Is there an optimum fin shape?
quadd...@googlemail.com
What shape would minimise drag inscrease stability? Are the current
trend of fins suitable for their purpose?
vet...@hotmail.com
> Well, what do you reckon? Is there?
Let me see if I can jump in before the usual heavies contribute;
Well, yes there is an optimum shape, which will minimise drag and
maximise stability, but .....
For what type of course (straight line 2k, twisty river, ...) ?
For what type of boat, and does it also have a rudder ?
For what shape of hull ?
Also, are you rowing in flat or rough water, calm/head/tail/cross
wind ?
Where should the fin be ?
Just to think really radically, is one fin optimum ? How about one at
the stern, one at the bow ? How about a continuous keel ? (Any UK
readers remember 'restricted' boats ?)
So there are lots of different best shapes. All we need is some clever
boat builder to supply their boats with a number of really easily
interchangable units (I'm thinking a few seconds here).
Even better; there are airoplanes that can change their wing shape/
profile whilst in flight. How about a variable geometry fin,
controlled by the cox, or a simple button on the end of the blade for
coxless boats :
'Swing-Fin' (copyright PJW 2008) - you heard it here first ....
Paul W
quadd...@googlemail.com
We're thinking on the same wavelength Paul! lol
My initial thoughts are for a 1x on a straight 2k with fair
conditions. Would an optimal fin actually give much advantage ie
quicker due to greater stability? and reduced drag? If it was long and
thin would it be effected by a cross wind.
Could a long thin fin give greater stability and actually also
increase steering resoponsiveness (if thats a word) on river courses?
Fin units are changed relatively quickly, but there a generally no
variation in shapes anyway.
Rob Collings
> maximise stability, but .....
Is there? I suspect you could design a fin for drag, stability, course
holding or for steering. All four will have differing requirements, so
anything you put on your boat will be a compromise
> Just to think really radically, is one fin optimum ? How about one at
> the stern, one at the bow ? How about a continuous keel ? (Any UK
> readers remember 'restricted' boats ?)
All for different requirements, as above. As for 2 fins, I forsee Carl
appearing with a tale about a boat with 9 men, a quadrennial
competition involving five rings, far more blazers and some
disagreement over advertising...
> So there are lots of different best shapes. All we need is some clever
> boat builder to supply their boats with a number of really easily
> interchangable units (I'm thinking a few seconds here).
I've seen some boatbuilders (empacher spring to mind) that supply a
deep fin (for straight courses) and a shallow fin for twisty ones. I'd
really like to see someone make an easily detachable fin mount near
the bow that had minimial effect on the flow when unoccupied...
> How about a variable geometry fin,
> controlled by the cox, or a simple button on the end of the blade for
> coxless boats :
Hmm... KISS. That sounds way to complex for it to be easily
maintainable. At least current fins don't really suffer from
breakdowns. But then most fins don't seem like a whole lot of thought
was actually put into them.
Rob.
James.
Whilst Im thinking Carl is best to answer this, surely it is going to
depend on the hull? We have a Ted Hale 8 that is wide but shallow, it
doesnt really need a fin for stability (voice of experience *cough*),
but maneuverability is hindered, whereas our stelph 8 rocks all over
the shop without the fin (I really wish I didn't know that...) as the
hull is narrower and sits much higher.
As far as I can see the old carbo design of chunky fins seems to be
coming back but as an aerofoil with a tiny rudder behind it. The depth
of most fins seem to be getting deeper which I guess should add to
stability as it has more leverage on the hull.
So basically, it depends on what hull you have, how much assistance
you need sitting it and what kind of course you are doing. A crew that
can sit the boat and only row on a straight lake really dont need some
of the large beasts out there, a novice crew on a river might benefit
from it, despite the drag.
quadd...@googlemail.com
In order to reduce many of the variables listed above......please
assume a competent sculler in a hwt 1x, training on a slow constant
speed river and racing on purpose built 2k venues.
J Flory
The Annual Fin Shape Discussion......
Here are my guesses.
OK, probably best to train with what you will race with, so optimize
skeg for the 2k venue.
If there is any chance of weed in the racing venues you'll want a
steeply sloping front edge (I learned this last season). Weed is a
remarkably effective brake.
But beyond that a lot depends on the hull shape, fin location,
sculler's need for lateral stability, and sculler's ability to
maintain a straight course without detracting too much from efforts to
maximize speed.
A hull with a flatter keel (many Empachers and Filippis) will track
better and so can get by with a smaller skeg than a boat with more
rocker. Some of us have reduced Empacher skegs to <1/3 their normal
size (same depth but the aft section cut away). The boat steers
easier but the improvement was less than I hoped for, probably due to
the inherent tracking of the hull itself.
A less stable boat (due to hull shape or high seat height) might
benefit from a deeper skeg to improve lateral stability. The further
aft the skeg is the smaller it can be, other things being equal.
As for the cross sectional shape of the skeg, a foil might be
desirable for straight racing because it maintains lift (and hence its
straightening moment) at increasing angles of attack better than a
flat plate, as has been pointed out here. Since turning in
unnecessary on a 2k course, this might be an advantage. But for
general purpose use a flat plate would be preferred because it stalls
beyond a low angle of attack and hence allows one to turn.
Things get much more interesting for head racing, where you have to
balance the ability to turn with the ability to track straight between
turns. And even more interesting for stake races, which have a sharp
180 degree turn in the middle, so time gained there might (or might
not) be worth compromises elsewhere on the course.
Carl Douglas
Are you concerned about fins for oar-steered boats like 1x & 2x, or
rudder/fin combinations usable by all other classes of boat?
Carl
--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Email: ca...@carldouglas.co.uk Tel: +44(0)1932-570946 Fax: -563682
URLs: www.carldouglas.co.uk (boats) & www.aerowing.co.uk (riggers)
quadd...@googlemail.com
> quaddav...@googlemail.com wrote:
> > Well, what do you reckon? Is there?
>
> > What shape would minimise drag inscrease stability? Are the current
> > trend of fins suitable for their purpose?
>
> Are you concerned about fins for oar-steered boats like 1x & 2x, or
> rudder/fin combinations usable by all other classes of boat?
>
> Carl
>
> --
> Carl Douglas Racing Shells -
> Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
> Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
> URLs: www.carldouglas.co.uk(boats) &www.aerowing.co.uk(riggers)
Carl,
Interested in oar steered boats mainly.
Thanks
quadd...@googlemail.com
> On 6 Feb, 11:32, Carl Douglas <c...@carldouglas.co.uk> wrote:
>
>
>
>
>
> > quaddav...@googlemail.com wrote:
> > > Well, what do you reckon? Is there?
>
> > > trend of fins suitable for their purpose?
>
> > Are you concerned about fins for oar-steered boats like 1x & 2x, or
>
> > Carl
>
> > --
> > Carl Douglas Racing Shells -
> > Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
> > Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
> > Email: c...@carldouglas.co.uk Tel: +44(0)1932-570946 Fax: -563682
> > URLs: www.carldouglas.co.uk(boats) &www.aerowing.co.uk(riggers)
>
> Carl,
>
> Interested in oar steered boats mainly.
>
>
> - Show quoted text -
Carl having seen your very detailed post from last year......below....
Would you say that Filippi's latest fin for their singles are spot on?
or as close as well get?
http://www.filippiboats.it/popupElement.asp?i=250
"The 1x fin's job is 2-fold:
1. Keep the boat tracking straight when that's what you want
2. Allow you to turn the boat without undue resistance when you need
to
All shells have a tendency to yaw (turn) out of line, because the
centre
of drag is ahead of the centre of gravity & the centre of effort.
You
can't help that, although some designs may do better than others when
they lose the fin.
Once the boat starts to yaw the rate of yawing will increase, which
is
what gives control problems if you have no fin. The fin acts to slow
the
rate of yaw by generating a small sideways "lift" as soon as the
stern
swings, which should allow you to maintain directional control
without
conscious effort.
When you want to turn, however, it gets pretty inconvenient if the
fin
continues to resist what you are doing. Whereas it is generally a
bad
thing for a foil (any lift-generating plate-like device) to lose its
lift at rather small angles of attack, with a fixed fin that's just
what
you do want when you need to make a turn.
A flat fin will only generate lift over a small range of angles of
attack, so it "stalls" (loses lift) quite easily when you make a
turn,
and recovers control only when you are nearly back on course. An
aerofoil section (narrow tear-drop cross-section as you view it from
the
tip towards the boat) generates increasing lift over wider range of
increasing angle of attack, so it resists turning and grabs back
control
rather earlier towards the end of the turn.
So much for cross-section, plate vs aerofoil: the flat plate is more
suited to the job in hand. Except that it should have a sharp
trailing
edge and a well-radiused leading edge, so being unduly thin is not
advisable & is also rather vulnerable to impacts - a bent leading
edge
really screws up the tracking of your boat by creating lots of one-way
lift.
Now profile: the deeper & narrower (front to back) the fin, the more
effective it is while still generating lift, because the tip end is
working in less-disturbed water, so the smaller it need be. And it
is
more effective to have a tapered profile than a square or
parallelogram
profile. In theory a semi-elliptic (think Spitfire wing) shape would
be
best, but the difference between that & a trapezoidal shape is
minimal
at best, especially in the fin's particularly messy flow environment.
For weed shedding (but for no other reason) you need a raked-back
leading edge."
Carl Douglas
<snipped>
>
> Carl having seen your very detailed post from last year......below....
>
> Would you say that Filippi's latest fin for their singles are spot on?
> or as close as well get?
>
> http://www.filippiboats.it/popupElement.asp?i=250
Well, you quote from my posting which is, I hope, reasonably easily
understood. So you may take it that I disagree with your Filippi
proposition for a number of reasons, given within that posting.
To help folk to ponder this issue, I have left my quoted text as follows:
Cheers -
Carl
--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
quadd...@googlemail.com
> URLs: www.carldouglas.co.uk(boats) &www.aerowing.co.uk(riggers)- Hide quoted text -
>
> - Show quoted text -
Just testing Carl ;)
J Flory
suited to the job in hand. Except that it should have a sharp
trailing
edge and a well-radiused leading edge, so being unduly thin is not
advisable & is also rather vulnerable to impacts - a bent leading
edge
really screws up the tracking of your boat by creating lots of one-
way
lift."
Exactly why carbon fins are a good idea. They don't bend and do not
nick as easily as metal ones do. They can be make in either a foil or
a flat cross section.
As Carl explained, the deep, foil-shaped, carbon ones made by both
Empacher and Filippi may be well suited for a straight course where
only slight turning is needed. For general purpose use a flat cross
section would be better.
The leading edge of the Filippi one at
http://www.filippiboats.it/popupElement.asp?i=250
would trap weed, but of course this would not be an issue on clean
water.
vet...@hotmail.com
You said
"The 1x fin's job is 2-fold:
1. Keep the boat tracking straight when that's what you want
2. Allow you to turn the boat without undue resistance when you need
to"
Is it also to keep the boat stable ?
Presumably a larger (longer or deeper ??) fin would be more stable*,
but produce more drag. What's the trade off likely to be ? I could
probably scull better if my boat was more stable**, but would a larger
fin slow me down more than the extra stability allowed me to speed
up ?
(* and do you make one ?)
(**yes, OK, I know I should improve my technical faults, but life is
short and I'm playing the percentages here).
Paul W
Carl Douglas
I believe that your boat has the old type of brass "Matt Wood" fin? A
deeper fin doesn't have to be bigger in area. Indeed, it can be smaller
in area yet more effective, since depth is much more important than area.
We hope soon to be able to offer you a deeper but neat stainless version
to retro-fit in your existing fin box. Then you could see if it works
better for you. The deeper blade should diminish the wobbles, perform
rather better than your present fin & not increase drag. BTW, not a lot
of folk know that a big source of surplus drag is that on the hull when
you can't keep the boat pointing in the way you are going.
Steven M-M
> "So much for cross-section, plate vs aerofoil: the flat plate is more
> suited to the job in hand. Except that it should have a sharp
> trailing
> edge and a well-radiused leading edge, so being unduly thin is not
> advisable & is also rather vulnerable to impacts - a bent leading
> edge
> really screws up the tracking of your boat by creating lots of one-
> way
> lift."
>
> Exactly why carbon fins are a good idea. They don't bend and do not
> nick as easily as metal ones do. They can be make in either a foil or
> a flat cross section.
>
> As Carl explained, the deep, foil-shaped, carbon ones made by both
> Empacher and Filippi may be well suited for a straight course where
> only slight turning is needed. For general purpose use a flat cross
> section would be better.
>
> would trap weed, but of course this would not be an issue on clean
> water.
The newer Filippi carbon fin is swept back 45 degrees:
http://www.filippiboats.it/popupElement.asp?i=247
J Flory
That looks much less aquacultural.