To Build a Cat

Discussion in 'Multihulls' started by Pity098, Apr 28, 2011.

  1. Pity098
    Joined: Apr 2011
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    Location: Texas

    Pity098 Junior Member

    Howdy,

    I have been lurking here for a while, and have decided to look for some input on a pet project of mine if anyone here will consent to give it.


    Background: I live in a landlocked and waterless hell known as the panhandle of Texas so all of this design speculation will more than likely be just that, speculation and fancy. My background is structural engineering; however the focus was steel and concrete so catamaran construction deals with foreign materials. Finally, my sail experience is limited to overtaking and sinking hobie cats in my blissful youth (yes, if you load them enough they sink); and to the design, construction, and racing of concrete canoes in various venues.


    Project: Design of an easily constructible and highly livable catamaran with a specific hull configuration, namely the one attached. Speed is secondary to comfort and safety. It will be sail, with double outboard/inboard for problems/windless days.


    Thoughts so far: Hull frames (called bulkheads?) at 32” O.C., and double thick frames at bridge deck entrances. Stringers between frames @ 16” O.C around frames, and the hull skinned with two layers of ½” plywood covered in fiberglass/epoxy.

    Deck/bridge-house extending from the rear of the catamaran for 36’, supported by 2x6 beams at 16” O.C. and all flooring/ exterior to be two layers of ½” plywood. Beams will again be coated in fiberglass/epoxy.

    Hulls are to be 22’ apart center to center.


    Requests: Any and all critiques or information without destroying me completely; I am doing this for the hell of it but learning is always a good thing. I know nothing about rigging, layouts, bilges, etc. so bring on the pain.

    1. *** Side note that I may build a 1/4th scale model once I figure out how I want to scale loads for testing. It’s a shame there isn’t a Reynolds number for composites and solids (at least that I remember).


    Thanks,

    Land Locked Aggie
     

    Attached Files:

    Last edited: Apr 28, 2011
  2. hoytedow
    Joined: Sep 2009
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    Location: North of Cuba

    hoytedow Wood Butcher

    Welcome, LLA. I, unlike you, am not a structural engineer, but your description is of a solidly constructed vessel. Have you any illustrations? What type of plywood will you be using? What kind of wood will the 2 by 6's be? What is your construction strategy? Where will you plan to build it and sail it? If you build it in the panhandle you will have a hard time getting it to the sea. Tell us more.
     
  3. Corley
    Joined: Oct 2009
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    Location: Melbourne, Australia

    Corley epoxy coated

    Hi and welcome, I'd question the sense of building a boat of the nature your describing in timber it will be very expensive if you use marine ply (which you should) and the scantlings you have mentioned are very heavy for a multihull and are closer to monohull scantlings.

    Modern materials that are favoured are composite foam cores or endgrain balsa and strangely enough these days you often spend less per square metre using these cores than you do on good marine plywood. Another good option is Western Red Cedar or Paulownia strip plank, In the size range you are proposing aluminium may be a valid choice also.

    There are some excellent books on design you might like to look at Ed Horstman has a construction manual for multihull foam sandwich boatbuilding. I also found Dave Gerr's Elements of Boat Strength an interesting read.

    You might like to check out the design of "Fallado" it's primarily designed with a strong focus on strength in composite not my cup of tea but interesting.
    http://www.fallado.net/maincontent/CatamaranFallado.htm
     
    Last edited: Apr 28, 2011
  4. sailsocal
    Joined: Jun 2006
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    Location: Los Angeles, CA

    sailsocal Junior Member

    Files

    I suggest attaching PDF or JPEG files to your post. Files in proprietary formats like XLS require expensive software which many readers do not have.
     
  5. Pity098
    Joined: Apr 2011
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    Location: Texas

    Pity098 Junior Member

    Howdy,

    Ok here goes an attempt at providing information. I apologize if response time lags, some areas of my work prohibit communication during my non-work life.

    Hoytedow: pictures are hopefully coming soon, however my drawing abilities are very limited. I design foundations normally which a two year old could (and do) draw.

    Wood choice is currently Douglas fir for all non-visible wood and an okoume (spelled correctly?) for any visible layers. Douglas fir is a wood I have some experience with.

    I would like to sail it (eventually after gaining much more experience) coastally and eventually in blue water. I realize even that if I had this boat ready made that would be a long way off however.

    Unfortunately I don’t have much of a construction strategy, because I have never built a boat before! Currently I am trying to ready any literature found on the subject; the vast and myriad amount of information is slightly overwhelming however.

    Building the boat in the panhandle would be crazy, but would it be possible to do gross construction there and finish work closer to the sea/water? I am related to many people in heavy equipment areas, so movement of large structures would be a possibility for me. If building out here in the wasteland isn’t feasible, once construction time comes I would relocate to Galveston, TX for the build.




    Corely: Truthfully I haven’t looked at other materials very closely, after reading your opinion and the information about Fallado I will have to peak at it. The scantlings I chose were based off of the perceived loads which could possibly be a complete load of dung and therefore changeable.

    I have just found a copy of Dave Gerr, and will be reading it as soon as time permits. Now I suppose I need to find Ed Horstman as well; the book stores cannot complain to me.

    Fallado, by the way, did pique my interest. Do you know if any more detailed information is available?




    Sailsocial: It never even occurred to me people would not be able to look at the XLS stuff, I suppose easy access has made me complacent. JPEG’s of the excel files have been added to the original post.

    Thanks,

    Land Locked Aggie
     
  6. hoytedow
    Joined: Sep 2009
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    Location: North of Cuba

    hoytedow Wood Butcher

    Several catamarans have been constructed and documented in this forum. One current one is being well documented by Catbuilder. You should visit these sites and see the impressive results. I am happy to know about corecell and balsa for hull construction and the modern techniques that accompany their use. You can save a lot of weight without sacrificing strength by utilizing this knowledge.
     
  7. Corley
    Joined: Oct 2009
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    Location: Melbourne, Australia

    Corley epoxy coated

    Moving large items by truck is not too much of a problem as long as they fit into legal size limits many people have constructed their base hulls and crossbeams on their farm or home and then trucked the parts to another location to assemble.

    The important thing to keep in mind is once your hulls are joined together and the boat is 22' wide everything becomes magnitudes more difficult and expensive when it comes to transport, some people would advocate making the boat demountable but that brings its own compromises. One good approach is to build the base components then assemble the hull, beam and bridgedeck near the launching site.
     
  8. rattus
    Joined: Feb 2008
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    Location: US

    rattus Señor Member

    Just download a copy of OpenOffice from http://www.openoffice.org/

    It's free and works well with many open and proprietary file formats, including .xls

    Available for Mac OS X, various Windows flavors, and, of course, Linux.

    Mike
     
  9. Sand crab
    Joined: Feb 2011
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    Location: Montana

    Sand crab Junior Member

    construction videos

    There are many cat and boatbuilding videos on Youtube. Kelsalls ( I think)KISS system that is the most interesting. Basically you build one of the hulls as a large flat sheet of glass and core material, cut a long tringular slice fore and aft a predetermined distance part way down the centerline then fold it together to the hull beam size which gives you the shape of the hull. The cutout edges are then a clamped together and glassed up tight which gives you the taper of the bow and stern. Crazy, but it works and gives you a nice light hull without all the complicated molding and forming and it's really fast. That video is in Youtube. Also, Offshore Cats in Florida fabricates light and fast 48' hulls using female molds and sells them on Ebay for $89,000. I can't find it now but there is an unfinished cat for sale in Texas and the guys already about 80% done. I agree with the other posters that your design will be needlessly heavy. You want to make sure that you build this so it will be attractive in various ways to the guy that you want to sell it to down the road and nobody wants one of the heaviest cat ever made. Anyway, Good luck. BOB
     
  10. garydierking
    Joined: Sep 2004
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    garydierking Senior Member

    Just for reference consider a 100' long sailing catamaran that has been operating commercially in Hawaii since the 1960's. The hull skin is two layers of 3/8" ply. Another 50' charter cat there has a single layer of 1/4" ply.
    Your 1" thick hull would be unnecessary extra weight.
    Gary
     
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  11. Sand crab
    Joined: Feb 2011
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    Sand crab Junior Member

  12. rberrey
    Joined: Oct 2010
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    Location: AL gulf coast

    rberrey Senior Member

    Ed Horstmans books can be bought from his web site, I have both trimaran , catamaran construction and foam fiberglass construction. His books and build method will help if you go the douglas fir path. As an old form carpenter I know you should be able to get a good deal on your wood. Russian birch or finn form is heavy but will do for bulkheads and frames, you may have to sand both sides before you epoxy. Building the hulls in the panhandle has some advantages , it would be a very dry build and could be done outside with no climate control need other than heat in the winter. I spent my teen years in Corpus , high humidty there is no differant than the Mobile area or anywhere else on the gulf coast. Unless your designing a bridge I would think you have plenty of time to design and build your boat, structural work has dryed up in this part of the world. rick
     
  13. Pity098
    Joined: Apr 2011
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    Location: Texas

    Pity098 Junior Member

    Howdy,

    So from what I am reading, building both hulls and the bridge-deck (as separate pieces) and then transporting would not be huge issue; the joining on site is the larger problem. Would most ship yards rent space for a short while to finish the construction?

    Clearly I need to revisit the numbers if there are cats operating safely with hull so much thinner; I do however like the idea of doubling the ply layers (say two 1/4th inch sheets) for the extra stiffness. Is there some huge advantage/disadvantage of doubling the plywood layers that I am missing? Weight being the obvious problem (however 1/4th sheet of ply ways about 1 lb/sf).

    The KISS system does look interesting and I shall research it more though for some reason I feel the need to sick out my tongue and waggle it when the subject comes up.

    Thanks rberrey, good to know it is not completely insane to contemplate construction up here. You are of course correct at the moment, structural work is dryer than a bone in the desert (part of the reason I am looking into designing myself a catamaran at the moment) and does not look to be picking up soon.



    On hull shapes, most catamarans (or boats in particular) seem to be semi-teardrop shaped; that is a narrow front that widens and then narrows back somewhat near the back. Is there a significant reason for this other than speed? I find myself wondering how it would compare to a narrow front that builds to a full body without tapering near the stern.


    Thanks,

    Land Locked Aggie
     
  14. sabahcat
    Joined: Dec 2008
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    sabahcat Senior Member

    Seeing as you are having issues on hull shapes and hull thickness I would suggest you SAVE your materials by purchasing some plans for an older style of vessel, where the plans are probably more affordable
     

  15. Sand crab
    Joined: Feb 2011
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    Location: Montana

    Sand crab Junior Member

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