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Steel yacht building questions


Frequently asked questions answered by Paul Fay


Why steel?

Steel yachts are growing in popularity around the world for several reasons. The strength and security of a steel hull gives great confidence. The ability of a steel hull to withstand damage when grounded or while moored with fishing boats or other commercial craft is becoming legendary.


The problems of steel rusting have been overcome as paint technology has improved. A steel yacht is now totally protected with modern paints, in fact many steel yacht owners think of their boats as a steel hull inside a plastic one. (The paint)

The improvements of paint have meant that the traditional extra thickness of steel allowed for corrosion is no longer needed. This has led to a reduction in the weight of modern yachts, so that modern steel cruising yachts can compare favourably with those built of other materials in both weight and performance.

The cost of a steel hull is very competitive when compared to other materials and the price of steel has remained constant for several years.


The cost of the materials alone to build a 35 to 40ft yacht hull and deck is around £4000.00 ($6400.00 US) (UK prices in 1998). This makes the cost of a steel hull particularly good for the home builder.


There are many other reasons why an increasing number of sailors are choosing steel, they are;


That steel is an easy material to use;


A yacht can be constructed quite quickly with steel;


The home builder can build in the open, saving on the cost of shelter;


The climate will have little affect on the steel during construction;


Very little previous experience with using steel is needed;


The maintenance required is not great;


The re-sale value compares favourably with other materials;


Steel is usually readily available although you should check in your local area;


Some companies offer 'kits' of pre-cut plates that can be assembled very quickly (although these tend to be expensive);


Some designers offer their plans with detailed sets of measurements allowing you to pre cut your own 'kit' of plates (this only adds a little to the cost of a set of plans)


An added advantage with steel is that if you are planning to visit 'out of the way places' then steel makes the perfect construction material as there is always a vehicle garage with welding and cutting equipment that can be utilized for repairs or modifications.


Alternatively, a modern 'inverter technology' welding set can easily be carried aboard. These welders are really small and light weight but are now being used in shipbuilding yards during the initial construction of yachts and fishing boats. They are easy to use and only need a suitable electric supply.


What about insulation?

Most steel boats are lined. Usually with plywood, which is fixed to battens fitted to the frames or stringers. This leaves an air gap that can be filled with some fire proof insulating material. There are several materials that are suitable. The very best insulation is to spray the inside of the hull with expanded polyurethane foam before fitting out begins. This is a good system that also adds to the protection of the steel. Where fittings need to be added on the deck the foam has to be cut away and then under the fitting the insulation must be made good with rockwool etc.

Commercial shipping and large yachts are lined with slab rockwool that has an aluminium foil fire/moisture barrier stuck to the inside face. This can be used to good effect on smaller yachts although it can soak up bilge water etc. so attention must be given to this. There must be a moisture barrier covering the rockwool otherwise condensation will pass through the rockwool and condense on the steel. This will either be the foil or plastic sheet can be used but ensure there are no gaps.

Fiberglass type insulation has sometimes been used but the continual movement of a boat can cause fiberglass dust to enter the accommodation and cause irritation to sensitive crew. Also the commonly available fiberglass does not have the density for good insulation.

Various other slab foams can be stuck to the steel but they are difficult to use as they tend to be rigid and the steel will have a curve.


Are steel yachts hot in the sun?

No, often steel yachts have better and thicker insulation than vessels built of other materials. As this is the case, by keeping a normal airflow through the yacht it will often keep very cool. Conversely in the colder months and climates a steel yacht is easier to keep warm.


How do you maintain the inside after building?

The interior surfaces of the steel will not need any maintenance, apart perhaps in the area of the bilge, if the vessel has been built and painted properly from the start. All stringers must have adequate drain holes to allow any condensation to drain into the bilge and the surfaces must be painted with a good coat of a modern paint, such as coal tar epoxy which is a really thick paint that gives excellent protection with minimum coats. Experience has shown that it is best to brush paint the inside as this ensures that the paint is forced into all the corners and behind any frames where they don't touch the hull plating.


What type of paint?

Wandering onto the subject of paint has bought us into a very technical area. But many of the problems start long before the paint is applied.

This is because it is not appreciated just how important sand blasting (see section later) and rounding off the corners is. If you take a winch handle and tap the middle of a painted sheet of steel you are unlikely to damage the paint. On the other hand tap the edge or a sharp corner and a large flake of paint will fly off. On the deck and especially in the cockpit area it is well worth rounding off all the corners by the use of round bar edges, or by using tube to form the corners of cockpit seats and coamings.


There are many types of paint available but at the moment the best seems to be to paint the whole yacht with some form of epoxy system and then apply a cosmetic UV resisting finish over the topsides and deck with an antifouling primer and antifoul below the waterline.


Some countries are now very environmentally sensitive and are restricting the types of paint that is available so a check with the suppliers will be needed. If you are building a steel yacht from scratch then it will be worth shopping around the commercial suppliers and buying in bulk as this will save money. (ask a local fishing boat skipper who the suppliers are)


Any experienced steel boat owner will tell you to use good quality paint inside the yacht. This is the area where it is very difficult to maintain the paint system. I always use coal tar epoxy inside as a good thickness is obtained with only a few coats. (We apply two thick coats by brush)

If you already own a steel yacht that has internal paint problems, I have often overcome them by spraying the bad areas with 'Wax oil'. Exactly the same as is sold to treat inside the panels of motor cars.


Another area that paint often fails is around the water-line. This can be caused because sometimes two different paint systems meet in this area. For instance, perhaps the topsides are painted with epoxy resin while below the water-line epoxy tar is used. As they are both epoxy you may think that they are compatible. WRONG! They often use different solvents and the epoxy resin needs to be taken well below the water-line. Allowed to cure and the solvent to evaporate for several weeks before being sanded and overlapped with the epoxy tar.

Take the paint manufacturers advice, and stick to it.


What about sand blasting?

There are several materials that can be used to blast the steel. Bead blasting is employed in large machines that plates of steel are fed through to pre-blast it before building. These machines normally also spray the steel with a holding primer. On the outside the weld seams will really need re-blasting and it is best if all of the outside is 'wash' blasted by standing back with the blaster and slightly roughening (refreshing) the blast primer prior to final painting. The seams and plate of any internal tanks will need treating in the same manner. The other internal seams will be fine if they are carefully wire brushed.

Sand blasting, as it's name implies uses some form of sand to prepare the steel. It normally produces a reasonable profile, depending on what sort of sand is used.

Grit blasting uses a very hard substance such as 'crushed copper slag' and produces the best surface. This should be to SA2.5 (near white metal. See Yacht photos 10)

When building a steel yacht from scratch it must be decided whether to order bare plate and grit-blast after construction, or to have the plate blasted and primed with a holding primer before delivery.


Working with bare plate is dirtier than with pre-primed plate, but you don't have the problem of having to 'stripe in' the areas of damaged primer at the end of each working day, or the worry of how long the primer will protect the steel, a consideration if building the hull is planned as a long slow project. With bare plate the whole hull and deck, inside and out, will need grit-blasting and priming.


Doing all the work yourself, hiring the equipment and buying the sand will cost considerably more than having the steel supplier pre-blast and prime it.

The inside of a hull built from pre-blasted steel can simply be cleaned and painted but externally around each weld, re-blasting is really the best way to achieve long term protection.


I have never found 'striping in' the areas where I have damaged the primer by cutting or welding to be a problem, I just allow a few minutes at the end of each work period for this job. In practice if the steel has a good coat of primer it will last for several months in the open, before there is any sign of rusting. Check with the primer manufacturer what the maximum over coating time is, as some primers need painting within 6 months or a year. If this is the case some primers can be re-activated by painting on a second coat of primer. This ensures that subsequent coats of paints adhere well. Don't forget to order extra primer when ordering the steel, probably about 25 Liters ( 5 gallons ) will be sufficient.



What about 'filler'?

The use of filler on steel yachts has acquired a bad reputation due to several reasons. In years past poor building methods leaving deep hollows to be filled, the use of polyester resin filler and poor application methods have often caused the filler to fall off.


With proper building techniques only a small amount of thinly applied filler should be needed in the area of welds etc. Above the waterline polyester seems fine but below the waterline epoxy filler should always be used. following the manufacturers procedure. This can entail painting the area to be filled first, then filling and fairing before painting the whole yacht. With the correct materials and procedure the use of filler is acceptable.


'DIY' Boat building, can 'I' do it?

The answer is a resounding YES! There are very few skills used in building a yacht that can't be utilized by the amateur. For instance the welding required to assemble a steel yacht is minimal. The whole hull can be 'tacked' together and when assembled then a professional welder can be employed, for a few days, to solidly weld all the joints.


How much will it cost?

This is a difficult question to answer as it depends on how much of the labour is supplied by the builder or the family. Without any labour charges it is surprising how inexpensively a good yacht can be built. For instance in 1995 I completed an ocean going yacht for myself that I subsequently sailed from England to the Azores, The Canaries and back to England, encountering several gales and two storms. This yacht cost £18000 ($28800) for materials, which included GPS and SW radios etc. This cost represented less than a third of the ultimate re-sale value of this yacht. Either by building yourself or by 'managing' the build you can make substantial savings. A recent 'Spray' constructed by Fay marine was 'managed' by the owner. This yacht ultimately cost less than £60000 ($96000) but is insured for more than £120000 ($192000), showing that even with no labour input a saving of 50% is possible.


How easy will it be to find people to do the work if I manage it myself?

There are several materials that are popular for yacht-building. Traditional timber or plywood construction can be difficult and expensive as there are not many really skilled craftsmen left.

GRP is popular for mould produced production yachts but generally makes for an expensive hull. For the one off yacht GRP is fairly expensive and is a messy difficult process.

Steel shipbuilders are prolific and easy to find at the moment as the worlds fleets of commercial shipping shrinks. Steel has been a major product in the 20th century and there are many people experienced in it's use and in the last 30 years the tools needed to work with steel have become easy to obtain and use.


How long will it take to build?

Another difficult question to answer as everyone works at their own pace. Different designers quote vastly differing numbers of hours to build yachts of similar sizes. Often this is because of the difference between the designs. A hull with only one chine will be much quicker to construct than a multi chine version. The problem here is the looks of the vessel and the re-sale value. The single chine boat will usually be less pleasing to the eye, sail less well and sell for less.

Talking to home builders I have generally found that it takes three to four thousand working hours to put a 35 to 40ft yacht in the water. This means that a couple working two evenings a week and weekends could have a yacht afloat in about eighteen months. In practice it usually takes a little longer due to holidays etc.

This is a very rough guide. Only you know how hard and fast you will work. Go and talk to other builders and try to envisage how you will tackle the project.


Will it be as good as a yard built yacht?

There is absolutely no reason why the home builder can't produce a yacht that is equally as good as a production vessel. Many people find that by building or managing the building themselves they obtain a yacht that is exactly what they want. Many amateur builders pay more attention to the details than is the case with production yachts, producing high quality boats. A production yacht is usually built to a price, it will be aimed at a certain price bracket, meaning that often the gear aboard is kept to a lower standard than is possible. The home builders can choose for themselves where cost savings should be made and where high quality gear is wanted. A good example in these days of 'Marina hopping' is the anchor winch. Often quite large sheet winches are fitted to the yacht but the anchor winch is woefully inadequate. Fine if the yacht only spends it's life in marinas. Potentially a disaster if the unsuspecting owner is caught in an anchorage with a rising wind and expects the winch break the anchor out. This is just one example of where the owner/builder can fit the correct gear.


Must it be surveyed?

When a yacht is home built in the UK it doesn't have to have a survey. This will have to be checked in the country where you are building.

However, it may well be a good idea to have the yacht surveyed for three basic reasons; The first is your own peace of mind, a surveyor should be able to point out anything you may have missed.

Your insurance company will probably want the vessel to have a survey.

Finally, when you come to sell your yacht a prospective purchaser will be pleased to find that the vessel was surveyed during the building.

To save on the cost of surveys, some builders just have the hull surveyed when the hull  is complete but not fitted out. The surveyor, insurance company and potential purchaser will appreciate this as everything will be easy to examine and a survey at this stage should be inexpensive but well worth it in future dealings.


Where will I find information?

Boat building information is easily available. There are discussion pages on the web where questions can be posted. These will often be answered by some of the worlds leading designers who seem to look at these pages on a regular basis.

There are many books that cover all aspects of yacht construction, in all materials. These are normally available from your library. To find them take a look at one of the on line bookshops such as The Armchair Sailor who are in Newport RI USA.

If you have real problems with information about steel yacht building that are not answered then e-mail me your question. Please be frugal with the questions, I receive many, it may take me a few days to reply.


Can I increase the plate thickness?

I am constantly approached by builders who are thinking of increasing the thickness of the plate, either all over the hull or just below the water-line. The thought is usually to increase the thickness by 1mm from the 3 or 4mm plate specified by the designer, to make the yacht stronger.


This has two adverse affects. The first is that the yacht will be overweight, in some cases by so much that the amount of ballast has to be reduced. This is often coupled with the other effect which is that by thickening the plate the centre of mass of the yacht is raised, reducing the stability.


In an effort to overcome this some builders over ballast their yachts and raise the water-line, while others just accept that the yacht is tender.

On one 40 foot steel yacht, the builder decided to increase the deck thickness from 3 to 4mm as this would help to reduce the welding distortion. Good idea you may think. But hold on, that is an increase in weight of 33%, or in real terms about 500lb. and this weight is acting about 4ft. above the centre of mass. This can be represented as 4 x 500 = 2000ft.lbs or nearly a ton extra acting against the righting moment, which meant that the yacht needed extra ballast.


DON'T INCREASE THE PLATE THICKNESS. Steel yachts are incredibly strong! Most 30 to 40ft yachts could be built from thinner plate. The reasons why thicker metal is used, is the difficulty of welding thin plate, also to resist denting while alongside quay walls etc. and traditionally as an extra margin for corrosion, which has now been overcome with modern paint systems.


When designing the Fay 32 we specified 3mm rather than 1/8 inch plate. Only 3% thinner, but this represents a saving of 160lb overall. Which in real terms means that an extra 180 tins of food can be carried for those long passages.


Should I weld all the frames to the hull Plating?

If you weld the frames to the plating this will give the yacht a hungry horse look showing every rib. The frames usually only need welding to the hull skin where they meet the chine bars. Longitudinal stringers which are always stitch welded along the length of the hull, join the skin to the frames. Any horizontal lines on the hull don't offend the eye the way a vertical line does.


On the subject of frames, an enormous amount of time and effort can be saved, by pre-drilling the inboard edge of the frame pieces, with a 4 or 5mm hole every 6 inches or so. This can easily be done on a pillar drill before assembly. These holes can be used to screw the frame pieces onto plywood, which helps reduce distortion while welding them together. Later on when fitting the timber inside the hull it saves lots of awkward drilling.


How can I form the chines?

There are several methods of forming the chines. Round bar, T bar, butting plates together and flat bar on edge. They are all successful apart from the flat bar on edge which bends, twists and sags between frames and won't take up a nice curve. When fitting the chine bars to the frames, only join them with a tiny spot of weld until the hull is complete. Otherwise they tend to kink around the frame rather than gently curving.


Can I make my own deck fittings?

Most people building their own yacht are doing it because of finance, they want to save money. On a trip around almost any marina you can see otherwise perfect steel yachts that are spoilt because the builder tried to save too much on the deck fittings.


Making your own cleats, stanchion bases and guard-rails from mild steel will save lots of money. The problem is that many builders weld them onto the deck and then paint them. It is almost impossible to stop them getting chipped, rusting and looking nasty and then making rust streaks across the paintwork. Much better is to make all the parts from stainless steel which has fallen in price in recent years and is now very cost effective.


If you must use mild steel then weld the fittings onto separate bases of 6mm plate. The whole lot can then be sent for galvanizing, which is very inexpensive. When you come to weld them on, the galvanizing is ground off the edge of the plate, which is then welded onto the deck and the weld painted over. This produces long lasting, rust free deck fittings at minimal cost.


What is the best method to cut the steel?

There are lots of ways to cut steel. Nibblers, Angle grinders, Plasma cutting, and either oxy-propane or oxy-acetylene have all been used by home builders.

Nibblers are noisy, difficult to handle, and often won't handle cutting the thicker plate. Angle grinders are noisy, dusty, slow, and curves are difficult to cut with them.


Plasma cutting is very clean, but difficult to use and not very versatile.


Oxy-propane is very similar to oxy-acetylene, but you can't use an A.S.M.N. nozzle, ( described next ).

The finest all round method that I have found is Oxy-acetylene. It is very versatile. During your project you will use it to cut plate of all thicknesses, making both straight and curved cuts easily. It is quiet and fairly clean, only needing a small amount of cleaning after with an angle grinder. It will also be used to heat and bend bars etc. and for heating areas of plate if the hull needs 'fairing'.


The oxy-acetylene bottles can normally be hired from a local supplier. Have the largest you can as this saves time waiting when you run out and is usually cheaper in the long run. Buy a pair of gauges, some long hoses, (30 foot hoses saves moving the bottles around) and a cutting torch. Three or four 'pepper pot' type burners spanning the range will be adequate, and an A.S.M.N. ( acetylene sheet metal nozzle ) type burner will be needed.


It is stated that an A.S.M.N. nozzle is only for plate up to 3mm thick, in fact they will cut 6mm easily, 9mm reasonably, and I have cut 12mm with difficulty by turning the gas pressures up high.

These 'step' nozzles work by actually sitting on the plate, which makes using them easy. The edge of the plate is heated red hot, then the oxygen trigger is pressed which starts the burning process and the torch is pulled slowly towards you making the cut. I always use a guide made from a piece of flat bar, 6mm by 50mm by 300mm long, with an old screwdriver welded on as a handle. This is held next to the line to be cut and the nozzle ran along it.


Can I gas weld the steel?

Often people enquire if a yacht can be gas welded, to which the answer is no. This puts too much heat into the steel and would cause massive distortion and so can not be considered.


How should I weld it?

There are two methods which are Mig or Stick welding. Mig has the advantage of causing slightly less distortion and being easier to use. But is more expensive, carrying the wire feed unit around is difficult, and it can not be used in a draught as the gas is blown away. This means that it can not really be used outside.


Stick welding takes slightly more skill, but is less expensive. It is not so critical of conditions so it can be used outside, and with a long cable it is easy to reach all around and inside the yacht. A 180amp welder will be fine for all the welding except the heavy plate around the keel. The high amperage needed in this area will cause the welder to overheat and cut out after only a few inches of weld. It may be cost effective to hire a larger welding plant to tackle these areas.


Can the hull be used as the negative conductor, like a car?

Absolutely not!! Keeping the electrics out of the hull is a must, don't use the hull as the negative, the way it is done on a car. This causes massive electrolytic problems.

Also while talking of electrolysis you may want to use the modern plastic ball type sea-cocks, rather than the traditional bronze. This also helps keep galvanic corrosion down.

A tip from an electronic engineer friend is, if you have a 240 volt supply, fit electronic circuit breakers, but don't earth the hull. This is because if there is a fault in the marina your hull may provide an earth for a large part of the marina, with the resulting electrolysis.


Can I build a round bilge yacht?

Amateur construction of steel hulls is generally limited to either a hull with chines, or the radius chine method.

The simplest, and quickest is a design with one hard chine, although these tend to be less ascetically pleasing than any other type. The difficulty of construction increases with the number of chines, the only exception is around the bow, where a greater number of chines spreads the heavy compound curvature often found in this area.


Radius chine is a reasonable proposition for home construction. With this method enough plates to form the chine on both sides are all rolled to the required radius. Differing widths are cut from this pre-rolled plate and used along the length of the yacht to form the chine. This sometimes needs a little cutting and fairing amidships to make it lie correctly. The rest of the hull is built from flat plate, but due to the rounded chine has the appearance of being round bilge.

The only method that I have seen of producing a true round bilge yacht without rolling the plate, is to diagonally plate from keel to toe rail with narrow plates. This is a rather difficult method of construction, which usually needs much work to achieve a fair hull. One professional constructor in the UK produced very expensive high class yachts using this method but to achieve a fair hull a team of plasterers would cover the hull with filler prior to fairing it by sanding. Currently there are some designs being promoted for this type of construction and plans are being sold to amateur constructors. To date I have no knowledge of how successful these have been and time will tell how people fare with them.





An Article by Paul Fay


The yacht had performed really well, they had sailed ten thousand miles in the last year with no problems and now that they were back on their half tide mooring on the West Coast, the owner was catching up on some maintenance. While walking around at low tide having a look at the underwater areas, he suddenly noticed with alarm that the anodes were disappearing very quickly. This needed immediate investigation, as the yacht is 37 feet, built of steel and normally only needed the anodes replacing every four to five years. The way they had suddenly started wasting away showed that something had gone dramatically wrong.

This was not an isolated incident or one that only happens to metal boats. In another case the yard that had hauled a new fiberglass yacht out of the water, found that the alloy sail drive leg was corroded away. The only thing keeping the water out was the internal rubber gaiter. A new sail drive leg that came with a new anode was fitted but three days after going back into the water the anode had corroded so badly that there were holes appearing large enough to insert a finger.

Horror stories like this are all too common and with our increasing reliance on electronics are being reported more and more.

These stories are all made worse by the almost mystical way that many people view galvanic and electrolytic corrosion. The marine industry has also compounded the situation because most marine engineers, both mechanical and electrical, were never taught the simple and easily understood theory behind the subject while they were being trained. When called upon to solve a corrosion problem they start casting around for all sorts of complicated solutions, often making the problem worse and costing the owner a small fortune.

What always seems to be misunderstood is that for there to be a galvanic or electrolytic problem there MUST BE A FLOW OF ELECTRICITY.

For there to be a flow of electricity THERE MUST BE A CIRCUIT.


Remembering our days at school and what we learnt in the science classes will remind us that if we hang two different metals in an electrolyte liquid and connect them then electricity will flow. This is a simple battery. One piece of metal will be corroded; the other will be virtually unaffected. This is galvanic corrosion.

Another form of corrosion happens when two pieces of metal are in an electrolyte and are then connected to an electrical supply. This can be either a battery or a mains supply. The metals can be different or the same. The corrosion is dependent on the current we are applying and the direction that we apply it. This is electrolytic corrosion.

There are several rules here that must be fulfilled for corrosion to happen.

To produce galvanic corrosion the metals must be different. The further apart in the galvanic series of metals then the higher the current that will flow and the faster the wasting away of the anodic metal.

Second, the metals must be in an electrolyte. At school we would have tried various conducting liquids and found that they each gave different results. The one that we are interested in is salt water, which is a good electrolyte.

Third, THE CIRCUIT MUST BE COMPLETE. This is the same as any circuit, if the metals, or battery plates, or anode and cathode, whatever the term used for them, are not electrically connected then NO CURRENT WILL FLOW.

This electrical connection can be made either by touching or by some other conducting path, such as a wire or the damp interior of a yacht.

This therefore means that if we can isolate the different metals that we use below the waterline then we have broken the circuit. We have stopped the flow of electricity and therefore stopped the corroding or wasting away of the metal.

The point that is to be made here is that when we have electrolytic or galvanic problems we need to LOOK FOR THE CIRCUIT AND BREAK IT.


In practice, there are certain places where it is virtually impossible to isolate the metals. For instance, we normally use stainless propeller shafts with bronze propellers; also very often mild steel rudders have stainless pins welded on as the bearing surfaces. When this is the case the two metals are chosen to be as close as possible in the galvanic series so that the corrosion is minimal.

The circuit in both of these instances is through the two metals that are touching, through the water and back into the metal.

To reduce the small amount of galvanic damage that will inevitably happen, even though the metals are close in the galvanic scale, a sacrificial anode is fitted. A sacrificial anode is a piece of metal that is low in the galvanic series and is attached to the part that needs protecting. They are usually made from an alloy of zinc and because of it’s low galvanic series properties this will be the first to corrode away, thus protecting the other metal.

Another place where it can be difficult to isolate the metals is in timber boats. Here the circuit is from a skin fitting, through the water to another skin fitting, then through the damp timber of the hull back to the first skin fitting.

It is common practice to connect many or all of the fittings together on wooden boats by joining them internally with copper wire. The copper wire is connected through the hull to a series of anodes that protect the fittings. These anodes need replacing at regular intervals. The only problem with this procedure on some wooden boats is that around the fittings being protected an alkali will be formed. Some timbers will be quickly damaged by this and expert advice should be taken when proposing to fit a new anode system to a wooden boat.

On early GRP hulls, the skin fittings were often joined in the same way as on wooden boats, but it is now more usual to leave the fittings isolated. This uses the insulating properties of the plastic hull to ensure that no circuit is formed, but care is needed to make sure that the fittings are not continually wet by the bilge water, as this would form an internal connection. On many modern GRP hulls, the only anode that needs fitting is to the propeller shaft, as all other fittings are well insulated from each other and so no circuits are formed.

On metal hulls, various methods are employed to minimize galvanic problems. There are now available excellent plastic sea cocks that can be fitted. These will totally cure the problem, as they are electrically inert.

If bronze fittings are used then they must be electrically insulated from the metal of the hull. This is achieved with plastic washers, gaskets and sleeves to form a barrier between the bronze and the hull, so that no circuit is formed.

Welding in the stern tube and then using a resin cased stern tube bearing; a rubber mounted packing gland and a plastic coupling between the gearbox and the shaft, will insulate the propeller and shaft from the hull. (Shaft bearings are available with either bronze or resin outer cases. Only a resin case should be used where ti will be in contact with the metal hull.)

These are just a few of the many ways that under-water circuits leading to corrosion can be avoided. With the boat out of the water, the resistance between fittings can be checked with a cheap multimeter, or even a simple battery, wire and a bulb. This should show that there is no contact between the fittings.

A commonly made mistake is to connect the meter between two fittings when the boat is afloat, to see if current is flowing. What actually happens is that the meter instantly makes a circuit. This produces an alarmingly high current reading, leading the operator to believe that there is a problem when in fact there may be none.


The alternative method to the galvanic corrosion that has just been discussed is to cause corrosion by introducing current to the fittings. On ships, this is often deliberately carried out by supplying controlled current to special anodes. This is so that the vessel's engineers are in direct control of the electrolysis. This is known as an impressed current anode system.

In the case of small boats and yachts, induced current normally happens by accident. A wire or electronic fitting fails and allows ‘stray’ electric current into the water through a seacock or other underwater fitting, usually causing fast and quickly noticed electrolytic corrosion.

It is fortunate for us boat owners that most corrosion problems are galvanic as when electrolytic corrosion happens it can be difficult to trace the circuit. The reason is because in this case the circuit will often be from the positive of the battery, through an instrument or radio; the instrument is connected to a skin fitting, keel bolt, or the hull to obtain a radio ‘ground’; the current then passes through the water to the engine block that has a heavy cable connecting it back to the battery. Although tortuous, the current will find it’s way round this circuit and even though the chassis of electronic gear is connected to the negative of the battery the electric leak can put different underwater fittings at a differing electric ‘potential’. Although this is only slight it is all that is needed to cause electrolytic corrosion.

It is a fact that the negative on many boats is connected to the water without the owner being aware of it, making it easy for any stray current to form a circuit.

This can be through the engine block, through control cables, and commonly via the outer braid or ground on VHF and SW radios and other radio receivers such as weather faxes.

An often missed, but common way for the circuit to be made is this: - The chassis of radio receivers is connected to the negative of the battery. The outer braid of the co-ax cable from your VHF is also connected to the chassis. The other end of the co-ax is connected to the antenna bracket, which is connected to an alloy mast. Many boat owners will have the mast or rigging connected to a skin fitting or keel bolt for lightening protection. This again connects the negative of the boats supply to the water thus making it possible for stray current to form a circuit.

As you can see locating an electrolytic circuit can be difficult. However, there will always be a circuit and experience has shown that these rarely just ‘happen’. There will normally have been some work on the boat that the problem can be traced to. Perhaps some new instruments have been fitted, or it could simply be that a shelf was put up and a screw has damaged a wire.

So what about the stories quoted at the beginning of the article?

A few weeks before noticing the problem, the owner of the steel yacht had checked the bolts holding a bronze stern tube in place and slightly over tightened them. This had caused the insulation between the bronze and the steel hull to fail, making a circuit. The insulation was repaired and the problem solved.

The GRP yacht with the corroded sail drive leg had a new weather fax fitted. The radio engineer had joined many of the underwater fittings together with wire to obtain a good ‘ground’ for the instrument. Between the alloy sail drive and the bronze seacocks this caused massive galvanic corrosion that cost the owner thousands of pounds to repair.

Most galvanic and electrolytic problems have a fairly simple solution. As boat owners and engineers we don’t need to have a deep understanding of the science that goes into the construction of marine grade metals, this can be left to the experts who make the skin fittings and anodes. All we need is a basic awareness that to avoid or cure these corrosion problems NO CIRCUIT MUST BE PRESENT.


Fay Marine are dedicated to producing Yacht designs for ocean cruising and to further ease the passage with which sailors can achieve the goal of crossing oceans, we here freely offer, for individual or club use, this information. This may be printed and used free, for personal or club use. Permission to re-print in magazines other than club magazines or on other internet sites must be obtained from the author.   

 ©Paul Fay 1999