28' sail cat

Discussion in 'Multihulls' started by mihari, Sep 4, 2009.

  1. Fanie
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    Fanie Fanie

    Hello Mihari,

    Congratulations on deciding to build your own boat. It's going to keep you busy :D Oh yeah, and could I say welcome to the world of lunatics, take fair warning that boat building is an incurable illness. Very few kick the habit after the first one... ;)

    Hint from me - When you finally decide what it is you want, you don't start the boat. You start to query every decision, from size to color to materials to everything else.

    Simple, once the build starts you want to be sure you are building the thing you REALLY want. Some ways in the build and you 'discover' something that you would rather prefer as to what you have, is going to be a cap on your enthusiasm. It's a long haul, you are going to need each bit you can gather.

    I would also suggest you go out on a boat smaller to larger than your own... just get a feel for it. You may (again) discover some things you weren't aware of before.

    Good luck. I will follow your build (if you don't come to your senses before you start :D)
     
  2. boat fan
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    boat fan Senior Member

    [​IMG][​IMG][​IMG]Mark Prescott Catamarans :)
     
  3. mihari
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    mihari Junior Member

    Sabahcat, thanx for the enlightening pics. The concave/convex section looks a lot more forgiving and more flexible than the tongue/groove one. But more wood wasted. I guess that the idea behind using machined timber is that you get greater area for the glue to bond and get a better fit than with the rectangular sectioned strips. (and maybe a fairer hull?)

    Jamez, that is a nice looking interior... I hope mine will look like that one day. Is that thing on the left a daggerboard pocket?

    Fanie, thanx for the inspirational post.

    Boat fan, two beautiful boats... Would it be safe to say that Australians have their own "school" of multihull design? I can only wish that my cat would look like that...
     
  4. jamez
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    jamez Senior Member

    Yes its the daggerboard case. On the Tennant boats the boards are on the 'inside' of the hulls. BTW Mark Pescott also has a nice 28' tubecat design.
     

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  5. boat fan
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    boat fan Senior Member



    I guess so mihari .......Australians really took a liking to catamarans some while ago. Many have , and are being built here still...I think we do have good designers here .Good designers elsewhere too though , as you have already discovered yourself.

    I think you could save some time and money if you combined strip planking ( knuckle ) with developable flat sheet sheet panels.( Plywood , and cedar , or all foam maybe ).

    Rob Denney uses Kiri on his innovative boats :

    http://www.harryproa.com/Elementarry/BuildingPhotos_1.htm

    http://www.google.com.au/url?sa=t&s...ir3RDA&usg=AFQjCNGhm2YRGiiH0YkyUdlXU5WhiSSUaA

    http://www.harryproa.com/Elementarry.htm

    My fav. Australian designers include

    Peter Snell ( Easys )
    Shionning
    Bob Oram (Mango)
    Prescott http://www.youtube.com/watch?v=BfOUDz-qb-0&feature=related
    Crowther
    Grainger
    Lidgard
    Tim Clissold ( NZ )http://www.tcdesign.co.nz/range.htm

    http://www.seeker.co.nz/bimaran/jasmin.htm


    International ,

    Chris White
    Nigel Irens
    Richard Woods
    John Shuttleworth

    Soooo many boats .....not enough time or money ......:D
     
  6. Petros
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    Petros Senior Member

    Milhari,

    Have you considered using Dura-Core? It comes in either sheets or strips, it is made of balsa core with hard wood veneer skins. It comes in different thicknesses, and you glass it once you have it assembled. It is supposed to be stronger than foam core and much less costly, especially if you buy large bundles of the bead and cove strips for making round hull shapes. If you are designing from scratch it is not a bad way to go, it gives you more design options.
     
  7. sabahcat
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    sabahcat Senior Member

    Does anyone use Duracore anymore?
    I thought Duflex was the norm if going an ATL product?

    Never liked duracore anyway as it was impossible to fair before glassing, meaning you end up with more filler than should be necessary.

    At least with cedar or kiri you can plane and sand the surface getting everything perfectly fair before glassing, meaning absolute minimal bog.

    To do the same with duracore maeans sanding planing through the hardwood veneer face, which is a reduction in strength.
     
  8. sabahcat
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    sabahcat Senior Member

    Yep, plenty of wood wasted using both the concave convex and T&G.
    I having used tongue and groove and square edge I could never see the advantage of using T&G over it due to cost and waste. T&G offcuts cant be used for stifenners/stringers for furniture stiffening as they have a T&G edge, so more waste.
    On the tongue and groove boat it was glued together with Purbond, not sure if I liked this over epoxy or not, tests done on planking showed it seemed as strong as cedar, so I suppose it's OK.

    I never had a problem with the square edges, I used long plastic strips and screws or staples to line planks up.

    Takes a bit more time, but saves many dollars in machining and wasted timber costs over T&G and concave.

    If you look at my earlier link to the 50ft hulls you can see we left a paint scraper gap between planks and rolled resin in, letting capillary attraction pull resin through the join, that and a bit of squeegee action at times.

    More Here and a couple more in same thread.

    You can see the plastic strips used to line up planks in some of the pics
     
  9. mihari
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    mihari Junior Member

    OK all, thanx for your input. I think I am leaning towards plywood hulls and maybe (just maybe) strips for knuckles. Round hull has to have a second layer of flooring (actual hull skin cannot be used comfortably for walking or standing in the galley). I don't think the extra weight, lost space, and extra effort is really worth it for my kind of scope for this cat.

    Next big topic: "Waterline shape"
    For most cats that I have seen, the waterline section (I would call it water line plan) of the hull is pretty similar, all just slightly varying in proportions. None will fit a NACA4. I was wondering if there is any general rule as to how the hulls are shaped. The rate of change from the tip of the bow to the mid hull and then back to the transom. I know that their overall proportions (length to width) pretty much categorize the boat as being a cruiser or racer (or a boat house). But what rules govern the rate of width change?

    I am also convinced of using a knuckle at the outer side to gain enough room for double bed, so the actual waterline width of the hull is not so important to be wide (as long at it can accommodate two people crossing)
     
  10. jamez
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    jamez Senior Member

  11. bob the builder
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    bob the builder novice

  12. mihari
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    mihari Junior Member

    Jamez, yes, I had seen that before. From what I gathered this guy worked with no plans. (so he says)
    And I think that it is true, because look at the beam supporting the mast... It doesn look like a box beam. And it doesn't look like steel. And it doesn't look like it is going to hold on for too long... I hope it does, but I don't think it is like what I am used to seeing with the usual monocoque construction.
    Other than that it is an overall nice looking boat, and not far from what I think mine will finally look like. At least as far as the hulls go.

    With what I have in mind, access to the hulls will be through a salon, and the salon will be able to accommodate a few people standing and/or seated.
     
  13. mihari
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    mihari Junior Member

    After searching a bit more, I have found the answer to the question "What waterline shape?"
    I am sure many naval architects have been searching all their lives for the golden rule of hull shapes. And I have found the answer in just a few days... "there is no rule".

    I have to build a shape that pleases the eye and accommodates the needed spaces, and then play around refining it to find the best form... time to dive deep into the world of hull software.

    Just a question before taking that deep breath... Rocker or no rocker? Most cats have rocker bottoms. I never thought that mine might be flat bottomed. I know there is an issue of beaching (that you get less draft with a no rocker) but how about attitude? Any experience to compare the two?

    Hope to post here soon...
     
  14. boat fan
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    boat fan Senior Member


    More rocker....easier to turn the boat...within reason.
    Too much rocker too deep extra surface area / drag.Not enough buoyancy in the ends...( hobbyhorsing ).

    No rocker...fast . you still need to turn up the " run " at the stern to prevent dragging that stern around .Unless its a fast power boat .
    These are gross generalizations.

    Need to find that happy medium for your displacement slenderness ratio l/b ...looking at successful boats that are close to your size and displacement is a good place to start.


    Fusion Catamarans say this :

    ""The trend toward flatter rocker in recent years has largely been induced by the desire to increase the prismatic coefficient (buoyancy in the ends) and thereby dampen pitching and optimise reaching performance. Designing rocker into a new hull is a delicate balancing act as it has to be considered in respect to the flatness of the underwater sections and the required buoyancy.

    For example if the sections forward are fairly flat or rounded (as opposed to a 'V' section) then it is important to ensure there is adequate rocker forward to avoid hard landings while beating in a seaway. The run aft should remain reasonably flat especially if some planing lift is being sought downwind. An excessively flat rocker line might also require that lost buoyancy be compensated by the beam to length ratio.""

    Shuttleworth :

    ""Many early multihulls were prone to hobbyhorsing, and pitching. This was caused by too much rocker on the hull profile, and fine V sections both fore and aft. As hull shapes improved tending towards more U shaped underbodies particularly aft, pitching still remained a problem, because the large width of the stern sections caused the sea to lift the sterns as the boat passed over the wave, driving the bow down. However we now know that pitching can be dramatically reduced by finer sections at the stern combined with the center of buoyancy being moved forward in the immersed hull, and aft in the lifting hull . This effect can be achieved in both cats and tris, giving a very comfortable and easy motion upwind. At the same time windward performance is improved, because the apparent wind direction is more stable across the sails.""

    Lightspeed say :

    ""Flat rocker with rounded sections forward transitioning into distinct U-shaped sections aft derives from tank testing. The round forward sections give a gentle motion. The flatter aft sections provide a planning surface and additional buoyancy where the crew weight is centered. Flat rocker provides for high top end speed.'''

    This may be helpful too...http://www.google.com.au/url?sa=t&s...mLS6BA&usg=AFQjCNHBmUnL8hv-3AJH1aPzj7djxHXUtg

    This Cruiser discussion maybe helpful :http://www.google.com.au/url?sa=t&s...mLS6BA&usg=AFQjCNH0f2ote_RKbnLa5gtDkX2KnZeknw


    It`s both art and science.Good luck.
     

  15. sailsocal
    Joined: Jun 2006
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    sailsocal Junior Member

    What does he mean, moving the center of buoyancy forward and aft?
     
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