Rudder profile and cross section suggestions
Bruce
I'm looking for feedback on rudder design. Specifically what aspect ratio
and cross sections make good rudders. How much balance should be in the
rudder? This is for a 25 foot centerboard sloop.
Thanks
Bruce
matt colie
You have asked what sounds like a simple question, but I could (and
have)write about three pages of answer to very similar.
This must not be a raced class or you would pretty much have to have the
original design, so in simple terms...
higher aspect ratio makes a more effective rudder, but too high becomes
too fragile...
balance may be nice, but much at all will reduce the usable load
feedback at the tiller...
the section almost universally used for rudders is in the NACA 0012-0015
area...
My advice from here:
Do not bother with balance unless it is easier to build that way - do
not go over 20% or it may be unstable.
Plan a 3-4 aspect ratio, but limit the rudder draft so the rudder does
not stike first in shallow water (If it is not a keel/centerboard boat
consider building it as a kickup).
If I can be more help, I'm here (or remove the .nospam).
Matt Colie A.Sloop "Bonne Ide'e" S2-7.9#1
Lifelong Waterman, Licensed Mariner and Perpetual Sailor
(Also a Marine Engineer and Naval Architect)
mike worrall
Specifically what aspect ratio and cross sections make good rudders...
Bruce:
See: http://www.boat-links.com/foils.html
Mike Worrall
Los Angeles
Bruce
Thanks for the info. The rudder is for a 1978 Venture 25. The replacement
rudder currently on the boat has a horrible shape and does not function very
well. All I know of the original is the a crude line drawing showing the
profile. I've had to guess at the correct dimensions and I have no idea
what the cross section of the original is. My research had pointed me to a
NACA 0009, 0012, or 0015. That's where it gets confusing, is there much
difference in performance between the 3 shapes? I'm not a racer but I do
like to go fast.
Bruce
"matt colie" <mco...@provide.nospam.net> wrote in message
news:3D8A066...@provide.nospam.net...
RJ Cook
stalling. Thinner sections have some bit less drag, but will provide less
lift. Bottom line is if the rudder is small and trim loads get high the
thinner section may not provide enough lift. I would tend to favor the 0012
or 0015 and reduce wetted area (make the rudder smaller) if made from wood,
as it would make for a stronger rudder, but with glass or metal it doesn't
matter as much. One advantage of the thinner sections is the tiller is more
responsive, because the angle you turn the rudder is smaller to steer/trim
the boat.
RJ
"Bruce" <jae...@cfl.rr.com> wrote in message
news:nCOj9.24743$R8.9...@twister.tampabay.rr.com...
matt colie
You will never notice the difference in boat speed between 0009 and
0015. The thinner section will stall and asperate (it's not going to
cavitate at your speed) sonner than the thicker.
Go with 0015....
I am laying out templates for a J-24 on my plotter as I write. Their
rudder design is 0013, and you can not get a lot crazier than that.
Matt
Bruce
that backs it up called AERO-HYDRODYNAMICS OF SAILING by C.A. Marchaj,
International Marine, heavy technical reading but it has all the nitty
gritty details you could want.
The next question would be how to make a mould to effectively reproduce the
0015 cross section.
Bruce
"matt colie" <mco...@provide.nospam.net> wrote in message
news:3D94F83A...@provide.nospam.net...
matt colie
If you are only making one, don't bother with a mold.
First - Decide how you want to anchor the shank. A way that works is to
weld two or three 6-8"*1/2 the shank.
Do some flat layup (24-36oz) and cut that to the sections at the pins
plus one each for the top and bottom. You will have to cut all of these
but the bottom to fit the shank. Then glass the two for the pin
sections to the pins (this will be the anchor-make it good.
Make a form out of anything handy (corragate works, my favorite is light
sheet metal) if you might want the form material back, remember the release.
Now fill it will foam and stand back. Hack, carve, file and sand the
foam to the edges of the glass sections. When it looks pretty, cover it
with glass and epoxy. If the last layer is a fine weave, you can
barrier coat over that and then anti-foul.
If you need more take out the nospam.
Fair Wind and Smooth Sea
Matt Colie
Bill Kreamer
similar to the 0015 family, but for lifting in one direction only? I can
get sections for aircraft, but are tweaks needed for H20?
Bill Kreamer
"matt colie" <mco...@provide.nospam.net> wrote in message
news:3D95D438...@provide.nospam.net...
Bob Walters
Others are correct when they say you won't be able to tell the difference
between a 0009 and a 0015 section.
However, unlike many, I would not try to reproduce the fabrication method
used at the factory or get fancy with making a foam cored rudder.
I don't know what the rudder attach is like on the Venture, but I'd be sure
that the shank is firmly attached to some sort of metal web, most likely
some bar stock welded to the shank. You don't want the shank turning inside
the rudder on some dark and stormy night.
If you liked your previous rudder, I'd make it the same aspect ratio and use
the same amount of balance. You should consider PHRF and class rules if you
intend to change the planform. I'd make up a marine plywood center part with
the plan view you decide to use.
This piece should be routed out to accept the metal web and cut out to
accept the shank. Then laminate (with epoxy) some mahogany lumber on each
side at 15% thickness and shape this to the proper profile. I like to use a
razor sharp low angle block plane here. If you're careful with the
laminations, the glue lines will provide good contour guides for your
eyeball.
Draw out the profile you want to shoot for so you know what it looks like.
Study it some more so you really know what it looks like. Perhaps you might
want to make a female template to check your work, but don't be too
concerned about making an EXACT copy of a NACA 0015 section. Smoothness and
fairness is MUCH more important than trying to reproduce a NACA four digit
section.
Remember, these simple sections were intended for early aerodynamic studies
and they were developed a long time ago. They're defined by very simple
mathematics to make shapes that looked something like good airfoils of the
day. By altering the numbers in a simple way one could produce cambered
airfoils of reasonable shape and it is important to note that by following
simple rules anyone in the world could reproduce these shapes and study
them. Don't forget, this was all in the days before computer modeling. The
idea was to come up with something simple to study. NACA was the first
group to conduct a large scientific study of airfoils and their lift/drag
properties.
So, then NACA studied them and determined the characteristics of each.
Later on, when they'd learned something, they developed and studied the 6
digit series, which are much more sophisticated and could be carefully
tailored to a particular need, i.e. low drag at low angles of attack, extra
high lift at high angles of attack, etc.
These days, airfoils are designed by powerful computers and certainly can't
be described by four numbers.
Anyway, the 0015 shape is good, but your best eyeball guess is likely to be
just as good for your application, once you know pretty much what the 0015
looks like, that is.
So try to carve something that looks much like a 0015 section and then spend
some time making it smooth and fair without any lumps or bumps.
After you're happy with the shape, put a strip of masking tape about 2
inches back from the leading edge on one side. Glass the other side wrapping
around the leading edge and a bit onto the tape. At the "cheddar cheese"
stage, knife trim along the tape line and trailing edge. Use a Surform tool
to knock down and taper the cloth at the leading edge a while later, but
before full cure.
Let it cure hard, clean off the blush using hot water and ammonia or
detergent. Scratch sand the leading edge. Then remove some wood at the
trailing edge and replace it with glass strands and epoxy mixed with flocked
cotton or chopped fiberglass. This will strengthen the thin trailing edge.
Use a Surform tool to fair this trailing edge reinforcement at the "cheddar
cheese" stage of cure and then glass the other side, wrapping the leading
edge so as to overlap the glass on the first side to form a double leading
edge.
Fill, fair, and paint and you're good to go, with the emphasis on making it
fair.
IMHO this method will produce an inexpensive, strong, easy to build rudder
which will also perform well. It might not be quite as light as a complex
composite rudder, but it is likely to be lighter and stronger than a typical
factory rudder.
Bob
Stephen Baker
> Smoothness and
>fairness is MUCH more important than trying to reproduce a NACA four digit
>section.
I'll second that. you could spend a fortune (in time or money) getting a
"perfect" aerofoil that had high lift, low drag, high stall angle, etc, etc,
characteristics, and your boat still won't travel without pitching and screwing
up the flow. If the flow is not perfect over the foil, all bets are off.
Fairness and smoothness are much more important, not to mention cheaper to
achieve ;-))
Steve
matt colie
A Whole lot of what you laid out is good, but plywood inside a glass
shell is a real bad idea. It sounds good, but there is no practical way
to make the assembled rudder completely waterproof. They will always
loose the integrity to the rudder shank (usually sooner rather than
later). Plywood has continues fiber in line, so water can run right
into the grain. Where there are internal voids, the water will cross to
the next layer and it will not come out.
Rest assured that your remaks about having the shank solid are very good
and should not be underestimated.
Matt Colie A.Sloop "Bonne Ide'e" S2-7.9 #1
Lifelong Waterman, Licensed Mariner and Perpetual Sailor
Meindert Sprang
> Bob,
> A Whole lot of what you laid out is good, but plywood inside a glass
> shell is a real bad idea. It sounds good, but there is no practical way
> to make the assembled rudder completely waterproof. They will always
> loose the integrity to the rudder shank (usually sooner rather than
> later). Plywood has continues fiber in line, so water can run right
> into the grain. Where there are internal voids, the water will cross to
> the next layer and it will not come out.
> Rest assured that your remaks about having the shank solid are very good
> and should not be underestimated.
> Matt Colie A.Sloop "Bonne Ide'e" S2-7.9 #1
> Lifelong Waterman, Licensed Mariner and Perpetual Sailor
A company here in the Netherlands manufactures plywood rudders sheated with
glass/epoxy, as a replacement for worn-out once-foam-filled polyester
rudders. They even produce stabilizer fins of about 3 meter for large
vessels. So it does not seem such a bad idea though....
Meindert
Bob Walters
> A Whole lot of what you laid out is good, but plywood inside a glass
> shell is a real bad idea. It sounds good, but there is no practical way
> to make the assembled rudder completely waterproof. <<
Well..........I'll have to disagree. I've been building rudders like this
for over 25 years and I've never had a failure. Others have had the same
success.
Of course, good workmanship is important, and good materials are required as
well. For example, water can't be trapped in any voids because there aren't
any.
Sealing the rudder to the shank is not difficult at all and making the
rudder completely waterproof is a piece of cake.
By the way, whenever I "imagine" something "might" be a good idea, but I
haven't tried it myself, I always make that clear in any of my posts.
Otherwise any advice I pass on is something with which I have had personal
experience and usually has stood the test of time.
Bob
matt colie
Up until about ten years ago I would have agreed but since then, I have
had plywood structures that I glassed some time ago - fail. Usually
due to rot. Submerged in sea water lasts longer the fresh water, but
all the parts that I tracked took on water weight. In spite of
significant effort, I could never get the epoxy/glass to stainless or
bronze to last more than four seasons.
Back when plywood was the prime boat building material for manufactures,
I could get good marine ply (no voids in the core). I have not managed
to get any in the last decade.
The non-wood core rudders can have their own problems. I have repaired
several that had a non-stainless anchor plate welded to the stainless
shank. One of these actually made so much scale that it split the
rudder core and skin.
Plywood is really the best material for a great number of applications,
but wherever you use it, be sure it can dry out if(when) water does to
it. I can build a boat that is much nicer and finished in much less
time in marine ply than any plastic (and have been since the late
fifties). It is still the best material for rudders and boards for
small boats particularly if you armor the leading edges with glass.
I have been at this a very long time, and have learned from a lot of
mistakes - some mine - some others. My big coup in the rudder building
was the replacement for a Red Wing 30. That was built to lose the
bottom third easily to limit damage and and with a removable shank so it
could easily be replaced (spare provided) with the boat in the water.
I wish you the best of luck.
Matt Colie A.Sloop "Bonne Ide'e" S2-7.9 #1
Lifelong Waterman, Licensed Mariner and Perpetual Sailor
Bruce
I have access to a 3d router set-up and a CAD program for the fabrication so
I'm thinking of making 2 female molds to get the outer surface. The
following posts brought up the insides which I hadn't given much thought to
yet so thanks for that info.
The rudder is a kick up design mounted on two pintle and guegeons. I don't
like the current rudder which is why I am building a new one. The shape of
the current rudder is terrible, it's essentially a slab with rounded edges.
Also the rudder stock(?) is a bit flimsy and it does a poor job of keeping
the rudder down where it needs to be to do it's job. I have a rough, and
small, line drawing of the original rudder that I am going by. It shows an
aspect ratio of about 3 with a trapazoidal swept back shape and a little bit
of balance.
I may be wrong here but I figure the original rudder was designed for the
boat and should perform well(or at least better than the existing) but I am
open to other shapes if it means better performance.
Thanks,
Bruce
"Bob Walters" <boban...@cox.net> wrote in message
news:4p%l9.58374$IL6.2...@news2.east.cox.net...
Schöön Martin
I hope you don't think any look-like-a-real-section section will work as
long as it is glossy enough.
An old NACA four-digit section is not a very clever choice if you are
concerned by the possibility of loosing steering due to air being sucked
down the low-pressure side of your rudder. There are sections out there
that are designed for the low Reynolds numbers our rudders see, foils
that have smoothed out low-pressure 'spikes' to lower the likelihood of air
suction and to decrease sensitivity to surface pertubations (dents or
whatever) and non-perfect situations in general.
Try to look for Eppler E386 at
http://amber.aae.uiuc.edu/~m-selig/ads/coord_database.html
(I wrote "try" because I had problems with the connection a few
minutes ago.)
--
========================================================================
Martin Schöön <Martin...@ericsson.com>
"Problems worthy of attack
prove their worth by hitting back"
Piet Hein
========================================================================
Stephen Baker
>I hope you don't think any look-like-a-real-section section will work as
>long as it is glossy enough.
No. You start with a "real" section, preferably suited to the application and
Rn, but if you mess up a little in the building it doesn't matter too much as
long as it's fair. Better fair and not-quite-right than right and not smooth
(if you see what I mean)
>Try to look for Eppler E386 at
>http://amber.aae.uiuc.edu/~m-selig/ads/coord_database.html
>
>(I wrote "try" because I had problems with the connection a few
>minutes ago.)
I have that whole database on disk, if you want the beast. E-mail me your
address and I'll cut one for you.
Steve
Stephen C. Baker - Yacht Designer
http://members.aol.com/SailDesign/private/scbweb/home.htm
Schöön Martin
> Martin writes:
>
> >I hope you don't think any look-like-a-real-section section will work as
> >long as it is glossy enough.
>
> No. You start with a "real" section, preferably suited to the application and
> Rn, but if you mess up a little in the building it doesn't matter too much as
> long as it's fair. Better fair and not-quite-right than right and not smooth
> (if you see what I mean)
This is my experience too. I doubt my pesky little brother would agree
though :-) (He is the hydrodynamic specialist in our family.)
>
> I have that whole database on disk, if you want the beast. E-mail me your
> address and I'll cut one for you.
>
Thanks for the offer but I have what I need and some through my brother.
Stephen Baker
>I doubt my pesky little brother would agree
>though :-) (He is the hydrodynamic specialist in our family.)
Only cure for a hydrodynamicist - tow them on the end of a long rope to study
hydrodynamics first hand. They don't take long to shut up ;-))
Steve
Bob Walters
> long as it is glossy enough.
>
"fair", not glossy, which is an entirely different thing. The short answer
is, if it looks pretty much like an airfoil and it's reasonabily fair, it
WILL indeed work and most likely work just fine.
> An old NACA four-digit section is not a very clever choice if you are
> concerned by the possibility of loosing steering due to air being sucked
> down the low-pressure side of your rudder. <<
This is a Venture 25 for goodness sake. It ain't an AC boat and it ain't
"rocket surgery", so a cleaver choice is not required where a reasonable
choice will do.
It's an ordinary sailboat we're talking about here and there is no need to
try to get by with the absolutely smallest rudder that MIGHT keep the boat
under control under most conditions. Highly loaded rudders with exotic
profiles have there place in sailing, but not on a Venture 25. Make it
strong enough, make it big enough, and make it fair and you'll be happy.
OTOH, trying to calculate just how small and highly loaded you can go with a
replacement rudder that MIGHT work may be a fun exersize for some, but I get
the idea that Bruce is interested in going down a different path.
>>There are sections out there
> that are designed for the low Reynolds numbers our rudders see, foils
> that have smoothed out low-pressure 'spikes' to lower the likelihood of
air
> suction and to decrease sensitivity to surface pertubations (dents or
> whatever) and non-perfect situations in general.
>
What you say is true, but I don't think it's good advice for what I think
Bruce has in mind. As you undoubtedly know, the more exotic the profile,
the more important it is to accurately reproduce the desired shape so as to
realize in actual practice the promised performance increase over what you
can expect from a primitive, but less demanding profile.
I find this stuff interesting but by the time Bruce digested the available
information on some Eppler profile, he could be well along with the
construction of a simple NACA 4 digit style rudder. Before long he might
actually be out there sailing his Venture, which I thought was the whole
idea.
Bob
Schöön Martin
> > I hope you don't think any look-like-a-real-section section will work as
> > long as it is glossy enough.
> >
> Actually.............more or less yes. But in my post I used the term
> "fair", not glossy, which is an entirely different thing. The short answer
> is, if it looks pretty much like an airfoil and it's reasonabily fair, it
> WILL indeed work and most likely work just fine.
>
>
> > An old NACA four-digit section is not a very clever choice if you are
> > concerned by the possibility of loosing steering due to air being sucked
> > down the low-pressure side of your rudder. <<
>
> This is a Venture 25 for goodness sake. It ain't an AC boat and it ain't
> "rocket surgery", so a cleaver choice is not required where a reasonable
> choice will do.
OK, dvelve on the following story from real life.
A colleague and friend of mine approached me some years ago on his brother's
behalf. The brother had a 23' sailboat and his rudder had stress cracks
along the leading edge near the lower pintle. Repair and beef up?
After having had a look at it I recommended him to build a new rudder
since the one at hand represented the typical crappy workmanship of
mass produced boats and it was very heavy by my standards. The shape
was fair enough but of unspecified origin - a typical look-like-a-foil
thing.
I just happen to have female moulds about the right size for this boat.
I made these moulds when I built my boat back in 1985-6 and we have used
these mould to make rudder for three cats, one tri, and a couple of monos.
The foil shape is a NACA 63-something and 15% thick. The cord is 30 cm
and the trailing edge is truncated so the actual rudder blade comes out
28 cm wide. This was slightly less than the original rudder but we could
make the new rudder as deep as the old one so it would be only slightly
smaller in area.
The new rudder was laminated using epoxy resin, stitched glass-'weave'
with fibres +-45 degrees to the blade and unidirectional glass. A fir
beam runs down the middle of the blade and the remaing volume was
filled with lightweight pink foam.
The resulting rudder came out at 1/2 weight and 3x stiffness compared
to the old rudder. NB no carbon, just glass, careful engineering and
workmanship.
So what about the difference in shape? Both rudders were about the same
thickness but one had a 'proper' foil shape and other had some kind of
foil shape. Did he notice any difference? You bet he did! This particular
boat has a very wide transom and had a nasty habit of spinning out of control
down-wind when the rudder lost grip which was quite common: Heeling over
the rudder was lifted out of the water and what remained in the water
became highly loaded when the helmsman tried to stay in control.
With the new rudder loosing control became a rare experence.
Hence, a proper foil section is a matter of safety rather than rocket
ship performance.
>
> >>There are sections out there
> > that are designed for the low Reynolds numbers our rudders see, foils
> > that have smoothed out low-pressure 'spikes' to lower the likelihood of
> air
> > suction and to decrease sensitivity to surface pertubations (dents or
> > whatever) and non-perfect situations in general.
> >
>
> What you say is true, but I don't think it's good advice for what I think
> Bruce has in mind. As you undoubtedly know, the more exotic the profile,
> the more important it is to accurately reproduce the desired shape so as to
> realize in actual practice the promised performance increase over what you
> can expect from a primitive, but less demanding profile.
Nope, don't know this to be generally true. In some cases this holds water
but I wouldn't use or recommend such a foil shape.
Cheers,