Re: Simplified foils
If a flat plate is ok, that's because you can live with the extra
drag. You can almost always come up with an airfoil shape that will
perform considerably better than a flat plat: less drag, more maximum
lift. At high angles of attack these effects will be somewhat masked
if you have a low aspect ratio (as in a short rudder hard over), but
reduced drag will be there the rest of the time.
I suppose that if you have a REALLY high drag boat (like our Brick)
the extra drag of a flat board will be masked, but remember all those
articles where Bolger states that when rowing you can really feel if
there's a board in the water? This says to me that board drag is
significant. Of course if you're under sail you might not really
notice 20% or 30% more drag.
I think the idea of the plate "pushing" on the water versus the foil
lifting is a false distinction. The plate may be big enough to have a
low angle of attack so the greater lift capability of the foil isn't
required, but a properly shaped foil will have less drag. If you have
a hull shape which can make a little leeway without a lot of drag, or
if you can cant the board a degree or so, a properly shaped foil can
probably be a lot smaller and still do a good job.
It's possible that the bent foil shape you had is low drag, but I
think it's very unlikely to be a "NACA" foil, as a certain amount of
precision is required that wouldn't be acheived by bending wood over
one spacer.
Perhaps most designs don't use fancy foils because that would become
the trickiest part of the construction, particularly with the
precision that is required for best results, but I think for anyone
who cares much about performance, here is a way to get it. Another way
is probably to clean up the daggerboard or centerboard slot. Lots of
draggy gaps here, usually. I think if you look at some of the foils on
windsurfers, you will see some good designs. Impressive how much
smaller their boards can be for the same sail area. Of course part of
that is the greater speed of the board.
On trailing edges, the worst thing is to round the corners. Tapering
to a sharp corner is probably best.
Got interested and just did some calculations. Looks like you can save
something like .7lbs drag with a proper foil section instead of a flat
plate. Don't dismiss that, because as you'll see, total drag is
probably not all that much larger. For you tech types, this is with a
2 square foot board (pretty small) at Cl of 0.3, Rn 300k, 4.9fps, Cd
of .023 for the flat plate and .009 for the foil (NACA 009 or several
others will do this). You could also use a smaller board, say 1.3 ft2,
because the max Cl of the flat plate is about .5, but it's .75 for
the foil. Effects on drag will be a bit more complicated, due to
increased induced drag, etc., but you get the idea. Since the side
force with the original flat plate board is only 14 lbs or so, the
thrust from the sail is probably comparable or less, so .7 lbs is
significant. You could point higher or go faster, and you could get
similar gains from the rudder. I tried to make this typical of a very
small boat going upwind, but there is some guesswork on angle of
attack, board size etc. The overall effect is clear. If you were
rowing, it would probably be something like 12% of your effort.
(assuming 1/20hp output of rower, which is a big assumption but
definitely in the ballpark).
If you got in a match race with otherwise evenly matched skippers and
boats, on a day when the wind was steady, I'm sure the effect would be
clear.
-Lincoln Ross, model airplane enthusiast
drag. You can almost always come up with an airfoil shape that will
perform considerably better than a flat plat: less drag, more maximum
lift. At high angles of attack these effects will be somewhat masked
if you have a low aspect ratio (as in a short rudder hard over), but
reduced drag will be there the rest of the time.
I suppose that if you have a REALLY high drag boat (like our Brick)
the extra drag of a flat board will be masked, but remember all those
articles where Bolger states that when rowing you can really feel if
there's a board in the water? This says to me that board drag is
significant. Of course if you're under sail you might not really
notice 20% or 30% more drag.
I think the idea of the plate "pushing" on the water versus the foil
lifting is a false distinction. The plate may be big enough to have a
low angle of attack so the greater lift capability of the foil isn't
required, but a properly shaped foil will have less drag. If you have
a hull shape which can make a little leeway without a lot of drag, or
if you can cant the board a degree or so, a properly shaped foil can
probably be a lot smaller and still do a good job.
It's possible that the bent foil shape you had is low drag, but I
think it's very unlikely to be a "NACA" foil, as a certain amount of
precision is required that wouldn't be acheived by bending wood over
one spacer.
Perhaps most designs don't use fancy foils because that would become
the trickiest part of the construction, particularly with the
precision that is required for best results, but I think for anyone
who cares much about performance, here is a way to get it. Another way
is probably to clean up the daggerboard or centerboard slot. Lots of
draggy gaps here, usually. I think if you look at some of the foils on
windsurfers, you will see some good designs. Impressive how much
smaller their boards can be for the same sail area. Of course part of
that is the greater speed of the board.
On trailing edges, the worst thing is to round the corners. Tapering
to a sharp corner is probably best.
Got interested and just did some calculations. Looks like you can save
something like .7lbs drag with a proper foil section instead of a flat
plate. Don't dismiss that, because as you'll see, total drag is
probably not all that much larger. For you tech types, this is with a
2 square foot board (pretty small) at Cl of 0.3, Rn 300k, 4.9fps, Cd
of .023 for the flat plate and .009 for the foil (NACA 009 or several
others will do this). You could also use a smaller board, say 1.3 ft2,
because the max Cl of the flat plate is about .5, but it's .75 for
the foil. Effects on drag will be a bit more complicated, due to
increased induced drag, etc., but you get the idea. Since the side
force with the original flat plate board is only 14 lbs or so, the
thrust from the sail is probably comparable or less, so .7 lbs is
significant. You could point higher or go faster, and you could get
similar gains from the rudder. I tried to make this typical of a very
small boat going upwind, but there is some guesswork on angle of
attack, board size etc. The overall effect is clear. If you were
rowing, it would probably be something like 12% of your effort.
(assuming 1/20hp output of rower, which is a big assumption but
definitely in the ballpark).
If you got in a match race with otherwise evenly matched skippers and
boats, on a day when the wind was steady, I'm sure the effect would be
clear.
-Lincoln Ross, model airplane enthusiast
--- In bolger@y..., pateson@c... wrote:
> Not a fluid dynamicists here but probably makes sense.
> At the relativly low speed and high drag of most sailboats,
> the added drag of a non foil shaped rudder or DB would
> probably be minimal. I read somewhere that basic flat plate
> DB's are perfectly OK on small sailboats. The lateral
> resistance at the low speeds is provided by the plate, pushing
> against the water, rather than the "lifting" effect of a true foil.
> "Lightnings" are relatively fast monohull boats, yet they
> use just flat plate boards.
> On the big sailing ships, strenght was likely more of
> a concern than drag. (don't want a big chunk of finely tapered
> rudder trailing edge breaking off at a bad time)
> On very fast, low drag, boats, such as a planing hulls
> or racing cats, it has been shown, in practice, that careful
> design and foil shape and a sharp trailing edge is important.
> Most "Bolger Boats" are neither very fast or low drag.
> I'm sure he has some designs which a foil board would benefit,
> but for the most part, "Flat" is fine.
> Low drag bottom paint, and careful polishing would probably
> do more good to reduce drag than foil shapes.
>
> I designed a "Foil" shaped leeboard for my "Elegant Punt".
> (the ultimate in low speed and high drag)
> It was an experiment in light weight, and high strenght.
>
> I modified it to be built of two outside layers of 1/4"
> ply, and a vertical spar inside. All covered with glass.
> (Took as long to build as the boat.)
> Kind of cool idea.
> Kind of like an airplane wing. Spar about 2/5 of the way
> from the leading edge, so gives a NACA Airfoil (of some number)
> shape to the board. Very important on an "Elegant Punt".
> Idea would be great for dagger or center boards.
> Very stong, and light weight, and hollow, so balast could
> be poured in to put it down low where it should be.
> 1/2-" leading and trailing edges. Leading rounded, trailing
> tapered.
> "For an "Elegant Punt"?
> I got a wild hair.
>
> Pat Patteson
> Molalla, Oregon
>
> --- In bolger@y..., William Samson <willsamson@y...> wrote:
> > Hi,
> >
> > Hannes's account of trying to get his DB to a foil
> > section brought to mind a conversation I had with a
> > fluid dynamicist a few years ago.
> >
> > I was thinking about building a Drascombe Peterboat at
> > that time, and it needed a steel/iron centreplate.
> > The options were to make a wooden pattern and get it
> > cast. The other was to buy some steel plate and grind
> > it.
> >
> > Obviously it would have been quite a job to grind a
> > big 3/4" thick steel plate to a perfect foil section.
> > My fluids friend told me not to worry about the taper
> > to the rear of the foil - just get the forward edge
> > ground to the right shape and leave the rear edge full
> > thickness and square. He said I probably wouldn't
> > notice any degradation in performance - something to
> > do with turbulence and boundary layers.
> >
> > Interestingly, looking at the rudders of tall ships,
> > like Cutty Sark for instance, there's no attempt to
> > taper the trailing edge of the rudder and yet Cutty
> > Sark was the fastest ship in the world at the time. I
> > suspect it wasn't just ignorance . . .
> >
> > Any fluid dynamicists out there who can refute or
> > support this approach?
> >
> > Cheers,
> >
> > Bill
> >
> > ____________________________________________________________
> > Do You Yahoo!?
> > Get your free @... address athttp://mail.yahoo.co.uk
> > or your free @... address athttp://mail.yahoo.ie
I had the same experience on the Frolic. I beveled the leading edge back
about 1/4 of the board width leaving the leading edge rounded about 3/8"
thick. The trailing edge was left rounded but full thickness. The board
would hum, then vibrate, then a top speed would start chattering until I
though it was going to self destruct. I went back to the plans a realized
the trailing edge was to be beveled too. I beveled it back about 1/3 the
width of the board and to about 1/4" thick. Now at any speed it's smooth
with no chatter or humming of any kind. The boat even handles better and
speed seems to have increase.
Jeff
about 1/4 of the board width leaving the leading edge rounded about 3/8"
thick. The trailing edge was left rounded but full thickness. The board
would hum, then vibrate, then a top speed would start chattering until I
though it was going to self destruct. I went back to the plans a realized
the trailing edge was to be beveled too. I beveled it back about 1/3 the
width of the board and to about 1/4" thick. Now at any speed it's smooth
with no chatter or humming of any kind. The boat even handles better and
speed seems to have increase.
Jeff
----- Original Message -----
From: <richard@...>
To: <bolger@yahoogroups.com>
Sent: Monday, August 20, 2001 8:08 AM
Subject: [bolger] Re: Simplified foils
> Not a fluid dynamica-sist, but I did have a flutter, humming, problem
> with the board on Entropy. I did my standard, round nose over,
> feather rear to 1/3 board width, leave trailing edge blunt and 3/8"
> thick deal.
>
> Could tell almost exactly how fast I was going by the tone from the
> board.
>
> Talked to JM&H, faired board, made it into a nice sweet foil profile
> with a bunt trailing edge 1/4" thick. Still hummed. Added weight to
> the bottom, still hummed.
>
> Finaly, at the suggestion of JM&H, from something he read in a 'zine,
> beveled the trailing edge to 35 degrees. No humming. Something about
> there being one vortex instead of two now I'm sure.
>
> My point? If you are going to play with square ended simple foils,
> you might consider beveling the trailing edge.
>
> Also, I've read in a couple of places that the drag is not much more,
> but that the stall angle is much lower for flat plates. You might get
> away with using one for lateral resistance, but probably should stay
> away from using a flat plate for a rudder.....
>
> --- In bolger@y..., William Samson <willsamson@y...> wrote:
> > Hi,
> >
> > Hannes's account of trying to get his DB to a foil
> > section brought to mind a conversation I had with a
> > fluid dynamicist a few years ago.
> >
> > I was thinking about building a Drascombe Peterboat at
> > that time, and it needed a steel/iron centreplate.
> > The options were to make a wooden pattern and get it
> > cast. The other was to buy some steel plate and grind
> > it.
> >
> > Obviously it would have been quite a job to grind a
> > big 3/4" thick steel plate to a perfect foil section.
> > My fluids friend told me not to worry about the taper
> > to the rear of the foil - just get the forward edge
> > ground to the right shape and leave the rear edge full
> > thickness and square. He said I probably wouldn't
> > notice any degradation in performance - something to
> > do with turbulence and boundary layers.
> >
> > Interestingly, looking at the rudders of tall ships,
> > like Cutty Sark for instance, there's no attempt to
> > taper the trailing edge of the rudder and yet Cutty
> > Sark was the fastest ship in the world at the time. I
> > suspect it wasn't just ignorance . . .
> >
> > Any fluid dynamicists out there who can refute or
> > support this approach?
> >
> > Cheers,
> >
> > Bill
> >
> > ____________________________________________________________
> > Do You Yahoo!?
> > Get your free @... address athttp://mail.yahoo.co.uk
> > or your free @... address athttp://mail.yahoo.ie
>
>
> Bolger rules!!!
> - no cursing, flaming, trolling, spamming, or flogging dead horses
> - pls take "personals" off-list, stay on topic, and punctuate
> - add your comments at the TOP and SIGN your posts, snip all you like
> - To order plans: Mr. Philip C. Bolger, P.O. Box 1209, Gloucester, MA,
01930, Fax: (978) 282-1349
> - Unsubscribe:bolger-unsubscribe@yahoogroups.com
>
> Your use of Yahoo! Groups is subject tohttp://docs.yahoo.com/info/terms/
>
>
> Finaly, at the suggestion of JM&H, from something he read ina 'zine,
> beveled the trailing edge to 35 degrees. No humming.I have heard that the hi-tech racers have determned that it is not
necessary to have a razor sharp edge, but you do want a edge, not a
flat on the trailing side.
A high-class Lightning sailor said in a seminar I attended that it
was important to make a smooth transition from the flat side of the
board to the shaped edge section. A lot of simple foils have a little
ridge there which can set up an eddy.
PHV
Not a fluid dynamica-sist, but I did have a flutter, humming, problem
with the board on Entropy. I did my standard, round nose over,
feather rear to 1/3 board width, leave trailing edge blunt and 3/8"
thick deal.
Could tell almost exactly how fast I was going by the tone from the
board.
Talked to JM&H, faired board, made it into a nice sweet foil profile
with a bunt trailing edge 1/4" thick. Still hummed. Added weight to
the bottom, still hummed.
Finaly, at the suggestion of JM&H, from something he read in a 'zine,
beveled the trailing edge to 35 degrees. No humming. Something about
there being one vortex instead of two now I'm sure.
My point? If you are going to play with square ended simple foils,
you might consider beveling the trailing edge.
Also, I've read in a couple of places that the drag is not much more,
but that the stall angle is much lower for flat plates. You might get
away with using one for lateral resistance, but probably should stay
away from using a flat plate for a rudder.....
with the board on Entropy. I did my standard, round nose over,
feather rear to 1/3 board width, leave trailing edge blunt and 3/8"
thick deal.
Could tell almost exactly how fast I was going by the tone from the
board.
Talked to JM&H, faired board, made it into a nice sweet foil profile
with a bunt trailing edge 1/4" thick. Still hummed. Added weight to
the bottom, still hummed.
Finaly, at the suggestion of JM&H, from something he read in a 'zine,
beveled the trailing edge to 35 degrees. No humming. Something about
there being one vortex instead of two now I'm sure.
My point? If you are going to play with square ended simple foils,
you might consider beveling the trailing edge.
Also, I've read in a couple of places that the drag is not much more,
but that the stall angle is much lower for flat plates. You might get
away with using one for lateral resistance, but probably should stay
away from using a flat plate for a rudder.....
--- In bolger@y..., William Samson <willsamson@y...> wrote:
> Hi,
>
> Hannes's account of trying to get his DB to a foil
> section brought to mind a conversation I had with a
> fluid dynamicist a few years ago.
>
> I was thinking about building a Drascombe Peterboat at
> that time, and it needed a steel/iron centreplate.
> The options were to make a wooden pattern and get it
> cast. The other was to buy some steel plate and grind
> it.
>
> Obviously it would have been quite a job to grind a
> big 3/4" thick steel plate to a perfect foil section.
> My fluids friend told me not to worry about the taper
> to the rear of the foil - just get the forward edge
> ground to the right shape and leave the rear edge full
> thickness and square. He said I probably wouldn't
> notice any degradation in performance - something to
> do with turbulence and boundary layers.
>
> Interestingly, looking at the rudders of tall ships,
> like Cutty Sark for instance, there's no attempt to
> taper the trailing edge of the rudder and yet Cutty
> Sark was the fastest ship in the world at the time. I
> suspect it wasn't just ignorance . . .
>
> Any fluid dynamicists out there who can refute or
> support this approach?
>
> Cheers,
>
> Bill
>
> ____________________________________________________________
> Do You Yahoo!?
> Get your free @... address athttp://mail.yahoo.co.uk
> or your free @... address athttp://mail.yahoo.ie
Not a fluid dynamicists here but probably makes sense.
At the relativly low speed and high drag of most sailboats,
the added drag of a non foil shaped rudder or DB would
probably be minimal. I read somewhere that basic flat plate
DB's are perfectly OK on small sailboats. The lateral
resistance at the low speeds is provided by the plate, pushing
against the water, rather than the "lifting" effect of a true foil.
"Lightnings" are relatively fast monohull boats, yet they
use just flat plate boards.
On the big sailing ships, strenght was likely more of
a concern than drag. (don't want a big chunk of finely tapered
rudder trailing edge breaking off at a bad time)
On very fast, low drag, boats, such as a planing hulls
or racing cats, it has been shown, in practice, that careful
design and foil shape and a sharp trailing edge is important.
Most "Bolger Boats" are niether very fast or low drag.
I'm sure he has some designs which a foil board would benefit,
but for the most part, "Flat" is fine.
Low drag bottom paint, and careful polishing would probably
do more good to reduce drag than foil shapes.
I designed a "Foil" shaped leeboard for my "Elegant Punt".
(the ultimate in low speed and high drag)
It was an experiment in light weight, and high strenght.
I modified it to be built of two outside layers of 1/4"
ply, and a vertical spar inside. All covered with glass.
(Took as long to build as the boat.)
Kind of cool idea.
Kind of like an airplane wing. Spar about 2/5 of the way
from the leading edge, so gives a NACA Airfoil (of some number)
shape to the board. Very important on an "Elegant Punt".
Idea would be great for dagger or center boards.
Very stong, and light weight, and hollow, so balast could
be poured in to put it down low where it should be.
1/2-" leading and trailing edges. Leading rounded, trailing
tapered.
"For an "Elegant Punt"?
I got a wild hair.
Pat Patteson
Molalla, Oregon
At the relativly low speed and high drag of most sailboats,
the added drag of a non foil shaped rudder or DB would
probably be minimal. I read somewhere that basic flat plate
DB's are perfectly OK on small sailboats. The lateral
resistance at the low speeds is provided by the plate, pushing
against the water, rather than the "lifting" effect of a true foil.
"Lightnings" are relatively fast monohull boats, yet they
use just flat plate boards.
On the big sailing ships, strenght was likely more of
a concern than drag. (don't want a big chunk of finely tapered
rudder trailing edge breaking off at a bad time)
On very fast, low drag, boats, such as a planing hulls
or racing cats, it has been shown, in practice, that careful
design and foil shape and a sharp trailing edge is important.
Most "Bolger Boats" are niether very fast or low drag.
I'm sure he has some designs which a foil board would benefit,
but for the most part, "Flat" is fine.
Low drag bottom paint, and careful polishing would probably
do more good to reduce drag than foil shapes.
I designed a "Foil" shaped leeboard for my "Elegant Punt".
(the ultimate in low speed and high drag)
It was an experiment in light weight, and high strenght.
I modified it to be built of two outside layers of 1/4"
ply, and a vertical spar inside. All covered with glass.
(Took as long to build as the boat.)
Kind of cool idea.
Kind of like an airplane wing. Spar about 2/5 of the way
from the leading edge, so gives a NACA Airfoil (of some number)
shape to the board. Very important on an "Elegant Punt".
Idea would be great for dagger or center boards.
Very stong, and light weight, and hollow, so balast could
be poured in to put it down low where it should be.
1/2-" leading and trailing edges. Leading rounded, trailing
tapered.
"For an "Elegant Punt"?
I got a wild hair.
Pat Patteson
Molalla, Oregon
--- In bolger@y..., William Samson <willsamson@y...> wrote:
> Hi,
>
> Hannes's account of trying to get his DB to a foil
> section brought to mind a conversation I had with a
> fluid dynamicist a few years ago.
>
> I was thinking about building a Drascombe Peterboat at
> that time, and it needed a steel/iron centreplate.
> The options were to make a wooden pattern and get it
> cast. The other was to buy some steel plate and grind
> it.
>
> Obviously it would have been quite a job to grind a
> big 3/4" thick steel plate to a perfect foil section.
> My fluids friend told me not to worry about the taper
> to the rear of the foil - just get the forward edge
> ground to the right shape and leave the rear edge full
> thickness and square. He said I probably wouldn't
> notice any degradation in performance - something to
> do with turbulence and boundary layers.
>
> Interestingly, looking at the rudders of tall ships,
> like Cutty Sark for instance, there's no attempt to
> taper the trailing edge of the rudder and yet Cutty
> Sark was the fastest ship in the world at the time. I
> suspect it wasn't just ignorance . . .
>
> Any fluid dynamicists out there who can refute or
> support this approach?
>
> Cheers,
>
> Bill
>
> ____________________________________________________________
> Do You Yahoo!?
> Get your free @... address athttp://mail.yahoo.co.uk
> or your free @... address athttp://mail.yahoo.ie
Hi,
Hannes's account of trying to get his DB to a foil
section brought to mind a conversation I had with a
fluid dynamicist a few years ago.
I was thinking about building a Drascombe Peterboat at
that time, and it needed a steel/iron centreplate.
The options were to make a wooden pattern and get it
cast. The other was to buy some steel plate and grind
it.
Obviously it would have been quite a job to grind a
big 3/4" thick steel plate to a perfect foil section.
My fluids friend told me not to worry about the taper
to the rear of the foil - just get the forward edge
ground to the right shape and leave the rear edge full
thickness and square. He said I probably wouldn't
notice any degradation in performance - something to
do with turbulence and boundary layers.
Interestingly, looking at the rudders of tall ships,
like Cutty Sark for instance, there's no attempt to
taper the trailing edge of the rudder and yet Cutty
Sark was the fastest ship in the world at the time. I
suspect it wasn't just ignorance . . .
Any fluid dynamicists out there who can refute or
support this approach?
Cheers,
Bill
____________________________________________________________
Do You Yahoo!?
Get your free @... address athttp://mail.yahoo.co.uk
or your free @... address athttp://mail.yahoo.ie
Hannes's account of trying to get his DB to a foil
section brought to mind a conversation I had with a
fluid dynamicist a few years ago.
I was thinking about building a Drascombe Peterboat at
that time, and it needed a steel/iron centreplate.
The options were to make a wooden pattern and get it
cast. The other was to buy some steel plate and grind
it.
Obviously it would have been quite a job to grind a
big 3/4" thick steel plate to a perfect foil section.
My fluids friend told me not to worry about the taper
to the rear of the foil - just get the forward edge
ground to the right shape and leave the rear edge full
thickness and square. He said I probably wouldn't
notice any degradation in performance - something to
do with turbulence and boundary layers.
Interestingly, looking at the rudders of tall ships,
like Cutty Sark for instance, there's no attempt to
taper the trailing edge of the rudder and yet Cutty
Sark was the fastest ship in the world at the time. I
suspect it wasn't just ignorance . . .
Any fluid dynamicists out there who can refute or
support this approach?
Cheers,
Bill
____________________________________________________________
Do You Yahoo!?
Get your free @... address athttp://mail.yahoo.co.uk
or your free @... address athttp://mail.yahoo.ie