Ballast a Steel Boat

There are a hundred wrong ways to ballast your steel boat – but only one Right Way, usually learnt only by painful experience.

There are many wrong ways to go about this fundamental part of building a steel sailboat, and since the penalties of using incorrect methods will have serious implications for your health, wallet, and boat stability, it pays to go about it the right way.

But what is the right way? It turns out to be an impossible task to research the correct methods, with precise instructions for every stage, since that information does not exist anywhere in print. There are plenty of hints and tips available, from many well-dispersed sources, but there is nowhere a single source of the whole procedure carried out in the most efficient manner from start to finish. In part, this is due to the length of the article or book section required – even a book on steel boats cannot necessarily devote the space to it that the subject demands. Another factor is that many writers on steel boats, including those who have written admirable books on and about the whole process, have not actually themselves gone through the task of building and ballasting a keel from start to finish. This job teaches more by doing it than any other department in steel boatbuilding.

Here we are then – perhaps the world's first full treatise on steel boat keel ballasting.

Whose responsibility?

Who can ballast a keel? The builder, the owner, or his contractors, when buying a bare hull. The first thing you will learn when you become familiar with the world of steel boatbuilding is that the standard builder's description 'bare hull' can mean all things to all men... At any rate it never includes the ballast, though, and the builder will charge several thousand pounds to do this work. Any competent handyman can carry out this job; though it can be hard graft, dirty, and in some cases dangerous if you are not sensible. Which is a fair description of any part of steel boat construction.

As a separate point, a more reliable description of a hull which is complete and suitable for fitting out is the term 'all steel work finished'. This is quite often a very different proposition from the the first term 'a bare hull', and can in some cases be double the cost.

The ballasting process is best broken down into sections:
   1.   Preliminary considerations.
   2.   Method to be employed.
   3.   Tools and equipment required.
   4.   The installation stages.
   5.   Final considerations.

Here are some drawings of the relevant structures:

                      keel and floor details

Preliminaries

Let's assume you are building a steel boat and are thinking about the ballasting stage. You must decide several things: at what stage of the building process to install the ballast, what type of ballast to use, how much of it, and how to install it.

Box keel construction is the normal choice in steel boatbuilding, since it has so many advantages. The term simply refers to the type of construction, and has no implications as to shape – a box keel can be of any shape whatsoever. It is actually one of the major advantages of building in steel: an aerofoil shape can be used, the keel structure is part of the ballast package, tankage is easily incorporated, the absolutely vital bilge sump every voyaging boat must have can be built-in, a heavy shoe plate can be used to make the keel stronger than any other type against grounding, and the keel structure is so strong it is always reliable. There are so few advantages to an external lead keel on a steel boat that this type can be ignored.

The ballast can be installed at any stage of building, up to the point where the cabin sole goes in; but is best tackled early on. The normal point is when the hull, deck, and keel have been completed and all steelwork is coming to an end. This is the most convenient time; although it is possible to build the keel separately, ballast it, then attach it to the boat. This would normally only be done due to height restrictions in a building shed, since it is difficult to handle a ballasted keel separately, and for various reasons the end result may not be as strong as single-unit construction. The floors for instance, which should resemble a large T-shape where they go down into the keel, cannot be built and installed in one piece.

Ballast materials

The boat designer will normally tell you which ballast material to use, though there is only one choice for optimum results: lead. Any other material either presents problems for the builder or the owner. Lead weighs 705 pounds per cubic foot, so it is a very dense material. In practice we can use a figure of 700 pounds since our lead is unlikely to be pure, however we may go about refining it in a boatyard. It is also usually a relatively 'inert' material in seawater (but see later), which some other materials are not. Although metric measurements are generally much easier to use in all areas of boatbuilding, there are particular reasons why this item is best described in pounds and cubic feet; a cubic metre of lead, for instance, doesn't bear thinking about.

Of the various alternatives, spent uranium is an even heavier material than lead (nearly twice the weight), and better in theory, but of course presents obstacles to the builder. The ballasting will have to be recalculated by the naval architect, it is hard to source, it is very expensive, it cannot be cast easily and will therefore need to be cast into ingots by a third party, it will have to be fixed into the keel anyway by poured lead or resin, and is bound to end up costing a huge amount more in one way or another (unless you live next to an army tank firing range and can hop over the fence at night – it's used in tank gun sabot rounds). Use of this material is normally the preserve of well-off racing syndicates, for external keels on racing yachts. The owner will benefit, though, from the lower centre of gravity; and a warm feeling around the feet on a cold night?

Other materials are too light, or are lower on the Galvanic scale than lead, and therefore have too many drawbacks for the future owner of the boat. Steel, in one form or another, is sometimes used: scrap plate, steel punchings, worn components, and so on have all been employed. At around 500 pounds per cubic foot, it appears to be a reasonable ballast material; however, there is no way of using solid material unless the ballast section is built from layers of solid plate. This seems unlikely due to the cost of the steel, which is only being used in the first place for cheapness. A weight of 400 pounds per cubic foot is quoted for steel / binder ballast, thus allowing a loss of 100 pounds per cubic foot for binder. It is therefore possible that the final resulting weight of steel scrap ballast may be little more than around half that of lead. This has two implications: the keel centre of gravity will be higher, therefore the boat's stability will be less; and the volume available for tankage in the keel will be either a lot less or non-existent. If the correct binder is chosen, it will be as effective against corrosion as plain lead ballast correctly installed; but choose the wrong binder or do the job poorly, though, and corrosion is much more likely.

Binders

The steel scrap is normally bound with poured bitumastic, resin, or concrete, before being plated over or otherwise stanchioned down. As a marine engineer, in repairing problems with steel-scrap-fill keels, I have found that bitumastic is an unsatisfactory material. Water has been found to permeate right down through the mix, even in a relatively new boat which appeared to have a solid bitumen fill, as a result of leaks which may have been due to any or all of the following: faulty keel welds, impact damage, incorrect fixing of items such as anodes, poor fitting of top inspection hatches, and so on. This may have been accelerated by diesel acting as a solvent.

Resin appears to be a more satisfactory binder, at a greatly-increased cost. A simple ortho polyester resin would normally be employed (cheap 're-manufactured' resin seems best for this), since using anything else would cost the same as doing the ballast properly with lead in the first place.

Finally, concrete as a liquid binder for the scrap steel has a lot to recommend it, if beer coupons are short. It seems to have a poor reputation in this area, which is why several steel boats owners of my acquaintance have had it removed while refitting, to check for the corrosion that was expected in the bilge area. This is always a difficult job, requiring pneumatic breakers and so on, and a lot of back-breaking labour.

In all these cases the keel or bilge area was perfect and uncorroded. The concrete had bonded perfectly to the steel, or had otherwise helped to stop any corrosion starting – perhaps by an alkali bias? It seems that similar good results have been found where Thames barges, planked in heavy oak, have also had their bilges concreted: on removal of the cement, after what must have been approaching a hundred years, these areas were found to be free from rot. Presumably there must be some boats somewhere that suffered badly as a result of concreting the keel or bilges, in order to generate this poor reputation; or is this just another boat myth?

There again, it may be possible that a badly-corroded boat had been concreted as a quick-fix or to disguise the fact. If a boat is widely corroded, and also has corrosion under concrete in the bilge, one feels that the concrete can hardly be blamed for what is obviously a systemic (overall) problem and not a topical (localised) one. It seems wise to offer the advice, therefore, that if concrete in a bilge may possibly be of recent origin, alarm bells should ring.

Since I have never encountered a boat that has demonstrably suffered corrosion as a result of being concreted (where the concrete was of always of many years' standing); but have seen and heard of many that were revealed to be in perfect condition after as much as a hundred years: on the evidence I have, concrete is a perfectly acceptable method to use. Of course, as a ballast component it is sadly lacking in weight. Concrete alone cannot be used since it is fairly light compared to the heavy metals, at around 200 pounds per cubic foot. It has to be combined with something else heavier.

In conclusion, scrap steel is a viable alternative if the binder is chosen carefully. Don't spend any money on the process, though, since you would be a lot better off with lead in the first place. It is almost worth having to forego the proper ballasting procedure, using scrap-iron weights as a temporary measure, and then simply adding lead to the keel box as and when money allows, rather than using the far less satisfactory alternatives – this is what I would do if forced to do so by financial constrictions. You are only building the boat once, and once it's done, you have to live with the results for a long time. If you find you have made a serious error, there is only one practical solution: cut the keel off and start again...

Stanchioning down

All ballast materials require fixing in, and/or fixing down. Several tons of solid material cannot even be allowed to move a millimetre, without nasty implications. There is also the important factor that steel box keel construction allows the installation of fuel or water tanks above the ballast, together with a sump for bilgewater; such items require a sealed plate above the ballast, to prevent leaks, and to maintain sump or tank integrity. This lock-in process starts in a steel boat by welding 'random shorts' onto the keel's internal plating, in the ballast area, before the ballast package is installed. These small pieces of scrap flat bar and angle, about four inches long, help to lock the ballast material to the keel plating.

The keel will be divided into sections by the floors, usually 16 to 20 inches apart. These are vertical plates running athwartships that give the keel rigidity, attach it immovably to the bottom plating, give the bottom of the boat strength, and can be used to divide the keel sections up for different purposes. The floors are best shaped like a T, with the vertical section reaching down to the keel shoe, and the horizontal arms extending out across the bilge and attached to the shell plate there; they are then extended out across the full width of the bottom plating to form the main floors. At the lower end of the floors, down on the bottom of the keel, the corners of the vertical plate are 'sniped off' (cut away), so that lead or binder can flow between the ballast sections and help to lock it in; and a similar arrangement is needed at the top as well. A large hole 3 or 4 inches in diameter is put in the centre of the plate between the ballast sections, to allow a through-flow and lock-in of the ballast. A plate is welded over the top of each ballast section to keep the ballast secure, to stop water entering, and to maintain the integrity of the tanks above.

Builders will install the keel floors / ballast capping plate intersection in different ways. The designer may specify the method.
1. The keel floors may be fitted in one continuous vertical length. In this case, the cap plates over each ballast section will be separate. Some way has to be found to allow a free run of air and/or lead at the top of the section during ballasting, as the section fills to the top.
2. The ballast capping plate may be fitted in one length along the whole run of the keel. Here, the keel floors are in two sections, with the section below the cap plate welded in first. There are several advantages to this method; the only disadvantage is that the floors are not then continuous, but ways can be found to alleviate this. In any case the structure will still be stronger than anything found anywhere else in the world of marine construction.

Form of lead

The type and method of lead placement is often debated. The simplest, easiest, quickest, and cheapest way of doing it is to melt scrap lead into the keel – though there is slightly more to it than that. Alternative methods are to place solid lead in the form of ingots or pigs (large ingots) in the keel and bind them with poured lead or one of the other liquid binders; or to use lead shot, and bind it likewise.

If the employment of any of these latter methods is contemplated, bear the following in mind: it will always be more difficult and expensive than straight pouring; the results are not guaranteed to be as satifactory, since the resulting ballast package weight may be lower due to air inclusions or the use of a lighter binder; and it is more time-consuming. Random shorts and a top plate welded on are almost always needed in any case.

Ingots can be self-made by pouring lead into moulds, and the best weight by far to aim for here is a ten-pound ingot. These are easy to make, easy to handle, easy to carry into the boat, easy to place in the keel, easy to remelt for pouring, easy to stack, and so on. Pigs are large ingots, the term coming from pig-iron, the precursor to steel which is made into these large ingots. The common weights for these are 50 or even 100 pounds. There is an equivalent of these in scrap iron: cast-iron fork lift truck weights, which are of fifty pounds weight, and do at least have a convenient carry handle formed into the top. You will find that lead pigs are in fact just that – real pigs to lift, carry, place and work with. They need some form of handle at the top, or will be impossible to shift; perhaps a wire handle. This will need cutting off once in place. Re-melting them takes forever, because of their large mass.

There is an argument that pouring lead into the keel will distort it. Placing ingots into the keel and then pouring lead to lock them in is often advised. This may have some validity in the case of a very lightly-built keel; but why build a keel light in the first place? The heavier it is, the better, surely? Provided that only reasonable amounts are poured at a time, in my experience this has no grounds in reality. Distorting an aerofoil-shape box keel built from 6mm or 8mm plate, with a heavy shoe, plus cross-ties or a tank-bottom plate halfway up, and floors throughout, will be difficult; it would probably require a single pour of at least half a ton or more to do it. This would obviously be rather foolish. It could be that this myth originates from someone who carried out a major pour into an unframed keel; in which case they got what they deserved.

In fact there may be an advantage to hot-melt pouring, in that the keel structure will be 'normalised', which is a gentle heat treatment process often used in industry to relieve welding and manufacturing stresses in a structure.*

You will find that lead ballasting by pouring small amounts into a random-short fitted, floor-divided, plated-over box keel cannot be beaten.

* When I worked in a machine shop, the drawings would sometimes state Normalise, as the process to be carried out after a metal piece part was machined. This meant the parts needed to be heated to a medium temperature and allowed to cool, which allows machining stresses to work themselves out. As a lad in my first job, the foreman said "We don't bother with that, there isn't time - just stick the crate over there in the corner, they can normalise overnight".

Keel drawings

Click this link for drawings of keel and floor details. The drawings open in a new window, which won't open if you have disabled pop-ups.

                                Drawings: keel and floor details

Design considerations

Your keel will probably have been designed for lead-melt ballasting, at least in terms of the volume provided for the lead, and the tank space remaining. There may be an alternative keel for steel scrap ballasting. There may also be alternative boat models, or different keel configurations. For instance, van de Stadt frequently offer alternative construction methods such as round bilge, multichine, framed, or frameless, together with shallow or deep keels. The vessel's weight and ballast package may vary. You may even be building a modified or customised version; or a digitised flat-pack steel-kit version. It is therefore quite important that you measure the actual keel ballast volume, and compare the resulting weight with that of your plan version. Measure the volume on the plans; check it against the specification for the ballast weight in your boat version; and then check the actual built volume. Mistakes here are rather difficult to rectify afterward.

A boatbuilder of my acquaintance reports several poor experiences with boat plans digitised for pre-cut kit building, from several different UK sources, so this ought to be taken into account. Personally, my own experiences have been better, but only through dealings with van de Stadt, de Groot, and customisation by a UK commercial steel ship Naval Architect.

So: calculate the weight resultant from the lead volume carefully, and check it is valid. A precept of boatbuilding is that you never ballast fully until the vessel has been launched and tested. This is still just as valid today for one-off construction as it was for wooden boatbuilding, though far harder to carry out. If at all possible, you should leave 50% of the furthest-forward and furthest-aft ballast compartments empty, for future trimming. Trimming means filling later, part-filling, or even leaving empty. This is always a valid exercise since a boat's balance, i.e. trim, can vary from the design optimum – perhaps due to a bigger engine, re-siting of tanks, and so on. In addition, steel boats are commonly over-built rather than under-built: they often come out too heavy and sit below their marks.

It is common to see a steel boat two or three inches down; six inches over- immersed is not uncommon; and I have even seen one nine inches down on her marks. Boats like this don't sail. They won't point up, they go downwind very slowly, and they generally act like a submarine, to the intense annoyance of their builders, who thought they were being so clever when they added a millimetre to every thickness of bulkhead and section. And they are usually sold quickly – so watch it!

From the plans, or by contacting the designer, check the precise freeboard (the deckedge-to-waterline height. Less than that shown on the plans is not a good sign.

Corrosion Limitation

This is a very important factor in all areas of steel boat construction. If you are doing it for the first time, unfortunately you are bound to make some errors here. Or perhaps the errors will be made for you, by designers or builders who do not appreciate or care about the resulting problems.

The first thing to note about keel construction is that there must on no account be any blind areas anywhere – areas that are boxed-in, with a dead air space, and cannot be accessed. These are just problem sites for the future, since they are very hard to keep water-vapour or water-ingress free. Every space in the keel must have some form of access: an inspection hatch, or even just an oil-filler cap if the void is small. As an example, the leading edge of an aerofoil box keel may be formed by a pipe, to which the keel sides are welded. This pipe may extend from the keel shoe, up through the shell plate, to the floors and sections above the keel that lock it to the hull. Sailboat keels extend (or should do) right up through the shell plate and well into the boat, to form a massive joint structure. The pipe will be capped-off at the top, and should therefore have a small oil-filler welded on: a short section of 1-inch pipe with a threaded end and a screw cap to fit. The pipe is then filled with oil, for two very good reasons: there must be no empty blind spaces; and there certainly shouldn't be any air voids down in the lower keel, since a keel will hardly perform its function properly if full of air. Imagine the lifting moments of the buoyancy when the boat heels...

Blind voids in the top of the keel section, perhaps above the shell plate level, are common at each end of the keel. The furthest-forward point at the top front of the keel, and the furthest-aft at the top of the narrow back edge, are often left empty. These should have an oil-filler fitted, but only need to be half-filled with oil or less, to maintain a corrosion-proof environment. Ensure that these areas are well-painted with epoxy, and allowed to dry, before welding shut and oil filling.

All external keel welds must be continuous and double-sided; all internal welds that divide sections with a different purpose, the same. Of course, this is impossible for some welds for example the top of the ballast capping plate - so the weld must be absolutely homogenous and of good thickness.

The entire ballast section should be pressure-tested if at all possible, after completion of the ballasting, though this is easier said than done. In any case, all and any keel fuel or water tanks must always be pressure-tested, whatever difficulties may try to prevent this; there are no exceptions. In order to avoid problems at that stage, it is better to have previously tested the ballast area below.

Pressure testing can be carried out by fitting an ordinary car tyre air valve to the tank or compartment surface. A simple electric car tyre pump, which has a built-in pressure gauge, can be used to take the internal pressure up to 10 psi. The pressure is checked again after an hour – there should be no loss at all. It is more accurate to use a separate tyre pressure gauge, rather than the pump's gauge. If using an airline, you will need the separate gauge; be careful not to pressurise the tank or ballast compartment too highly, since there is no advantage in taking the pressure up above 10 psi. In fact, it is dangerous to do so: people have been killed and injured by explosions when doing this. Truck tyres and separate stainless steel tank structures are famous for this; a stainless tank should never be taken above 10 psi.

I have seen stainless fuel tanks pressurised in a workshop to such a high pressure that the tank lids were blown off the tops of the baffles, when the worker involved connected it to an air compressor and walked away. I used to keep well away when a tank was about to be tested; people are killed by this sort of procedure.

Common sites of corrosion

If corrosion occurs in a steel box keel, there are three likely sites:

    1. In blind areas;
    2. In the ballast section;
    3. At the top of a diesel tank.

We have dealt with blind areas. The ballast section should be entirely conceived, built, and filled, with corrosion prevention in mind. All welds should be continuous and double-sided (where possible - see above). The capping plates must be air and water-tight. The idea is to keep all external fluids out: either sea water from outside, or tank water and diesel from above. If this can be achieved, then corrosion will be prevented.

Even if there is some inward leakage, perhaps due to impact damage if a rock is struck at full-tilt, the situation is not too bad if the following procedure is followed:-

Leave a 2-inch airspace free of lead, above the ballast, in just one of the sections before it is welded shut. Fit an oil-filler to the cover plate on this section (which will probably be within a tank). If the airgap below is 16 inches long by 16 inches across by 2 inches deep, then it is around one-third of a cubic foot in volume. A cubic foot equals 6.25 gallons (Imperial, not US measure), so the airspace will be of just under two gallons. Fill this with oil.

The reasoning behind this is as follows:
   1.   In theory, the ballast area should be protected against corrosion in every way possible, including the coating of the internal steel surfaces with paint or some other substance to protect the steel. In practice, this is easier said than done – any paint will probably be burnt off the steel by the hot lead when melted in. Bitumen-type coatings are unsuitable for the same reason. It would be interesting to test a steel tank model with various coatings, to see if any could resist melted lead. Perhaps there is a form of cement (we have seen proof that concrete is a satisfactory coating), or an epoxy paint that would survive. A very good candidate for this is the 'Weldarite' type of epoxy blast primer that can be welded without burning. These paints, although thin, are heat-proof so that that they do not burn away from weld zones. If you are using pre-blasted and primed steel for your project, you can specify this paint in the first place. It is a fairly thin, dark red, coating that is often sufficient for several months' protection even if building outside without shelter. A second coat inside the keel area cannot do any harm, therefore; but in practice we should assume that the steel surfaces will end up entirely bare and unprotected.

   2.   Poured lead shrinks fractionally on cooling, so there is likely to be a tiny air gap in places, between the lead and the plating. The lead is unlikely to bond solidly to the steel everywhere. This seems especially likely with placed ingots, bound by poured lead. That is why random shorts are vital, as they help to lock the ballast in place. Assuming there are air gaps and no paint, some way of mitigating corrosion due simply to water vapour is needed. It is extremely difficult – and probably impossible – to prevent water vapour existing in, or entering into, 'sealed' areas in a marine environment. Anyone who thinks it is possible is an incurable optimist (and probably trying to sell you something). An oil-loaded atmosphere is a standard industrial way of combatting this problem. Even a small amount of oil introduced at the top of the ballast will help; the vapour, and probably the oil itself, will seep down into any voids, and even along the length of the keel. After all, this is exactly what water does if allowed in. The oil level can be checked and refilled if feasible, perhaps at refit intervals of five years.

   3.   Mild steel plating and lead ballast are quite close to each other on the Galvanic scale (although you may read that they are not). Check the table at:
              www.pelaginox.com/data/d-galvanic.html

For practical purposes, only cast iron separates them. Steel is the lower and most anodic or base; cast iron is higher or more noble; lead is next higher in the table, followed by the various brasses such as manganese bronze (which of course is not a bronze at all). Lead survives well in sea water (witness all the bare external lead keels existing), and it is higher in the table than cast iron, which we again know survives well when used for external keels. These iron keels survive despite often being unpainted, since antifouling cannot qualify as a paint here. Lead is sometimes described as being inert in seawater, though this cannot be strictly true as it is fairly low down in the table; given the wrong conditions it will suffer from electrolytic corrosion. (However, a major component of galvanic corrosion is quite simply mass - a huge mass of several tons may be suffering from mild corrosion effects, but it will take a couple of hundred years before you even notice.) Steel will suffer worse, since if you place some bare steel and lead next to each other and connect them with seawater, corrosion is bound to affect the steel. This must be prevented.

Our third corrosion site is at the top of a diesel tank in a box keel. Although diesel itself, being an oil, stops corrosion occurring below its surface, it is vulnerable to water contamination. Condensation occurs in the empty tank space above the diesel, water forms on the steel surfaces, and corrosion may result if the coatings fail. A high-quality two-pack epoxy paint, proof against fuel solvent action, must be carefully applied in the diesel tanks. Where this has not been carried out, the top of a diesel fuel keel tank can corrode badly after twenty years or so, and the whole keel structure is therefore at risk. You can see that the corrosion will therefore occur all round the top of the keel, just where it joins the hull. Whoops – cut on the dotted line.

This problem is well-known in the world of commercial shipping, where seawater ballast tanks can obviously suffer badly from this problem. When empty or partially empty, corrosion can run riot. The solution is to part-fill the tanks with oil, using a product called Levelite. This light oil floats on the surface of the seawater, going up and down with the level in the tank, and coats the steel surfaces to help prevent corrosion. That is why the same approach in our lead ballast spaces is entirely valid.

Leaving some space ballast-free for oil filling will obviously result in the loss of ballast weight. This amount should be calculated, and the same value in ingots can be used as trimming ballast. The ingots could even be stanchioned down on top of the ballast cover plate, right above the oil space, if required. For various reasons it is better to perform all these oil-fill, trimming, and sump construction operations at the back, or nearer the back, of the keel than the front. In other words, given a choice, this will all occur somewhere around the bottom of the companionway, and usually just in front of the engine; not at the forward end of the saloon.

Types of oil
If you choose to employ an oil-based anti-corrosion method, as above, what type of oil is best for boatbuilders as against shipbuilders? If you are a ship builder you can ask Shell Oil to solve the problem for you, but yacht builders don't need a 2,000-gallon tanker delivery - 2 or 3 gallons is just fine.

My choice is a light motor oil. This has the precise characteristics we need:
1. Long life, even when contaminated
2. Provable anti-corrosion properties even when contaminated - must coat surfaces and prevent corrosion
3. Immiscible (doesn't mix with water)
4. Reasonably light oil - a heavy oil would not be so good here

There are any number of alternatives, such as cooking oils, engine flushing oil, 2-stroke oil, diesel oil, machining oil (cutting oil), and machining emulsion oil. Let's take a look at each of these in turn and discuss.

Cooking oils: this would include vegetable oil (rapeseed oil etc), corn oil, and olive oil. These are interesting to consider - and very kind to the hands - but I don't see any positives. How good are they at preventing corrosion? This is an unknown quantity and I can't see that their performance would be as good as a mineral oil. These oils are basically immiscible (they don't mix with water), which I see as a good thing.

However the big negative with these oils has to be that they have a finite lifespan. They will die off, and fairly soon. Organic, natural materials cannot possibly last as long as mineral-based ones. Durability and extended lifespan has to be a big factor - and these fail. They will go off, rot, and fail. You will end with a stinking, rotting mess in the keel, that has no corrosion inhibiting value, and maybe creates some gas pressure.

Engine flushing oil: this is a definite candidate - it's that light, yellow-colour oil you use after you drain your engine oil. You refill the engine, run it for 5 minutes, then drain it again and throw that oil out. After that you refill with the new, correct engine oil. You do that, don't you? :)

Well, that's what we used to do in the days when everything was done properly. Those days are long gone of course but you can still buy flushing oil. The pros: it's a light grade, which is good. It's not expensive, because it isn't designed to have any kind of lifespan. The cons: it isn't designed to have any kind of lifespan... Also, anti-corrosion properties will be lacking because it isn't built to protect an engine from corrosion over an extended period of time. So, on balance, this is not the right oil.

2-stroke oil or 2-T: OK, it's available, and it's oil - but apart from that it doesn't really fit the bill. It's designed to lube an engine and get totally burnt off and disappear after a time, when it will be replaced by more of the same. It doesn't have the kind of extended lifespan while being contaminated with aggressive adulterants that we want. It's kind of diesel with a bit extra. Not what we need.

Diesel: it's an oil I guess. But not the type we need, for sure. It may just about hold corrosion back under the surface of diesel in a tank, but it certainly doesn't do that in the air gap above the oil - we know for a fact that this is one of the worst places for corrosion in a steel boat. Diesel absorbs water and we don't want that. Absolutely none of the properties we need.

Machine cutting oil: we're talking about machine tool oil here, lathe cutting oils. Their main operating requirement is to be able to take high pressure loads, and also have a high ignition point. They cool and lubricate the cutting tools on lathes and auto-lathes etc. These oils are fairly light and look interesting - but they aren't cheap and may not have the characteristics we need. Long-term anti-corrosion is unlikely to be their strong point - we change the lathe oil after a time and throw it.

Machine tool oil emulsion: this is what we old engineers call 'pigeon's milk'. It's an emulsion oil, meaning it mixes fully with water to create a white liquid that resembles milk. It's very good at its designed job: to be a much cheaper solution than pure oil; to lube the cutting tools and not ignite under heat and pressure; and to last long enough that it's economical. But 'long enough' is only 3 months. It only has to last a short time (comparatively) before changing out. After that, the oil-water mix starts to lose its corrosion preventing qualities rapidly, and the mix goes off and starts to rot. Any machinist can tell you this. The machine starts to smell bad and rust up, so you change out the oil emulsion for new. It has a short design lifespan and works well during that time. We need a product with an extended lifespan.

Therefore, as you can see, light engine oil has the perfect profile and it's the #1 choice. The only thing it isn't designed to do is float up and down on top of seawater - but it will do that in any case. I don't like the idea of emulsion oils because all the ones I know of have a limited lifespan. In addition, an oil that sits on top of water and coats exposed surfaces is the proven commercial solution.

This area is well worth considering because all the rusty steel boats I've seen had in fact rusted out from the inside. From my own experience, steel boats rust out from the inside, if and when they rust. An exception is below the waterline, where poor maintenance over many years can result in thin plate due to lack of care. But the rusty boats I've seen had an internal problem, not external. Anyone can see rust on the outside, and fix it - but rust on the inside is hidden. Also people don't take as much trouble with epoxying etc on the inside - but that is a bad mistake. If anything it needs to be better on the inside than out. People figure it won't hurt to skimp on the inside paint because it's just going to be condensation or whatever. That may be true but poorly-coated surfaces still rust like hell, it doesn't seem to make much difference if the condensation is fresh or salt water. I think this may be because of the salty atmosphere - fresh water cannot exist at sea, the vapour and the condensation gets contaminated with salt, which then draws more salt in as it is hygroscopic. Pour some salt on the saloon table, leave it, then look at it the next day - it's converted itself into a pool of water, salt water. And salt gets in everywhere on a boat, it's in the atmosphere and probably cannot be excluded even from 'sealed' areas.

On my own steel sailboat the most expensive paint, and the most care, and the heaviest coatings, went into the bilge - ie the inside hull, low down. I reckon that's money well spent.

I believe that if the entry of oxygen to an enclosed area can be reduced in volume, and surfaces can be coated with an oil with an extended lifespan and that has good performance even when contaminated, then rust can potentially be reduced to very low levels. The devil is in the detail of course, but if a builder wants to take the trouble, a very high standard of build quality can result.

Keel tanks
Again, there is a choice to be made between siting diesel or water tanks in the keel. There are an awful lot of pros and cons to examine here, and it would certainly be possible to cover the subject in a thousand words or so – but not less. Suffice it to say that most people end up with their main diesel tank here, even though there are sufficient reasons for a water tank instead.

Continued in Part 2: Ballasting Methods and Tools

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