Advice on multihull design

Discussion in 'Multihulls' started by Dwilson83, Sep 14, 2019.

  1. Dwilson83
    Joined: Sep 2019
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    Location: Uk

    Dwilson83 New Member

    Hello everyone,
    I’ve recently decided to build myself a Bruce Roberts Euro cat 1900. I am completely new to boat building, I know I am taking on a huge project but am fully looking forward to the challenge. I have brought the study plans, however as I have never worked with metal (plans are for metal only) I have decided to make it using marine plywood a wood epoxy sandwich with a fibreglass layer.
    Could someone please advise me on the thickness of plywood I would be better off using for the frame/bulkheads please. Thank you
  2. oldmulti
    Joined: May 2019
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    Location: australia

    oldmulti Senior Member

    I know of several guys who have completed large aluminium cats of 60 feet and above. One did it by himself 14 hours per day and over 4 years with 3 days per YEAR off. Easter, Xmas and a day at the boat show. Translation over 20,000 hours by him and his wife also added her time. Aluminium is the fastest way to build a big cat. Converting the boat to another material is a specialist job that needs to be done by a Designer who will be the cheapest $10,000 you will ever spend. I am adding the specs of an old plywood trimaran to give you a feel for some of the structures but this is no guide to what you may need. Its information only.

    “ I will give you the structure of the Piver Empress trimaran. The boat is 64 x 32 foot weighing 44000 lbs carrying 1500 square foot of sail in a ketch rig. This boat would displace 60,000 lbs. Piver sold home build plans and several were built by professional boat builders. No home builts that I know of. This boat was a charter machine with huge internal space 18 berths and 2 floors in the main hull etc. Float and main hull 15 mm ply sides 18 mm ply bottom. 30 x 140 frames on 12 mm ply at 70 to 950 mm centre lines. Stringers 30 x 140 mm on flat in main hull Float stringers 25 x 95 mm flat at 400 mm centre lines. Decks 12 mm ply with deck beams and stringers of 30 x 140 mm. Cabin roof and sides 15 mm ply with frames and stringers. Underwing 18 mm ply wood. The crossbeams are box plywood 950 mm high by 850 wide. The top and bottom are 3 x 18 mm plywood. The webs are 2 x 18 mm ply. Rudder shaft 1.5 meters long by 65 mm solid stainless steel. This boat is not fast. Or you could build a Cross 80 footer, better design, faster boat if one was ever finished. It only took 18 sheets of 9 mm ply per bulkhead. These boats are massive and consume large amounts of material. For an Empress you would be using at least 500 sheets of 12 mm plus plywood and tons of timber let alone the several 44 gallon drums of glue etc. Anything is possible with enough money and time (20,000 plus hours or 10 years full time).” Also look at the following page for a build story 6 years long with several helpers. Multihull Dynamics, Inc. - News Article

    Attached Files:

  3. Corley
    Joined: Oct 2009
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    Location: Melbourne, Australia

    Corley epoxy coated

    I'm not wanting to be rude but just trying to understand why you are considering building such an enormous boat? Are you planning to run a charter business?
  4. Dwilson83
    Joined: Sep 2019
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    Location: Uk

    Dwilson83 New Member

    Thank you very much for your reply, it has given me a lot to think about. Also thank you for the link, it was a good read. I’d hope that by living in the UK I wouldn’t have half the problems he had with deliveries etc.
  5. Dwilson83
    Joined: Sep 2019
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    Location: Uk

    Dwilson83 New Member

    The reason I am considering something of this size is I will be living on this full time, it’s not going to be a hobby more a home. Other smaller catamarans just hasn’t caught my imagination like this one has. Plus having something of this size, each hull could have its own apartment type set up with a shared lounge/ kitchen on the bridge deck. One side for myself the other for potential rental when moored at home. Or for rental when travelling the world.
  6. Deering
    Joined: Feb 2005
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    Location: Juneau, Alaska

    Deering Senior Member

    You would be far ahead in money and time building a houseboat (or terrestrial house) for a home. Then buying a used boat for traveling the world. You’ll spend years (note plural) of your life building the thing, while paying rent/mortgage on a place to live in while you’re doing it, not to mention the rental income you’ll miss out on. Given the enormity of the project and your lack of experience the outcome is very uncertain. You could spend a lot on materials and time only to have to abandon it before it’s finished.

    Dip your toe in the water. Buy plans for a dinghy and build that first. Then maybe a bit larger boat. That will give you some experience as well as an appreciation for the scale of the job you’re considering.

    The whole thing about renting out one of the hulls while you travel the world with it makes no sense to me. Where are you going to find a renter who wants to get involved with that?
    rwatson likes this.
  7. catsketcher
    Joined: Mar 2006
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    Location: Australia

    catsketcher Senior Member

    Gday D
    You need to build a boat as stated above (a tender would be good) and learn about the square cube rule. Doubling the size of a boat makes it 8 times more expensive - you double each dimension - 2x2x2 = 8
    The good thing is, you don't know much about boats. That is good because you need to know that you can sail around the world in a 23ft Wharram. Two different sets of friends had a great time circumnavigating in 10 metre Crowther cats. You can buy one of these for about $100 000. We lived for 3 years as a family on our modest 38 footer. Get the simplest boat you can unless you have heaps of money. As for renting out space on the water, here in Australia, many waterfront owners are not too happy to share their views with liveaboards so boaties can get hassled by authorities pleasing rich people.
    bajansailor and rwatson like this.
  8. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Please, don't even think about it - it is a crazy idea. Especially as the cat would have to be completely re-designed for plywood construction, as opposed to aluminium. And I mean COMPLETELY. It is not just a case of increasing the plywood thickness.
    Here is a link to the Bruce Roberts Euro 1900 -
    Bruce Roberts, CATAMARAN boat plans, CATAMARAN boat building, boatbuilding, steel boat kits, boat kits

    As mentioned above, your best bet would be to start with building a dinghy. If you are still keen on completion, then maybe move up to a cruising cat - but buy plans that are drawn specifically for the construction material / method that you want.
    Personally, I think that Richard Woods' designs are light years more attractive than that Eurocat - have a look at
    Sailing Catamarans - First Choose a Design
    Richard has built many (if not most) of his designs himself, so he knows what he is talking about when he says "We do not recommend home builders to attempt a boat over 40 ft unless experienced and want a boat for charter or long term cruising. Most families will find that boats under 40 ft will comfortably meet their needs.".
    Take heed - this is very sage advice.
    rwatson likes this.

  9. Ike
    Joined: Apr 2006
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    Location: Washington

    Ike Senior Member

    In most western countries this is called passengers for hire. That requires a license. I suggest you consult the Maritime and Coast Guard Agency before you even think about doing this.
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