and waiting for the lifeboat? Would it be better to take the crew off and leave the boat? Has the coastguard been told?

If the casualty is in danger (a lee shore close by, people injured), the skipper should be making a Pan-Pan or Mayday so the coastguard can send a lifeboat. A friendly tow does not replace the rescue services.

Communication

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Agree before the line goes across:

• a primary VHF working channel and a backup (and mobile phone numbers);
• who is in charge (normally the towing skipper);
• intended course, speed and destination;
• hand signals for slow down, speed up, stop and "slip the tow", for when the radio fails or the crew cannot hear;
• an emergency abort signal, for example several short blasts on the horn, meaning stop and slip the tow;
• how the tow will be shortened, transferred to alongside, and released at the end.

Keep listening on the agreed channel and on Channel 16 throughout.

Rigging the Tow

Strong Points

Cleats on many yachts are not designed for towing loads. Use the strongest fittings available: the anchor windlass base, a samson post, mast foot (keel-stepped), sheet winches or properly backed cleats. Spread the load across several points.

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On the towing vessel the line should lead through a fairlead, or over the quarter, to a strong attachment that can be released under load. Avoid leads that cross sharp edges.

The Bridle

On the towing yacht, a bridle from both quarters spreads the load and keeps the towing point ahead of the rudder, which means the towing boat can still turn. A tow from a single stern cleat pins the stern and can make the towing yacht almost impossible to steer.

On the towed yacht the line usually goes to the bow, where a bridle to both sides keeps the load centred and the boat tracking straight.

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Practical tips:

• Lead the bridle legs round winches, mast or windlass, not just a single cleat.
• Protect against chafe at every fairlead, stemhead and edge with hose, leather or cloth. Chafe is the commonest cause of a parted towline.
• Make sure the bridle can be released under load from the deck, for example by leading it to a winch so that it can be eased, or using knots like a round turn and two half hitches. A bowline under heavy load is very hard to undo.
• Use nylon (polyamide) for the towline because it stretches and absorbs shock. Polyester stretches less, Dyneema hardly at all. Floating polypropylene is useful as a messenger and stays clear of propellers but is weak.

A round turn and two half hitches is the classic knot for a towline because it can be tied and released under load (surge it round the fitting first), and it holds firm when the line is wet and tight. The photograph below shows the finished hitch. Tie it round a strong fitting, not just a thin rail.

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Image: USCG PTC Developer, CC BY-SA 4.0, via Wikimedia Commons

On the towed boat, the line can also be made up on a cleat with a round turn and figure-of-eight turns, locked with a final hitch if it will not be released under load. The photograph shows a correctly tied cleat hitch; for a towline add an extra round turn first, so that the line can be eased under tension.

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Image: BenFrantzDale, CC BY-SA 3.0, via Wikimedia Commons

Catenary and Towline Length

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The towline should sag in a curve (the catenary) between the two vessels. As the boats surge and the line tightens, the curve straightens first, cushioning the load. A bar-taut line delivers every surge as a snatch load that can pull out a cleat or part the line.

• A long line gives more catenary and more stretch. In open water, use as much line as you have: at least 3 to 5 times the combined length of the vessels, more in a seaway. Typical yacht tows offshore use 50 to 100 m.
• Adjust length so the two boats are "in step", that is both on the crest or both in the trough of waves at the same time. When out of step, one boat is surging down a wave while the other climbs, causing snatching.
• Weight in the middle of the line (a length of chain or the anchor) increases the catenary.
• In harbour approaches and confined waters shorten the tow, or change to an alongside tow.

Passing the Line and Taking Up the Tow

1. Approach the casualty from downwind or down-tide so that you can stop alongside under control, or pass close to windward and let the line drift down. Keep your propeller well clear of lines in the water.
2. Pass a light messenger first with a heaving line or a float, then pull the heavy towline across with it. Never throw a heavy line at people.
3. Make the line fast on the towed boat as agreed. The towed boat confirms "line fast" by radio or signal.
4. Take up the slack gently at dead slow ahead, paying out to the agreed length. The line should come taut slowly while the casualty starts to move.
5. Increase speed slowly. A displacement yacht under tow should rarely be pulled faster than about 5 knots, and usually less.
6. Nobody stands in the bight of the line, or in line with it near the fittings: a parting line under tension can snap back and injure severely.

Controlling the Tow

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• Speed. Start dead slow. Increase gradually and watch the line and the tow. Excessive speed increases loads sharply and makes the tow sheer and yaw. A rule of thumb is not to exceed the hull speed of the smaller vessel and usually to tow well below it.
• Course. Make alterations slowly and in small steps, warning the towed vessel first. A sharp turn pulls the tow sideways and can capsize a small boat or part the line.
• The towed vessel should steer to follow the towing vessel's wake. If she cannot steer (lost rudder), she can trail a drogue or warps astern to reduce yaw.
• Sea state. Head into or away from the predominant swell rather than across it. A tow beam-on to the swell rolls heavily and snatches.
• Surfing. In following seas the tow may surf up behind and overrun the towing vessel. Trail a drogue from the towed vessel's stern and lengthen the line.
• Check regularly for chafe, fittings working loose and water coming in, at least every half hour. Rotate crew on watch at the towline.

Alongside Tow

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For entering harbour, picking up a mooring or berthing, a tow astern gives too little control. Instead, lash the two vessels together alongside:

1. Put out plenty of fenders between the hulls.
2. Position the towing vessel with her stern aft of the towed vessel's stern, so her propeller and rudder are clear and she has better steerage.
3. Rig a bow line, a stern line and, most importantly, springs: the forward-leading spring from the towing vessel takes the load when going ahead, and an aft spring when going astern. Make all lines as tight as possible so the two hulls move as one.
4. Angle the towing vessel's bow slightly towards the tow (toe in), to counter the tendency to turn away from the tow.
5. Manoeuvre slowly. Turning towards the side with the tow is harder; plan turns away from the tow where possible.

The alongside tow is for calm water only. In any sea the two hulls grind together, the lines snatch and fenders burst.

Emergency Release

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Both vessels must be able to slip the tow at any time without help from the other. Situations include: the towed vessel sinking or catching fire, the towline fouling a propeller or rudder, the tow being swept towards danger, or someone caught in the line.

• Keep a sharp knife (or an axe for heavy line) at each towing point, ready to cut.
• Make fast in a way that can be released under load: a round turn on a cleat or winch that can be surged, or a slip.
• Agree the abort signal in advance and keep crew clear of the line.
• After release, recover the line carefully on the towing vessel, keeping it clear of the propeller, and inspect for damage before reuse.

Lights, Shapes and Sound Signals

When towing, both vessels must show the correct lights and shapes (COLREGs Rule 24):

The length of tow is measured from the stern of the towing vessel to the after end of the tow. For a yacht that cannot show the towing lights, Rule 24(i) allows a vessel not normally engaged in towing to show what she can, illuminating the towline when unable to show the lights. In restricted visibility the towing vessel sounds one long and two short blasts at intervals of not more than 2 minutes; a manned towed vessel sounds one long and three short immediately after.

Salvage and Liability

Accepting a tow can have legal consequences. If you are towed by a commercial vessel or another yacht without agreement, the tow-er may claim salvage, an award based on the value of the vessel saved and the danger involved. To avoid disputes:

• agree terms on the radio before the line is passed, for example "tow to Falmouth at a fixed cost" or "no cure, no pay" Lloyd's Open Form if offered by a professional;
• use your own towline if possible, which strengthens the argument that it was a tow, not salvage;
• record everything in the log;
• tell your insurer as soon as possible.

RNLI lifeboats do not claim salvage for saving lives, although they may tow you in when lives are at risk.

Worked Example: A Second Scenario, Dismasted Yacht Far Offshore

Scenario. You are skipper of a 10 m yacht with a crew of four, 40 miles offshore. You come across a dismasted 12 m yacht, drifting with three uninjured crew. The forecast is for the wind to increase to Force 6 within six hours, from a direction that will drive her towards a lee shore 15 miles downwind. You have 60 m of 16 mm nylon warp and no dedicated towing equipment.

Decision framework.

1. Call the coastguard first. Pan-Pan with both positions, the situation and your intention. Ask whether a lifeboat or tug can be tasked. A tow in these circumstances should be a holding action, not a long-distance rescue.
2. Risk assessment. A dismasted yacht has little windage once the wreckage is cleared, but she has no steerage, may have rigging round her propeller and rudder, and is heavy. Your 10 m yacht towing a 12 m yacht in building wind is at the edge of its capability. Nylon is the right material (elastic), but 16 mm is marginal for sustained heavy weather, as the loads table above shows.
3. Options. (a) Tow now while conditions allow, towards the nearest safe haven or simply to keep her off the lee shore; (b) take her crew aboard your yacht and abandon hers if the tow is unsafe; (c) stand by and wait for a tug or lifeboat, keeping her out of danger by towing slowly; (d) ask her crew to make sure the wreckage is cleared and that any rig trailing in the water is cut away.
4. Decision. Attempt the tow now, while the sea is manageable, with explicit abort criteria. If the tow cannot be controlled within 30 minutes, or if the line or fittings show signs of failure, you transfer her crew to your yacht. Tell the MRCC the plan and agree reporting intervals.
5. Rig. Make the bridle from her strongest points (windlass base, both bow cleats and the mast stump if it is solid) with maximum chafe protection, and double the line if the load looks high. A bridle on your own yacht from the primary winches through the quarter fairleads. Knives at both ends.
6. Execute. Dead slow ahead, 3 knots through the water at most, heading into the swell. Accept slow progress over fast failure. If the casualty steers wildly, ask her to stream warps from the stern.
7. Review at each half hour. Chafe, fittings, weather, fuel, crew fatigue, ETA of assistance. Hand over to the lifeboat or tug as soon as one arrives.

The examiner's question underneath is always: do you consider whether the tow is safe before you rush in, and do you have a way out?

Examiner Expectations

The Yachtmaster Offshore examiner may test towing in practice (rigging a tow and taking up the tension, or coming alongside another yacht) or in the oral. They are looking for:

• a calm, ordered risk assessment before any line is passed;
• knowledge of the physical principles: catenary, shock loading, bridle geometry, the effect of speed and wave step;
• a sound rig with chafe protection and the ability to release under load, with a knife in reach;
• clear communication with the other vessel and with the coastguard;
• awareness of the legal position (lights, signals, salvage, insurance);
• the humility to say when you would decline or abandon a tow.

A common oral question is "how long should the towline be and why?" The answer: as long as practical, so that there is plenty of catenary and the boats can be in step, shortening only in confined waters.

Quick Reference Checklist

Worked Example

Scenario. You skipper a 10 m yacht with 4 crew. A 12 m yacht has been dismasted 15 miles off a lee shore. The wind is south-west Force 6 and rising. Her engine will not start because the rigging went round the propeller. The coastguard knows and the lifeboat is 2 hours away. The casualty asks you to tow her out to sea, away from the shore, while waiting.

1. Risk assessment. Your engine is 30 hp; the casualty displaces about 9 tonnes. Towing to port into the sea is not realistic. But holding her off the lee shore at slow speed, heading into the swell, is possible. You agree to try, with abort criteria: if the tow is out of control after 30 minutes, you will slip and stand by to take off the crew.
2. Communications. Agree Channel 72 primary and 77 backup, the abort signal (five short blasts), and that you are in charge.
3. Rig. On your yacht, a bridle from both primary winches through the quarter fairleads, padded with hose. On the casualty, the line to the windlass and both bow cleats, with chafe protection at the stemhead. Knives at both ends.
4. Pass the line. Pass close to windward of the casualty and float a messenger down on a fender; they haul the 60 m nylon towline across.
5. Take up slack. Dead slow ahead, paying out to 60 m until the catenary forms.
6. Increase revs in small steps to about 3 knots through the water, heading into the swell.
7. Monitor. The tow yaws badly; the casualty streams warps from her stern, which steadies her. Every 30 minutes check the chafe points. You keep sending updates to the coastguard.
8. Handover. The lifeboat arrives, takes over the tow, and you release the line on request, recovering it clear of your propeller.

The decision to accept slow progress and set abort criteria is the key to doing this safely.

Common Mistakes

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• Towing from a single cleat, which can be torn out of the deck. Use a bridle to several strong points.
• Too short a line, so there is no catenary and every surge snatches.
• Too much speed. Start dead slow and increase gradually.
• No emergency release plan. Have knives ready and a way to slip under load at both ends.
• Towing beam-on to the swell. Align the tow with the swell.
• Ignoring chafe. Check at least every 30 minutes and re-protect.
• Lines in the water near the propeller when passing or recovering the tow.
• Crew standing in the bight or in line with a loaded towline.
• No agreement on salvage terms before accepting a tow.

Knowing when not to tow is as important as knowing how.

Summary

• Assess first: power, displacement, weather, distance to safety, crew and alternatives. Inform the coastguard.
• Agree communications, signals and who is in charge before passing the line.
• Use a bridle to strong points with chafe protection; nylon towline; releasable under load.
• Long line with good catenary, vessels in step with the waves.
• Pass a messenger first, take up slack at dead slow, increase speed gradually.
• Alter course gently, keep the tow in line with the swell, check for chafe regularly.
• Use an alongside tow for harbour manoeuvres, with springs and the towing vessel's stern aft.
• Both vessels must be able to release at any time; keep a knife ready.
• Show correct lights and shapes, and agree terms to avoid salvage claims.

Check Your Understanding

1. Give four factors you would consider before agreeing to tow another yacht.

Answer: Any four of: your engine power compared with the tow's displacement and windage; weather and sea state; distance to safety and hazards such as a lee shore; experience and number of crew on both boats; strength of fittings; and whether the rescue services would be a better option.

2. Why use a bridle on the towing vessel rather than a single stern cleat?

Answer: A bridle spreads the load across strong points and keeps the towing point ahead of the rudder so the towing vessel can still steer. A single stern cleat may be torn out and pins the stern, making steering difficult.

3. What is catenary and why does it matter?

Answer: The sag of the towline between the vessels. As the boats surge the curve straightens before the line comes taut, absorbing shock loads. A bar-taut line snatches and may part or tear out fittings.

4. What does it mean for the two vessels to be "in step" and how do you achieve it?

Answer: Both vessels are on the crests or in the troughs at the same time, so they do not surge in opposition. Adjust the length of the towline to match the wavelength.

5. In an alongside tow, where should the towing vessel be positioned and which lines are most important?

Answer: With her stern aft of the towed vessel's stern so her propeller and rudder work effectively, bow slightly toed in. The springs are the most important lines, taking the load ahead and astern, plus bow and stern lines, all tight.

6. What lights does a power-driven vessel show when towing astern with a tow length under 200 m?

Answer: Two masthead lights in a vertical line, sidelights, a sternlight and a yellow towing light above the sternlight.

7. Why should you agree terms before accepting a tow from a commercial vessel?

Answer: Because without an agreement the tow-er may claim salvage, an award that can be a large proportion of the value of the vessel. Agreeing a fixed price or recognised terms, using your own line and logging everything reduces the risk of disputes.

8. What must be available at both ends of a tow, and why?

Answer: A sharp knife (or axe) and a way of releasing the line under load, so that each vessel can slip the tow instantly and independently if the tow becomes dangerous.

9. You are being towed by an RNLI lifeboat. How should you prepare, and who is in charge?

Answer: The lifeboat coxswain is in command. Prepare the strongest forward fitting (windlass base, samson post or mast foot), pad the lead against chafe, keep a knife ready, put the crew in lifejackets and keep them off the foredeck, steer to follow the lifeboat, and have an anchor ready. Follow the crew's instructions but report any problem you can see.

10. A 16 mm nylon line has a new breaking load of about 3,600 kg. What working load is sensible when towing in a seaway?

Answer: Allow for wet strength (about 10 to 15 per cent less) and for knots and ageing, then work to no more than about one fifth of the new breaking load, so roughly 600 kg here. If the predicted snatch loads exceed that, use a longer line, lower speed, a doubled line or a stronger arrangement.

11. Your propeller is fouled by the towline. What do you do?

Answer: Stop the engine and put it in neutral immediately, slip the tow and, if near danger, anchor. Clear the line only when it is safe to do so, and do not restart the engine until the propeller is verified clear, because a fouled shaft can break the coupling or stern gland.

12. What are the lights of a manned vessel being towed, and what sound signal does she make in restricted visibility?

Answer: Sidelights and a sternlight, and a diamond shape by day if the tow is over 200 m. In restricted visibility she sounds one prolonged and three short blasts, immediately after the signal of the towing vessel (one prolonged and two short at intervals of not more than 2 minutes).