A. Only structures immersed in water can be protected by sacrificial anodes. For onshore structures a coating such as Zinga can be used.

A. You can use either zinc plated or Stainless 316 mounting bolts. The stainless bolts will place more of a demand on the anode but this is not a problem. With a stainless bolt it is normally easier to undo the nut to change the anodes as the end of the season. It is extremly important to ensure no bilge water is allowed to wet the bonding connection on the inside of the boat, whether stainless or zinc plated the bonding cable may start to corrode and develop high resistance. Make sure you cover the connection with vaseline or similar to protect the connection.

A. The number, type and positioning of zinc anodes for full underwater hull protection is dependent upon the following factors:

1. Area of immersed plating and structure.
2. Efficiency of underwater paint coatings.
3. Anode life required.
4. Type and area of other underwater metalwork.
5. Type of anode considered most suitable.

The following anode selection table, assumes conventionally designed vessels - whether pleasure or working craft, having good underwater paint coatings with minimal paint loss (less than 10% over 24 months).

To select the correct type and size of anode, use our hull anode selector tool.

Wetted Surface Area Calculations

Using the main dimensions of the vessel in metres, i.e. length (waterline), breadth and draft (mean loaded), calculate area by the following formula: L W L x (Breadth + Draft). The resulting area applies to tugs, trawlers ferries, motor cruisers, launches and full bodied sailing craft.

For vessels with a wetted surface area larger than 110 square metres, please contact us.

A. You should change anodes if you are moving between fresh water and salt water.

• Zinc anodes should be used in salt water
• Aluminium anodes should be used in salt & brackish water
• Magnesium anodes should be used in fresh water

A. The anodes can only be matched on the drive and not the engine. If you have a DP-E drives you will need;
2x CMV290DPKITM for fresh water use.
You would also need 4 x triangle anodes but these are not available in magnesium.
You could also use a pear shape hull anode you will need a MD77 for fresh water use.

A. You will need the CMV290DPKIT (zinc for salt water use) plus you will need the 2 of the small triangle anodes CM872793Z.

A. The selection of materials is of prime importance in the construction of craft. Generally naval architects and boatbuilders ensure that they select metals which are, as far as possible, suitable for marine applications and that any combinations of different metals are compatible. When this is not possible metals must be isolated from one another. There will always be occasions when fittings or metalwork require replacement or repair and it is important that the same criteria is adopted. In particular ensure that fastenings and split pins are compatible and of the highest quality.

The paint system on any boat is an important first barrier against corrosion. Seek advice from the paint manufacturers for their recommendations on the most appropriate coating system and follow the application instructions completely. Ensure that a good anti-corrosive primer is applied if anti fouling is to be used. When using a copper based anti-fouling none of the paint must be applied directly to bare metal surfaces. Vegetable oil based paints, although far less widely available than in the past, should not be used with cathodic protection systems as the paint tends to saponify.

A. Cathodic protection is an electrochemical process which halts the natural reaction (corrosion) of metals in a particular environment by superimposing an electrochemical cell more powerful than the corrosion cell.

Sacrificial Anodes are fitted or bonded to the metal to be protected, this results in an electrical potential difference and the metal becomes cathodic causing the sacrificial anode to waste instead. In a correctly installed MGDUFF Cathodic Protection System corrosion only occurs to the sacrificial anode which is replaceable. The number and size of anodes is determined by the type of material and the surface area being protected.

Several factors determine the type of cathodic protection system fitted. Firstly the environment in which the vessel is operating, secondly the size and type of construction and finally the length of time that the vessel is likely to be afloat before the next maintenance slipping.
MGDUFF recommend the use of Zinc anodes for vessels permanently kept on a salt water berth.

A. Electro Chemical Decay is the breakdown of the cellulose structure in timber which occurs as a result of the flow of electrical current through water or a water saturated material between two connected electrodes immersed in the water. These electrodes may occur in the form of a natural anode and cathode in a galvanic or electrolytic coupling of dissimilar metals, or the elements of a cathodic protection system. In this process, known as “Hydrolysis”, water is ‘broken down’ into its component parts of hydrogen and oxygen. The liberation of hydrogen at the cathode results in an increased level of alkalinity which can cause the breakdown of the cellulose structure in timbers local to the cathode. Normally the alkalinity generated at the cathode is openly exposed to copious amounts of water and will constantly be diluted by that water. However the protective current from the anode will flow to all parts of the cathode surface where it is in contact with water including where the water is contained within saturated timber through which the cathodic item or its fastenings pass or are embedded. In these circumstances there is no dilution and levels of alkalinity can become concentrated enough to cause the timber to breakdown!

The effect of the breakdown is to allow a greater amount of water to enter the timber but not sufficient to materially reduce the alkalinity. Eventually leaks begin to occur along the path of the fastenings or fittings passing through the timber. The ingress of water is not initially great but contains a high level of alkaline salts in solution. On meeting the atmosphere within the vessel the water evaporates and the salts form crystals on the surfaces around the point of leakage, often building up to form a significant deposit. There will be very high alkaline conditions beneath the crystals (pH11 to pH13 are common) and the timber can break down into a fibrous woolly texture with little or no structural strength. Fastenings securing load bearing items such as shaft brackets begin to pull through the timber.
The first obvious indication of this condition is the formation of crystals around various metal items within the vessel. If these are found and the items are believed to be in contact with some form of anode the contact must be broken and the crystals washed off with fresh water. If the damage to the timber is slight it may be possible to avoid further deterioration by repeated washing of the internal surfaces with fresh water to dilute the alkalinity If the condition is detected in the early stages and dealt with in this way no further action may be needed but if the timber is softened structural repair will be required.

Probably the biggest single factor that encourages the conditions necessary for this process to develop is the drying out of the vessel when brought ashore for winter storage as joints can open and sealants dry out. Whilst the joints close again on re-floating the vessel the sealants do not always recover especially if the are themselves susceptible to alkalinity. In some instances repair may be extensive, particularly in the case of vessels constructed on the old system of double or triple skin planking where usually an impregnated cotton material is placed between the skins. In such vessels the cotton around the cathode also decomposes and acts as a reservoir for the highly alkaline water which it disperses throughout the skins to a far greater extent than if single skin planking were used. The multiple skins are much thinner than a single skin and degradation of the timber soon renders an area of the skin structurally weakened. Because the condition is usually worse within the vessel due to the concentration of salts brought about by evaporation it is often the inner skin of multi-skin vessels that requires most extensive repair and this usually involves removal of large areas of the external skins to afford access. Associated framing and supporting structures also often need repair. A further factor with this construction is the deterioration of the fabric membrane between the skins which will result in a small gap forming and some difficulty in maintaining a fully watertight hull thereafter. Differing types of timber react to the presence alkaline solutions to a varying extent but in general the heavy resinous types are more resistant than the hard wood types. A heavy single skinned vessel planked with a very resinous pine will be least effected whilst a light displacement multi-skinned vessel such as a double diagonal motor boat planked in African mahogany is likely to be severely effected.

This condition can be associated with galvanic corrosion or electrical installation defects as well as being an undesirable side effect of cathodic protection. However it can really only exist where some deficiency is already present in the vessel’s structure. That is not to say the vessel is un-seaworthy but the defect is more a natural process whereby the timber around fastenings, particularly in load bearing areas, becomes increasingly waterlogged as time passes. When constructed to good practise all metal fittings are bedded down on a sealant and the fastenings are made watertight by sealant around the external heads. The timber usually has some residual resin content and does not absorb much water in the early life of the vessel due to the protective effect of surface paint coatings etc. The condition is usually quite slow to start but becomes progressively worse. When large build ups of crystals are discovered internally it can be assumed structural damage to the timber exists. The extent of such damage will depend on the construction of the vessel and the type of timber used.

A. Keep all wiring, connections and junction boxes above the bilge area and other areas likely to become wet.

Make sure that when fitting additional equipment that the work is carried out in accordance with the manufacturer's instructions. The polarity of connections should be correct and each circuit must be correctly fused. Where necessary use an earthing plate isolated from the hull.

Electrical and electronic work is best carried out by a qualified marine technician.