Novice - questions about cat hull design and beam

Discussion in 'Boat Design' started by T Lamb, May 29, 2022.

  1. T Lamb
    Joined: May 2022
    Posts: 1
    Likes: 0, Points: 1
    Location: Shanghai, China

    T Lamb New Member

    So, I am a complete novice, but a crazy bit of a handyman, I like to take on big projects like building my own house, etc. I've built factories for the last 20 years as part of my job, so plans, following plans, and the details are all something I'm comfortable with.

    That said, thru all that, I've always had someone else to do my CAD work, so, I've never become adept at it, and have suffered a little trying to learn.

    But, I have some questions as I've looked at lots of study plans, designs, and the accomodations and decks and I thought I'd toss some out here and see if anyone can answer them for me.

    1) I like Wharram designs for the simplicity, but I find that they truly were designed around 'sea camping' and not cruising. So, I've pondered over changes that I'd make to something like the Tiki 46.

    Original specs are Overall length 46ft
    Length WL 38.1ft
    Beam overall 24ft
    Beam -hull 6'1"
    Empty wt 4.5 - 5 ton
    Draft 3ft
    Load capacity 3-4 ton
    Sail area 1000ft2 (soft wingsail schooner)

    Diplacement / Length = 93
    Sail area / Displacement = 1.4
    Stability = 30knots Static / 18 knots Dynamic (o.6 x static)

    2) so if a boat is 46ft long, and 38ft at the waterline, is canoe shaped in principle and currently has a beam in each hull of about 6ft, what if any issues occur if i decide to modify the design and expand the hull to 8ft? If the geometry still flows from the keel, seems you should still be able to match added displacement to counter the added weight, and still maintain load capacity, if not improve it slightly. I get this might slow it down a little, but so much you can't effectively sail?

    Also, is anyone familiar with the Wharram designs and the method for connecting the deck trusses? I note from video and plan reviews that the deck pods are basically set in between the trusses and connected. Anyone know how much actual flex the trusses have? seems like a deck pod could still be mounted within the deck area, but expand it to cover the two primary hatch entrances as long as that weight is still supported on the trusses and not the hull decks themselves.

    THanks in advance.
  2. bajansailor
    Joined: Oct 2007
    Posts: 3,559
    Likes: 1,531, Points: 113, Legacy Rep: 37
    Location: Barbados

    bajansailor Marine Surveyor

    Welcome to the Forum T Lamb.

    First of all - a 46' catamaran is a HUGE building project to undertake, never mind that you have built houses and factories in the past.

    Secondly, IMHO, there are 'better' designs than Wharrams out there now - and especially so if you are looking for more displacement / load carrying ability.

    Have a look at Richard Woods' catamaran designs - he started off working for James Wharram, and later branched out on his own, and never looked back.

    If you really want to build a 46' cat (and Richard advises strongly on his site to build the minimum size that you actually need) there are many other very nice designs (also) available - do some googling.
  3. fallguy
    Joined: Dec 2016
    Posts: 7,554
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    Location: usa

    fallguy Senior Member

    The critical ratio is length of waterline to beam, generally.

    So, 38/6 is only a ratio of 6.33, and 38/8 only 4.25, if my late nite math is close.

    And these are not exceptionally good numbers as cats go. The 6 is already a 'fat' cat and the 4 may as well be a monohull. I am rather tired and will let others explain all the problems of wider hulls and how the purpose of the catamaran is lost this way.

    If you are exploring different boats amd can afford or don't need to slip a large boat; there are others to consider, some larger Wood's designs are designed well. And Harry Proas also an option with nice accomodation and amenities at 50'.

    Personally, I wanted a slip boat and finishing a Woods Skoota 32 and getting close to survey after over 4 years of build time. Even slipping this boat has been hard to find affordable and wide enough slips. One marina told me I'd habe to rent their 70+ foot slip to get to 18' beam, which is actually really narrow for me. I finally found a 19'6" wide slip.
    bajansailor likes this.
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