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If sacrificial anodes are disappearing very quickly this can indicate there is a galvanic or electrolytic problem caused by electrical currents, which means an electrical circuit is being made for some reason. In order to understand what could be going on, it is worth trying to get to grips with the differences between these two forms of corrosion.

Galvanic corrosion is an electrochemical reaction between two or more different metals, in the presence of an electrolyte (note salt water is a good electrolyte). In other words, one piece of metal will be corroded, known as the anode, and the other piece of metal will be unaffected, known as the cathode. Common examples of galvanic corrosion are where an aluminium outdrive comes into contact with a stainless steel propeller, causing the aluminium to corrode, or where aluminium comes into contact with copper or bronze fittings. The first signs of galvanic corrosion is blistering of paint below the waterline and the formation of a white powdery substance on exposed metal. The exposed metal areas become deeply pitted as the metal is eaten away by the corrosion.

Electrolytic corrosion, also known as stray current corrosion, is an electrochemical process where a metallic surface is corroded by another metal it is in contact with, due to an electrolyte and the flow of an electrical current between the two metals, caused from an external source of electromotive force (EMF). In other words, electrolytic corrosion is caused by a current such as a boat’s battery or a shore supply. Metal hull boats are particularly at risk because their hulls are good conductors and stray wires or connections use the hull as a ground.

Preventing electrolytic corrosion requires good electrical installation where marine grade wiring should be insulated return with two wires rather than earth return. Metal hulls must never be used as the earth return.

Risks from electrolytic and galvanic corrosion

Aluminium boats are at risk from electrolytic and galvanic corrosion. Aluminium boat owners need to take precautions that would not be of concern to owners of GRP boats. For example, it is wise not to stay too long berthed next to a steel-hulled boat in a marina, where both vessels are connected to shore power. The reason for this is an electric circuit can form between the two vessels with the aluminium hull acting as an anode and the steel hull acting as a cathode, resulting in galvanic corrosion of the aluminium hull.

Owners also need to make sure that the shore power polarity is correctly wired in marinas as this can lead to corrosion too. They also pay particular attention to the condition of their wiring and electrical systems, to protect the boat from electrolytic corrosion.

Some aluminium boats are equipped with current leakage meters which display real time corrosion potential that warns of problems with electrolytic and galvanic corrosion.

Protecting an aluminium hull

  • Electrolytic action – all stray currents must be removed from the boat’s electrical system.
  • Anodes – zinc anodes should should be positioned on the outside of the hull and replaced when 80% consumed.
  • Stern gear – to protect the stern gear, the propeller shaft should be fitted with an electro-eliminator brush and earthed to the hull.
  • Insulation – insert plastic seals between aluminium and other metals. For example, ensure gaskets are fitted between aluminium and the flange of a stainless steel or brass valve. The same applies to lead or iron keels, where insulating washers and gaskets between the hull and keel.
  • Painting – while aluminium topsides do not need to be painted, antifouling coating beneath the waterline does need to be applied. Only use coatings that are designed for use with aluminium.

 

Sailing at the touch of a button

Easier and more controlled sail handling can also be achieved by powering up a furling mast. I came across some interesting solutions at the Southampton Boat Show this week on the Selden Mast stand, where they were running demos of their E40i electric winch and SMF furling system.

Jester Challenge 2022 – Sailing single handed from Plymouth UK to the Azores: Part 2 – Weather

Jester Challenge – A modern experiment in old-fashioned self-reliance, self sufficiency, and personal responsibility. This is the second of a 10-part post where solo sailor, Bernie Branfield, shares his first hand account of his single-handed, 2022 Jester Challenge, from Plymouth, UK to the Azores, in his 26′ Invicta Mk2, Louisa.

Jester Challenge 2022 – Sailing single handed from Plymouth UK to the Azores: Part 6 – Communications

Jester Challenge – A modern experiment in old-fashioned self-reliance, self sufficiency, and personal responsibility. This is the sixth of a 10-part post where solo sailor, Bernie Branfield, shares his first-hand account of his single-handed, 2022 Jester Challenge, from Plymouth, UK to the Azores, in his 26′ Invicta Mk2, Louisa.

How to operate a winch

Winches are drum shaped mechanical devices used to handle halyards, sheets and control lines. One of the important crew...

Antifouling for leisure boats – Part 2

To prepare for antifouling, as soon as your boat has been lifted out and pressure washed, you need to check all the surfaces of the hull below the waterline, remove any remaining barnacles and check for blisters.

Wooden Hull Repairs

While wooden boat hull maintenance is mostly straightforward, it is always a good idea to take expert advice on any repair job needed doing to a wooden boat, unless you have done the job before and know what you are doing.

Jester Challenge 2022 – Sailing single handed from Plymouth UK to the Azores: Part 10 – The Return Trip

Jester Challenge – A modern experiment in old-fashioned self-reliance, self sufficiency, and personal responsibility. This is the final instalment of a 10-part post where solo sailor, Bernie Branfield, shares his first-hand account of his single-handed, 2022 Jester Challenge, from Plymouth, UK to the Azores, in his 26′ Invicta Mk2, Louisa.

First aid at sea basics

At least one person on board should be trained in first aid and know how to administer the contents of the first aid kit, ensuring there are adequate supplies for the planned duration of the trip.

Steel and Aluminium Hulls

The two metals used for hull construction are steel and aluminium. These are both very strong materials and will last a long time as long as they are cared for, which primarily means protecting steel boats from rust and aluminium boats from electrolytic action.

Navigating narrow channels

Navigating narrow channels can present challenges for boat operators due to limited space, restricted manoeuvrability, and potential hazards such as shallows, rocks, or other vessels. To ensure safe passage through narrow channels, it’s essential to follow specific strategies and techniques.

Leaking decks

Leaking decks are perceived as a nuisance by some boat owners, but if leaks are ignored a much more serious situation may well be developing, especially in the case of boats with balsa or plywood deck cores. So deck leaks do need to be investigated and dealt with.

Sending distress signals

In an emergency situation at sea, it is a top priority is to know how to send and receive emergency radio calls and alert others of your predicament. Likewise, if you receive a distress signal, you must be ready to go to the help of others.

Keeping boat records and doing checks

A boat’s records should provide information about maintenance schedules, when major work was done and when equipment was replaced or added to the boat. Without this information you are left guessing when things are likely to need replacing in the future and also what the costs are likely to be.

Estimating and plotting your position at sea

This post covers how we go about estimating and plotting our position using traditional methods, when out of sight of land – covering Dead Reckoning, Estimated Position, Tidal Streams, Leeway and more…

Common medical emergencies at sea

A medical emergency aboard a boat at sea requires immediate attention to ensure the safety of the casualty and the crew in general. The skipper needs to know which crew members, if any, have had medical training or have a first aid qualification. All boats should carry first aid handbooks to help an untrained crew cope with a medical emergency.

Marine engine electrical system

The typical basic electrical system associated with a marine engine includes a dedicated engine starting battery, a starter motor, a charger in the form of an alternator, a solenoid and some engine sensors and instruments.

Keel design – options to consider when choosing a yacht

Keel design is constantly evolving and nowhere is this more apparent than in modern racing yachts such as the Imoca Open 60 class. These fast...

Top 5 Reasons Why an Inflatable SUP Should Be Your Next Yacht Accessory

In this article, inflatable paddle board expert Jason Paul gives the top 5 reasons why an inflatable SUP should be your next...

Finding your way at sea: waypoints

Any sea voyage needs a certain amount of planning before it is undertaken. It makes sense to think about where you are going, how you will get there and what factors might influence your plan. Planning the route itself is also critical. One of the essential parts of modern navigation is the use of waypoints.

Boating Rules of the Road – International ColRegs

    International ColRegs Rule 7: Risk of Collision Anyone who is responsible for a vessel at sea, from the...

Fixing position at sea using traditional methods

This post looks at some traditional methods used for fixing a vessel’s position at sea, within sight of land. Electronic fixes using chart plotters are very straightforward to record, but if for some reason a vessel’s electronics are faulty it is essential that a skipper knows how to use traditional methods.

Safety at sea principles

Safety at sea is not as simple as just spending money adding shiny new emergency equipment such as liferafts, danbuoys, distress flares, EPIRBs and so on.

Cutless bearing replacement

Cutless bearings can last for many years but if the propeller shaft is out of alignment they will wear through more quickly. If you have noticed a clunking sound when motoring then it could be a worn cutless bearing that is causing the problem.

Boat surveys

A full boat survey assesses the condition of the hull, mechanical gear and means of propulsion. The survey is carried out with the boat...

Estimating your position at sea

Navigators use a combination of techniques to estimate their position at sea. The primary method used today is GPS (Global Positioning System), a network of 24 satellites that became fully operational in the 1990s and was originally restricted for use by the United States military.