Reverse Engineering (conversions And Modifications)

Discussion in 'Boat Design' started by viking north, Dec 25, 2010.

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    BATAAN Senior Member

    The only SS we used much in my boatyard days was "Aquamet", a shafting material. This was utilized for replacement ballast keel bolts on a very wide range of craft from 60' ocean racing sleds with enormous bulb keels to mundane things like a Spidsgatter, Folkboats and such. Hard to cut threads but a canny old machinist suggested using CocaCola for cutting fluid and it worked! We used a plumber's pipe cutting machine, with straight dies though of course. I seem to remember having to replace many SS tanks on yachts built in the Far East, usually with SS again at the owner's insistence.
    Fishboat and most commercial fuel, water and waste tanks are almost universally mild steel of adequate dimension to give 20 year life. Yachts are seen as different and needing exotic materials, probably due to an aversion to rust stains, which are inevitable.
    My 40 foot yawl, BERTIE, has two, fifty gallon Pepsi syrup shipping drums as water tanks. These are some food-grade SS and show no change at all in 30 years mounted on deck with much salt water over them. There's no place to trap water so no crevice corrosion. Fuel tanks are two, twenty six gallon aluminum tanks in a dry place below. These are off-the-shelf, tested, mass manufactured items from a maker in Florida and were bought through West Marine. A fifty gallon plastic fuel tank in a plywood and epoxy box lives on deck also. This doesn't mind salt water and seems to condense less internally than a metal tank might.
    BERTIE is a modified SPRAY and can tolerate great amounts of deck weight because the deck is so close to the CG anyway.
    A cement wash inside a water tank is good and all steel tanks are coated outside with epoxy paints, and carefully installed to not trap water at the mounts, on the top etc.
    ABYC recommends aluminum for fuel and water. I'm not sure of their attitude towards blackwater tanks.
    I knew a guy who was engineer on a Sea Shepherd activist vessel (160' ex-FV) some years ago and he hooked up their blackwater tank to the pressure fire system and used it to repel Faroese CG boarders when SS was fleeing after documenting illegal whale kills. Nothing like having your inflatable filled with blackwater to discourage you from coming closer. This was after the CG guys shot out the pilothouse windows trying to get them to stop before the 12 mile limit. It worked, they quit in unimaginable disgust and SS got away.
  2. BertKu
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    BertKu Senior Member

    John, just learning and soaking information up. What Michael has written is very interesting. I love SS, but I am starting to wonder. I was planning to make the keels from stainless steel, but have now second thoughts.
  3. viking north
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    viking north VINLAND

    Bertku, i know several people who have used 1/2 to 1in stainless plate as drop centerboards, there was never any problems, the only problem as i can see is if you are using it as a keel shell, but even in that case where it is antifouled i still can't see a problem. I have use stainless as part of my keel shoe holding the lower rudder bearing on full keels and in 20 yrs. never seen any problems. I know big fishing boats that use a stainless keel shoe and never any problems, i wouldn't worry about it unless as Michael has pointed out it's use on internal tanks, or as keel bolts where there is a possibility of water getting at them . Geo.
  4. Scunthorp
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    Scunthorp Hull Tech

    Why Stainless Steel?

    The use of stainless steel as a building material insures a high quality, virtually maintenance-free storage facility, and years of use. Bolted stainless steel tanks designed to meet AWWA standards have a product life expectancy of over 40 years. Stainless steel is durable, attractive, and resistant to corrosion; in contrast, mild steel is ferrous and will rust. Stainless steel tanks have a longer life expectancy than epoxy-coated or glass-lined tanks. They do not have to be repainted, re-gasketed or welded. Stainless steel has a residual value, which far exceeds that of other building materials. Stainless steel is strong, can be formed to meet individual preferences, is watertight, and is very resistant to cold weather. In addition, stainless is conducive to alteration without distortion.

    Stainless steel is a combination of nickel, chromium, magnesium, and other alloys; this combination provides stainless steel's corrosion resistance and durability through passivation, which makes it an extremely attractive building material. Since there is no coating to scratch, chip, or wear off, stainless steel does not have the built-in disadvantages of coated steel.

    Industry consolidation has also led to a lack of competition, which has significantly increased coated tank prices. In comparison, there is greater competition in the stainless steel market, which results in extremely competitive pricing between tanks constructed of stainless steel and tanks constructed of coated steel.

    Stainless steel is widely accepted as a building material in many industries and by many regulatory agencies. Stainless is commonly used in the food processing industries because of its ability to meet FDA standards. As the population grows and the clamor for clean drinking water increases, the demand for stainless steel as a building material will also increase.

    Visit for more stainless steel information.
  5. viking north
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    viking north VINLAND

    John, the problem with stainless occurs with oxygen starvation, I.E. it is deprived of exposure to oxygen. It's the exposure to oxygen that builds up a surface film that gives it it's great resistance to corrosion. The other problem is there is so much poor grade stainless on the market(both 304 and 316) so one has to be vigilant of the manufacturer. However i have never heard of a problem using it with fuel or fresh water tanks but have read about problems with stainless keel bolts corroding (crevise) at the hull to keel joint with bolt on keels.I suspect that even in those cases most of the problems could be with poor workmanship and poor quality stainless . I guess it pays to be vigilant and where keel bolts are concerned i would hesitate in using it where there is any possibility of water intrusion. As for stainless tanks in my 40 yrs. of messing around boats, boat yards never heard of a problem, again i suspect three factors could be combined resulting in problems, poor quality stainless, oxygen starved water(warm climates)(brakish water)(water containing certain chemicals) and electrolises due to floating grounds and maybe some dissimilar metal activity. Geo.
  6. Scunthorp
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    Scunthorp Hull Tech

    Because of its superior corrosion and oxidation resistance, good mechanical properties and fabricability, SX 316 has applications in many sectors of industry. Some of these include:
    Tanks and storage vessels for corrosive liquids.
    Specialised process equipment in the chemical, food, paper, mining, pharmaceutical and petroleum industries.
    Architectural applications in highly corrosive environments.

    SUBJECT: Corrossion problems associated with stainless steel 4-1

    The rotating equipment business uses a great deal of 300 series stainless steel, and as a result we often experience several types of corrosion:

    General corrosion
    Galvanic corrosion
    Inter granular corrosion
    Chloride stress corrosion cracking
    Erosion- corrosion
    Concentrated cell or crevice corrosion
    Selective leaching
    Micro organisms
    At the end of this aticle is a page titled, "The Galvanic Series Of Metals and alloys". I'll be referring to this chart during our discussion.

    The basic resistance of stainless steel occurs because of its ability to form a protective coating on the metal surface. This coating is a "passive" film which resists further "oxidation" or rusting. The formation of this film is instantaneous in an oxidizing atmosphere such as air, water, or other fluids that contain oxygen. Once the layer has formed, we say that the metal has become "passivated" and the oxidation or "rusting" rate will slow down to less than 0.002" per year (0,05 mm. per year).

    Unlike aluminum or silver this passive film is invisible in stainless steel. It's created when oxygen combines with the chrome in the stainless to form chrome oxide which is more commonly called "ceramic". This protective oxide or ceramic coating is common to most corrosion resistant materials.

    Halogen salts, especially chlorides easily penetrate this passive film and will allow corrosive attack to occur. The halogens are easy to recognize because they end in the letters "ine". Listed in order of their activity they are:

    astatine (very unstable.)
    These are the same chemicals that will penetrate Teflon and cause trouble with Teflon coated or encapsulated o-rings and/ or similar coated materials. Chlorides are one of the most common elements in nature and if that isn't bad enough, they're also soluble, active ions; the basis for good electrolytes, the best conditions for corrosion or chemical attack.


    This type of corrosion occurs when there is an overall breakdown of the passive film formed on the stainless steel. It's the easiest to recognize as the entire surface of the metal shows a uniform "sponge like" appearance. The rate of attack is affected by the fluid concentration, temperature, fluid velocity and stress in the metal parts subject to attack. As a general rule the rate of attack will double with an eighteen degree Fahrenheit rise in temperature (10° C.) of either the product or the metal part.

    If the rotating portion of the seal is rubbing against some stationary component, such as a protruding gasket or fitting, the protective oxide layer will be polished off and the heat generated will increase the corrosion as noted above. This explains why corrosion is often limited to only one portion of the mechanical seal, metal casing.

    There are many good publications available to help you select the proper metal for any given mechanical seal application. As a general rule, if the wetted parts of the equipment are manufactured from iron, steel, stainless steel or bronze, and they are showing no signs of corrosion, grade 316 stainless is acceptable as long as you do not use stainless steel springs. (see chloride stress corrosion below)


    If you put two dissimilar metals, or alloys in a common electrolyte, and connect them with a voltmeter, it will show an electric current flowing between the two. (This is how the battery in your automobile works). When the current flows, material will be removed from one of the metals or alloys ( the ANODIC one) and dissolve into the electrolyte. The other metal (the CATHODIC one) will be protected.

    Move down to the end of this aticle and look at the Galvanic Series chart The further apart the materials are located on this chart, the more likely that the one on the ANODIC end will corrode if they are both immersed in any fluid considered to be an electrolyte.

    Salt water, is one of the best!

    Example #1.

    A ship has lots of bronze fittings and a steel hull. Note that steel is located seven lines from the ANODIC end, and bronze is listed at twenty seven rows from the same end. Sea water is a perfect electrolyte, so the bronze fittings would immediately attack the steel hull unless something could be done to either protect the steel ,or give the bronze something else to attack.

    The classic way to solve this problem is to attach sacrificial zinc pieces to the hull and let the bronze go after them. Again, looking at the chart, you'll note that zinc is found on line three from the top of the chart. In other words the zinc is further away from the bronze than the iron, so the galvanic action takes place between the zinc and the bronze, rather than between the steel and the bronze. Zinc paint is used for the same reason.

    Example #2

    Nickel base tungsten carbide contains active nickel. When this face material is used in dual seal applications it is common to circulate water or antifreeze between the seals (as mentioned in the beginning of this paper, water can be an excellent electrolyte because of the addition of chlorine and fluorine). You'll note that active nickel is located twenty one rows from the top of the chart. Passivated 316 stainless steel is positioned nine rows from the bottom. This means that the stainless steel can attack the nickel in the tungsten carbide causing it to corrode.

    The rate at which corrosion takes place is determined by :

    The distance separating the metals on the galvanic series chart
    The temperature and concentration of the electrolyte. The higher the temperature, the faster it happens. Any stray electrical currents in the electrolyte will increase the corrosion also.
    The relative size of the metal pieces. A large cross section piece will not be affected as much as a smaller one.
    Many metal seal components are isolated from each other by the use of rubber o-rings or similar materials and designs. Shaft movement that causes fretting of the 316 stainless steel rubs off the passivated layer and exposes the active stainless to the electrolyte until the metal part becomes passivated once more. This is one of the reasons we see corrosion under o-rings, and Teflon wedges. In the following paragraph I'll be discussing another cause of corrosion under rubber parts.

    This is an accelerated form of chemical attack in which the rate of corrosion is greater in some areas than others. It occurs when the corrosive environment penetrates the passivated film in only a few areas as opposed to the overall surface. As stated earlier, halogens will penetrate passivated stainless steel. Referring to the galvanic chart you'll note that passivated 316 stainless steel is located nine lines from the bottom and active 316 stainless steel is located thirteen lines from the top. Pit type corrosion is therefore simple galvanic corrosion, occuring as the small active area is being attacked by the large passivated area. This difference in relative areas accelerates the corrosion, causing the pits to penetrate deeper. The electrolyte fills the pits and prevents the oxygen from passivating the active metal so the problem gets even worse. This type of corrosion is often called "Concentrated cell corrosion". You'll also see it under rubber parts that keep oxygen away from the active metal parts, retarding the metal's ability to form the passivated layer.


    All austenitic stainless steels (the 300 series, the types that "work harden") contain a small amount of carbon in solution in the austenite. Carbon is precipitated out at the grain boundaries, of the steel, in the temperature range of 1050° F. (565° C) to 1600° F. (870° C.). This is a typical temperature range during the welding of stainless steel.

    This carbon combines with the chrome in the stainless steel to form chromium carbide, starving the adjacent areas of the chrome they need for corrosion protection. In the presence of some strong corrosives an electrochemical action is initiated between the chrome rich and chrome poor areas with the areas low in chrome becoming attacked. The grain boundaries are then dissolved and become non existent. There are three ways to combat this:

    Anneal the stainless after it has been heated in this sensitive range. This means bringing it up to the proper annealing temperature and then quickly cooling it down through the sensitive temperature range to prevent the carbides from forming.
    When possible use low carbon content stainless if you intend to do any welding on it. A carbon content of less than 0.3% will not precipitate into a continuous film of chrome carbide at the grain boundaries. 316L is as good example of a low carbon stainless steel.
    Alloy the metal with a strong carbide former. The best is columbium, but sometimes titanium is used. The carbon will now form columbium carbide rather than going after the chrome to form chrome carbide. The material is now said to be "stabilized"

    If the metal piece is under tensile stress, either because of operation or residual stress left during manufacture, the pits mentioned in a previous paragraph will deepen even more. Since the piece is under tensile stress cracking will occur in the stressed piece. Usually there will be more than one crack present causing the pattern to resemble a spider's web. Chloride stress cracking is a serious problem in industry and not often recognized by the people involved. In the seal business it is a serious problem if you use stainless steel springs or stainless steel bellows in your seals. This is the main reason that Hastelloy C is recommended for spring material. Here are some additional thoughts about chloride stress cracking that you'll want to consider:

    Chlorides are the big problem when using the 300 series grades of stainless steel. The 300 series is the one most commonly used in the process industry because of its good corrosion resistant proprieties. Outside of water, chloride is the most common chemical found in nature and remember that the most common water treatment is the addition of chlorine.
    Beware of insulating, or painting stainless steel pipe. Most insulation contains chlorides and piping is frequently under tensile stress. The worst condition would be insulated, steam traced, stainless steel piping.
    If it's necessary to insulate stainless steel pipe, a special chloride free insulation can be purchased, or the pipe can be coated with a protective film prior to insulating.

    Stress cracking can be minimized by annealing the metal, after manufacture, to remove residual manufactured stresses.
    Never replace a carbon steel bolt with a stainless steel one unless you're sure there are no chlorides present. Bolts can be under severe tensile stress.
    No one knows the threshold values for stress cracking to occur. We only know that you need tensile stress, chlorides, temperature and the 300 series of stainless steel. We do not know how much chloride, stress or temperature.
    Until I figured out what was happening I had trouble breaking stainless steel fishing hooks in the warm water where I live in Florida.
    Many cleaning solutions and solvents contain chlorinated hydrocarbons. Be careful using them on or near stainless steel. Sodium hypochlorite, chlorethene. methylene chloride and trichlorethane are just a few in common use. The most common cleaner used with dye checking material is trichloroethane, explaining the reason we sometimes experience cracks after we weld stainless steel and dye check it to inspect the quality of the weld.

    This is an accelerated attack resulting from the combination of mechanical and chemical wear. The liquid velocities in some pumps prevents the protective oxide passive layer from forming on the metal surface. The suspended solids also remove some of the passivated layer increasing the galvanic action. You see this type of corrosion very frequently at the eye of the pump impeller.


    This type of corrosion is easily seen on the pump shaft or sleeve. You'll see the damage on the shaft under:

    The grease or lip seal that is supposed to protect the bearings.
    The packing used to seal the fluid.
    The dynamic Teflon or elastomer used in most original equipment seals.
    The vibration damper used in rotating metal bellows seals.
    The rubber boot used in low cost seals, if it did not attach to the shaft properly.
    As mentioned earlier, 300 series stainless steel passivates its self by forming a protective chrome oxide layer when ever it is exposed to free oxygen. This oxide layer is very hard and when it imbeds into a soft elastomer it will cut and damage the shaft or sleeve rubbing against it. The mechanism works like this:

    Oxygen passivates the active stainless steel forming a protective ceramic layer.
    The seal or packing removes the oxide layer as the shaft or sleeve rubs against it.
    The ceramic passivated layer sticks into the soft elastomer turning it into a "grinding surface".
    The oxide reforms when the active metal is exposed and the process starts all over again.
    A visible groove is cut into the shaft, or sleeve that will cause seal leakage and "hang up".

    This corrosion occurs any time liquid flow is kept away from the attacked surface. It is common between nut and bolt surfaces, under O-rings and gaskets, and between the clamps and stainless steel shafts we find in many split seal applications. Salt water applications are the most severe problem because of the salt water low PH (8.0&endash;9.0) and its high chloride content. Here is the mechanism:

    Chlorides pit the passivated stainless steel surface.
    The low PH salt water attacks the active layer that is exposed
    Because of the lack of fluid flow over the attacked surface, oxygen is not available to passivate the stainless steel.
    Corrosion continues unhampered under the rubber and tight fitting clamp.
    The inside of the o-ring groove experiences the same corrosion as the shaft or sleeve.

    The process fluid selectively removes elements from the piping or any other part that might be exposed to the liquid flow. The mechanism is:

    Metals are removed from the liquid during a de-ionization or de-mineralizing process.
    The liquid tries to replace the missing elements as it flows through the system.
    The un-dissolved metals often coat them selves on the mechanical seal faces or the sliding components and cause a premature seal failure.
    Heat accelerates the process.

    These organisms are commonly used in sewage treatment, oil spills and other cleaning processes. Although there are many different uses for these "bugs", one common one is for them to eat the carbon you find in waste and other hydrocarbons, and convert it to carbon dioxide. The "bugs" fall into three categories:

    Aerobic, the kind that need oxygen.
    Anaerobic, the kind that do not need oxygen.
    Facultative, the type that goes both ways.
    If the protective oxide layer is removed from stainless steel because of rubbing or damage, the "bugs" can penetrate through the damaged area and attack the carbon in the metal. Once in, the attack can continue on in a manner similar to that which happens when rust starts to spread under the paint on an automobile.


    ALUMINUM 5052, 3004, 3003, 1100, 6053
    ALUMINUM 2117, 2017, 2024
    302, 303, 304, 321, 347, 410,416, STAINLESS STEEL (ACTIVE)
    60 NI-15 CR (ACTIVE)
    80 NI-20 CR (ACTIVE)
    COPPER (CA102)
    MONEL 400, K500
    60 NI- 15 CR (PASSIVE)
    80 NI- 20 CR (PASSIVE)
    302, 303, 304, 321, 347, STAINLESS STEEL (PASSIVE)
  7. viking north
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    viking north VINLAND

    O0000K, That should answer most questions re. Stainless, what did you do buy an electronic book and plug it in the stainless port on your computer,(smiley face) Geo.
  8. BertKu
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    BertKu Senior Member

    That was very informational. Thanks for the trouble to give us a lecture.
    In my case, I am using stainless 316 spring washers. I may have to re-consider this.
    Thanks John
  9. michael pierzga
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    michael pierzga Senior Member

    The question to ask is what is the best metal...or material ...for each task. With metal, Ive always been taught that stainless is used above waterline and bronze or steel below.

    Im not sure what Vikings construction material will be. Plastic hulls have plastic tanks...metal hulls have metal tanks.

    Tanks in general take some thought. Huge bolt on removable tops for inspection and maintenance is very important. Venting the poo poo tank is always a challenge. Ideally its a portable hose that can be run and vented into the next door neighbors boat...the guy who runs his generator all night, listens to the BeeGee's at full volume, and puts his kids on jet skis to tear around and raise hell in the harbor.

    . Dont try mast venting...your whole boat will stink . Seems waterline venting is the best compromise. Swimmers in Bikinis dont like it. Be sure to run both port and starboard vents. Any positive pressure in the Poo poo tank will stink up your boat.

    I have seen SS keels and Ive seen bronze keels. These were trailer sailors.

    With stainless Ive never..NEvER..been able to successfully bond paint to it underwater. I have a ss shaft bearing P bracket hub...impossible to bond to.
  10. Scunthorp
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    Scunthorp Hull Tech

    "Best" Hmmmmmmm
    A boat made out of 3m5200.
    Best is knd of hard.
    Industry srandard might not be best.
    Then we are talking money.

    If I am unsure I read
    "Knowledge informs us practice convinces"
  11. michael pierzga
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    michael pierzga Senior Member

    Best is always a combination of physical properties, workabilty, durability and cost. The end result is always somewhere in the GREY zone of best. Its up to the builder, designer , customer to consider options .
  12. rwatson
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    rwatson Senior Member

    No doubt, but the Centre of Lateral Resistance will not change, apart from the rudder angle effect.
  13. Pierre R
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    Pierre R Senior Member

    Monel beats stainless on boats around salt water in every way except in price and strength.
  14. hoytedow
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    hoytedow Senior Member

    How you can have a neg rep after this post is beyond me. This is an excellent tutorial on electolysis.

  15. michael pierzga
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    michael pierzga Senior Member

    Whats a negative rep ? Some kinda steel alloy ?
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