Shell construction article

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JD

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Mar 25, 2011, 10:03:19 AM3/25/11
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http://www.theboatrace.org/article/features/rowingshelldesign

By Mark Mills, a prominent sailor and yacht designer, on hull shapes
and drag issues. Nice overview.

Carl Douglas

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Mar 28, 2011, 2:27:34 PM3/28/11
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Very interesting, so I awaited comments. And I waited......

Actually, I think the article was confusing & bitty.

It ignored the bugbear of shell design - the fluctuating velocity -
while making passing reference to pitching. WRT pitching, it appeared
to favour a more flared bow section

It left the reader confused on wave drag. It said wave drag was tiny,
so surface area should be reduced bringing shorter hulls: “If you do
something that increases your wetted surface by one square foot you
better be good because that is a price you will be paying.” It got into
a bit of a knot over semicircles without explain what stopped us from
using hemispherical hulls. And that came after noting Lazauskas &
Tuck's evidence that wave drag, while relatively small, did matter &
indicated longer hulls with inevitably higher wetted surface: "their
work suggested that even longer and crucially narrower hull shapes than
currently popular theoretically appeared to be the way forwards".

It mentioned the effect of beam on stability, then advocated removing
the rudder. Did it mean removing rudder & fin? The fin enhances roll
stability, while the varying effects of wind & imbalances of
individuals' rowing forces affect steering at all times. A very little
yawing off-line will cost far more in parasitic drag than the possible
drag of the decent steering foils which would keep the boat straight.

There was worthwhile mention of aerodynamic drag; sailors know well
that you get a faster boat through lift enhancement & drag reduction.
We rowers know we go slower into a headwind, but ignore all known ways
of meaningfully reducing wind drag (e.g boundary layer trips on
oar-shafts) & do nothing to address the other sources of windage.

I'd be interested to know which "recent shell designs appear to be
looking to exploit new solutions". Don't think we saw any of that in
the last Boat Race.

Maybe the problem is that you can't cover any of that meaningfully in a
few hundred words?

Cheers -
Carl

--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Find: http://tinyurl.com/2tqujf
Email: ca...@carldouglas.co.uk Tel: +44(0)1932-570946 Fax: -563682
URLs: www.carldouglas.co.uk (boats) & www.aerowing.co.uk (riggers)

Henry Law

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Mar 28, 2011, 2:34:42 PM3/28/11
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On 28/03/11 19:27, Carl Douglas wrote:
> On 25/03/2011 14:03, JD wrote:
>> http://www.theboatrace.org/article/features/rowingshelldesign
>>
>> By Mark Mills, a prominent sailor and yacht designer, on hull shapes
>> and drag issues. Nice overview.
>
> Very interesting, so I awaited comments. And I waited......

We were all waiting too. For you :-)

>
> Actually, I think the article was confusing & bitty.

I too was confused, not that I know much about hull design, but if the
intention was for me to know more about it when I'd finished reading the
article pretty much failed.

Nor was I any the wiser about what, if anything, the author was
advocating as a response to the various factors outlined. Disappointing.


--

Henry Law Manchester, England

Tink

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Mar 28, 2011, 5:12:17 PM3/28/11
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Wasn't this article on-line last year? Or something very very
similar? I seem to remember reading something on boat design by a
yacht designer on the boat race website last year. I seem to remember
not being very impressed with it then.

zeke_hoskin

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Apr 2, 2011, 5:52:42 PM4/2/11
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> Email: c...@carldouglas.co.uk  Tel: +44(0)1932-570946  Fax: -563682
> URLs:  www.carldouglas.co.uk(boats) &www.aerowing.co.uk(riggers)

This article only showed up on my computer on April 2. If that
happened elsewhere, it might explain the wait.
I'd kind of expect flared bow and stern sections above equilibrium
waterline to be an advantage, since they seem to be forbidden by the
no-convex-hull-sections rule. I'm pretty sure that the extra drag of a
directionally-stable hull shape would cost more than a proper fin. A
drop-in rudder might very well be worth trying.
Me, I'm fixing up my Frontrower. Nearly hit a swimmer last year.

grahamca...@gmail.com

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May 14, 2018, 10:02:08 PM5/14/18
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A single scull moves about 45 tonnes of water aside over 2ks.
Go smaller c/s area under water
Go sharp bow,reaching full shape near halfway.
Go about 20kg boat weight to reduce pulsing.
Go smaller shaped hydrodynamic keel at the stern. Try without!
Have fun.

Peter

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May 15, 2018, 5:06:21 AM5/15/18
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I'm confused over this 45tonnes. Boat + me = 120Kg and that's all I displace however far I row. Of course if it's an 8m scull moving 2Km then 2000/8 * 120 = 30Tonnes.
If I'm moving 45 tonnes at a geriatric pace of 2K over 8mins at 26 spm then that's 45 tonne/208 = 216kg per stroke..or remove my weight and it's the same as me lifting 96kg 26x per minute..
On the other hand if I'm only shifting that weight a micron or so in distance as boat slides then perhaps not that many newtons dropping apples on my head while I row?

While ponderng this (and other nonsense) it occurs to me that the pulse action of a boat rasing and lowering itself could be countered with a weight sliding in a keel tube. My 105kg moving 1 metre (averaged in design to 1/2m) countered by a 13kg sliding mass in an 8m tube.....which presumably means Mr Newton will chuck another apple at me.


pgk

carl

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May 15, 2018, 7:43:28 AM5/15/18
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If the boat has a maximum cross-sectional area of about 0.02 sq m &
travels 2000m, then it might be said to have displaced 2000 x 0.02 = 40
tonnes of water. Of course, that water flows around the boat &,
although temporarily displaced, returns to roughly its original position.

More important is the energy given to that water through skin friction,
which drags some of it along with the boat & which is largely dissipated
in various liquid shearing processes to end up as heat.

Deriving a functional hull shape is far more complex than what might be
called "the general principles" indicted by the OP.

There is certainly an optimum boat mass, below which the consequent
increase in the boat's cyclic velocity fluctuations will tend to
increase overall drag. But that optimum will be very dependent on
sculler technique, smoother & lighter scullers being likely to benefit
from lighter boats than the heavy-weight wood-choppers.

As for the "smaller shaped hydrodynamic keel at the stern"? There are
certainly good & bad designs & positions for the essential fin/skeg on a
sculling boat, but those words tell us nothing. I would say, however,
that foil-based shell control systems based on proper fluid dynamic
principles have been available from my company for the last 18 years,
and they consistently improve steering, stability and race performance
by more than any other of the more widely discussed shell design &
equipment options.

Cheers -
Carl

--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Find: tinyurl.com/2tqujf
Email: ca...@carldouglasrowing.com Tel: +44(0)1932-570946 Fax: -563682
URLs: carldouglasrowing.com & now on Facebook @ CarlDouglasRacingShells

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