9m Steel Catamaran; thoughts?

Discussion in 'Boat Design' started by rustybarge, May 20, 2016.

  1. rustybarge
    Joined: Oct 2013
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    Location: Ireland

    rustybarge Cheetah 25' Powercat.

    Hi All,

    Can steel make a good materials choice for a small 30' Cat for home building, , and what plus/minus points does 3mm plating pose?

    http://www.bodenboatplans.com/popup_image.php?pID=23&image=0

    http://www.bodenboatplans.com/popup_image.php?pID=23&image=1

    BB260 "Steel Catamaran 9 Metre" $595.00

    Length 9.0 m
    Displacement 6500-7500 kg
    Material Steel
    Hull Draft 0.85 m
    LWL 7.9 m
    Hull Weight 4500 kg steel
    Beam 3.8 m
    Max Speed 12 kn
    Motor capacity 2 x 60 hp
    Fuel Type Diesel
    Fuel Capacity 700 litres

    [​IMG]


    100mm x 50 x 3mm cross beams folded plate
    Fabricated frames 3mm flanged plate.
    30 x 3 mm stringers.
    2.5 mm superstructure .

     
  2. Stumble
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    Location: New Orleans

    Stumble Senior Member

    Let's see.... The design displacement is roughly four times what a 30' cat should weigh, and the speed is pretty marginal.

    Setting aside the difficulty in welding steel this thin, you would be far better buying a 30' fiberglass sailing catamaran and adding bigger engines. It would be pretty inefficient compared to a designed power cat, but still far better than this.... Thing.
     
  3. rustybarge
    Joined: Oct 2013
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    Location: Ireland

    rustybarge Cheetah 25' Powercat.

    I've welded 3mm steel roof onto a 60' barge and it welds OK ; admittingly cutting it with a torch wasn't much fun as it distorted very easily. I tacked the sheets in place, then cut them to get around the problem.

    Just looking at few examples of cruising cats and their displacement ...

    PDQ 34' at 7 tons twin 75hp tops out at 18 kts ...

    Length overall: 34' 6"
    Length waterline: 33' 11"
    Depth: 2' 4"
    Beam overall: 16' 10"
    Height above waterline: 12' 3"
    Displacement: 15,000 lbs.
    Fuel Capacity: 184 US gal.
    Water Capacity: 80 US gal

    [​IMG]
     
  4. fredrosse
    Joined: Jan 2005
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    fredrosse USACE Steam

    Actually no difficulty welding this steel thickness with a wire feed machine. Stick welding would be difficult for me, but I am no professional welder. With a quality MIG machine even I could weld very well for a thin steel hull. Cutting the steel with an abrasive wheel is also easy, with virtually no distortion.

    Steel weight for 3mm thickness is about the same as 35mm plywood, whereas in plywood you could use 20mm thickness. So the hull plating in steel is considerably heavier than with plywood, but most of us know that already.

    Steel hulls, with a good coating system, can hold up very well, and if that is your preference, then proceed to get what you want.
     
  5. rustybarge
    Joined: Oct 2013
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    Location: Ireland

    rustybarge Cheetah 25' Powercat.

    My project is to home build a small coastal cruising cat on a small budget. The advantage of steel is easy welding; alloy needs an experienced expert .

    The materials for a 30' plywood cat come to about $20k using good quality marine ply.

    Here in Europe steel is at an all time low of £300/ton because of Chinese dumping and oversupply from European mills.

    I estimate £2k would cover the material costs in steel.
     
  6. Richard Woods
    Joined: Jun 2006
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    Richard Woods Woods Designs

    I doubt if you will get 14 knots with twin 60hp on a 7.5T catamaran. Has someone achieved that speed with one?

    I recall you have posted before about the powercat you were building. So I am pretty sure you know my Skoota 28, which is, as Stumble says, nearly a quarter the weight of the steel boat. The hulls are 6mm, the decks 9 and 12mm. Yet has proven to be a successful live aboard cruiser. My Skoota 32 is currently being built and will be under 3T in the water

    If you think that a steel boat will be cheaper remember to add in the cost of bigger engines and the running costs. I suspect the bridgedeck will slam as heavy boats need higher clearance

    The 9m Catalac doesn't weigh 7T!

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  7. rustybarge
    Joined: Oct 2013
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    Location: Ireland

    rustybarge Cheetah 25' Powercat.

    Hi Richard,

    You're right I was building a cheetah 23' , but was looking for something a bit bigger. I've had a look at some of the speeds achieved on power cats and most seem to get about 15kts on 20hp/ ton. So an extra ton or two would not require much extra HP to push it along at 10-15kts.

    The Roger Hill designs seems to have low bridge deck clearance , and Australian waters have a reputation for steep seas, as do the cheetahs. I think closely spaced hulls compress waves and act like a shock absorber that stops slamming at higher speeds.

    You are quite right the catalac is 7000 lbs not KG's ...
     
  8. rustybarge
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    rustybarge Cheetah 25' Powercat.

  9. waikikin
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    waikikin Senior Member

    Rustybarge,

    Steel wins as a low cost start to fabricate, the other materials soon catch up when you factor in abrasive blasting and paint system....welding consumables/gas/ abrasive discs are a minor add on but add up, then you need to build the interior which generally means lining the habitable areas which in effect is building a thin ply/laminate boat inside much of the steel one.
    It would be one tough little cat, one was for give away incomplete in Wollongong a few years back.
    At the other end of the project you would have a tough, comfy, slow piece of paradise..... on resale which you may not care about could be very low.
    Cecil E was a respected designer of commercial craft and some pleasure craft. If he was designing today in the market available that boat might be very different than presented.
    I'd never say don't build it but keep your eyes wide open at the big pic.
    Al the best from Jeff.
     
  10. SamSam
    Joined: Feb 2005
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    Location: Coastal Georgia

    SamSam Senior Member

    It's in your moniker rustybarge...rust. In thin steel, a corrosive pit is halfway through the skin. The worst is from the inside out, from condensation. I imagine creating and maintaining a protective coating in all the nooks and crannies inside those narrow, deep hulls would be a chore.
     
  11. rustybarge
    Joined: Oct 2013
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    rustybarge Cheetah 25' Powercat.


    On the plus side a steel boat doesn't need a climate controlled shed for the epoxy to cure; so some savings can be gained there, and also with the very low materials cost.

    From the preamble it seems to suggest it might have been an acedemic design project to see if a steel cat could actually be viable build material. My thoughts are that a plywood superstructure/steel hulls might save a substantial amount of weight, along with modern lightweight engines.

    Just as an example of the cost of a10mtr GRP Cat, the BW seacat comes as a hull moulding, wheel house moulding and kit of parts inc. Shafts, rudder but no windows doors or deck fitting at £100,000 plus vat of 20% for private buyers.

    http://www.findafishingboat.com/bwseacat-ltd-grp-bwseacat-ltd-grp/ad-36620

    So .... £120k versus say £3k for a steel cat fabricated with industrial two pack finish.

    And when you come to sell the steel boat has absolutely zero value,but the GRP hull maybe retains half its value.
     
  12. rustybarge
    Joined: Oct 2013
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    rustybarge Cheetah 25' Powercat.

    This problem had occurred to me, especially if there are parts of the frames with doublers . how do you stop corrosion once its got into two thin steel plates spot welded together?

    I suppose after good layer of two pack paint, some sort of wax treatment like waxoil could be injected into overlapping sections of the steel work. But is plywood any better if it gets wet and starts rotting, and how would you dry it out in narrow hard to get at sections?
     
  13. waikikin
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    waikikin Senior Member

    Well you need to seal weld the perimeter, I'm assuming you're indicating the gussets? lapping the framing.
    The "secret" with steel or timber for that matter is ventilation and limbering(drainage holes), an effective paint scheme is very important to steel, access to or elimination of void areas as well.
    The 3K structurally complete cat is fantasy if including coatings even in Pound$.
    A plywood or composite cabin may be ok if the landing/upstand/coaming is in staino, the fastenings required generally compromise coatings to steel & eventual results blow up with oxygen mixing with steel;)

    Jeff.
     
  14. whitepointer23

    whitepointer23 Previous Member

    what about alloy glued instead of welding. i read somewhere that alloy boats can be glued together just like plywood. i think the bond is also a lot stronger than glued ply bonds. even a combination of epoxy and rivets like an aircraft.
     

  15. rustybarge
    Joined: Oct 2013
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    rustybarge Cheetah 25' Powercat.

    I'm not familiar with ' staino', got no google results?

    The 2.5mm steel plating would be difficult to weld fittings to and I imagine bolting through with backing plates would be the only way to support any sort of load; as you say rust will start to form around bolts and other fixtures which will move with the flexing of the boat, then allowing moisture behind the plating.


    But giving due consideration to steel construction its quick, strong, cheap ....but heavy and prone to rusting.

    Whereas plywood is expensive, slow and tedious to construct ..but also very vulnerable to water ingress and delamination from small area of underwater damage that may be overlooked until the annual haul out.
     
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