Quick Reference
| MOB Rule | Tether on BEFORE leaving the cockpit — no exceptions |
| Tether Length | 1-meter tether: you stay aboard. 2-meter: you may go over. |
| PLB vs. AIS MOB | PLB alerts coast guard (hours); AIS MOB alerts nearby vessels (minutes) |
| Lifejacket | Crotch strap essential — prevents ride-up and injury |
| Night Rule | Always tethered after dark, no exceptions |
| Medical | Medlink via Iridium for at-sea consultation |
Man overboard is the offshore emergency that kills the most sailors. It is also the emergency most amenable to prevention. Unlike a knockdown, a dismasting, or a hull breach — which arrive without warning — falling overboard almost always results from a sequence of decisions that could have been interrupted at any point. Wearing a harness. Clipping on before moving from the cockpit. Maintaining three points of contact. Using the right tether length. These are not instinctive behaviors; they are trained habits that work only when applied consistently, including on the passages where it feels calm enough that they seem unnecessary.
The MOB Prevention Mindset
The fundamental rule of offshore watch-keeping is clipping on before leaving the cockpit — not after you reach the foredeck, not when conditions deteriorate, but before you step out. The clipping-on discipline is only useful as an absolute habit because it is precisely in the moments when you think it is unnecessary that the unexpected happens: the unexpected wave, the accidental gybe, the slippery foredeck at 3am when you are not fully alert.
The "one hand for the ship" principle — always maintaining a grip on a fixed part of the vessel when moving on deck — predates harnesses and jacklines and remains valid even when fully tethered. It reduces the force loading on your tether and harness by preventing the fall that the tether then has to arrest. Tethers and harnesses stop you from going over the side; grip and balance prevent the situation that would require them to do so.
The most dangerous moment on an offshore passage is the seemingly benign one: flat seas, moderate breeze, watch change at 2am, sixty miles offshore. Fatigue and familiarity erode discipline faster than bad conditions do. Experienced offshore sailors report that MOB incidents and near-misses most commonly happen in conditions that seemed manageable, not in the serious weather where everyone is already alert and clipped in.
Jackline Setup and the Tether Length Argument
Jacklines — lifelines running fore-and-aft along the deck that the tether clips to — should allow you to reach every part of the working deck without unclipping. On most cruising boats this means two jacklines, port and starboard, running from cockpit to bow, with attachment points in the cockpit for the tether before you move forward. The jacklines should be positioned inboard of the lifelines, not along the toerail. A jackline at the toerail allows you to go fully overboard while still attached; a jackline inboard of center keeps you on deck when you fall.
Tether length is one of the most consequential and least-discussed decisions in offshore safety kit. A 2-meter (double-length) tether, which is the standard length in most offshore safety kits, allows you to travel far enough from a jackline attachment point that a fall can place you fully in the water before the tether becomes taut. This is a documented MOB mechanism even for tethered sailors: they go overboard, the boat continues moving, and dragging a person through the water at boat speed is a serious and frequently fatal situation. A 1-meter tether keeps you on deck or at the rail when a fall is arrested. The tradeoff is reduced mobility, which is why most sailors carry both lengths. The rule for most offshore watch-keepers: clip the short tether when forward of the mast, use the long tether in the cockpit where mobility is less constrained.
Check jackline attachment points at the beginning of every passage. Soft jacklines — webbing rather than stainless wire — should be inspected for chafe, UV degradation, and stitching integrity. Replace soft jacklines annually or when wear is visible. Stainless wire jacklines last longer but are harder on hands and unforgiving on tether hardware. Both are used in offshore fleets.
Personal EPIRB (PLB) vs. AIS MOB Beacon
Every offshore crew member should carry a personal emergency signaling device on their harness or lifejacket. The two main options — Personal Locator Beacons (PLBs) and AIS MOB devices — work differently and serve different purposes.
A PLB transmits on the 406 MHz COSPAS-SARSAT frequency. The signal goes to satellites, which relay it to a ground station, which notifies the relevant Maritime Rescue Coordination Centre. The response time from activation to coast guard alert is typically in the range of thirty minutes to several hours depending on satellite geometry and ground station processing. PLBs provide global coverage including areas where no vessels are nearby. Registration with your national authority links your PLB to your vessel and personal information, which significantly speeds rescue response.
An AIS MOB device broadcasts on the AIS frequencies (156.8 MHz, 162.025 MHz), transmitting a distress signal that appears on the AIS display of any vessel within approximately five nautical miles. The response is local and nearly immediate — within seconds of activation, your mothership (if AIS-equipped) and any other vessel in range sees a MOB alert with your position. The weakness is range and coverage: in mid-ocean, there may be no vessels within five miles. The strength is the immediate alert to the vessel you just fell off.
The answer to the "which one" question is both. A PLB provides the long-range, coast-guard-alert function. An AIS MOB device gives your boat crew the best chance of finding you in the first critical minutes. Carry both, clipped to your harness or integrated into your lifejacket. The weight and cost are modest relative to the coverage they provide.
Lifejacket Inflation Mechanism and Fit
Offshore lifejackets are inflatable, not foam-block. Inflatable jackets pack flat and are tolerable to wear continuously; foam-block offshore jackets are unwearable for long periods and therefore not worn when they should be. The inflation mechanism — how the jacket inflates if you go overboard — is a decision with real consequences.
Automatic inflation jackets use a water-soluble bobbin that dissolves on immersion, triggering inflation without any crew action. This is the correct choice if the wearer might be unconscious — knocked out in a fall, incapacitated before hitting the water. The failure mode is nuisance inflation: automatic jackets can trigger in heavy spray, in pouring rain, or when washed by a wave, inflating on deck when you do not want them inflated. This is inconvenient and wastes the CO2 cylinder, but is not dangerous.
Manual inflation jackets require the wearer to pull a cord. They do not false-inflate in spray, but they require a conscious, functional person to operate. For sailors who are confident they would be conscious after going overboard, manual is a reasonable choice. Many offshore sailors use automatic jackets for night watches and switch to manual for daytime passages in rough weather where spray triggering is more likely.
Harness and lifejacket fit is non-negotiable. The crotch strap — a leg loop that prevents the jacket and harness from riding up over the shoulders when loaded — must be fitted and must be worn. A lifejacket without a crotch strap can, under the load of a tether arrest, ride up to neck level and cause serious injury or even decapitation in an extreme scenario. Check the crotch strap position before every offshore watch. Loose harness chest straps also ride up on inflation; the jacket should be snug enough that it cannot shift significantly under load.
Night Safety Discipline
The two absolute rules of night safety on an offshore passage: always tethered after dark; always tethered forward of the mast in daylight while underway. These rules are absolute because the conditions that feel safe enough to break them are precisely the conditions in which MOB incidents occur. An experienced delivery skipper's formulation is useful: "If you're on deck, you're clipped on. There is no exception. Not for two minutes, not to check the anchor, not because it's calm."
Night passages require additional awareness of silhouette and visibility. A crew member working on the foredeck at night is invisible to the helmsman unless deliberately illuminated. Red headlamp use preserves the helmsman's night vision; white light should be used sparingly. Call out before moving forward so the watch-keeper knows where you are.
Warm clothing and movement restriction contribute to MOB risk. Cold reduces dexterity and reaction time; bulky foul-weather gear restricts movement and reduces tactile feedback through hands and feet. Budget time before going on watch to dress properly rather than rushing on deck half-prepared. A watch-keeper who is shivering and moving awkwardly is more likely to lose footing than one who is comfortably dressed.
Alcohol and Watch-Keeping
The offshore zero-tolerance alcohol policy is defensible on purely practical grounds. Alcohol impairs balance, reaction time, and judgment in exactly the conditions that matter most on a night watch — fatigue combined with unfamiliar motion, reduced sensory input, and low illumination. A crew member who has consumed two drinks at dinner and is then called to watch at 2am for a sail change in building conditions is impaired by both the alcohol and the interrupted sleep. This is not a moral position; it is a mechanical one.
The reality in most cruising boats is a moderated rather than strict policy — drinks in harbor, no alcohol at sea. Most experienced offshore sailors arrive at this position naturally rather than through imposed rules. The question to ask before departure on any blue-water passage is whether the skipper is willing to defend the alcohol policy in a post-incident inquiry.
Medical Emergencies Offshore
Medical emergencies at sea are primarily managed through prevention (crew health assessment before departure, reasonable medical kit, basic first aid training) and through access to professional consultation rather than self-treatment improvisation. Three resources matter most when someone is seriously unwell offshore.
Medlink — a medical consultation service operated by MedAire — provides physician consultation via satellite phone or SSB radio to vessels at sea. The service is subscription-based and available through most satellite phone providers. A physician with access to your patient's symptoms, vital signs, and medical history can guide treatment, advise on medication use from your onboard kit, and help assess whether helicopter evacuation is appropriate. This service has managed appendicitis, cardiac events, severe infections, and trauma from offshore vessels. If you plan offshore passages, carry a Medlink subscription alongside your medical kit.
DAN (Divers Alert Network) is specifically relevant for boats carrying divers and for sailors who free-dive at anchor. DAN provides emergency consultation for diving-related injuries including decompression sickness, which requires hyperbaric treatment not available at sea. DAN's 24-hour emergency line connects to physicians specializing in dive medicine.
The helicopter evacuation threshold is "life-threatening only" — and this standard should be applied genuinely rather than conservatively. Helicopter evacuation is dangerous for both the patient being transferred and the crew of the rescue aircraft. Conditions that typically meet the threshold: uncontrolled hemorrhage, stroke, myocardial infarction, severe trauma, bowel obstruction, signs of serious infection not responding to antibiotics. Conditions that typically do not: fractures of non-critical bones, moderate infections responding to treatment, seasickness, non-cardiac chest pain. Medlink physicians are experienced at making this assessment and should be consulted before requesting evacuation. Get medical advice early; do not wait until the situation has deteriorated beyond the point where you have treatment options.
Offshore Security Hub
Return to the full security and safety guide collection — piracy risk areas, passage security protocols, and anchorage crime prevention: Offshore Security & Piracy Awareness →
← Anchorage Security | All Security Guides →
Frequently Asked Questions
Why does tether length matter so much for offshore MOB prevention? A 2-meter (double-length) tether — standard length in most offshore safety kits — allows travel far enough from a jackline attachment point that a fall can place you fully in the water before the tether becomes taut, and dragging a person through the water at boat speed is a serious and frequently fatal situation even for tethered sailors. A 1-meter tether keeps you on deck or at the rail when a fall is arrested, at the tradeoff of reduced mobility. Most offshore sailors carry both lengths and use the rule: short tether when forward of the mast, long tether in the cockpit where mobility is less constrained.
What is the difference between a PLB and an AIS MOB beacon and should offshore sailors carry both? A PLB transmits on the 406 MHz COSPAS-SARSAT frequency to satellites, which relay to a ground station that notifies the Maritime Rescue Coordination Centre — response time from activation to coast guard alert is typically thirty minutes to several hours but provides global coverage including areas with no vessels nearby. An AIS MOB device broadcasts on AIS frequencies transmitting a distress signal that appears on the AIS display of any vessel within approximately five nautical miles, giving the mothership a nearly immediate alert with your position — but in mid-ocean there may be no vessels within five miles. The answer is both: a PLB for the long-range coast-guard-alert function and an AIS MOB device for the immediate alert to the vessel you just fell off.
Why is a crotch strap on a harness and lifejacket considered essential rather than optional? A lifejacket without a crotch strap can, under the load of a tether arrest, ride up to neck level and cause serious injury or even decapitation in an extreme scenario. The crotch strap — a leg loop that prevents the jacket and harness from riding up over the shoulders when loaded — must be fitted and must be worn on every offshore watch. Loose harness chest straps also ride up on inflation; the jacket should be snug enough that it cannot shift significantly under load.
Where do offshore MOB incidents most commonly occur and when is clipping-on most critical? Experienced offshore sailors report that MOB incidents and near-misses most commonly happen in conditions that seemed manageable — not in serious weather where everyone is already alert and clipped in. The most dangerous moment on an offshore passage is the seemingly benign one: flat seas, moderate breeze, watch change at 2am, sixty miles offshore. Fatigue and familiarity erode discipline faster than bad conditions do, which is why clipping on before leaving the cockpit must be an absolute habit rather than a conditional one applied only when it feels necessary.
What medical consultation resources are available to offshore sailors when someone is seriously ill at sea? Medlink — a medical consultation service operated by MedAire — provides physician consultation via satellite phone or SSB radio to vessels at sea and is subscription-based and available through most satellite phone providers. A physician can guide treatment, advise on medication use from the onboard kit, and help assess whether helicopter evacuation is appropriate; the service has managed appendicitis, cardiac events, severe infections, and trauma from offshore vessels. DAN (Divers Alert Network) is specifically relevant for boats carrying divers, providing 24-hour emergency consultation for diving-related injuries including decompression sickness requiring hyperbaric treatment.