How Long a PLB Will Really Transmit Offshore

Key Takeaways: A modern PLB might promise twenty four hours of transmission at minus twenty degrees, but offshore reality is rarely that tidy. Cold water, battery age, spray, and how well the antenna stays clear of the surface all chip away at the headline figure. A well maintained unit will usually manage somewhere between eighteen and twenty two hours in proper offshore conditions, but there are scenarios where you may see less. The trick is understanding what affects endurance and how to stack the odds in your favour.

How Manufacturers Arrive at the Twenty Four Hour Claim
What Actually Reduces Transmission Time at Sea
The Role of Water Temperature
Antenna Exposure and Why It Matters So Much
Battery Age, Storage, and Self Test Behaviour
How Sea State and Spray Influence Endurance
The Effect of GNSS Acquisition on Power Use
What Offshore Sailors Report in Practice
How to Maximise Your PLB’s Transmission Time
Summing Up

How Manufacturers Arrive at the Twenty Four Hour Claim

The twenty four hour figure printed on the box comes from a controlled laboratory test. The unit is placed in a chamber at minus twenty degrees and run until it no longer meets the minimum radiated power requirement. It is a fair benchmark for comparing one model with another, but it is not a prediction of what you will see in the water. The test assumes a fresh battery, a perfect antenna angle, and no wave action. Offshore sailors rarely enjoy such tidy conditions.

What Actually Reduces Transmission Time at Sea

Once you move from the lab to the ocean, several factors start to nibble away at the headline endurance. Cold water draws heat from the battery faster than cold air. The antenna is rarely held perfectly upright. Spray and waves interrupt the signal. A battery that is a few years old will not behave like a brand new one. All of these influences are small on their own, but together they can trim hours off the expected run time.

The Role of Water Temperature

Water conducts heat away from the battery far more efficiently than air. Even in temperate seas, the battery cools quickly once the unit is immersed. A cold battery delivers less current and reaches its cut off voltage sooner. In practice, this is one of the biggest reasons why offshore endurance tends to sit in the eighteen to twenty two hour range rather than the full twenty four.

Antenna Exposure and Why It Matters So Much

A PLB only performs properly when the antenna is upright and clear of the water. Even partial immersion reduces radiated power and forces the unit to work harder to maintain a stable signal. If the antenna is repeatedly knocked flat by waves or held at a poor angle, the power amplifier draws more current and the battery drains faster. This is why buoyancy pouches and raised mounting sleeves make such a difference in a person overboard situation.

Battery Age, Storage, and Self Test Behaviour

A PLB battery ages even when the unit is not in use. Heat accelerates this process, so lockers that warm up in the sun are not ideal storage locations. Frequent self tests also nibble away at the reserve. Manufacturers design the battery to meet its rated endurance at the end of its service life, but that assumes ideal storage and minimal testing. A unit that has lived in a hot grab bag for five years will not match the performance of one kept cool and tested sparingly.

How Sea State and Spray Influence Endurance

Heavy spray and breaking waves interrupt the signal and force the PLB to repeat bursts more often. This increases current draw. A calm sea allows the antenna to stay upright and dry for longer periods, which is why endurance figures from coastal rescues often look better than those from offshore incidents. The harsher the sea state, the more the battery has to work.

The Effect of GNSS Acquisition on Power Use

The first few minutes after activation are the most power hungry. The GNSS receiver works hard to acquire satellites and calculate a position fix. If the antenna is shielded by the casualty’s body or repeatedly submerged, the receiver may take longer to lock on. That extended acquisition period uses more energy and shortens the overall transmission time.

What Offshore Sailors Report in Practice

Reports from offshore sailors tend to cluster around the same numbers: eighteen to twenty two hours for a well maintained unit in cold water, sometimes a little more in warmer seas. There are outliers where a PLB has run for longer, but these usually involve calm conditions and a casualty able to hold the antenna clear of the water. At the other end of the scale, units with ageing batteries or poor antenna exposure have stopped transmitting several hours earlier than expected.

How to Maximise Your PLB’s Transmission Time

There are a few simple habits that make a noticeable difference:

Store the PLB somewhere cool and dry.

Keep self tests to the recommended schedule.

Replace the battery on time rather than stretching the interval.

Use a buoyancy pouch or mounting sleeve that keeps the antenna upright.

Practise deploying the unit so you know how to hold it clear of the water.

These small steps help the battery deliver its best performance when you need it most.

Factor	Effect on Endurance
Water temperature	Cold water drains heat from the battery quickly and shortens run time.
Antenna exposure	Poor angle or partial immersion reduces radiated power and increases current draw.
Battery age	Older batteries deliver less current and reach cut off voltage sooner.
Storage conditions	Heat accelerates battery ageing and reduces endurance.
Sea state and spray	Waves and spray interrupt the signal and force repeated bursts.
GNSS acquisition time	Longer acquisition periods use more power early in the activation.

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Summing Up

A PLB’s published endurance is a useful benchmark, but it is not a promise. Offshore conditions are messy, cold, and rarely kind to electronics. If you assume eighteen to twenty two hours from a well maintained unit, you will be in the right ballpark. Look after the battery, keep the antenna clear, and give the unit the best chance to do its job when everything else has gone wrong.

This article was written by Dick McClary, RYA Yachtmaster and author of the RYA publications 'Offshore Sailing' and 'Fishing Afloat', member of The Yachting Journalists Association (YJA), erstwhile member of the Ocean Cruising Club (OCC) and owner/administrator of 'Liveaboard Sailboats for Sale'.