Re: fairing the rudder
Let me make a simplified point for those who have not be following
the details of the NACA foil sections, etc.
If a rudder (or any foil) is at a high angle of attack, the flow
around the rudder can break down ("stall") so that all you get is
drag, with very little lift, i.e. steering. If you can manage to find
a dock with good current flowing by, you can easily experiment with
an oar or paddle.
In order to be able to turn the rudder as far as possible without
stall, you need a fairly fat rudder blade. The fatter the rudder, the
more gentle the corner the water has to run around to flow smoothly
over the back side. Of course, a fat blade has more drag when not
turned at all, so there is a tradeoff.
Experience has shown that the best degree of "fatness" is about 10-
12% of the length in the direction of flow (the chord). In order for
this to pay off, the leading edge really needs a nice parabola-shaped
nose. The back edge should taper, but the exact profile of the taper
is less important. So, a rudder that is 1 foot front to back should
be about 1 1/4" thick, with the thickness in the proper location as
discussed in the preceeding posts.
All this is true of centerboards too, but centerboards should never
work a high angle of attack, so it is possible to have a thinner
board, thinner centerboard case, etc.
This will matter on some boats more than others. In a sloop, the two
sails can be adjusted to get a neutral helm, i.e. so the rudder has a
low angle of attack. However, a catboat always has to have some lift
from the rudder, and I should think that a good foil would be
critical in a cat. It surprises me that although Bolger has
championed the end plate in cat rudders, he has not stress foil
shapes. I would think they would be equally important.
As noted, these comments apply to rudders without a skeg or keel on
the leading edge.
Peter
the details of the NACA foil sections, etc.
If a rudder (or any foil) is at a high angle of attack, the flow
around the rudder can break down ("stall") so that all you get is
drag, with very little lift, i.e. steering. If you can manage to find
a dock with good current flowing by, you can easily experiment with
an oar or paddle.
In order to be able to turn the rudder as far as possible without
stall, you need a fairly fat rudder blade. The fatter the rudder, the
more gentle the corner the water has to run around to flow smoothly
over the back side. Of course, a fat blade has more drag when not
turned at all, so there is a tradeoff.
Experience has shown that the best degree of "fatness" is about 10-
12% of the length in the direction of flow (the chord). In order for
this to pay off, the leading edge really needs a nice parabola-shaped
nose. The back edge should taper, but the exact profile of the taper
is less important. So, a rudder that is 1 foot front to back should
be about 1 1/4" thick, with the thickness in the proper location as
discussed in the preceeding posts.
All this is true of centerboards too, but centerboards should never
work a high angle of attack, so it is possible to have a thinner
board, thinner centerboard case, etc.
This will matter on some boats more than others. In a sloop, the two
sails can be adjusted to get a neutral helm, i.e. so the rudder has a
low angle of attack. However, a catboat always has to have some lift
from the rudder, and I should think that a good foil would be
critical in a cat. It surprises me that although Bolger has
championed the end plate in cat rudders, he has not stress foil
shapes. I would think they would be equally important.
As noted, these comments apply to rudders without a skeg or keel on
the leading edge.
Peter
>John Welsford's answer about rudder sections:John has it about right. Even if he says NACA foils are not hard to
>
>
>> One of the issues on rudder blades is that it is often
>difficult to build a rudder blade, especially a kickup one of the
>type that most small boats use, with a thick enough section for real
>efficiency. For a general purpose boat rather than a race boat,
>given a choice and without that constraint I use a modified NACA10014
>14% section with a slighly fatter leading edge. This gives a section
>with a very high stall angle that will hang on in slow speed tacking .
>> Otherwise I specify a flat plate section with an elliptical
>leading edge of the largest size that I can get on the thickness of
>the blade and then run a taper over the last 1/3rd to the trailing
>edge and perhaps a 3 or 4mm square back edge which does not reduce
>the effiency much and makes the blade much more robust. Its the
>leading edge radius that seems to determine the stall angle and most
>of the resistance.
>> Not fancy stuff, but it works, its the raceboats that get the
>rocket science.
>>
>> Incidentally, these small amateur build boats are the only ones
>where I use NACA sections, they are very effective and very easy to
>make, they are not sensitive to mistakes in making the shape and are
>thick enough to be very strong.
>> Benedic and JSRA supplied the foils on the last two serious
>perfomance boats that I have done. Both fiddly and sensitive.
make, they are relatively hard to make compared to a rounded/tapered
"fake foil". But they are nice and fat.
Information on all these in my Foils FAQ (Seehttp://boat-links.com/)
from Lo these Many Years Ago.
A flat plate is actually a very good rudder if you don't plan on
using it much (least drag when going straight) but is only useful at
very small deflections (5 degrees perhaps) so ti requires a whole
different steering philosophy.
If you're obsessed with these things try stealing a rudder from a
Hobie or a windsurfer daggerboard (or find a discarded one).
--
Craig O'Donnell
Sinepuxent Ancestors & Boats
<http://freepages.genealogy.rootsweb.com/~fassitt/>
The Proa FAQ <http://boat-links.com/proafaq.html>
The Cheap Pages <http://www2.friend.ly.net/~dadadata/>
Sailing Canoes, Polytarp Sails, Bamboo, Chinese Junks,
American Proas, the Bolger Boat Honor Roll,
Plywood Boats, Bamboo Rafts, &c.
_________________________________
-- Professor of Boatology -- Junkomologist
-- Macintosh kinda guy
Friend of Wanda the Wonder Cat, 1991-1997.
_________________________________
---
[This E-mail scanned for viruses by friend.ly.net.]
John Welsford's answer about rudder sections:
type that most small boats use, with a thick enough section for real
efficiency. For a general purpose boat rather than a race boat,
given a choice and without that constraint I use a modified NACA10014
14% section with a slighly fatter leading edge. This gives a section
with a very high stall angle that will hang on in slow speed tacking .
the blade and then run a taper over the last 1/3rd to the trailing
edge and perhaps a 3 or 4mm square back edge which does not reduce
the effiency much and makes the blade much more robust. Its the
leading edge radius that seems to determine the stall angle and most
of the resistance.
make, they are not sensitive to mistakes in making the shape and are
thick enough to be very strong.
> One of the issues on rudder blades is that it is oftendifficult to build a rudder blade, especially a kickup one of the
type that most small boats use, with a thick enough section for real
efficiency. For a general purpose boat rather than a race boat,
given a choice and without that constraint I use a modified NACA10014
14% section with a slighly fatter leading edge. This gives a section
with a very high stall angle that will hang on in slow speed tacking .
> Otherwise I specify a flat plate section with an ellipticalleading edge of the largest size that I can get on the thickness of
the blade and then run a taper over the last 1/3rd to the trailing
edge and perhaps a 3 or 4mm square back edge which does not reduce
the effiency much and makes the blade much more robust. Its the
leading edge radius that seems to determine the stall angle and most
of the resistance.
> Not fancy stuff, but it works, its the raceboats that get therocket science.
>where I use NACA sections, they are very effective and very easy to
> Incidentally, these small amateur build boats are the only ones
make, they are not sensitive to mistakes in making the shape and are
thick enough to be very strong.
> Benedic and JSRA supplied the foils on the last two seriousperfomance boats that I have done. Both fiddly and sensitive.
>
> John W
Technically correct, Craig, but I'd guess he was referring to foil sections
designed to maximize laminar flow; e.g. the B-24's wing versus more tolerant
sections such as the B-17's which allow turbulant flow to begin further
forward on the chord. Laminar flow sections offer significantly lower drag,
but they require rigorous attention to eliminating roughness on the surface
such as ice and barnacles.
The other "gotcha" in this discussion is that rudders and centerboards are
almost always symetrical, while airfoils usually are not, at least at
moderate speeds.
Roger
derbyrm@...
http://derbyrm.mystarband.net
designed to maximize laminar flow; e.g. the B-24's wing versus more tolerant
sections such as the B-17's which allow turbulant flow to begin further
forward on the chord. Laminar flow sections offer significantly lower drag,
but they require rigorous attention to eliminating roughness on the surface
such as ice and barnacles.
The other "gotcha" in this discussion is that rudders and centerboards are
almost always symetrical, while airfoils usually are not, at least at
moderate speeds.
Roger
derbyrm@...
http://derbyrm.mystarband.net
----- Original Message -----
From: "craig o'donnell" <dadadata@...>
> >Lincoln,
> >Could you elaborate on the meaning of a "laminar section"?
> >Peter
>
> No such thing. I think he means "foil section".
> Craig O'Donnell
>Lincoln,No such thing. I think he means "foil section".
>
>Could you elaborate on the meaning of a "laminar section"?
>
>Peter
--
Craig O'Donnell
Sinepuxent Ancestors & Boats
<http://freepages.genealogy.rootsweb.com/~fassitt/>
The Proa FAQ <http://boat-links.com/proafaq.html>
The Cheap Pages <http://www2.friend.ly.net/~dadadata/>
Sailing Canoes, Polytarp Sails, Bamboo, Chinese Junks,
American Proas, the Bolger Boat Honor Roll,
Plywood Boats, Bamboo Rafts, &c.
_________________________________
-- Professor of Boatology -- Junkomologist
-- Macintosh kinda guy
Friend of Wanda the Wonder Cat, 1991-1997.
_________________________________
---
[This E-mail scanned for viruses by friend.ly.net.]
Lift and drag from the rudders section (as opposed to induced drag) are
two different things, so a reduction in drag is not of itself going to
make your rudder less responsive. In fact, an airfoil section ought to
make your rudder hang on longer before it stalls (as in a broaching
situation or at very low speeds). Airfoil shaped rudders work great on
Hobie Cats.It might be worthwhile, if you care about speed, to do your
homework and pick a real airfoil shape and try to build close to it. May
I suggest NACA 009 or one of that series (i.e. NACA 008, 010 etc.)? Not
the latest whiz bang but should help a lot compared to a flat one. If
you are a really precise builder then it might be worthwhile to really
dig into the subject. Even if you are, don't use a laminar section
unless you want to worship your rudder, stopping to clean it off every
hour or two and storing it in it's own little bag, and wet sanding every
year. Otherwise it won't do any good compared to a less sensitive one,
and maybe it would stall easier. I think a lot of people make this
mistake. In fact, I seem to recall seeing what resembles a laminar
section on a Bolger plan!
I think it might also speed up the boat a bit if you could make the
centerboard, leeboard, daggerboard or whatever in a section like this.
Another way to get more responsive, less draggy steering is to use a
long, skinny rudder instead of a short fat one. However it makes the
stall sharper and takes more draft. Same applies to daggerboards, etc.
two different things, so a reduction in drag is not of itself going to
make your rudder less responsive. In fact, an airfoil section ought to
make your rudder hang on longer before it stalls (as in a broaching
situation or at very low speeds). Airfoil shaped rudders work great on
Hobie Cats.It might be worthwhile, if you care about speed, to do your
homework and pick a real airfoil shape and try to build close to it. May
I suggest NACA 009 or one of that series (i.e. NACA 008, 010 etc.)? Not
the latest whiz bang but should help a lot compared to a flat one. If
you are a really precise builder then it might be worthwhile to really
dig into the subject. Even if you are, don't use a laminar section
unless you want to worship your rudder, stopping to clean it off every
hour or two and storing it in it's own little bag, and wet sanding every
year. Otherwise it won't do any good compared to a less sensitive one,
and maybe it would stall easier. I think a lot of people make this
mistake. In fact, I seem to recall seeing what resembles a laminar
section on a Bolger plan!
I think it might also speed up the boat a bit if you could make the
centerboard, leeboard, daggerboard or whatever in a section like this.
Another way to get more responsive, less draggy steering is to use a
long, skinny rudder instead of a short fat one. However it makes the
stall sharper and takes more draft. Same applies to daggerboards, etc.
>MJeff Comfrey <mike78612003@...>
>wrote:
>does it mater that much to anyone racing or for beter control in following sea when crusing, If you fair out the rudder to be more wing shape hopefully for less drag will you get better steering results or will the boat have less responsive steering due to the decrease in Resistance. along its following edge.
>
Shouldn't it be said that this assumes a free-standing rudder, well
separated from the keel/deadwood/skeg/hull? Otherwise it's closer to
flap/aileron design; i.e., one has to look at the whole surface with a
varying chord line and also consider "slot" effects.
I'm also confused as to what effect the radically different Reynold's
numbers have on "foil" choice. If I remember right; size, speed, and fluid
density are all factors. My copy of "Theory of Wing Sections" seems to
imply that the tests were aimed at WWII aircraft and are not really
applicable to either very slow flight (ultra-lights) or trans- & super-sonic
regimes. (Bumble bees don't fly, they climb thru molasses.)
I think we're well into black magic here, given the effects of turbulance,
rudder lift to windward when sailing with a lee helm, etc.
A well-rounded leading edge won't stall as abruptly. A thin trailing edge
won't drag as much turbulance. Beyond that, ????
Roger
derbyrm@...
http://derbyrm.mystarband.net
separated from the keel/deadwood/skeg/hull? Otherwise it's closer to
flap/aileron design; i.e., one has to look at the whole surface with a
varying chord line and also consider "slot" effects.
I'm also confused as to what effect the radically different Reynold's
numbers have on "foil" choice. If I remember right; size, speed, and fluid
density are all factors. My copy of "Theory of Wing Sections" seems to
imply that the tests were aimed at WWII aircraft and are not really
applicable to either very slow flight (ultra-lights) or trans- & super-sonic
regimes. (Bumble bees don't fly, they climb thru molasses.)
I think we're well into black magic here, given the effects of turbulance,
rudder lift to windward when sailing with a lee helm, etc.
A well-rounded leading edge won't stall as abruptly. A thin trailing edge
won't drag as much turbulance. Beyond that, ????
Roger
derbyrm@...
http://derbyrm.mystarband.net
----- Original Message -----
From: "Lincoln Ross" <lincolnr@...>
> Lift and drag from the rudders section (as opposed to induced drag) are
> two different things, so a reduction in drag is not of itself going to
> make your rudder less responsive. In fact, an airfoil section ought to
> make your rudder hang on longer before it stalls (as in a broaching
> situation or at very low speeds). Airfoil shaped rudders work great on
> Hobie Cats.It might be worthwhile, if you care about speed, to do your
> homework and pick a real airfoil shape and try to build close to it. May
> I suggest NACA 009 or one of that series (i.e. NACA 008, 010 etc.)? Not
> the latest whiz bang but should help a lot compared to a flat one. If
> you are a really precise builder then it might be worthwhile to really
> dig into the subject. Even if you are, don't use a laminar section
> unless you want to worship your rudder, stopping to clean it off every
> hour or two and storing it in it's own little bag, and wet sanding every
> year. Otherwise it won't do any good compared to a less sensitive one,
> and maybe it would stall easier. I think a lot of people make this
> mistake. In fact, I seem to recall seeing what resembles a laminar
> section on a Bolger plan!
>
> I think it might also speed up the boat a bit if you could make the
> centerboard, leeboard, daggerboard or whatever in a section like this.
>
> Another way to get more responsive, less draggy steering is to use a
> long, skinny rudder instead of a short fat one. However it makes the
> stall sharper and takes more draft. Same applies to daggerboards, etc.
>
> >MJeff Comfrey <mike78612003@...>
> >wrote:
> >does it mater that much to anyone racing or for beter control in
following sea when crusing, If you fair out the rudder to be more wing shape
hopefully for less drag will you get better steering results or will the
boat have less responsive steering due to the decrease in Resistance. along
its following edge.
> >
>
>
>
> Bolger rules!!!
> - no cursing, flaming, trolling, spamming, or flogging dead horses
> - stay on topic, stay on thread, punctuate, no 'Ed, thanks, Fred' posts
> - Pls add your comments at the TOP, SIGN your posts, and snip away
> - Plans: Mr. Philip C. Bolger, P.O. Box 1209, Gloucester, MA, 01930, Fax:
(978) 282-1349
> - Unsubscribe:bolger-unsubscribe@yahoogroups.com
> - Open discussion:bolger_coffee_lounge-subscribe@yahoogroups.com
>
> Your use of Yahoo! Groups is subject tohttp://docs.yahoo.com/info/terms/
>
>
>
John Welsford, the KIWI designer has the following comments on
centerboard shape. We can probably get a response from him on whether
he would use the same for rudders:
On foils, I have experimented with all sorts of stuff and come back
to this simple formulae. For a non racer centreboard, the thickness
should be 12% of chord, the leading edge radius should be 1/6th the
thickness, the maximum thickness should be at 40% back from the
leading edge, the trailing edge should be square across 5% of the
maximum thickness. Make a nice fair curve from leading to trailing
edge. This gives a high lift section with a very high stalling angle
which will assist the boat to tack reliably in very light or very
rough conditions when the low drag sections tend to stall and lose
lift. It is also not prone to damaging the leading edge if you hit
anything. It is a thick section, though, and on some boats would not
fit the centrecase and you may have to scale the proportions down to
suit the thickness available. John Welsford
centerboard shape. We can probably get a response from him on whether
he would use the same for rudders:
On foils, I have experimented with all sorts of stuff and come back
to this simple formulae. For a non racer centreboard, the thickness
should be 12% of chord, the leading edge radius should be 1/6th the
thickness, the maximum thickness should be at 40% back from the
leading edge, the trailing edge should be square across 5% of the
maximum thickness. Make a nice fair curve from leading to trailing
edge. This gives a high lift section with a very high stalling angle
which will assist the boat to tack reliably in very light or very
rough conditions when the low drag sections tend to stall and lose
lift. It is also not prone to damaging the leading edge if you hit
anything. It is a thick section, though, and on some boats would not
fit the centrecase and you may have to scale the proportions down to
suit the thickness available. John Welsford
Lincoln,
Could you elaborate on the meaning of a "laminar section"?
Peter
Could you elaborate on the meaning of a "laminar section"?
Peter
> does it mater that much to anyone racing or for beter control infollowing sea when crusing, If you fair out the rudder to be more wing shape
hopefully for less drag will you get better steering results or will the
boat have less responsive steering due to the decrease in Resistance. along
its following edge.
I would shape the rudder to be more wing like. On my Frolic I had a flat
blade made of aluminum and at real slow speeds the rudder would stall out
and I'd have little control of the boat. I could drop the leeboard and be
better off but even then it would stall. I believe the current owner has
added plywood cheeks to the aluminum rudder and shaped it better with good
results, it no long stalls and controls the boat as long as there is water
moving across the rudder.
Jeff
does it mater that much to anyone racing or for beter control in following sea when crusing, If you fair out the rudder to be more wing shape hopefully for less drag will you get better steering results or will the boat have less responsive steering due to the decrease in Resistance. along its following edge.
---------------------------------
Do you Yahoo!?
The New Yahoo! Shopping - with improved product search
[Non-text portions of this message have been removed]
---------------------------------
Do you Yahoo!?
The New Yahoo! Shopping - with improved product search
[Non-text portions of this message have been removed]