FISA Regs on shell surface finish
tjhc*
Was intrigued to see a shell being carried to the boating pontoons at
Dorney on Sunday which had a 'non-smooth' surface finish over the
whole wetted surface of the boat. As they passed I reached up to
touch the surface at the stern section and found that while not as
rough as sandpaper, there was a definite texture as if the shell had
been gently sand-blasted. It rather put me in mind of the 'shark-
skin' swim suits that are currently proliferating in top-end swimming.
The FISA regs say that "no substances or structures capable of
modifying the natural properties of water or of the boundary layer of
the hull/water interface shall be used". Given that no surface finish
is specified (in the rules of racing that I found
http://www.worldrowing.com/medias/docs/media_360788.pdf), can anyone
see a reason why a 'non-smooth' surface would be disallowed and, if it
were to have the game-changing effect that we've seen in swimming, why
it's not appearing on more modern racing shells?
Regards,
Tom
Kit
> Dear All,
>
> It rather put me in mind of the 'shark-
> skin' swim suits that are currently proliferating in top-end swimming.
>
> The FISA regs say that "no substances or structures capable of
> modifying the natural properties of water or of the boundary layer of
> the hull/water interface shall be used". Given that no surface finish
> is specified (in the rules of racing that I foundhttp://www.worldrowing.com/medias/docs/media_360788.pdf), can anyone
> see a reason why a 'non-smooth' surface would be disallowed and, if it
> were to have the game-changing effect that we've seen in swimming, why
> it's not appearing on more modern racing shells?
>
> Regards,
>
> Tom
Surface finishes like riblets had a definite organised structure to
them, ie lines of tiny ridges aligned perpendicular to the water flow.
These are banned.
I think it would be difficult to legislate against a randomised rough
finish however since it is impossible to define the threshold for how
the rough finish of eg a bad paint job differs from that of a
deliberate attempt at drag reduction.
But I would be surprised if random roughness reduced drag anyway. Are
you sure it wasn't just an incomplete refurbishment?
Carl Douglas
There are certain forms of regular, aligned surface texturing, the best
known of which is Riblets(tm), which have the potential to reduce fluid
drag by stabilising the characteristics of the boundary layer. You
could liken the surface they create to that of a vinyl record.
It is also possible to replicate the regular pattern of denticles found
on shark skin & this may have a similar effect.
It is those kinds of modified surface which fall under the FISA ban, as
does adding materials to the water flowing over the hull to similarly
alter water's fluid properties & reduce drag.
There are those who like to think that sanding the hull surface will
somehow replicate the Riblet effect. I doubt that it does, & I think
that rowing more intelligently would be more profitable, but others
might know more.
Cheers -
Carl
--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Find: http://tinyurl.com/2tqujf
Email: ca...@carldouglas.co.uk Tel: +44(0)1932-570946 Fax: -563682
URLs: www.carldouglas.co.uk (boats) & www.aerowing.co.uk (riggers)
JD
> URLs: www.carldouglas.co.uk(boats) &www.aerowing.co.uk(riggers)- Hide quoted text -
>
> - Show quoted text -
Long ago, a friend, one of the premier mathematicians in the field of
chaos theory suggested he could get me some of the paint used on US
fighter jets, which has a rough surface due to the inclusion of
microspheres. He explained that it reduced drag becasue the spheres
cause vortices that increase laminar flow down the lenth of the
aircraft, and would have done the same for our shells.
Smart guy. He would go to Vegas and come back with lots of money.
fastyacht
>
> > - Show quoted text -
>
> Long ago, a friend, one of the premier mathematicians in the field of
> chaos theory suggested he could get me some of the paint used on US
> fighter jets, which has a rough surface due to the inclusion of
> microspheres. He explained that it reduced drag becasue the spheres
> cause vortices that increase laminar flow down the lenth of the
> aircraft, and would have done the same for our shells.
>
> Smart guy. He would go to Vegas and come back with lots of money.
Ar you sure it wasn't initiating the transition to a turbulent
boundary layer, which has a much greater resistance to stall
initiation? Jets are so fast, the Reynolds Number is too high to
maintain laminar flow. On the other hand, competition gliders are
designed with full laminar wing sections, and even the accumulation of
dust in flight has an effect on performance.
Tony
>
> > > - Show quoted text -
>
> > Long ago, a friend, one of the premier mathematicians in the field of
> > chaos theory suggested he could get me some of the paint used on US
> > fighter jets, which has a rough surface due to the inclusion of
> > microspheres. He explained that it reduced drag becasue the spheres
> > cause vortices that increase laminar flow down the lenth of the
> > aircraft, and would have done the same for our shells.
>
> > Smart guy. He would go to Vegas and come back with lots of money.
>
> Ar you sure it wasn't initiating the transition to a turbulent
> boundary layer, which has a much greater resistance to stall
> initiation? Jets are so fast, the Reynolds Number is too high to
> maintain laminar flow. On the other hand, competition gliders are
> designed with full laminar wing sections, and even the accumulation of
> dust in flight has an effect on performance.
I suspect that is the case. Several glider pilots have found
increased performance by determining where the flow transitions from
laminar to turbulent and then strategically placing zig zag tape or
some other form of turbulator to keep that flow attached. attached
turbulent flow is better than nothing
Carl Douglas
>>
>>>> It is those kinds of modified surface which fall under the FISA ban, as
>>>> does adding materials to the water flowing over the hull to similarly
>>
>>>> There are those who like to think that sanding the hull surface will
>>>> that rowing more intelligently would be more profitable, but others
>>>> might know more.
>>
>>>> Cheers -
>>>> Carl
>>
>>>> --
>>>> Carl Douglas Racing Shells -
>>>> Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
>>>> Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
>>>> Find: http://tinyurl.com/2tqujf
>>>> Email: c...@carldouglas.co.uk Tel: +44(0)1932-570946 Fax: -563682
>>
>>>> - Show quoted text -
>>
>>> Long ago, a friend, one of the premier mathematicians in the field of
>>> chaos theory suggested he could get me some of the paint used on US
>>> fighter jets, which has a rough surface due to the inclusion of
>>> microspheres. He explained that it reduced drag becasue the spheres
>>> cause vortices that increase laminar flow down the lenth of the
>>> aircraft, and would have done the same for our shells.
>>
>>> Smart guy. He would go to Vegas and come back with lots of money.
>>
>> Ar you sure it wasn't initiating the transition to a turbulent
>> boundary layer, which has a much greater resistance to stall
>> initiation? Jets are so fast, the Reynolds Number is too high to
>> maintain laminar flow. On the other hand, competition gliders are
>> designed with full laminar wing sections, and even the accumulation of
>> dust in flight has an effect on performance.
>
> I suspect that is the case. Several glider pilots have found
> increased performance by determining where the flow transitions from
> laminar to turbulent and then strategically placing zig zag tape or
> some other form of turbulator to keep that flow attached. attached
> turbulent flow is better than nothing
Which takes us back to Vortex edges? Well, best not.
The prime function of Riblets(tm), AIUI, is to stabilise the base level
of the boundary layer. When a fluid passes along a surface, at the
start you'll get laminar flow - a steady variation in velocity over a
thin, but thickening, section of the flow as you move from the surface
(water moving at boat speed) to the as-yet undisturbed body of the
water, with all flows parallel to the surface, i.e. flowing as laminae
or thin sheets. As you move further along the boat, the laminar BL
thickens steadily & also becomes unstable, with flows wanting to curl
back on themselves (tiny vortices). Tiny bursts of vortices, resembling
tornadoes, can also start spontaneously from the hull surface & then
break away. Riblets stabilise these bursts within their grooved surface
structure, so that they don't grow & shed into the rest of the BL.
This is a good thing as, once a certain level of vorticity develops
within the laminar BL, the smooth structure thereof breaks down & is
largely replaced by a far messier, & thicker, turbulent BL. When energy
is transported purely by interlaminar shear within the laminar BL, the
fluid drag is (roughly) proportional to the inverse of the BL thickness,
so is relatively low & falls along the boat. After the
laminar-to-turbulent transition, energy is exchanged across the
turbulent BL in a much more vigorous & haphazard fashion, & the more
vigorous this process, the higher the drag. So there's a jump in skin
friction at the transition, followed by a rather slower fall as the
turbulent BL itself thickens. Delaying the transition thus reduces
frictional drag on the hull.
Even after transition, there remains a (thinner) laminar sublayer, right
against the hull. Riblets may help to better stabilise this sublayer,
allowing it to thicken more than might otherwise occur, limiting the
shear occurring in the turbulent layer outside it.
Things which stimulate an earlier laminar-turbulent transition can
include any too rapid change in pressure distribution over the surface,
so laminar-flow aerofoils (e.g. on gliders, but we've built one as a
sail for a racing catamaran) tend to have their thickest section further
astern that conventional airfoils. And things which can disrupt the
laminar BL include seemingly trivial objects as adherent dust, surface
defects & impacted flies (see Tony's remarks above). You don't use
laminar airfoils on commercial or military jets as a laminar flow wing
losing laminar flow is not what you want - it's altogether too dodgy.
However, even conventional airfoils can be vulnerable to flow
disturbances which muck up the BL & disrupt the overall flow pattern.
Flows tend to adhere to & follow smooth surfaces (Coanda effect) & this
is important to the proper functioning of airfoils. What can happen,
especially when operating at the higher angles of attack (flying slowly
& still needing to stay up in the air - i.e. retain lift), is a pressure
reversal on the upper surface of the wing, allowing a part of the flow
to double back & travel back up the BL. Then the BL (which is
turbulent) is disrupted & the overall flow separates from the surface,
causing a serious loss of lift, AKA Stall. Counterintuitively (all of
fluid dynamics is a bit counterintuitive), it is possible to re-attach,
or keep attached, the flow onto the surface by locally disrupting it
soon enough in a way which mixes in a bit of higher-speed air from
further away from the surface, thus keeping the velocity at the BL high
enough to prevent that reverse flow from starting - sometimes called
energising the BL. You sometimes see turbulence-promoters, in the form
of a line of skewed, upstanding blades set a moderate distance back from
a wing's leading edge. These are known to extend the operating envelope
for wings which have otherwise to work at the edges of their safe range.
I remain unconvinced that such devices, as sometimes seen in rowing,
have relevance to this use but others may disagree ;)
I hope that explains some of these processes in sufficiently plain terms
for the technical lay person?
Cheers -
Carl
--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Find: http://tinyurl.com/2tqujf
Tinus
> Dear All,
>
> Was intrigued to see a shell being carried to the boating pontoons at
> Dorney on Sunday which had a 'non-smooth' surface finish over the
> whole wetted surface of the boat. As they passed I reached up to
> touch the surface at the stern section and found that while not as
> rough as sandpaper, there was a definite texture as if the shell had
> been gently sand-blasted. It rather put me in mind of the 'shark-
> skin' swim suits that are currently proliferating in top-end swimming.
>
> The FISA regs say that "no substances or structures capable of
> modifying the natural properties of water or of the boundary layer of
> the hull/water interface shall be used". Given that no surface finish
> see a reason why a 'non-smooth' surface would be disallowed and, if it
> were to have the game-changing effect that we've seen in swimming, why
> it's not appearing on more modern racing shells?
>
> Regards,
>
> Tom
I see nice stories about the function of surface irregularities but
does anyone know the reason why they would be disallowed (if they are)?
Tinus
> I see nice stories about the function of surface irregularities but
> does anyone know the reason why they would be disallowed (if they are)?
I mean, what are the natural properties of a boundary layer and how
are they disturbed? (or is there another rule limiting the use of
irregularities)