Lack of data about steel multihulls

Discussion in 'Multihulls' started by Muzammal, May 18, 2021.

  1. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Only if you wish to travel fast/high Fn.

    We have done several 20m steel catamarans, worked out fine, as that's what the client wanted. :cool:
     
  2. Alik
    Joined: Jul 2003
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    Alik Senior Member

    Yes, if catamaran is needed mostly as a platform, it will work. But hydrodynamical efficiency of catamaran requires the ratio I mentioned above.
     
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  3. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    I have a blueprint of a hand drawn GA drawing - somewhere, I have been trying to find it - from about 30 years ago of a steel workboat powercat designed by an English naval architect (but I cannot remember his name now). The cat had assymetric hulls (as if you cut a monohull down the centreline, and separated the halves) and could be built in various lengths. On the drawing he proposes building the hulls separately, using the flat inboard side of the hull as a base for setting up the hull frames on.
    Can anybody on here recollect who this naval architect might be?

    I designed a 12 metre steel power cat for an English gentleman here 20 years ago - he wanted to set up a beach bar, but the cost of beach front land here is ridiculous, so I suggested to him that we could build him a floating beach bar instead. He especially wanted to provide some competition to a little beach bar that had been his favourite watering hole, but which had decided to go 'up-market' (whereby they doubled the prices of all their drinks, and put tablecloths on the tables).

    The hull and framework for the superstructure was built of steel, which in retrospect was a mistake - the owner was worried about costs, and the steelwork was (I think) about 60% of what the same vessel would have cost to build in aluminium (not including any outfit). Yet by the time all of the outfitting had been completed, and the hulls painted inside and out with many gallons of paint, the cost difference had reduced significantly.

    Here are a few photos of her -

    All the steelwork has been completed, and she is about to be taken down to the boatyard for fitting out.
    Rumpy Pumpy _0001.jpg
    Here she is in the final stages of outfitting - she was fitted with 2 x 40 hp O/B motors.
    Rumpy Pumpy _0002.jpg
    She would put out a stern anchor to hold her off, along with a couple of bow anchors embedded in the beach, ands drop her bow ramp to allow thirsty drinkers to easily walk on board. And yes, the name given to her by her owner was 'Rumpy Pumpy'.

    Rumpy Pumpy _0003.jpg

    Rumpy Pumpy _0004.jpg

    This is a scan of a postcard that was available at the time - the beach bar competition is partially hidden in the trees, to the left of the umbrellas.
    RP at Mullins.jpg

    She was subsequently sold about a year later, and the new owner carried out some 'improvements', including extending the bluff garvey type bows into pointed stems, and adding a glass box between the hulls for underwater viewing.
    Some years later she was sold again, and the new owner was even more ambitious - he extended the top deck fore and aft, fitted larger 90 hp four stroke engines, cut limber holes in the watertight bulkheads to allow water to flow freely aft to the bilge pumps, and announced that he now had the capacity to carry 100 thirsty drinkers on board (I had originally designed her for 40, along with a few crew).
    The photos of the Mark III version are below - I had told the 3rd owner that the boat did not have the extra buoyancy required for all the work he was doing, but he ignored me, and found out the hard way when he re-launched the vessel, and the engines were almost underwater.
    He then set about adding some more buoyancy in the stern, but soon after 'ran out of steam' - the boat has been resting ashore, rusting away and apparently abandoned now, for the past 6 years :(

    In contrast, the 15 metre aluminium powercat in my avatar was built just before the steel cat, and she is still working well today, and has needed a minimal amount of maintenance (relatively) over the past 20 years.

    DSCN4294.JPG

    DSCN4295.JPG
     
    Last edited: May 19, 2021
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  4. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    You have received advice from 3 serious designers about the possibility of a steel 50 foot power tri. Yes, I am sure its possible, but please understand you are going to pay for a large engine, much fuel and many hours of detailed construction. Steel can create a shape quickly but the sand blasting, painting and insulation of steel hulls is not a small task. The maintenance is also very important both outside and internally. It is probably easier and cheaper to use other materials. The following jpegs may give an idea of what can be done, but please speak to a designer, what your trying to design and build is going to be hard to get right. PS look at the "multihull structure thoughts" thread page 119 for some further input.
     

    Attached Files:

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  5. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

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  6. guzzis3
    Joined: Nov 2009
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    guzzis3 Senior Member

    I'm a mechanical engineer, but also a motor mechanic. I used to weld aluminium cans together with an oxy aceteline welder. Steel work like that is trivial.

    I wouldn't build in steel. Catamaran under about 12 meters it's hard to make the hull light enough. Above 12 meters the maintenance becomes prohibitive. It makes sense for commercial boats that are designed for regular maintenance and finite life but not for a private vessel. Trimarans are a much harder case than a cat. I haven't run the numbers but the boat would have to use the floats as load carriers, as the US navy did with their experimental trimaran warships. Otherwise the boat would have to be unfeasibly big.

    I really hate to discourage anyone from experimenting but a steel trimaran is a really tough brief.
     
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  7. Muzammal
    Joined: Dec 2020
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    Muzammal Junior Member

    Hahaha. Yes thats too old fashioned but may be if built in steel, that was the only option for the builder
     
  8. Muzammal
    Joined: Dec 2020
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    Muzammal Junior Member

    Thanks. I got it. I had idea of this before posting to the forum but you know, a larger platform with lesser horsepower is attractive to everyone who wants to travel at higher speeds. With that in mind, I searched for steel due to building ease and economy but unfortunately, steel is not feasible for my expected size. It is confirmed now. Thanks alot
     
  9. Muzammal
    Joined: Dec 2020
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    Muzammal Junior Member

    Thats right to the best of my little knowledge, too.
     
  10. Muzammal
    Joined: Dec 2020
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    Muzammal Junior Member

    That is great for a floating bar, but cannot be used to travel longer distances. Anyways, it gives an idea about the use of steel as demi hulls, and with varying purposes, modifications in size are definitely required.
     

  11. Muzammal
    Joined: Dec 2020
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    Location: Pakistan

    Muzammal Junior Member

    Thanks alot old multi, I am sure now that if I want to build a vessel in steel, it should be a monohull for the size I want.
     
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