STEEL HULLS with Composite Superstructure / Topsides

Discussion in 'Boatbuilding' started by brian eiland, Jun 16, 2013.

  1. Ad Hoc
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    Ad Hoc Naval Architect

    Significantly!

    I've done enough reviews for clients, and based upon our as-built designs of steel and ally, that show if you look at base capital material cost, i.e. what you're paying up front right now, sure steel looks the winner. But if you're savvy and look at total through life service/maintenance cost of the whole vessel, ally wins hands down after about 5 years.
     
  2. michael pierzga
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    michael pierzga Senior Member



    Its the inside of a steel boat the rots..not the outside. Small metal boats are trouble because space under the floorboards is so small and inaccesable that you cant keep the area clean, ventilated and painted. Aluminium is much more forgiving of abuse.
     
  3. MikeJohns
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    MikeJohns Senior Member

    I see very little difference in the cost of maintenance of leisure boats between Alloy and steel providing they have been designed and built with some thought towards longevity. There are several ancient steel boats I’ve been involved with one 65 footer well over 100 yrs old still a very sound vessel but needed the anchor locker replating. Another 50 footer built 1935 is in quite pristine condition but needed some minimal replating. There are numerous other boats around built more recently that have no wastage and are well built.

    Class societies recognise that yachts fare better and allow us to dispense with the required 25% wastage allowance that steel work boats are required to have, the difference being usually uncaring hard use and economic return over any maintenance of coatings.

    Painted alloy yachts can be a pain to maintain as they get older as the cracked or damaged paint allows chloride ions in and then the paint disbonds easily and quickly and the adjacent disbonded paint traps moisture, it’s surprising how large a patch it can affect around a small obvious blister.

    I’d give a steel hot zinc sprayed yacht hull built to my standards a 25 yr guarantee against internal corrosion, and in reality I’d expect it to last 150 yrs or more with a bit of patching here and there over the years. Sounds incredible but it’s the reality we are seeing from well maintained yachts.

    Workboats are almost always better in alloy and will give a neater cleaner ship for their working life but they should be left unpainted wherever sensible. But alloy as AdHoc will tell us is not a good material for an inexperienced yard to build in. I’ve seen very expensive failures. With Alloy look for a good yard, a good designer, and built to full class approval (not just plan approval).

    For home builders and less experienced yards steel is a better choice. I’ve surveyed a myriad of steel boats that will never give problems, keep the water out and touch up your exterior paint if it gets damaged. Well applied epoxy coatings are lasting indefinitely so long as they are protected from UV. With Steel it's important to stop water pooling and to allow coamings to drip clear rather than wetting from openings such as ports and hatches. If you follow a fre rules you can make steel pretty much as maintenance free as a well found GRP craft.
     
  4. michael pierzga
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    michael pierzga Senior Member

    If people take care of their steel boats they cause little trouble.

    Who takes care of their boats?

    Im sailing an alloy boat now. 20 years old and very heavy usage. The boat is full of paint dings, bare metal ....yet the boat looks pretty good. No rust bleeds. At the end of the season Ill break out my grinder, touch up the dings and she will look perfect. You cant do this with steel...metal filings in everything.

    Tanks are particularly high maintence on a steel boat
     
  5. michaeljc
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    michaeljc Senior Member

    I have to agree with you ally guys.

    We have learnt. Once was the time ally boats were knocked up and shoved out with no coating in the expectation they would be corrosion free. The basic precautions required are now well realised and implemented. If you were to take the same attitudes towards coatings as is the case with steel then, man, you have something very special. Coating in alloy are more about insolation from electrolysis than chemical corrosion. We had a boat come in recently with a gauge earthed to the body. It had accelerated corrosion in a range of locations. Tantalised wood can eat out a 6mm hull inside 12 months. Tools left in the bilge do the same. This is why I believe that sound coatings are as important on alloy as they are on steel.

    I was looking at 2 trawlers in Raglan the other day. One was steel. It had the typical flaky surface below the copious paint. It had not long been painted but rust stains were seeping down from behind the rubber wheelhouse window seals. For all the precautions over coatings if just one cm along the edge of a steel plate is chipped, this is what happens and it is progressive. What would you need to do in case like this? strip out all the windows, grind or sandblast back to clean metal and re-coat. Inside a year you will be doing the same somewhere else. It is relentless and only gets worse. In a fresh water environment maybe OK.

    Along side the steel trawler there a wooden one. She was probably 40 years old, looking as prim as you like.

    m
     
  6. michael pierzga
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    michael pierzga Senior Member

    Steel is a fantastic material for big boats...100 footers. And a fantastic material for industrial use workboats.

    The OP want to build a small lightweight yacht. I dont like it.
     
  7. brian eiland
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    brian eiland Senior Member

    Just another reason that I DO NOT want steel in the superstructure,...lots of multiple 'edges' to worry about. Not that many loose ends/edges in the basic hull shell.
     
  8. pdwiley
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    pdwiley Senior Member

    We had one of our 9m ally work boats painted when built. Paint started bubbling & flaking around dings (inevitable when you work in ice). We had the hull soda blasted back to bare metal, end of problem.

    I was advised by both the designer of my vessel and one of my staff - an ex-Austal welder - not to build my own boat from aluminium due to the nature of the material and the higher level of skill needed for a successful result. I paid attention.

    As for a steel hull with a composite topsides, I see no reason why this wouldn't be quite successful provided careful attention is paid to the bonding joint. I personally would use a steel deck and a raised lip to attach the superstructure.

    PDW
     
  9. MikeJohns
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    MikeJohns Senior Member

    Tanks these days should be separate and are often of polythene or similar construction and are no problem at all. Integral steel and integral alloy water tanks and holding tanks are all problematic and I never design them so.

    Most people I know with a yacht take great care of their boats ! And you can work in a rub rail/sponson on the side of a boat to great advantage. Galvanised steel half pipe sponsons work well to take the abuse, you can use stainless steel too for many of the parts that would otherwise suffer from paint chipping.
    There's many ways to make a steel boat lower maintenance even when it gets knocked around. A lot of production steel boats from Europe gave steel a bad name, they were poorly detailed and seamlessly fitted out with glued interiors, integral tanks and often with teak over steel decks. Their life span was about 20 years before they needed a lot of expensive work. I had a production van de Staadt 57 once that was like that. If they'd built them in alloy they would have lasted a lot better.


    Interestingly I saw several integral fresh water tanks erode through on Alloy boats. It's also excacerbated with amounts of copper in the water dissolved from copper pipes in the city supply, it accelerates corrosion. We found a similar result with Alloy gutterings and copper flashings.

    Alloy is at a very high energy state and is prone to some very rapid corrosion when conditions are met. It's also 'activated' by both sides of the pH scale. So it's important to have some knowledge of the material.

    I've seen some quite shocking damage from a leaking submerged power cable when the alloy boat was moored and earthed , it nearly sank after a few months and the extent of the damage surprised me. A steel cray boat adjacent lost much of it's propeller but the hull was pristine. Both were earthed. The paint was stripped off the alloy boat. That was a bit of an eye opener and I always tell people how to check for an electric field in the water now.
     
  10. michael pierzga
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    michael pierzga Senior Member

    Hmm...copper in the water. Never heard that.
     
  11. MikeJohns
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    MikeJohns Senior Member

    It's apparently a common warning now to civil Architects, don't mix copper flashings or roofing with alloy gutters or downpipes of any alloy. It's not direct contact, it's the minute amounts of dissolved copper in the rain water.

    Other major corrosion problems have been side exit of diesel exhaust and also Internal in stern tubes . Both of which caused near full penetration of adjacent plate/tube in one or two seasons. It's another reason you need an experienced designer and building yard.
     
  12. michaeljc
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    michaeljc Senior Member

    Our practice with coating ally: Very light sandblast, special ally etch coat, 2 pack primer/undercoat, final coat. I think we could probably get way with final 2 pack over etch primer. We have seen no bubbling or failure.
     
  13. michaeljc
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    michaeljc Senior Member

    Brian: If you can give the weight of steel in your bare hull and hull plate thickness, I can give you a cost comparison with building in ally on a percentage basis. Cost of welding gas and wire is higher with ally but this is more than offset by ease of construction (hours) due to weight difference and ease of cutting. The alloy plate and sections will be thicker but lighter. There is also high tensile aluminium which can be thinner but it is a ***** to weld.

    Yes, ally construction is very specialised. Weld quality and distortion are the main challenges. Top of the range welding plant is essential.

    I recently looked over a lovely 12 m craft in Fiji. It had not been painted. It that been donated by the Australian Govt to the Fijian Customs Department.

    Lovely boat. Pity about the welding, it was diabolical. It would not have passed a build survey in NZ.
     
  14. MikeJohns
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    MikeJohns Senior Member

    Only after there's breach in the paint from damage or from inevitable paint deficiencies. All paint need touching up after a period of time, but on alloy even very well prepared surfaces show a rapid paint disbondment when a paint breach is left untreated. And usually all that is visible is a raised uneven blistering.

    Under the disbonded paint the alloy surface is activated and readily corrodes, the corrosion product is also hygroscopic so the result eats away a thin layer of metal under the paint, then can proceed to pit corrosion closer to the paint breaches. No level of preparation or brilliant coating chemistry can stop this since it's not a failing of the paint bond, it's a loss of the substrate the paint is covering. People with painted alloy sailboat masts will be familiar with this. While anodised masts usually fare much better.
     

  15. MikeJohns
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    MikeJohns Senior Member

    Just a note here:

    AdHoc has an expert level of knowledge on alloy construction and would be worth contacting for anyone intending to build in alloy .
     
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