Sanding hulls
Leon Miguel
: But your theory has plenty of supporters, during the America's Cup
: regatta at Fremantle that rogue of the water Dennis Conner had a
: special film made by 3M laminated to his hull. It had minute
I remember reading about a new fin that has these micro-laminations on
the surface. It was claimed to increase speed (in testing tanks) by 2%,
which doesn't sound much, but was enough for Dennis Connor, and is
probably the difference in a place or two.
Leon
--
Leon Miguel < lmi...@uniwa.uwa.edu.au >
Centre for Legumes in Mediterranean Agriculture (CLIMA)
University of Western Australia
+ 61 9 380 1982
Tim Sercombe
>
> I understand that if boat hulls are sanded lightly with very fine sandpaper,
> that this will lead to a thin layer of water adhering to the hull and reduce
> friction. Has anyone tried this with a windsurfer? Does it work, and if so
> what technique is required?
>
> Any ideas appreciated,
> Thanks, BobYeah this is supposed to work - heard of quite a few guys doing it.
Don't use anything coarser than 400 Grit.
Happy Sailing
Tim
Bob Zuur
dcoveney
the adhesion of the film next to the hull. Who knows if it really is effective?
Tim Sercombe <t...@minmet.uq.oz.au> wrote:
Ian Knight
>
> I understand that if boat hulls are sanded lightly with very fine sandpaper,
> that this will lead to a thin layer of water adhering to the hull and reduce
> what technique is required?
>
> Any ideas appreciated,
> Thanks, Bob
Boundary layer theory assumes that a very thin layer of fluid adheres
to the surface, no matter how smooth the surface is. Thus the effect
you propose is already taking effect. Sanding could help take
the odd large lump off the surface which would increase drag.
I read somewhere of a definition of aerodynamically smooth, and
I suppose hydrodynamically smooth. I seem to remember that once a
surface is smooth enough to be classified as hydrodynamically smooth
no further sanding or polishing will help. Can someone else help
here before I go back to the library?
But your theory has plenty of supporters, during the America's Cup
regatta at Fremantle that rogue of the water Dennis Conner had a
special film made by 3M laminated to his hull. It had minute
etchings resembling shark skin, or so the newspapers said. He won
the cup back but I think most people attribute his success to skill
on the water rather than the special surface preparation.
We need more on this subject ..Ian
Glenn Ramsey
b...@mfe.govt.nz (Bob Zuur) wrote:
> I understand that if boat hulls are sanded lightly with very fine sandpaper,
> that this will lead to a thin layer of water adhering to the hull and reduce
> friction. Has anyone tried this with a windsurfer? Does it work, and if so
> what technique is required?
>
> Any ideas appreciated,
> Thanks, Bob
Bob,
I think they do this to trip the boundary layer (layer of
fluid close to the surface of the hull) to make it become
turbulent, this decreases the pressure drag on the hull.
This is also why golf balls have dimples.
There may be another reason for the rough surface but if
this is the reason then on a planing windsurfer hull the
boundary layer flow would most likely be fully turbulent
anyway so making the surface rough would have little effect.
If you try it let us know how you get on.
g
--
Glenn Ramsey The views expressed here are f.y.i only &
gle...@es.co.nz should not be used for design. so there!
Volker Wedemeier
On Tue, 30 Jan 1996, Bob Zuur wrote:
> I understand that if boat hulls are sanded lightly with very fine sandpaper,
> that this will lead to a thin layer of water adhering to the hull and reduce
> friction. Has anyone tried this with a windsurfer? Does it work, and if so
> what technique is required?
Well, that's why windsurfers used to sand their fins before a race until
some years back.
But a scientific examination has shown that all this is only esoteric hype.
With fins you'll even increase the number of cavitaion centers when you
roughen the surface and thus the fin will spin out earlyer while the drag
is not decreased at all.
Well, the study was for fins only, but I guess the same would apply for
whole boards (and boat hulls).
Regards,
Volker
sailquik_us_3704
Bob:
Ever get to look at the bottom of an F2 Sputnik World Cup Edition?
They are almost "sharkskin" and are still the fastest production boards
available last time I looked.
I can assure you that putting the sharkskin surface in the molds is extremely costly
so if it wasn't a good idea I'm sure F2 wouldn't do it.
Roughing the surface of the board with 400-600 grit wet and dry
is a good idea if your board is shiny on the bottom.
The principle here is water molecules roll along a very smooth polished
surface creating turbulence.
A surface that is dulled by wet sanding will "wet out"
and trap a layer of water molecules on the sanded surface.
This "wet"surface reduces friction and creates a stationary
boundary layer.
The "intermolecular" boundary layer creates less drag and turbulence
than having the molecules move across a polished surface.
This same principle applies to fins. Concrete Wave makes fins for
F2 boards and they invariably work at least twice as well after wet
sanding the glossy surface off the wetted portions of the fin.
They probably put the gloss on to make them sell better , but dulling the finish definetly
makes them "work" better.
sailquik US 3704
F2/CFX/North Sails/North Masts/True Ames/Rainbow Duffy Series/
Chinook/Wavelength/Body Glove/SSA/ Free Advice on all of above.
Email= sail...@atc.ameritel.net
Phone= (301) 872-9459
Ian Knight
> I think they do this to trip the boundary layer (layer of
> fluid close to the surface of the hull) to make it become
> turbulent, this decreases the pressure drag on the hull.
> This is also why golf balls have dimples.
>
> There may be another reason for the rough surface but if
> this is the reason then on a planing windsurfer hull the
> boundary layer flow would most likely be fully turbulent
> anyway so making the surface rough would have little effect.
> --
> Glenn Ramsey The views expressed here are f.y.i only &
> gle...@es.co.nz should not be used for design. so there!
>
>
I've found a book Ëlementary Fluid Mechanics by Venard and Street.
It discusses laminar and turbulent boundary layers. At the
leading edge of a flat plate (fin ?) a boundary layer starts off
laminar but after a distance the instability of the strongly sheared
layers triggers a transition to a turbulent boundary layer. The
thickness of the turbulent layer then thickens with distance from the
leading edge. But sandwiched between this turbulent layer and the
boundary is what's called a Viscous sublayer. The diagram in the book
shows this viscous sublayer as being thinner than the laminar layer
in the short non turbulent region at the leading edge.
"A boundary surface is said to be smooth if its projections or
protuberences are so completely submerged in the viscous sublayer
that they have no effect on the structure of the turbulence
..roughness heights larger than about 1/3 of the sublayer thickness
will augment the turbulence "
Unfortunately the book doesn't give any numbers on how thick this
sublayer is likely to be. More than the scratches from 400 grit
sandpaper I suppose.
The text says transition from a laminar to a turbulent flow occurs when
the Reynolds Number exceeds 500,000. The Reynolds number of a sailboard
fin with 10 cm chord going 10 m/sec is 900,000. Which is fairly close
to the critical value for laminar flow. But Reynolds number stuff is
only a rough indicator. Who knows does a sailboard fin have
laminar flow? The hull certainly wouldn't, being longer and having
the already turbulent surface water at it's leading edge.
Ian
Volker Wedemeier
On Thu, 1 Feb 1996 sailquik@.MISSING-HOST-NAME. wrote:
> Ever get to look at the bottom of an F2 Sputnik World Cup Edition?
> They are almost "sharkskin"
Are you sure you the board wasn't upside down and you looked at
the deck grip? :-))))
(Sorry, couldn't resist!)
(And then, I did! And they were smooth. - Well they were from 1992
and '93, so ... ??)
> I can assure you that putting the sharkskin surface in the
> molds is extremely costly so if it wasn't a good idea I'm sure F2
> wouldn't do it.
Sure, it _is_ a good idea! Now F2 can tell people, that this makes the
board faster and so it sells better. That's why they do it. Anyway, it's
you who pays the price in the end, not F2.
> A surface that is dulled by wet sanding will "wet out"
> and trap a layer of water molecules on the sanded surface.
> This "wet"surface reduces friction and creates a stationary
> boundary layer.
> The "intermolecular" boundary layer creates less drag and turbulence
> than having the molecules move across a polished surface.
> This same principle applies to fins.
Well, nice theory and there may even be something about the shark-skin
effect.
I only wonder, why dolphins and other fish don't have it.
And then there is still the fact, that the guys from 'Surf' mag have
actually tried it out. They put original and sanded fins into a water
channel. And what _they_ found on their _instruments_ is that the sanded
fins did not have less drag (no measurable difference). They only spun
out slightly earlier. So according to that, sanding your fin is even
counterproductive while sanding your board probably doesn't hurt.
> Concrete Wave makes fins for
> F2 boards and they invariably work at least twice as well after wet
> sanding the glossy surface off the wetted portions of the fin.
> They probably put the gloss on to make them sell better,
> but dulling the finish definetly makes them "work" better.
Are you sure, it's not a placebo-effect, that you see there?
I did sand two of my fins, too. But only partly to smooth out small
notches in the leading edge (which can lead to earlier spinouts). But I
did not notice any boost in performance.
Actually it was more like no difference at all.
Volker
Volker Wedemeier
On Thu, 1 Feb 1996, Glenn Ramsey wrote:
> I think they do this to trip the boundary layer (layer of
> fluid close to the surface of the hull) to make it become
> turbulent, this decreases the pressure drag on the hull.
Hmm, a turbulent flow has a higher drag. The theory is, that by trapping
some water in the rough surface, the water-water boundary flow gets more
laminar.
> This is also why golf balls have dimples.
I thought this was only to make the spin of the ball more effective. That
way you can slice a golf ball around a tree for example. It actually
_increases_ the drag. But as long as the balls are the same for all
players, this doesn't matter for the game. (Well, that way, golf courses
can be kept a little bit smaller. :-))) )
Volker
Kevin R. Clark
>
>> I think they do this to trip the boundary layer (layer of
>> fluid close to the surface of the hull) to make it become
>> turbulent, this decreases the pressure drag on the hull.
>>
I remember from my fluid dynamics class in college a video
which examined the effects of a rough surface vs. smooth
as it pertains to air drag. They took 2 balls of identical
diameter and weight and put them at the ends of a long rod.
They then put a pivot joint in the middle to create something
much like a balance only it was horizontal. They put all
this in a wind tunnel and observed that at slow speeds the
rough ball had more drag and hence moved back (pushing the
smooth ball forward). At higher speeds, the turbulence
caused by the rough ball did in fact lower the air resistance
and the rough ball moved forward (pushing back the smooth
ball). I don't remember exactly at what speed the rough
surface began to have an advantage, but it was somewhere
close to 100 MPH! Therefore, dimples on golf balls will
reduce drag, dimples on your bicycle will slow you down.
I don't know how this would translate from air to water, but
I'm sure there is a correlation. Maybe roughened hulls work
for Bjorn at 40+ MPH, but slow down us mere mortals...
BTW, I was into bicycle racing back then and found it quite
humorous that some racers had heard that the dimples on
golf balls lessened drag, and decided that all leading edges
of their time trialing bicycles should be roughened. I
even read this in a book. It just goes to show you that a
little bit of knowledge can be dangerous sometimes. :^)
|
Pray for Wind! /| Kevin Clark
/ | Design Engineer
/ | Alcatel Network Systems
/ | Dallas, Texas
----| Email: cl...@aud.alcatel.com
------------
sailquik
..Ian
>
>Check out the bottom of an F2 Sputnik or most other
F2 boards built in the last 3 years. They have machined
or etched the molds to add a definite tooth pattern to
the bottom surface of the finished board. These are very
fast (perhaps the fastest production) boards.
F2 would not spend the money to etch/machine the molds if there
wasn't some advantage to doing so.
I think the point here is that if you wet sand the bottom you will
get the surface to"wet out" and hold a stationary layer on the surface
which "lubricates" the adjacent boundary layer. I know absolutely that
this works on fins, but I don't have any research material that tells
me exactly what the dynamics are.
Glenn Ramsey
cla...@aud.alcatel.com (Kevin R. Clark) wrote:
> >On Thu, 1 Feb 1996, Glenn Ramsey wrote:
> >
> >> I think they do this to trip the boundary layer (layer of
> >> fluid close to the surface of the hull) to make it become
> >> turbulent, this decreases the pressure drag on the hull.
Which isn't quite correct, in ths case anyway, sorry, 'cos
I don't think pressure drag doesn't come into it.
> >>
> >> This is also why golf balls have dimples.
Where pressure drag does have an effect.
<snip>
> close to 100 MPH! Therefore, dimples on golf balls will
> reduce drag, dimples on your bicycle will slow you down.
>
> I don't know how this would translate from air to water, but
> I'm sure there is a correlation. Maybe roughened hulls work
> for Bjorn at 40+ MPH, but slow down us mere mortals...
The correlation is provided by the Reynold's number, which
is density x velocity x characteristic length / vicosity.
A paper airplane book that I had put it this way: "a bee
flying in air is like a person swimming in honey"
Water is much denser than air which goes a long way toward
explaining why a 300mm fin can generate enough lift to
balance that generated by 5 m^2 sail.
In article <217cc$122d2.2cc@NEWS>,
sailquik US 3704 (you) wrote:
> The principle here is water molecules roll along a very smooth polished
> surface creating turbulence.
> A surface that is dulled by wet sanding will "wet out"
> and trap a layer of water molecules on the sanded surface.
> This "wet"surface reduces friction and creates a stationary
> boundary layer.
> The "intermolecular" boundary layer creates less drag and turbulence
> than having the molecules move across a polished surface.
Don't agree at all with this. I think what I said before
about the rough surface tripping the boundary layer and
making it become turbulent is more applicable. I don't know
under what conditions the turbulent boundary layer causes a
reduction in drag (or skin friction) but there are some
cases where it does (well according to my fluids text
anyway, which doesn't say any more than that). I'd guess
that its got something to do with the speed at which the
board is going.
cheers
g
Ian Knight
of the boundary layer. A smaller wake is formed and consequently less
drag.
A boundary layer separates if it is decelerated to a stop by
a positive pressure gradient. By using dimples to promote mixing
of faster air closer to the surface, ie. a more turbulent boundary
layer, the point where the air flow stops and the wake tucks in
underneath is moved closer to the trailing " ėdge " of the ball.
This is not the case with fins because a tapered traling edge
is used to reduce the wake. Dimples on a fin may reduce spin
out by delaying separation of the boundary layer but I suspect
surface friction would increase and make them too slow?.
Some aeroplane wings use a similar principle. Those little 1 inch
high rectangles near the trailing edge of wings are also to mix
faster air close to the surface and delay separation in
the region of increasing pressure gradient.
Ian
Bill Crick
>In article <4eu10q$e...@news01.aud.alcatel.com>,
> cla...@aud.alcatel.com (Kevin R. Clark) wrote:
>
>> >On Thu, 1 Feb 1996, Glenn Ramsey wrote:
>> >
>> >> I think they do this to trip the boundary layer (layer of
>> >> fluid close to the surface of the hull) to make it become
>> >> turbulent, this decreases the pressure drag on the hull.
>
>Which isn't quite correct, in ths case anyway, sorry, 'cos
>I don't think pressure drag doesn't come into it.
>
>> >>
>> >> This is also why golf balls have dimples.
>Where pressure drag does have an effect.
>
><snip>
>> close to 100 MPH! Therefore, dimples on golf balls will
>> reduce drag, dimples on your bicycle will slow you down.
>>
>> I don't know how this would translate from air to water, but
>> I'm sure there is a correlation. Maybe roughened hulls work
>> for Bjorn at 40+ MPH, but slow down us mere mortals...
coating to reduce drag. This was apparently a plastic film with small ridges,
much like you would get form sanding. These boats only go 15 knots or so, and
I assume that anyone who is going to wallpaper a yacht hull must know
it is going to have an effect?
Bill Crick
With enough surgical tubing, and wet veneer, you can veneer a bowling ball;-)
Sting-a-ling-a-ding-dong
Rob Darman <dar...@ix.netcom.com> wrote:
>What about the idea that the fin is the only wetted surface on a
>planing board, with the board riding on a thin cusion of air? Maybe
>the ridges help hold air bubbles under the board???
I hate to start another thread here but, isn't this what those "air"
pockets are for on some boards? What are those big "dimples" called. Is it
for planing purposes or are they to make a board "looser" in the waves, or
what? Angulo, are you out there...? :)
>
>Rob
>
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Glenn Ramsey
sail...@atc.ameritel.net (sailquik) wrote:
> I think the point here is that if you wet sand the bottom you will
> get the surface to"wet out" and hold a stationary layer on the surface
> which "lubricates" the adjacent boundary layer. I know absolutely that
What a load of crap. Boundary layer theory assumes that
there is a stationary layer of fluid next to the surface
anyway.
> this works on fins, but I don't have any research material that tells
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> me exactly what the dynamics are.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
wonder why that is? ;-)