Adding amas for stability?

Discussion in 'Multihulls' started by soffi, Aug 7, 2010.

  1. soffi
    Joined: Jul 2010
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    Location: Reykjavik, Iceland

    soffi Junior Member

    Hi there! Long time lurker first time poster.

    I really really really like this boat:

    http://www.common-sense-boats.com/indiscreet.html

    It's called 'Indiscreet' and it's a 40' flat bottomed liveaboard/cruiser.

    Now what I was wondering, would it make any sense to add amas on each side to add stability? And if yes, how long should the supports stick out (ie. what should the beam end up like). And what kind of buoyancy / length should I be looking at?

    muchos regards!
    soffi
     
  2. ThomD
    Joined: Mar 2009
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    Location: TO

    ThomD Senior Member

    Doesn't jump out as a great idea. When you say "would it make any sense" what do you have in mind? Rather than having us hallucinate various options that might be of use to someone.

    A flat bottomed boat with 9 feet of beam probably would get along on it's own for the most part. If one was after more stability it would be normal to have a little more draft and a lot less beam to allow the new boat to have far less resistance than the current version. This would affect the boat's overall carrying capacity, but on the plus side a better shape would allow lighter materials so some of the loss might be offset. At the end of the day you end up with something like this:

    http://www.multihulldesigns.com/designs_stock/38tri.html
     
  3. soffi
    Joined: Jul 2010
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    Location: Reykjavik, Iceland

    soffi Junior Member

    What I was thinking is a more offshore capable boat, not that I am questioning the current version I just wanted to gather some thoughts :)

    The KH 38 Tri plans alone are 2500$ and the building method is much more costly using the cylinder moulding method. The Indiscreet is pretty much stitch and glue. (did i mention the plans are 2401$ cheaper ;))

    br,
    soffi
     
  4. DennisRB
    Joined: Sep 2004
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    Location: Brisbane

    DennisRB Senior Member

    The plans are the cheapest part. If I was building I wouldn't hesitate to buy expensive plans and end up with a much better boat of known pedigree than spend $150K and 5 years of my life building a little known or owner botched boat to save 2k. Specially when you consider resale value, that 2K saving could become a 100k loss. I'm not attacking a particular design here just pointing out some facts. Even taking on the task of building a boat in this economic climate is also something that should be carefully considered when a better boat than what you can build would probably be able to be purchased cheaper on the second hand market (in the case of a motor cruiser anyway)
     
  5. ThomD
    Joined: Mar 2009
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    ThomD Senior Member

    +1. There is a reason why people pay more for some plans than others. Kurt's sets are massively detailed and well drawn in 3 d cad, real numerically comprehensive cad, not just someone's Sketchup fantasy. His manual and videos are also pretty helpful to the builder.

    Also, CM is not all that expensive. To start with, a properly designed multihull is quite light. Rounded bottoms require far less wood that flat bottoms, as well as being pretty much better in every other way, though there are a few advantages to flat bottoms. Originally, CM boats where designed to use doorskins, and these are still available in some locations with waterproof glue in them. This is about the cheapest form of boatbuilding there is, with great longevity, and is very fast to build. Build wise the whole half hull goes together in one glue cycle. And the prep prior to that stage is pretty modest. His design is more complicated to a degree, but some of that would be up to you, you could complete it with more or less flat panels, or go all voluptuous and curvy. But the key issue is, and this is pertinent to your question, the time you spend building on this boat is to gain performance at every step, not to recreate the sense of a boating culture gone by, etc...

    A flat bottomed boat will require more hardware to assemble, more plywood, probably more than twice the materials for a given width, and the amount of fuel required to push it around, and the motor size will be larger. Seamotion worse.

    I would guess the Hughes style boat is much more seaworthy. It is somewhat similar to Yanmar endeavour which was the first outboard to cross the pacific without refueling.

    I would personally prefer not to take a flat bottomed boat far from shore, with the exception of some like appropriate sharpies that operate healed and therefore are not always flat to the seas. Flat bottomed boats can be noisy at anchor, though maybe not at that weight. That said, it turns out with the bandwidth of the internet, that almost everything is being done by someone, and is considered the best of all possible options at the same time... :)

    By the way, back to your question. Adding amas may make for a more seaworthy boat, but I would be really wary of combining two dissimilar types. For instance imagine this big wide thing starts to climb it's bow wave. What if that diminishes the functionality of the amas which are designed for level operation, what has been gained then. But if the whole boat has been designed from the get go to work as a package then all aspects should be complimentary.
     
  6. pogo
    Joined: Mar 2010
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    Location: Germany Northsea

    pogo ingenious dilletante

    Wot ?

    pogo
     
  7. ThomD
    Joined: Mar 2009
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    ThomD Senior Member

    That's the fast version for quick learners. Technically there is a flat bottomed multihull shape that is very close to the circular shape. That shape will have more surface area for the same displacement, more chines, more stringers, and possibly a thicker bottom. An example would be the flat bottomed version of the Newick tremolino, vs the curved. The flat bottomed version has a 1" full width timber full length, if I recall, not as cheap as the less long circumpherence, less thick, strip bottom. The kurt version of that boat had 6 sheets of 1/8" ply in it, that cost me 7 bucks each at the time. However, that isn't the comparison I had in mind.

    Typically it takes longer to build the curved version, but in the world of free labour home projects that is not at issue, and in the case of stuff like KSS, CM, Stressform, and certain ways of building strip, it can be faster than some flat bottomed slogs. A lot of work has gone into fast build methods for multihulls. Consider the likely heavy glass/zynol that will need to be wet out on something like a Ruel Parker Sharpie hull vs the 4-6 ounce on a Hughes. The OP seemed mostly concerned with out of pocket costs.

    Down to specifics, this particular flat bottomed boat uses a large stack of 1/2 ply, and at a guess I would imagine the bottom is layers thick and involves a lot of nailing (hardware) this is simple work, but it isn't cheap if you are putting down a sheet of 1/2 for every sheet of 1/8 that might be in the Hughes. Bronze ringnails? Kurt will find other ways to spend you money, but mostly they will be things that will save you some money on fuel (cored decks), and add to resale.

    For what it is worth, a Kurt hughes hull is a stich and glue boat (magic words, apparently) while I would be surprised if all that 1/2" ply was being built without frames.
     
  8. david@boatsmith
    Joined: Aug 2008
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    Location: Jupiter Fl USA

    david@boatsmith Senior Member

    I thought Iceland was a rock in the North Atlantic where if you fell in the water you only had minutes to live. This design IMHO is more suited to more protected waterways. This would be with or without amas. jes sayin
     

  9. DarthCluin
    Joined: Mar 2009
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    Location: Florida

    DarthCluin Senior Member

    Soffi,
    Pick up a copy of "Buehler's Backyard Boatbuilding" by George Buehler. It has plans for seven boats, permission to build from them, and an interesting construction philosophy. None of these boats are multihulls or flat bottomed. They are all heavy, deep draft, single chined keelboats, some power, some sail, built with lumberyard materials. The book is $17.13 on Amazon.
    His website is here:
    http://www.georgebuehler.com/
    Indiscreet looks to be inspired by Phil Bolger's Idaho, which was intended for river cruising.
     
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