>Thanks Kieran and Carl,
>I am still having some issues really understanding this however:
>Kieran, can you please explain your comment "the flow of water over the
>hull may separate from the surface" for me please. You mention that
>this type of design will cause eddies, however I would have expected
>that this would only occur after the water had passed the hull and was
>converging. As the hull was no longer in that area it shouldn't have
>an impact, should it (I wouln't have expected it to be like
[I'm reposting this because, as some may have noticed, my ISP sometimes
disappears postings, as it has done with my initial reply here, only to
send them out weeks later. No doubt the original of this one will pop
up in due course, when we've all forgotten what it was all about]
Kieran will respond in his own words, but a few points anyway: Water is
a fluid & so is air. At all times your boat is enveloped by these 2
fluids. As the boat moves forwards it parts both and as it passes they
both close on it (not behind it) because they remain always in contact.
As a first approximation, where the water is most displaced by the boat
its internal pressure is least. There being an air/water interface, the
water level everywhere reflects its internal pressure, so the surface
falls under this internal pressure decrease. And as the boat passes the
water expands to fill the space available (you don't leave a hole!) and
regains pressure, so the surface level rises.
All of that is fine if the flow is very smooth & if all streamlines flow
mutually parallel at all parts near & far from the hull surface. But
water cannot be compelled to fill the space vacated by the boat as fast
as we might hope - there is a bit of a lag. If this lag is significant
(whatever that may mean) the local pressure imbalances stir up the flows
around the boat while the main flow streams past rather ignoring the
boat's afterbody shape. Then you get messy flow (big energy loss) &
much reduced pressure recovery.
>If this is the case then wouldn't the reduction in wetted surface area
>of a blunt stern provide a better alternative?
Sometimes, yes. But the result is the sum of many subtle &
inter-related effects. As in all compromises, the optimum solution
depends on circumstances. So with some shapes a transom stern may work
better, especially if it gives a sailboat better stability, or an
effective re-alignment of its underwater hull shape when heeled. With
others, it does not.
>Carl, You mention pressure recovery which makes sense but during race
>conditions I would have expected that the speed of the boat would
>outrun any pressure differential. For example if the stern was
>straight and square and the skiff was travelling at a speed that left a
>indentation in the water of the hull shape for a small period of time
>before the waters converged then even if the stern was wedged shape
>there wouldn't be any advantage should there?
As I said above, it is a complex compromise. And you do not leave the
sort of indentation in the water that you appear to describe, which is
the sort of thing you may see astern of a powerboat which is operating
way above what we loosely term "hull velocity". We lack the power to do
that in a rowing shell - unless our name is Asterix the Gaul. Simplistic
design answers remain just that - they only seem obvious because one
ignores (at one's cost) other important factors which say "go this way".
The optimum shape is never that which optimises only one of a range of
> I have also thought
>about the recovery of the stroke and if the water did catch up it
>should exert positive movement on the hull as it hits the square stern.
The water does not catch up. Nor would you want it to, because the
energy penalty of making it accelerate & chase you would be intolerable
for the rower & there is no fluid dynamic mechanism that would make it
>I agree with you about the pitch problem, however I think I have a
>solution to that that I am working on.
>I am not an expert on this so I may be way off but appreciate any
We are all of us somewhat inexpert, even the most expert of us. At a
conference some 12 years ago on shell design I began by telling my
audience that I was there to tell them how very little we all knew, &
then to explain what we did know & where that led us. Others told the
audience it was all understood and that shell colour was the most
important thing :^)
Anyway, wouldn't life be dull if there was nothing else left to learn?
What does disturb me is those who build them but don't have a clue about
the fluid mechanical design of rowing shells, & don't let that stop them
from pretending that they are expert designers. And then there are
those who actually swallow that stuff.
Maybe my next lecture should be on the subject of peristalsis?
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing low-drag Riggers/Advanced Accessories
Write: The Boathouse, Timsway, Chertsey Lane, Staines TW18 3JY, UK
URLs: www.carldouglas.co.uk (boats) & www.aerowing.co.uk (riggers)