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Re: [AtkinBoats] Tunnel Hulls & Scaling

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  • Ronald Fossum
    I think you have the idea down well. My only other comment is to note that the tunnel , once it reaches the propeller, takes a downward curve, thus narrowing
    Message 1 of 2 , Jan 19, 2005
      I think you have the idea down well. My only other comment is to note that the "tunnel", once it reaches the propeller, takes a "downward curve, thus narrowing the height of the aperture at the transom (I think it's called "wedging"). This creates something of a venturi effect, accelerating the flow/velocity of water and thereby giving increased thrust. The more I look at the "Atkin Tunnel Stern" (it should certainly be named after them) the more impressed I am with their design ability.

      As to scaling... Most early wooden boat builders used this technique. The had a set of molds of a hull "This is our standard 18' model, but we can build it in lengths from 16' to 20'. They simply moved the molds closer together or further apart. My understanding is that 10% was the recommended maximum, although up to 15% might be done (depending on other aspects of the hull design). Beyond that one made molds for the for the standard 24' hull, etc. Stability, center of buoyancy, and all sorts of other factors come into play and, while the boat builders of old might not have used those terms, they understood what was going on and the corrective changes needed to accomodate a longer hull form.

      The version of Rescue Minor I'm planning will be 21' 6" LOA (that's a 10% "stretch"). I will be altering the shape above the waterline somewhat to accomodate allow glued plywood lapstrake construction with a small trunk cabin forward. I'm very carefully watching the weight distribution, to keep it within what Atkin had. As I'll be using 3/8" plywood planks instead of the 3/4" plywood that Billy specified, I should be OK.

      You can point out the "error of my ways" this Saturday at the Library Messabout".

      Ron Fossum

      ----- Original Message -----
      From: Leo
      To: AtkinBoats@yahoogroups.com
      Sent: Wednesday, January 19, 2005 7:16 AM
      Subject: [AtkinBoats] Tunnel Hulls & Scaling


      Hmmm.this is a new wrinkle that I had not yet explored in my quest
      for an efficient hull form. So let's see if I understand this
      concept correctly. Please do correct me if I have a misunderstanding.

      Up until this point I'd thought of a tunnel hull form as just that,
      essentially an arch recessed into the after ~1/3 of the hull. A
      tunnel could be used with either twin or single props. The primary
      purpose is to reduce the draft that a large prop would take. The
      rest of the hull outside the tunnels remained conventional, in that
      it still had a `standard' hull form, i.e., either flat or with some
      degree of conventional deadrise.

      Now with this recent discussion about Atkin's designs, my
      understanding of Atkin's version of a `tunnel hull' is to use a
      Seabright Skiff flat bottom and bring the sides of this flat bottomed
      protrusion together somewhere in the area of station 6 or 7 (on a
      conventionally laid out plan).

      >From this aft point, instead of conventional deadrise - where the
      centerline of the keel is deeper (lower) than the chines, like this
      V - Atkin used negative deadrise, i.e., an inverted V - like this ^ -
      recognizing of course the limitations of text to convey visual
      principles and that the deadrise and inverse deadrise angles are much
      less that depicted with these letters and symbols.

      The result of this inverted deadrise is that instead of the stern
      squatting as power is applied; there is instead a dynamic lift
      component present that actually causes the stern to either remain
      level or to actually rise as speed is built.

      The end result of this negative deadrise is an increase in propulsion
      efficiency. This efficiency defined in at least two ways; 1) by
      increasing the speed obtained from X amount of horsepower and 2) a
      more gentle parting and bringing together the various water streams
      as the hull moves forward, resulting in (much?) less parasitic drag
      from vortices and other associated turbulence.

      Are there other benefits that I haven't seen yet?

      On the front page of this group is a disclaimer that any
      modifications to any Atkin's plans as drawn nullify any performance
      assurances from the folks that are selling the plans.

      Again, correct me if I've misunderstood. My understanding
      of `conventional' boat plans is that one can usually scale the plans
      up or down by 10%-15% without affecting performance. It's also my
      understanding that a 20% scaling is pushing the envelope and that 25%
      is almost unheard of - in essence, a 25% stretch is a brand new
      design and needs to be re-calculated accordingly.

      Using 15% as the maximum permissible percentage, a 20' boat could be
      stretched to 23' - a 30' boat to a maximum of 34.5' and a 35' boat to
      a maximum of 40.25'.

      My question is why would not these same rules of thumb apply to any
      of Atkin's designs too?

      This recent discussion has been very interesting and very
      enlightening. Thanks to everyone for your insightful comments.



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