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Re: Light Skiffs (was Once, in a Blue Moon,.....)

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  • Gavin Atkin
    Graeme I d say the project was in two forms - 16ft and 19ft. I has to look like a traditional skiff, even if it is lighter in form. Speed-wise, it has to feel
    Message 1 of 44 , Jul 3 5:56 AM
      Graeme

      I'd say the project was in two forms - 16ft and 19ft. I has to look
      like a traditional skiff, even if it is lighter in form. Speed-wise,
      it has to feel exciting to row for someone who may be only moderately
      fit and may only get to row a few times a month. For those short
      periods, it would be nice to make the owner feel like an athlete! I'm
      not talking about a boat with riggers, but I guess it might be fitted
      with a sliding seat as an option.

      I'm groping towards defining a shape that might represent a small
      improvement on existing skiffs, not an out-of-the-box solution. That
      would be a project for another day!

      I've read Bolger's argument and I don't have any trouble with the
      vortices idea. I'm happy with the implication that increasing the
      angle between the sides in one of several ways (flare, vee-bottom,
      arc-bottom) will help to reduce them. I can see also that a curved
      run will help - though its rotundity will clearly be limited by the
      required within the confines of the required displacement.

      Where I do get a bit puzzled by your comments is up at the bow. I've
      never experienced the difficulty in steering that Bolger talks about,
      and I'm glad to say that as far as I can work out Ben Crawshaw's
      Light Trow isn't troubled in this department even though the foot of
      its bows were drawn to just kiss the water. In a way, it's a little
      like the destroyer bow seen in modern racing yachts, I've come to
      realise.

      Gav






      --- In boatdesign@yahoogroups.com, "graeme19121984"
      <graeme19121984@...> wrote:
      >
      >
      >
      >
      > I think it's a given that a fluid flow going around a sharp vertice
      > will have turbulence downstream of that sharp obstruction - eg
      > vortices. Bolger says at the bow on a hard chined boat if water
      > pressure build-up causes water to flow around the chine and under
      the
      > bottom then the lowered pressure there from the fast angular
      > (spinning) flow in those vortices will result in the bow sinking
      > still deeper which will only result in a kind of feed-back
      escalating
      > the whole problem increasingly (= deeper the bow > more cross chine
      > flow > deeper the bow > etc). Now, this mostly makes the boat
      > difficult to steer. This drag is directed mostly downwards and
      > sideways and so doesn't scrub much speed off by itself. What scrubs
      > the speed off is the rudder in being used to correct the unwanted
      bow
      > steering behaviour. (On lots'a boats the rudder is often used as a
      > brake.)
      >
      > In the quarters of a hard chined boat where the bottom is kinda
      flat
      > or entirely so relative to the sides, then the vortices are spun
      off
      > the sides after flow crosses the chines from the botttom. This
      > doesn't escalate like it does at the bow, as the bottom is mostly
      > not "sucked" up or down, but rearwards by the sides only. The
      > cumulative drag vector is mostly straight aft, slowing the boat
      only
      > so much, without the cascading steps, and the rudder is needed much
      > less for correction, if at all. A curve in the run of the aft
      bottom
      > reduces the cross-chine flow there.
      >
      > To get on the plane a usual (eg skiff) boat has to get over
      > the "hump". The hump is the range where a lot more power is
      required
      > than at lower speed, for not much extra speed. A sailboat has only
      so
      > much power, due to various limitations, and any way that resistance
      > can be lowered, or energy recovery can be got here is of great
      help.
      > A small arc in the aft bottom is better than flat. Is that what
      Gerr
      > means by deadrise in the aft bottom? A lot of times "deadrise"
      means
      > kinda vee-d and I don't get that as surface area is increased.
      >
      > Few human powered boats of any type will ever plane on this planet
      > (disregarding the help from surfing etc.), and possibly even fewer
      > humans could be such a power source! I guess planing speed is not
      > what Gavin meant by fast.
      >
      > Pretty - Light - Fast??
      >
      > Ways to get your ordinary flat-bottomed skiff rowboat fast might
      be:
      >
      > -Lightness (already on the wish list);
      >
      > -Long as possible ( Gavin do you mean fast under athelete types for
      > 1000m, or fast under average types all day long?);
      >
      > -Narrow.
      >
      > I'd say all the permutations have been done, so something out of
      the
      > box is required: Offsetting the weight by a sufficiently sized
      > hydrogen filled volume could help with "light". If the hydrogen
      > filled volume is above the boat then this reverse ballast will
      > stabilise the boat and allow it to be as long and narrow as wished.
      >
      > Another out of the box suggestion would be multihull-aided. The
      > bottom could be narrow like that of a canoe or light dory. The
      > stability assisting floats could just be upright thin foam plates.
      So
      > thin that shaping is almost not required - they wouldn't have to be
      > very long to have enough useful volume, nor rigged far from the
      main
      > hull, as it is not for sailing. They could be stored and deployed
      > from the bow, allowing a passenger to be still carried in an aft
      seat.
      > They would be ok in flat water, decreasing to lousy in surf.
      >
      > Graeme
      >
      > --- In boatdesign@yahoogroups.com, "Ed Bachmann" <edbz@> wrote:
      > >
      > > Dave Gerr wrote that the curve in the run helps. Another
      statement
      > he makes
      > > is that a flat bottom without some curve in the run produces
      > vortexes that
      > > increase drag. For boats that plane he seems to want a small bit
      of
      > deadrise
      > > to reduce the drag from vortexes. Not sure how that works at
      rowing
      > speeds
      > > but I think it makes sense.
      > >
      > >
      > >
      > > For weight distribution the requirements seem to me to:
      > >
      > > 1 ) Require that the transom be above or at the waterline.
      > >
      > > 2) Given that requirement, the trim should be set to maximize
      > overall
      > > length of waterline (figuring a variable the displacement).
      > >
      > >
      > >
      > > Ed B
      > >
      > >
      > >
      > > >>Gavin Atkin wrote:
      > >
      > > Graeme:
      > >
      > > A kind of Mouse??? Thanks for your thoughts - I'm breathless!
      > >
      > > You make some interesting points, and I see what you mean from
      page
      > > 333. I also found myself thinking about the Oarmouse I drew up
      some
      > > years ago. It hasn't been built, but it conforms to some of the
      > points
      > > you have made.
      > >
      > > But a flat-bottomed skiff is what's been asked for. One of my
      > > questions is how to best distribute the displacement - I notice
      that
      > > the traditional skiffs regularly had quite a pronounced curve in
      > their
      > > runs, and have long imagined that this was to give carrying
      capacity
      > > when required. Now, I suddenly ask myself, does it contribute to
      > speed
      > > also?
      > >
      > > What would be the ideal fore and aft distribution of the
      > displacement
      > > for a boat like this?
      > >
      > > Gav
      > >
      > >
      > >
      > >
      > >
      > > [Non-text portions of this message have been removed]
      > >
      >
    • pvanderwaart
      NACA foils are used for lift, and for foils expected to operate at a measurable angle of attack, neither of which apply to rowboat hulls.
      Message 44 of 44 , Jul 7 6:33 AM
        NACA foils are used for lift, and for foils expected to operate at a
        measurable angle of attack, neither of which apply to rowboat hulls.
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