Marcus Ward asked...
In my quest to avoid work, but to refinish my blades to a level
high enough to satisfy myself, I did some research (tons actually) about surface finishes
for foils. A lot of people advocate the 600 grit finish. Some advocate unsanded paint,
some advocate a mirror finish. After reading Frank Bethwaite's book, High Performance
Sailing, I'm afraid that mirror finish wins. He did an actual experiment with foils,
one sanded to 1200 grit, and one polished to a mirror finish, and found a 53lb difference
in lift generated at 11 knots. Since I know the 505 can sail at much higher speeds, I'm
sure this is of interest to everyone. I've copied the pertinent section of the book, as
well as scanned the diagrams in and posted them on my web pages for all to see. I am doing
this without permission, so I hope I'm not breaking any laws. I do recommend purchasing
the book if you're into the theory behind going fast though, it's a VERY good read, and
full of very good info.
If anyone thinks this info is wrong, wants to talk about it, or
discuss it, knows great ways of getting mirror finishes, etc, please post it to the list,
as I'm sure many people would be interested.
Thanks,
Marcus
*************************************************
Marcus Ward
505 US 7569
Central Missouri State University
*************************************************
Jesse Falsone
Marcus,
Surface roughness is an age old issue. The best thing about really
smooth finishes ("mirror finish") is that contaminants don't stick as easily.
Remember, Frank B's experiment was not done under real world conditions (i.e. - little
mixing of flow velocities, low contamination, free surface effect, etc). His claim of a 53
lb. increase in lift at 11 knots is highly speculative at best, so take it with a grain of
salt. Suffice it to say, most people would agree that the smoother the finish, the better.
Jesse Falsone
Bill Hamn
Marcus,
Ok, I'll bite :), how did you do this test? Not many towing tanks
around big enough to put a full sized 505 into.
Bill Hamm
[email protected]
Bill Hamn
Marcus,
Ok, now that I've thought about this a bit more....
The finish, assuming the board is fair, shouldn't marginally effect
the lift, but certainly should effect the drag. A finish of much over 220 grit should be
similiar to a polished blade. Also just the lift figure is only partially helpfull, really
need lift / drag. Not much use in having and increase in lift if in getting it your
substantially increase drag.
The research into finishes was done a number of years ago in
Scotland, at University of Glasgow I believe, but wasn't directed towards boats, rather
into a problem that aircraft with canards were facing, water beading on canards in rain
causing a loss of lift. It was found that if the finish was sanded to a 220 grit, the
water "sheeted" on the foil rather than beading, thus didn't effect the surface
dimensions materially.
Great book that you mentioned btw, altough some of the reasearch in
it is dated.
Bill Hamm
[email protected]
Griffin Boatworks
Chris Coleman
Funny you should mention Bethwaite's book because I have just begun
re-reading it from the beginning in an effort to gain a greater understanding of the
concepts and issues raised. Two copies (one mine, one my skipper's) of "High
Performance Sailing" made the roadtrip to Florida for the midwinters a much more
enjoyable ride, as well as providing a benchmark against which to measure our (miserable)
performance in St. Pete on the way home.
For those of you who don't already own it, go get it now. Bethwaite
provides insights not only into foil design and finishing, but also sails, hulls weather,
waves motions, and tactics. Among other things, he provides the clearest and most concise
explanation of the Coriolis Effect that I have ever read.
For those picking it up for the first time; don't try to read it
straight through from beginning to end as you would a novel. Skip around a bit and use it
as you would any reference book. Once you have gained a foothold on his ideas, then give
it a shot from page one.
Oh yea, Marcus, you probably are breaking the law.
Chris Coleman.
Peter Epstein
Marcus and Jesse,
This past summer I spent hours on finishing the bottoms and blades
on our 5o5 and Albacore. I took them down to about 1000 grit but did not have the
opportunity to buff them to a mirror finish. I agree with Jesse, the smoother the better!
Regards,
Peter Epstein
CAN 7823
Bransford Eck
Have not read magical book, but speed and lift as such are not real
important in drawing conclusions unless you know actual test hardware geometry, Reynolds
Number, and lift/drag ratio at a relevant angle-of- attack. Most imperfections will be
buried in the laminar boundary layer at typical 505 beating speeds. I did some work with
3D trips a long time ago. It is sometimes a good idea to trip the boundary layer turbulent
before it gets too thick. This is useful because most 505 foils work at a Reynolds of
greater than 250000. When speeds are high (reaches), foil wetted area is small relative to
total wetted area. Windward work makes lift and drag important because L/D ratio is
cotangent of pointing angle. Suggest you find very specfic treatment of 505 foils in 1976
"Tank Talk".
Bransford Eck, US 7067
Ed. The two articles Bransford mentions are available on this web
site at:
They are PDF files which will require the Acrobat reader (free
download) to view.
Marcus Ward
sanded to a 220 grit, the water "sheeted" on the foil
rather than beading, thus didn't effect the surface dimensions materially.
The problem with this study is that the wing wasn't flying through
water. The sheeting of the water made the wing more fair for the airflow that it actually
needed, increasing lift.
I have a friend who is a aviation instructor and he and I were
talking about this. The boundary layer in air is so small that it's almost a non-entity,
but he agreed that mirror smooth is best.
As to an increase in drag for a smoother foil (don't know who
mentioned this), I'm afraid that would contradict everything they told me in physics
class. Smoother = less drag, especially in water.
As to how this test was done, read what I posted, it was rudder
foils, and Frank Bethwaite doing the towing.
As to real world validity, I think this test has a very high level
of external validity. EVEN if the foils in question were super clean and sailing through
distilled water in a tank, because the foil is submerged, and not subject to waves
(something we can't duplicate in a tank), the difference between glass and 1200 would
still apply, and any claims of 'it's not real world' are just smoke.
reese
with reference to:
As to an increase in drag for a smoother foil...
My understanding is that there are basically two forms of drag:
`surface' drag and 'form' drag. Surface drag is due to the amount of energy lost in
satisfying the 'no-slip' condition at the surface of the body immersed in the flow.
Surface drag is affected by surface roughness and the characteristics of the boundary flow
(laminar/turbulent).
Form drag, on the otherhand, relates to the amount of energy it
takes to bend the bulk of the flow around the body.
For non-faired bodies like golf balls, form drag is the dominant
component.
These bodies see a dip in the overall drag as the boundary layer trips into turbulence. So
given a specific velocity range, they are designed to be operating in this dip. Surface
roughness can be manipulated to achieve this; I think this is why golf balls have surface
dimples.
For faired bodies like foils, surface drag is the dominant
contributer to drag. Laminar flow for these bodies is more efficient. (Also, since they
are often used as control surfaces, laminar flow lends stability) Here smoothness prolongs
laminar behaviour and keeps the flow attached to the surface of the body.
Reese
Mike Doell
One of the engineers in our firm keeps telling me to put 40 grit
sand paper on the leading edge of my foils. He has an old text book with a comparison test
using a bowling ball. The ball is dropped into a tank of water twice, once with the sand
paper once without. The one with the paper seems to have better flow. Has anyone heard of,
or tried this? Not the with the bowling ball but with their foils :)
Mike Doell
Industrial Designer
Bloorview MacMillan
Graham Alexander
Just a quick comment.
Bethwaite's book beats everything else hands down. He has the
interest, technical knowledge, practical experience, and has done a lot of good testing.
Be sure and read it before commenting. He hits some old wives tales pretty hard, like
smoothness past a certain roughness doesn't matter. But he has done the the experimental
work, and backs it up with good theretical explainations as to why former assumptions may
be incorrect. He and his sons and daughter(s) have definitely backed it up on the water.
Graham Alexander
[email protected]
Columbus, Ohio
Sail fast but sail smart.
US7685 and US4593
Nigel Lott
Mike Doell wrote: One of the engineers in our firm keeps telling
me to put 40 grit sand paper on the leading edge of my foils. He has an old text book with
a comparison test using a bowling ball. The ball is dropped into a tank of water twice,
once with the sand paper once without. The one with the paper seems to have better flow.
Has anyone heard of, or tried this? Not the with the bowling ball but with their foils :)
Mike
No I haven't tried it, and I wouldn't recommend it. Putting 40 grit
on the front of a bowling ball is classic text book stuff for tripping the boundary layer
from laminar to turbulent, and therefore increasing the attachment length of the boundary
layer (before gross separation) on a bluff body. See earlier e-mail by Reese.
It may work if your foil is really badly shaped, for instance with a
HUGE leading edge radius, maximum thickness right at the leading edge.
Either the engineer is pulling your leg, or needs to brush up on his
basic fluid mechanics.
Nigel
AUS 6483.
Nigel Lott
Marcus Ward wrote:
sanded to a 220 grit, the water "sheeted" on the foil
rather than beading, thus didn't effect the surface dimensions materially. The problem
with this study is that the wing wasn't flying through water. The sheeting of the water
made the wing more fair for the airflow that it actually needed, increasing lift.
I have a friend who is a aviation instructor and he and I were
talking about this. The boundary layer in air is so small that it's almost a non-entity,
but he agreed that mirror smooth is best.
As to an increase in drag for a smoother foil (Don't know who
mentioned this), I'm afraid that would contradict everything they told me in physics
class. Smoother = less drag, esp in water.
As to how this test was done, read what I posted, it was rudder
foils, and Frank Bethwaite doing the towing.
As to real world validity, I think this test has a very high level
of external validity. EVEN if the foils in question were super clean and sailing through
distilled water in a tank, because the foil is submerged, and not subject to waves
(something we can't duplicate in a tank), the difference between glass and 1200 would
still apply, and any claims of 'it's not real world' are just smoke.
So you have polished your foils, but were they the correct planform
and thickness distribution to start with............?
Nigel
AUS 6483.
Tom Price
One of the engineers in our firm keeps telling me to put 40 grit
sand paper on the leading edge of my foils. He has an old text book with a comparison test
using a bowling ball. The ball is dropped into a tank of water twice, once with the sand
paper once without. The one with the paper seems to have better flow. Has anyone heard of,
or tried this? Not the with the bowling ball but with their foils :)
Hmmm, but how do we get these bowling balls in the centerboard case?
Our model shop makes the towing tank models and wind tunnel models
for testing in our labs (380' x 22'x 20 deep - hmmm, maybe a 505 would fit - - let's call
it a Air Water Interface, Differential Pressure Propelled Surface Combatant.)
Ed. Well they certainly are good at intimidating anyone not
familar with them. Maybe congress needs to include building a bunch of these things in the
next defence budget.
Anyway, the Hydro guys have determined that anything over 400 grit
is just showing off, and not within their realm of making a measurable difference. Plates
with different surface finishes were towed at some point in time and the 400 grit was
decided on as the standard finish for models. We have done some work in skin friction
reduction techniques, but most additives just leave an unsightly ring around the tank,
without much drag reduction after the model has been run a bit. The most promising scheme
(done on a rowing shell) was to build in a belt of porous stainless steel at the
stagnation point and blow out bubbles. These bubbles were microscopic and made the water
look like milk. Don't know how the stainless steel "foam" was made, but it was
pretty neat stuff. A measurable decrease in drag was consistantly measured. (Leave your
bailers open?) The big problem is the pressure it took to blow through the porous steel.
Sorry, no practical application for 505's unless you can hook up to dozens of scuba tanks.
Our 505's pitch so much, I'd be surprised if there was much degree
of actual laminar flow on the foils. Under test conditions, it's hard to achieve. The
smallest irregularity spoils it. Most concerns are to keep it consistant, as opposed to
randomly separating and attaching. The neat trick is to get it to re-attach. Lots of, turn
lead into gold schemes to try an accomplish that.
Bottom line is that rather than spending much time polishing to an
impressive finish, be sure the shape is truly fair, the leading edge is really the right
shape, and the foil is symmetrical.
Regards,
Tom Price
8351
Bransford Eck
There seems to be some confusion between "laminar flow"
and a laminar boundary layer. I doubt that there is laminar flow over any 505 foil. When
Prandtl did his laminar flow work, they had to shut off the elevators in the building
because the vibration triped the bulk flow turbulent. What stabilizes the boundary layer
and causes it to remain laminar is a positive velocity gradient. As soon as you hit the
max thickness point, the boundary layer usually trips. The NACA 6-series gets a deep
low-drag bucket at very small angles of attack (alpha). These foils try to keep the
boundary layer laminar by moving the max thickness aft to prolong the positive velocity
gradient, hence the surface area over which the boundary layer remains laminar. This
produces a low drag bucket in the alpha vs drag coefficient curve. Unfortunately, this
bucket is very narrow and these foils are very unforgiving. The Davis air foil cost a lot
of people their lives. As do some current turboprop foils which stall easily when ice
trips the laminar boundary layer before the designed separation point. If you try to keep
the boundary layer laminar while changing speeds (gusts) and going over waves (yaw to the
foil) you will get high drag fast. At low Reynolds Number, under non-steady state
conditions (505 real conditions) at typical windward speeds, I propose that it is better
to move the max thickness forward and perhaps to trip the boundary layer turbulent with a
double row of 3D trips (say triangles several mm thick -- pointy end forward) just prior
to the max thickness point. This is a variation on the bowling ball riff. What you don't
want is stall. With typical 505 foil areas and righting moments, the foil alpha will be
about 5 degrees at windward speeds. This is way beyond the low drag bucket of laminar
foils so it seems reasonable to design a foil which gives you a bucket that is wider but
not as deep as a laminar foil will have. It is better to get 90% 90% of the time than to
get 100% 20% of the time amd 20% 80% of the time.
Also, to quote a learned doctor "Upside down is slow". You
have to be way good to use any of the advantages of any of this, so it's better for most
of us to have forgiving foils with a lot of low-drag vs angle-of-attack space. This will
give us good acceleration out of the tacks. If this general approach is correct, only the
leading edge and the surface up to the max thickness point make much difference. The
leading edge is especially important because disturbances there propagate in 3D. As the
speed goes up and the boundary layer thins, there may be some value in a smoother surface,
but not at 505 beating speeds. If it wets, as someone said, it's good enough.
When saying things about air and water foils remember that Reynolds
number is dimentionless. That is its power. You can't say "yes, but that was for
air" if you don't know the Reynolds Number under which the tests were conducted.
Guess I'll have to read magical book. Met the author at the 1978
Worlds. He did a lot better than I, so it's probably not a good idea to speak without know
basis of claims. Will visit posting (Marcus) and crawl back under my rock for another 10
years.
Bransford Eck, US 7067
Nigel Lott
Can somebody tell me how Bethwaite tested his foil finishes? I've
just dug out an old article as follows: Dinghy Hull Design. Bethwaite, F. Australian
Sailing Aug 1987. In this article he describes towing hulls (yes, real ones) through
different water conditions and at different trims. But oops, he doesn't say anything about
leeway angle.....
Some time ago a sailing Aerodynamicist Neil Pollock had some short
well written articles published in Australian Sailing. I recommend them. They are as
follows:
Boundary Layer Bafflement. Pollock N. Australian Sailing Jan
1987. In this article he describes the usual, but goes on to discuss surface waviness:
"A highly polished wavy or ripply surface will cause transition just as effectively
as a good growth of barnacles." Indications of what is 'bad' waviness is given.
Section Shapes for Foils. Pollock N. Australian Sailing Feb 1988.
Sail Section Shapes. Pollock N Australian Sailing Feb 1990.
There is also one on planform shapes of foils, but I've lost it. Can
anybody help out?
Nigel
AUS 6483
Nigel Lott
Marcus Ward wrote:
As to an increase in drag for a smoother foil (Don't know how
mentioned this), I'm afraid that would contradict everything they told me in physics
class. Smoother = less drag, esp in water. (Nigels emphasis)
Nope....? What about the riblets that modify the lower layers of a
turbulent boundary layer to make it less turbulent (reduce the mixing length I think I
should say)? Obviously no good for boats, and I think also worked out to be no good for
aeroplanes. But a contradiction to simple principles no doubt.
Nigel.
Tom Price
Heyyyy....., I had one of those drag buckets at St Pete. It was both
deep and wide.
TP
Scott MacKay
Actually, I think one needs to define what "no good for
boats" means.
As I recall, 3M used to (still does?) make a riblet film that, when
applied to hulls, increased boat speed. I understood that the film had a triangular cross
section and was applied lengthwise along the hull. I believe this was demonstrated on the
AC boats, then promptly outlawed. I understood tht this was one of the
&Quot;tricks" Bransford was talking about to attach flow along the length of the
surface, but I could have it backward and it was used to trip the flow early.
This film may not be applicable to foils though.
Any insight Bransford?
Scott Mackay
US7606
Mike Doell
I guess the sand paper idea is sub-optimal... What about the Buddy
Melges (I think) trick of caking dishwashing detergent onto his hulls several days before
a regatta? Would this work on foils?
Mike Doell
Industrial Designer
Bloorview Macmillan
Mike Doell
Thomas C Price wrote:
be sure the shape is truly fair, the leading edge is really the
right shape, and the foil is symmetrical.
Regards, Tom Price 8351
Tom, all,
I have what I think is "the right shape" as far as cross
sections are concerned, but I have not found any information on side profiles. Do you know
of any sources?
Thanks,
Mike
Peter Epstein
Heyyyy....., I had one of those drag buckets at St Pete. It was
both deep and wide.
TP
Tom,
Malcolm and I also experimented with creating a different hull shape
on Sunday at St. Pete's. We used a small 1.5" hole on our Starboard side to disrupt
the flow when we roll tacked. We tried to see if that made us faster on one tack than the
other. Unfortunately we forgot about the weight of water in the bow tank effect.
Regards,
Peter Epstein
CAN 7823
Tom Price
On Thu, 15 Jan 1998, Jonathan Phillips wrote:
Tom, until I read your message, I was going to tell the server list
that smoothness was merely a social construct created by smoothness advocates to make
rough surfaces appear inferior. In essence, rough surfaces have been oppressed for years.
This a sort of a false blending of literary and feminist theory all rolled into one. It
makes about as much sense as a few things written in the past day or so.
I was also going to mention that sailing is far too important to
leave to the engineers (which is certainly correct) but you have restored my faith in the
technical profession.
Cheers, Jonathan
So you mean I did Ali a favor by borrowing his board and jamming in
my trunk, where it got stuck and I scratched the hell out of it while pulling it out? Oh
good - I'm sure he will appreciate that.
By the way, about feminist theory. I always assumed that centerboard
boats were male, what with our obviously obsessive behavior in our concern for their
performance and the apparently universal practice of frequent polishing of the appendage.
But no.... I read that The local Chesapeake Bay watermen knew their local craft called
Pungy's, as she and he Pungy's. The he Pungys being keel ones and the centerboard ones,
having the cavity for the centerboard to be housed in, being the she variety.
Freud was right. Everything is about Sex.
Regards, Tom Price 8351
Tom Price
On Thu, 15 Jan 1998, Mike Doell wrote:
I guess the sand paper idea is sub-optimal... What about the Buddy
Melges (I think) trick of caking dishwashing detergent onto his hulls several days before
a regatta? Would this work on foils?
Mike Doell
You'd be super fast on the way to the start.
Overheard at local seafood resturants "Honey, my fish tastes like soap"
John Fracisco
The 3M built riblets, that were applied to 12 Meter class yachts,
were designed for the fluid phenomenon encountered at the typical maximum upwind speed of
a 12 Meter yacht. I don't know if the riblets were designed to produce counteracting
vortices that would "wipe out" the vortices present in the mean flow (and hence
reduce drag), or to try to keep the boundary layer attached longer and produce laminar
flow on the hull (also to reduce drag).
Why triangular riblets? In fluids, different phenomena and fluid
structures (i.e. streamwise vortices, crosswise vortices, boundary layers, etc.) have a
characteristic length scale (a.k.a. self-similarity). For the typical max. upwind speed of
a 12 meter, given that you already know the fluid (duh and the large length scale (the
hull or chord of the keel or something along those lines), one can determine the
characteristic length of the fluid phenomena. The riblets are somehow scaled to reduce
drag. (Hey, it's early, I'm on my first cup of coffee, and have been away from my fluid
mechanics for about a year. I was working on reducing drag by producing counteracting
vortices in a boundary layer with a micron-size flap for awhile.) Heck, they may have just
copied an organic phenomena (i.e. shark skin or fish scales or dolphin skin or etc...) and
scaled it to a 12 meter.
A 12 meter does not plane, and I think that once it's trucking along
it's speed is going to be pretty consistent (I didn't say anything about tacking duels).
The riblets aren't going to work near the waterline (that pesky air/water interface), and
aren't going to help on a planing boat where mean hull speed can vary a lot. They also are
going to make boat maintenance a lot more intense (a lot more surface area for organic
growth).
I need to spend more time sailing, and less time working on my boat.
John Fracisco
no 5o5 yet
Laser 162136
Nigel Lott
Marcus Ward wrote:
As to an increase in drag for a smoother foil (Don't know how
mentioned this), I'm afraid that would contradict everything they told me in physics
class. Smoother = less drag, esp in water
Nope....? What about the riblets that modify the lower layers of a
turbulent boundary layer to make it less turbulent (reduce the mixing length I think I
should say)? Obviously no good for boats, and I think also worked out to be no good for
aeroplanes. But a contradiction to simple principles no doubt.
Nigel.
John Fracisco
I guess the sand paper idea is sub-optimal... What about the
Buddy Melges (I think) trick of caking dishwashing detergent onto his hulls several days
before a regatta? Would this work on foils? Mike
I think the dishwasher soap is used to prevent growth, oils, and
whatever yuck is in the water from sticking to the hull. We tend to apply soap along the
waterline (kinda heavy) and all over the bottom (quite a bit lighter) on the J/24 before a
weekend series. I sail in Marina del Rey, and the marina just holds onto all of that
2-stroke oil, styrofoam, etc. This past summer, with the extended warm water temperatures
that we had in Southern California, we would get marine growth starting to form on the
hull after being in the water for less than 48 hours (the bottom is epoxy primer with an
Imron overcoat that has a Teflon polish applied every few months and soap before
splashing).
John Fracisco
Rick Leir
thickness forward and perhaps to trip the boundary layer
turbulent with a double row of 3D trips (say triangles several mm thick -- pointy end
forward) just prior to the max thickness point.
The olympic swimmers just discovered that they could attach these
chevrons to their butt and swim faster. They don't have the option of fairing their body
to dolphin shape, at least not until we learn more about genetics.
cheers -- Rick
Ali Meller
grinning, ducking and running
To all you guys trying to determine the optimum centerboard surface,
section, area, planform, etc..... having no advanced physics, hydrodynamics or fluid flow
background, and no current experience in the test tank, I cannot comment intelligently....
but I almost certainly practice in the 505 more than you do..... see you on the
water...;-)
Ali
P.S. In the unlikely event that someone develops a
"breakthrough" centerboard, we'll all just copy it....
P.P.S. I use standard Waterat gybing CBs, wet sanding to 600 or 800
before the Worlds... NACA 00 family, approximately 2.6 degree gybe angle.
Marcus Ward
As to Ali's latest comment, he's right. Practicing in the boat is
much more important than foil finish. The only reason I asked originally was that I'm
refinishing mine and don't want to do it halfway while I have time (because of weather) to
do a really premium job.
Now if we could clone Ali, put one in a boat sanded with 800, and
one in a boat polished and have them race......
M
Allan Johnson
Having spent a fair bit of time in the boat with ali, Trust ME,
Cloning him is a very bad idea!
To use one of his phrases, "Grinning, ducking and
running".
- -allan
Tom Price
Tom, all,
I have what I think is "the right shape" as far as cross
sections are concerned, but I have not found any information on side profiles. Do you know
of any sources?
Though not as pragmatic as Ali, I don't think you could go far wrong
in copying the Waterat planform.
The planform relates directly to the section shape. If you do
anything but a straight taper you are making the shaping very difficult indeed and the
theoretical advantages of elliptical lift distribution don't necessarily translate into an
elliptical shape. Keep in mind that your greatest benefits will be derived from accurately
producing the foil section you have determined is best. Making an eliptical foil to any
degree of real accuracy without CNC cutting is a challenge and the performance benefits
are sure to be lost in the noise of inaccurate shaping.
I'd certainly be interested in anyones thoughts on building foils.
By the way, there's been little work written on low speed foils, but
a highly recommended source to wade through is Hoerners, "Fluid Dynamic Lift",
and "Fluid Dynamic Drag". Neat stuff on Airships and planes with swasticas on
them.
Have fun, Tom Price 8351
Chad Price
To all you guys trying to determine the optimum centerboard
surface, section, area, planform, etc..... having no advanced physics, hydrodynamics or
fluid flow background, and no current experience in the test tank, I cannot comment
intelligently.... but I almost certainly practice in the 505 more than you do..... see you
on the water...;-)
motion seconded.
Although, as all of the colorado folks who beat us last fall know,
I'm no ace 505 sailor, I've raced other sailing vessels, and my observations seem to point
at the driver/crew as the primary factor, not the boat; and particularly so in 1-design
(or something approaching it) classes. I can only see the foil finish (or boat bottom, or
whatever) as being critical if there are 2 or more extemely evenly matched skipper and
crew combinations.
However, please continue the debate (debait?). I do find it
interesting.
Chad Price
US 5169
Marcus Ward
Though not as pragmatic as Ali, I don't think you could go far
wrong in copying the Waterat planform.
Does anyone know what method Waterat uses to make their foils?
M
Andy Williams
Can you imagine the amount of email two Ali Mellers would generate
??
Nigel Lott
What with all that polish and soap, it must make it harder for
someone else to keep the bottom of their boat clean! I hope you guys/gals use a low
phosphorus soap........
Nigel
AUS 6483.
Bryan Largay
Soap is a pollutant which promotes algal growth under many
environmental conditions.
Nigel Lott
What does a Water Rat planform look like. Can some one list the
planform as offsets from a straight spanwise datum for me? Even just an area would be
nice.
At the Nationals (Pre-worlds) in Townsville in 1996, I noticed that
several European boats had centre boards that were relieved at the root.
I have a logical explaination for why this would be done for a keel
boat, but not a dinghy that should be sailed flat. Any ideas? The only thing I can guess
at is that perhaps a horseshoe vortex would exist at the cb/hull connection, and that
relieving the span of the board might make the vortex smaller, but the end plate effect
must be lost to some extent and hence the aspect ratio reduced. So is it just for looks?
Is there a Fluids person out there that can fill me in.
With respect to planforms, I have come to the conclusion that one of
the faults of the 505 is that the CB case does not allow selection of an optimum planform
while at the same time maintaining a typical span of say about 1200mm.
I agree completely with Tomas Price: Shape boards with straight
leading and trailing edges. If you want something closer to the classic elliptical
planform (I'm not going to enter into discussions of wether it is worth it or not, there
is plenty written about that), shape it as a double taper.
I have only a lousy poor thickness distribution, poor planform
British gybing board that does not gybe. After two years in between my job and small
children my new board is nearly ready to sheath. Its a bit different, if it works you can
copy it! It will not make up for the skipper and crew of course.
Nigel
AUS 6483.
Nigel Lott
The 3M built riblets, that were applied to 12 Meter class yachts,
were .....
Can you tell me where to find a summary of riblets and how
counteracting vortices are made and hence influence the flow?
Nigel.
AUS6483
Nigel Lott
If anybody is interested, I have the following:
A bit of LISP and C source that I wrote, that will create a DXF file
of line segments that approximate a NACA 4 digit uncambered section. The length of the
line segments is based on a local error approximation using the curvature of the section.
In a CAD program, an outline of a template can then be added (picking up the 0,0 position
of the LE can be a bit tricky), the proposed cutting tool radius offset applied, and the
resulting DXF file of the outline trotted down to your local engraver who has a CNC
machine. The type of engraver that you need to speak to is the type that manufactures
control panel faces etc. Watch out for the LE radius versus tool radius!
They should be able to read the DXF file. With a bit of emphasis on
the correct tooling, they can cut the template in say 2mm acrylic. The result is not as
good as off a CNC mill, but better than shaping from a paper plot. Saves a lot of hassle.
The set of three templates that I am very slowly shaping a board to
cost me AUD65. Aluminium costs more.
Similar version to spit out basic G codes for CNC machine tools also
available, and another real coding mess still in Pascal (YUK) that in AutoCAD will put a
Uniform Rational B-Spline actually 'through the dots' instead of using them as the
'control points'. (Can use for other than NACA 4 digit sections) Perhaps AutoCAD will do
this now? Wonders never cease? I haven't investigated.
I wrote this stuff for R9? AutoCAD. I last set this up on R12 DOS
AutoCAD, and it still worked.
The code is a mess, and I have no time to clean it up. It is short.
I'm a mechanical engineer, so no correspondence from puritan programmer/analysts entered
into. I have enough trouble from structural engineers in my life! Couldn't resist that.
The above may be double Dutch to some, but those that understand may
be able to impliment and assist those that are local to their area.
Nigel.
AUS 6483.