Re: Light Skiffs (was Once, in a Blue Moon,.....)
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
--- In firstname.lastname@example.org, "graeme19121984"
> 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
> bottom then the lowered pressure there from the fast angularescalating
> (spinning) flow in those vortices will result in the bow sinking
> still deeper which will only result in a kind of feed-back
> the whole problem increasingly (= deeper the bow > more cross chinebow
> 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
> steering behaviour. (On lots'a boats the rudder is often used as aflat
> In the quarters of a hard chined boat where the bottom is kinda
> or entirely so relative to the sides, then the vortices are spunoff
> the sides after flow crosses the chines from the botttom. Thisonly
> 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
> so much, without the cascading steps, and the rudder is needed muchbottom
> less for correction, if at all. A curve in the run of the aft
> reduces the cross-chine flow there.required
> 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
> than at lower speed, for not much extra speed. A sailboat has onlyso
> much power, due to various limitations, and any way that resistancehelp.
> can be lowered, or energy recovery can be got here is of great
> A small arc in the aft bottom is better than flat. Is that whatGerr
> 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.be:
> 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
> -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?);
> I'd say all the permutations have been done, so something out of
> box is required: Offsetting the weight by a sufficiently sizedSo
> 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.
> thin that shaping is almost not required - they wouldn't have to bemain
> very long to have enough useful volume, nor rigged far from the
> hull, as it is not for sailing. They could be stored and deployedseat.
> from the bow, allowing a passenger to be still carried in an aft
> They would be ok in flat water, decreasing to lousy in surf.statement
> --- In email@example.com, "Ed Bachmann" <edbz@> wrote:
> > Dave Gerr wrote that the curve in the run helps. Another
> he makesof
> > 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
> > to reduce the drag from vortexes. Not sure how that works at
> > 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
> > 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
> > 333. I also found myself thinking about the Oarmouse I drew upsome
> > years ago. It hasn't been built, but it conforms to some of thethat
> > 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
> > the traditional skiffs regularly had quite a pronounced curve incapacity
> > runs, and have long imagined that this was to give carrying
> > when required. Now, I suddenly ask myself, does it contribute to
> > also?
> > What would be the ideal fore and aft distribution of the
> > for a boat like this?
> > Gav
> > [Non-text portions of this message have been removed]
- 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.