Anodes or Cathodic Protection

Play Video

Cathodic Protection in Yachts

If you have two different metals physically or electrically connected and then you immerse them in seawater, they become a battery. Some current flows between the two metals and the electrons that make up that current are supplied by one of the metals giving up bits of itself-in the form of metal ions-to the seawater. This is called galvanic corrosion and can destroy underwater metals. The obvious casualty is the bronze or aluminium propeller on a stainless steel shaft.

In order to counteract galvanic corrosion you need to add a third metal into the circuit, one that gives up its electrons first. This piece of metal we call a sacrificial anode, and usually it is zinc. It cannot be emphasised enough the importance of maintaining the zinc anodes on your boat. When an anode is dead, the metal component it was installed to protect begins to die.

professional yacht surveyor will check your yacht has cathodic protection.

Factors to bear in mind
The larger the surface area of a zinc anode the more protection, although this does depend on the metal it is protecting. Replace an anode when half gone do not nickel and dime.

Check your anodes at least annually and replace those that are half depleted.

For protection it is CRUCIAL that an anode must be in electrical contact with the metal being protected. Low-resistance, metal-to-metal contact is required so you can either by mounting the zinc directly to the metal being protected or connect the two with a wire.

Ensure that when an anode is mounted directly to the protected metal, e.g bolted to the side of a metal rudder, the surface that the anode is connected to must be bare and bright. Do not paint anodes.

What can go wrong?
Read about Skin fitting failures here

This Westerly Pageant had a nice shiny new anode but someone had removed the connection to the stern gear. The result? Dezincification of the propeller and the need for a replacement. Note the pink appearance? When sounded with a hammer the propeller did not ring like a bell, and when struck on it’s edge with a file a large piece of propeller blade came away.

Boat electrics

Electrical Boat Surveys


Stray Current (an explanation)
Stray current is electricity that is flowing where it’s not supposed to — through water, fittings on your boat, wet wood, damp surfaces, etc. It can be a shock hazard and it can cause corrosion (technically known as electrolytic corrosion). Stray current corrosion is caused by a power source such as your batteries or the shore power connection. It is unlikely for serious corrosion to be caused by stray currents flowing through the water, without a metallic path to your boat. Because of the relatively high driving voltages, stray current corrosion can act far more quickly than the corrosion caused by dissimilar metals in contact (galvanic corrosion).

Note: The word electrolysis is often mistakenly used to describe various kinds of corrosion. Electrolysis actually refers to the bubbling off of gases that occurs with electrolytic corrosion.

Common Ground Point (ship’s ground) Grounds from batteries, engine, switch-panel negative bus bar, bonding system, auxiliary power generator, underwater ground plate, ship’s 120 Volt safety-ground, and LORAN signal ground all meet at one point.This point must be a heavy bus bar or bracket with bolted connections.Note: When referring to 12 Volt wiring, ‘ground’, ‘negative’ and ‘ground return’ are all equivalent terms.

  • Easy to access and located as far above bilge levels as practicable.
  • Labelled as Common Ground Point.


WARNING! The hydrogen gas in and around lead-acid batteries is explosive and the acid can burn skin and eyes. Avoid sparks and wash well after handling your battery.

  • Acid (electrolyte) level is up to plastic liner inside holes. Letting the acid level go below the top of the plates will kill a battery quickly. Use distilled water to top up batteries. If distilled water isn’t available, tap water is OK if it’s clear, not ‘hard,’ and not highly chlorinated. Let the cold tap run for a minute to clear metal ions out of the pipes and use a well rinsed, glass or plastic container to transfer.
  • fully charged specific gravity is 1.245 to 1.300 in each cell.
  • In a partially discharged battery, specific gravity of each cell does not vary by more than 0.050 from the other cells. Battery cells sometimes charge with uneven specific gravities but after discharging about 25% (from a full charge) they should even out.Note: If it’s been awhile since charging the battery, acid may have settled to the bottom leaving a lower specific gravity electrolyte on the surface. If you overfilled the battery then the electrolyte may be diluted. Either of these situations can result in abnormally low readings and they don’t necessarily indicate a weak battery cell.
  • ‘At rest’ battery voltage is 12.1 to 12.8 Volts. A battery is ‘at rest’ when it isn’t being used and hasn’t received a charging current for at least 12 hours. A voltage above 12.8 Volts indicates that the battery is still settling after a charge. A voltage below 12.1 Volts indicates either a weak cell or a battery charge below 50% of capacity.Note: Standard batteries have their life span shortened drastically by deep discharges, even to the 50% level. True deep cycle batteries (see below) function well with 50% discharges.Engine cranks properly for 5 seconds with each battery alone — battery voltage is above 9.5 Volts and steady while cranking. Perform this test only after engine has been running so that protective oil has circulated. Disconnect coil ‘+’ wire or engage diesel fuel shut-off mechanism to keep engine from starting. It’s possible for batteries to fail this high current test while still being able to provide good storage capacity at lower currents.Note: If engine doesn’t crank properly and battery voltage remains high, then there is a problem with the starting circuit or starter motor.
    Note: Starter or electric winch motors will normally ‘pull’ a battery’s voltage down to 9 or 10 Volts while they’re operating. The battery should recover most of its ‘at rest’ voltage within seconds.
  • Batteries draw not more than a few amps of charging current once they are charged except during conditioning (see below), water loss is at most a few ounces (50 to 100 ml) per cell, per year. Significant water loss indicates a problem. If the water loss occurs evenly in the cells, alternator or battery charger voltages may be too high. Water loss in only one or two cells indicates weak or shorted cells.
  • Batteries are true deep cycle type if used for anything but starting.
  • Specify that you want ‘golf cart’ batteries because most marine/RV ‘deep cycle’ batteries are only marginally better than automotive batteries for deep cycling. True deep cycle batteries will provide many hundreds of charge/discharge cycles instead of only a few dozen.Note: Avoid discharging deep cycle batteries below 50% of their capacity. A 50% discharged battery has an ‘at rest’ voltage (see above) of 12.1 to 12.2 Volts.
  • Top surfaces clean and dry.
  • Cables in good condition – ends are soldered and correct size for terminal connectors. Check cables for broken or corroded strands, especially at the ends.
  • Only one cable to each terminal. In particular, avoid small wires in battery compartment. Run them to the battery switch and switch-panel negative bus bar instead.
  • No connection depends on spring tension (i.e., no alligator clips).
  • Connections cleaned and sealed.
  • Positive terminals have insulating cover. Negative cables go directly to Common Ground Point. Many systems have the negative cable running directly to the engine as part of the starting circuit. This means that other negative connections need to be at the engine, or in the battery box, which can cause corrosion problems.
  • Positive cables go directly to nearby battery switch.
  • No batteries wired in parallel.Paralleled batteries tend to fight each other when they are at rest — this causes premature discharge and a shortened life span. It’s OK to parallel batteries temporarily with the battery switch, while charging, starting and running the engine – just avoid leaving the switch on ‘BOTH’ when no power is being drawn. If you require a large battery capacity, connect several 6 Volt or even 2 Volt cells in series instead of wiring 12 Volt cells in parallel.Note: Two batteries are in parallel if their positive terminals are connected and their negative terminals are connected.
  • Ventilation is provided for cooling and for venting the gases produced by batteries. Batteries produce hydrogen, oxygen and corrosive sulphide gases. The lighter-than-air hydrogen must be able to rise naturally through a venting system, with or without a blower.
  • Batteries can be conditioned with an equalizing current. After a normal full charge, conditioning consists of applying a reduced charging current (2 to 5 amps for most batteries) either for a few hours or until battery voltage rises to 15.5 – 16.5 Volts — this takes the lead sulphate ‘crust’ off the battery plates and helps maintain full storage capacity. Check the acid level when finished because this process causes bubbling and fluid loss. Condition batteries every month when they’re being used heavily. Conditioning requires either an override on the alternator’s standard voltage regulator or a battery charger with a conditioning or ‘equalising’ option.Note: Don’t condition batteries when they are in parallel or one battery may take most of the conditioning current.
    Note: Shut off all electronic equipment during conditioning because of the high battery voltage.
  • Inlet vent below batteries.
  • Outlet vent as high as possible in battery compartment.
  • if using an electric blower for battery venting, the motor is not in the air stream.
  • Ventilation system is for batteries only.
  • Batteries strapped down and prevented from shifting.
  • Battery compartment protected against acid spills.
  • Easy to access and located as high above bilge as practicable.
  • If batteries are not being used, they are given a full charge at least once every 3 months.Lead acid batteries will self-discharge over a period of months so they should be charged periodically to ensure that they don’t completely discharge. This is especially important during freezing weather because a discharged battery can freeze develop cracks in the case.

(NOTE: if you are not able to understand any of the above then please refer to a Boat Electrical Surveyor)

Boat Osmosis

Boat Osmosis

Slide 1 Heading
Click edit button to change this text. Lorem ipsum dolor sit amet consectetur adipiscing elit dolor
Click Here
Slide 2 Heading
Click edit button to change this text. Lorem ipsum dolor sit amet consectetur adipiscing elit dolor
Click Here
Slide 3 Heading
Click edit button to change this text. Lorem ipsum dolor sit amet consectetur adipiscing elit dolor
Click Here

Information about Boat Osmosis

What is it boat osmosis  ?

Fibreglass osmosis  is the process of a lower density liquid moving into a higher density liquid through a semi permeable membrane.

In lay mans terms it is a natural process where two dissimilar liquids try to mix. the weaker trying to mix with the stronger. This effect results in a chemically induced pressure rise in the area of the high density liquid.

A Glass Reinforced Plastic( GRP) boat hull is a made up of a mixture of polyester resin reinforced with glass fibres, formed in layers which make up a laminate. This laminate has small voids or  air pockets and micro cracks within the resin. These occur at the interface between the resin and the glass fibres. Water diffuses into and then passes through the gel coat and the laminate in the form of water molecules, not a liquid. Water may pass slowly through a Glass Reinforced Plastic hull in this way or the water molecules can collect and condense within these voids. These are the blisters and you you burst one it will smell of strong vinegar. Within the voids/laminate are various water soluble components. These are solvents and unreacted constituents from the manufacturing process.

The water within the voids dissolves and reacts with these components. (Hydrolysis). The ongoing (Hydrolysis) will continue within the voids enlarging the cavities and forming a solution. The solution is water absorbing (“hydroscopic”) and once the solution is released in the voids it accelerates the rate of water absorption into the laminate. This process cannot be reversed simply by taking the boat out of the water. As this continues the voids are increased in size and the pressure within increases. At some point the pressure may become too high for the surrounding material to support and a blister is formed. As this process continues, moisture continues to be absorbed, the laminate break down accelerates and multiple blisters form. In time some larger blisters may develop within the laminate as well as those more commonly occurring between the gel coat and laminate.

Eventually at this stage, treatment will be required as the structural integrity of the hull can be effected.

If you inspect a hull yourself you can detect them. Look closely at the example pictures I have put on the page. However it is better that a skilled person does this for you. Any  professional boat surveyor will recognise the early signs of boat osmosis.


In the Mediterranean Sea a boat is usually safe from osmosis if it has been anti fouled correctly and not left in the water for more than three years(rule of thumb only)

No yacht has sunk from boat osmosis blisters,  not that I am aware of anyway, but big boat osmosis blisters weaken the laminate and ultimately affect the seaworthiness of the boat.

Boat osmosis treatment cost

The cost of boat osmosis repair can be high 6000 pounds for a 35 footer and five months out the water.

Boat osmosis repair

The water and the contaminants in the laminate have to be removed and whilst drying will remove the water it leaves many of the contaminants behind so for this reason steam cleaning and washing the hull surface is important but not sufficient. Depending on how big and how many blisters you have, the decision on how much work is down to you. A few isolated blisters can be carefully cut off with a chisel, washed out and filled with epoxy resin. Even large blisters can be treated this way.


Play Video

Boat osmosis blisters


Peeling the gel coat is common but extreme and on its own often adds to problems. Blisters will return, even after peeling, if there are still voids in the laminates and uncured gel coat,  more blisters will occur. So if you are the owner and don’t intend to sell, my personal view is deal with them individually as they occur. If you are planning to sell, yes the fact is, boats with untreated osmosis are harder to sell, so you will need to get a professional, warranted job done.

The gel coat is a very effective water barrier and does not allow the passage of glycols so it has to be removed usually by the use of a “Gel Peeler”. This removes a controlled thickness of gel coat/laminate leaving an even surface.

To promote drying and to abrade the surface left by the “Gel Peeler” the peeled surface should be “grit blasted” after peeling, for maximum effect this work should be done immediately after the end of the season. Thereafter the hull should be steam cleaned and washed regularly for a period of time to ensure that the solutions are washed out. At this stage, I would recommend HOTVAC treatment. This cures the uncured resin and reduces he chance of re-occurrence. When the surface is found to be neutral, the drying process can commence. Initially this can be air dried but eventually the hull will need to be heated to reduce moisture content to a very low level. Once the hull is dry a new coating can be applied. 

15 Steps to Buying a used boat in Spain.

15 Steps to Buying a used boat in Spain

 Buying a used boat in Spain is often a significant financial commitment. This guide will ensure you make the right decision:

  1. Which boat? – Decide what you want your boat for before you start looking. Cruising/racing? Do you have a young family? Port hopping or cruising round the Canary islands? or even Charter?
  2. Budget – Review your budget and don’t forget mooring and marina fees, fuel, slipping and winter storage fees, life jackets. If you are thinking of chartering, the cost of setting up a company and having the boat ‘coded’ to the MCA requirements.
  3. Where there are so many different ports around Spain make sure you make the right choice. Near to an airport, good security, reasonable fees, interesting sailing or boating nearby?
  4. Gather information – Get on the net watch videos , read reviews. Take your time. Talk to friends, people at the sailing club, etc about the types of boats you are considering. Read reviews from the magazines – a number of magazines have libraries of reviews.
  5. Scan the internet, look at what boats are advertised Get an idea of pricing, look at the exchange rate..
  6. When you find a boat you like, talk it through with the broker or seller and then make an offer based on what you have seen and understand and state clearly that your offer is subject to survey. The broker will provide a Sale & Purchase Agreement that both parties sign. Any amendments must be agreed by both parties. This is a binding contract so be sure before you sign. 
  7. The Deposit – You will need to place a deposit “arrhes” (usually 10%), which the broker must put in his Client Account (not his usual business trading account). This is your money held by the broker as stakeholder.
  8. The Survey – You should get the vessel surveyed by a professional boat surveyor before completing the purchase. The broker may not recommend a surveyor but may have a list of local surveyors. ( ask for a list)
  9. Read the survey report through carefully and discuss any queries with the surveyor. Advise the broker of any important defects. If they are substantial (“material”) you may consider withdrawing under the terms of the contract, revising the price to complete the sale, or requiring the vendor to provide a repair or piece of equipment. The contract will have a time frame for actions to be taken – be clear what these are. You will be responsible for lifting fees and the surveyor’s fees.
  10. Finance – If you require finance ensure you contact a finance house in good time, as the money needs to be available when you reach completion.
  11. Sea Trial – If you want a trial sail, advise the broker early so he can set this up, and allow for changes due to weather, for instance. You will be responsible for any slipping or skipper’s fees.
  12. Final checks – Decision made, check the inventory and that the title documentation is lodged with the broker to save any problems later, including the VAT documentation as appropriate.
  13. IBAN or Electronic Transfer set up the electronic transfer to the Broker’s Client Account in good time. Once the funds have cleared, completion can take place. The broker will give you the documentation, including the Bill of Sale transferring title from the vendor to you, and the keys to the boat.
  14. “Aseguro” better an English company. Insure it! Don’t forget to insure the boat before you take possession. You will almost certainly need a survey for this, so don’t be tempted to skip the pre-purchase survey, which can be used for this purpose too.
  15. Training – Remember buying a used boat in Spain you need the appropriate license (Day skipper, Power boat, e.t.c. British qualifications are accepted. If you haven’t sailed or driven a powerboat before do get some training before heading off. It is essential you know the ‘Rules of the Road’ and have adequate safety equipment. The RYA has a wide range of courses to assist you.buying a used boat in spain

Add Your Heading Text Here

Add Your Heading Text Here

Add Your Heading Text Here