Stern design question?

Stern design question?

Post by Geof » Sun, 15 May 2005 23:32:42


Why do rowing skiffs have a long trailing edge at the stern?  The
design is usually almost symetrical to the bow in length and very
similar in shape.  This sort of design is not seen in many other marine
applications such as yachting (excluding catamarans).  So why in rowing
couldn't a wider (providing more stability), shorter, stubbier stern be
used?

Regards,

Geoff.

 
 
 

Stern design question?

Post by Kiera » Mon, 16 May 2005 03:35:01

Quote:

> Why do rowing skiffs have a long trailing edge at the stern?  The
> design is usually almost symetrical to the bow in length and very
> similar in shape.  This sort of design is not seen in many other marine
> applications such as yachting (excluding catamarans).  So why in rowing
> couldn't a wider (providing more stability), shorter, stubbier stern be
> used?

> Regards,

> Geoff.

Geoff,

It seems you understand intuitively why the bow would be knife-shaped.
The stern needs to be shaped similarly, because if its profile changes
too quickly, (as in the case of a blunt/wide stern) the flow of water
over the hull may separate from the surface.  Separated, or detached
flow is very inefficient.  Separated flow allows for eddy currents which
cause water to actually flow forward.  This all creates an enormous
amount of drag.  It's much better to maintain an attached flow with a
gradually changing hull profile.

Yachts do not use this, I assume, because they NEED to be very wide.
Therefore to maintain attached flow, the stern would be REALLY long, and
heavy and expensive, and possibly add more to drag (wetted surface, etc)
than the blunt shape.  But rowing shells can be (and are) very narrow,
so they can maintain a good profile at the stern, without becoming TOO long.

Every boat design is an iterative optimization process, where many
different factors result in different hull shapes for different
applications.

Hope this helps.

-Kieran

 
 
 

Stern design question?

Post by Carl Dougla » Mon, 16 May 2005 04:48:36


writes

Quote:

>> Why do rowing skiffs have a long trailing edge at the stern?  The
>> design is usually almost symetrical to the bow in length and very
>> similar in shape.  This sort of design is not seen in many other marine
>> applications such as yachting (excluding catamarans).  So why in rowing
>> couldn't a wider (providing more stability), shorter, stubbier stern be
>> used?
>>  Regards,
>>  Geoff.

>Geoff,

>It seems you understand intuitively why the bow would be knife-shaped.
>The stern needs to be shaped similarly, because if its profile changes
>too quickly, (as in the case of a blunt/wide stern) the flow of water
>over the hull may separate from the surface.  Separated, or detached
>flow is very inefficient.  Separated flow allows for eddy currents
>which cause water to actually flow forward.  This all creates an
>enormous amount of drag.  It's much better to maintain an attached flow
>with a gradually changing hull profile.

>Yachts do not use this, I assume, because they NEED to be very wide.
>Therefore to maintain attached flow, the stern would be REALLY long,
>and heavy and expensive, and possibly add more to drag (wetted surface,
>etc) than the blunt shape.  But rowing shells can be (and are) very
>narrow, so they can maintain a good profile at the stern, without
>becoming TOO long.

>Every boat design is an iterative optimization process, where many
>different factors result in different hull shapes for different
>applications.

>Hope this helps.

>-Kieran

Good explanation from Kieran.  Just a few points to add:

You taper the stern of a boat to allow the water to close back together
sweetly into the hole the boat made.  In this process you get a degree
of pressure recovery, the reverse of the pressure loss on the frontal,
expanding section of the boat.  Imagine pushing a greased wedge between
your squeezed fingers - you have to push to force the wedge in & that's
the pressure loss at the bows.  Then while you keep squeezing suppose
you stop pushing - the wedge is squeezed out & the work done in that
process is the pressure recovery.  You never get complete pressure
recovery, but every little helps.  And you can afford a longish stern
because on such a long boat the local surface friction per unit area
near the stern is much lower than it is further towards the bows

That's fine for rowing shells, & keeping the boat long also helps reduce
their tendency to pitch - that little bit of variable displacement a
long way from the centre of gravity being a very effective stabilising
factor

However, water finds it rather hard to follow a rapidly contracting
stern taper - such as you get on most boats which are, relatively, short
& fat. On fat, rapidly tapering sterns the main flow becomes "detached"
(can't follow the shape) & your pressure recovery is less or lost.  That
makes it worth reshaping the hull to shorten the stern & make it
broader, giving you the chopped off transom stern, but you may still
taper up from underneath for pressure recovery, instead of from the
sides.

As Kieran says, hull design involves many compromises.

HTH
Carl
--
Carl Douglas Racing Shells        -
     Fine Small-Boats/AeRoWing low-drag Riggers/Advanced Accessories
Write: The Boathouse, Timsway, Chertsey Lane, Staines TW18 3JY, UK

URLs:  www.carldouglas.co.uk (boats) & www.aerowing.co.uk (riggers)

 
 
 

Stern design question?

Post by Geof » Mon, 16 May 2005 09:11:50

Thanks Kieran and Carl,

I am still having some issues really understanding this however:

Kieran, can you please explain your comment "the flow of water over the
hull may separate from the surface" for me please.  You mention that
this type of design will cause eddies, however I would have expected
that this would only occur after the water had passed the hull and was
converging.  As the hull was no longer in that area it shouldn't have
an impact, should it (I wouln't have expected it to be like
aerodynamics)?

If this is the case then wouldn't the reduction in wetted surface area
of a blunt stern provide a better alternative?

Carl, You mention pressure recovery which makes sense but during race
conditions I would have expected that the speed of the boat would
outrun any pressure differential.  For example if the stern was
straight and square and the skiff was travelling at a speed that left a
indentation in the water of the hull shape for a small period of time
before the waters converged then even if the stern was wedged shape
there wouldn't be any advantage should there?  I have also thought
about the recovery of the stroke and if the water did catch up it
should exert positive movement on the hull as it hits the square stern.

I agree with you about the pitch problem, however I think I have a
solution to that that I am working on.

I am not an expert on this so I may be way off but appreciate any
comments.

Regards,

Geoff.

 
 
 

Stern design question?

Post by JD » Mon, 16 May 2005 09:47:02

Don't sailing hulls measurements have to follow complex and rigid
displacement, length or other measurement formulas? I understood that
the chopped off sterns were a way of getting around these formula by
the marine architects & designers.

Racing shells are not subject to these design limitations and, with the
exception of a couple builders, none have transoms.

-JD

 
 
 

Stern design question?

Post by Richard Gladwel » Mon, 16 May 2005 13:10:02

Yacht design is generally subject to some sort of rating or class rule.

If there was no length restriction on yachts, then they would probably have
tapered sterns like rowing boats.

Yachts also need hull form stabilty to help offset heeling moment from the
rig - hence they are wider than they would otherwise be. The cut-off stern
design is trade-off amongst several factors, but is as low drag as possible.
Further yachts are often planing hulls rather than rowing skiffs which are
displacement hulls.

The nearest comparision is with catamarans - which have much more tapered
ends, or the International Canoe - which is sailed by one person on the end
of a sliding plank/seat - and those have a tapered stern - similar to a
rowing skiff.
Catamarans rely on beam fro stability. The Int. Canoes only need for hull
form stability is to support the crew between tacks and prove a base for
support of the sliding plank.

Where is is no need for form stability the hulls will get narrower and
narrower (depending on the class rule). On area to watch will be the
foiul-bourne Moths (11ft long) which are very narrow and gain stability when
they are foild bourne and it is to their advantage to get flying as quickly
as possible. So a low drag hull shape is required - which is now evolving as
they can now be got onto foils in s loittle as 5-6kts of breeze.

RG


Quote:
> Don't sailing hulls measurements have to follow complex and rigid
> displacement, length or other measurement formulas? I understood that
> the chopped off sterns were a way of getting around these formula by
> the marine architects & designers.

> Racing shells are not subject to these design limitations and, with the
> exception of a couple builders, none have transoms.

> -JD

 
 
 

Stern design question?

Post by plasticgu » Mon, 16 May 2005 14:25:55


Quote:
> Thanks Kieran and Carl,

> I am still having some issues really understanding this however:

> Kieran, can you please explain your comment "the flow of water over the
> hull may separate from the surface" for me please.

Let me take a shot at it.  There are 2 types flow of a liquid over a hull.
Laminar flow and turbulent flow.  Laminar flow is characterised by the
absence of a boundary layer.  The boundary layer in turbulent flow is
fluid moving more slowly than the normal flow of the water past the object.
Consider the hull a stationary object and the water moving past it.  Laminar
flow
(very low drag) moves along the hull at the free stream velocity.  Turbulent
flow
moves slower than free stream flow.  It can be caused by hull form, surface
roughness
or a number of other factors.  Basically the water slows down because it is
"sticking"
( a very un-technical term) to the hull and causing drag at the skin of the
vessell.
There is another high drag junction between the boundary layer and the
laminar flow
where those two sheets of water interact.  A shell uses a very fine entry
and stern exit
in an attempt to not "trip" a boundary layer event by causing a local
surface pressure spike.
The stern shape MUST allow for attached flow to exist to keep hull drag to a
minimum.
Stern shapes like "IOR sugar scoops meet rating rules but pay large
penalties as they
do not reattach flow as well as other finer hull forms.   I hope this helps.

Scott.

 
 
 

Stern design question?

Post by Geof » Mon, 16 May 2005 15:27:05

Thanks Scott, but if the vessel is travelling at a speed that means
that the water is converging well after the face of the stern (in event
of square cut off on stern) then the turbulance would occur in an area
of water the hull is not longer in contact with.

Your thoughts??

Geoff.

 
 
 

Stern design question?

Post by neuhar » Mon, 16 May 2005 23:41:40

I'd like to clear up a few things, if I might.

Quote:
> Let me take a shot at it.  There are 2 types flow of a liquid over a hull.
> Laminar flow and turbulent flow.  Laminar flow is characterised by the
> absence of a boundary layer.

Imagine the boat standing still, and the water flowing past the boat at the
speed
of the boat. (The is the perspective the coxwain has while sitting in the
boat.)
The boundary layer is the region close to the surface where the velocity of
the
fluid flowing past the surface goes from zero at the surface to the full
boat
velocity. If the boat is moving, the boundary layer always exists, and is
either
laminar, turbulent, or in transition from laminar to turbulent.

Quote:
>The boundary layer in turbulent flow is
> fluid moving more slowly than the normal flow of the water past the

object.

Laminar flow in the boundary layer also moves more slowly than the normal
flow of water past the object.

Quote:
> Consider the hull a stationary object and the water moving past it.
Laminar
> flow
> (very low drag) moves along the hull at the free stream velocity.

No, see above.

Quote:
> Turbulent flow
> moves slower than free stream flow.

Yes, as does the laminar boundary layer flow. However, the turbulent
boundary layer has more energy, so it actually exhibits slightly faster,
than laminar, flow nearer the surface.

Quote:
>It can be caused by hull form, surface
> roughness
> or a number of other factors.  Basically the water slows down because it
is
> "sticking"
> ( a very un-technical term) to the hull and causing drag at the skin of
the
> vessell.
> There is another high drag junction between the boundary layer and the
> laminar flow
> where those two sheets of water interact.

Again, the boundary layer is either laminar or turbulent or in transition,
so this
phenomenon doesn't exist. No offense intended.

Quote:
>A shell uses a very fine entry
> and stern exit
> in an attempt to not "trip" a boundary layer event by causing a local
> surface pressure spike.

The "fine" stern exit can reduce the level of the pressure recovery, and ,
as you
say, minimize the pressure-rise-induced transition from laminar to turbulent
flow. However, this gradual (shallow) stern closure angle also helps reduce
the tendency of the flow to stop following the shell surface, called
boundary
layer separation. This flow separation is caused by the increase in pressure
that the boundary layer flow encounters as it flows past the maximum
width of the shell (point of minimum pressure) and moves toward the stern.

If the pressure recovery isn't complete, due to the flow no longer following
the contour of the shell, there is a net pressure difference between the
fore
and aft parts of the shell, and this results in what is called pressure
drag. So,
you want a long, slender shell shape to minimize flow separation, so that
you minimize pressure drag.

Quote:
> The stern shape MUST allow for attached flow to exist to keep hull drag to
a
> minimum.
> Stern shapes like "IOR sugar scoops meet rating rules but pay large
> penalties as they
> do not reattach flow as well as other finer hull forms.   I hope this
helps.

> Scott.

I hope no offense is taken in my comments. If you want substantiation, any
basic fluid mechanics textbook will further illuminate you on this topic.

Best wishes.

 
 
 

Stern design question?

Post by neuhar » Mon, 16 May 2005 23:46:22


Quote:
> Thanks Scott, but if the vessel is travelling at a speed that means
> that the water is converging well after the face of the stern (in event
> of square cut off on stern) then the turbulance would occur in an area
> of water the hull is not longer in contact with.

> Your thoughts??

> Geoff.

You're right. The wake region aft of the stern has little effect on the
boat.
The drag due to a cut off stern is due to the pressure imbalance fore and
aft. The cut off stern doesn't allow for a gradual pressure recovery, to
balance the pressure on the forward part of the boat.

See my comments after Scott's post above.

 
 
 

Stern design question?

Post by Carl Dougla » Tue, 17 May 2005 03:47:55


Quote:
>Thanks Kieran and Carl,

>I am still having some issues really understanding this however:

>Kieran, can you please explain your comment "the flow of water over the
>hull may separate from the surface" for me please.  You mention that
>this type of design will cause eddies, however I would have expected
>that this would only occur after the water had passed the hull and was
>converging.  As the hull was no longer in that area it shouldn't have
>an impact, should it (I wouln't have expected it to be like
>aerodynamics)?

Kieran will respond in his own words, but a few points anyway:
Water is a fluid & so is air.  At all times your boat is enveloped by
these 2 fluids.  As the boat moves forwards it parts both and as it
passes they both close on it (not behind it) because they remain always
in contact.

As a first approximation, where the water is most displaced by the boat
its internal pressure is least.  There being an air/water interface, the
water level everywhere reflects its internal pressure, so the surface
falls under this internal pressure decrease.  And as the boat passes the
water expands to fill the space available (you don't leave a hole!) and
regains pressure, so the surface level rises.

All of that is fine if the flow is very smooth & if all streamlines flow
mutually parallel at all parts near & far from the hull surface.  But
water cannot be compelled to fill the space vacated by the boat as fast
as we might hope - there is a bit of a lag.  If this lag is significant
(whatever that may mean) the local pressure imbalances stir up the flows
around the boat while the main flow streams past rather ignoring the
boat's afterbody shape.  Then you get messy flow (big energy loss) &
much reduced pressure recovery.

Quote:
>If this is the case then wouldn't the reduction in wetted surface area
>of a blunt stern provide a better alternative?

Sometimes, yes.  But the result is the sum of many subtle &
inter-related effects.  As in all compromises, the optimum solution
depends on circumstances.  So with some shapes a transom stern may work
better, especially if it gives a sailboat better stability, or an
effective re-alignment of its underwater hull shape when heeled.  With
others, it does not.

Quote:

>Carl, You mention pressure recovery which makes sense but during race
>conditions I would have expected that the speed of the boat would
>outrun any pressure differential.  For example if the stern was
>straight and square and the skiff was travelling at a speed that left a
>indentation in the water of the hull shape for a small period of time
>before the waters converged then even if the stern was wedged shape
>there wouldn't be any advantage should there?

As I said above, it is a complex compromise.  And you do not leave the
sort of indentation in the water that you appear to describe, which is
the sort of thing you may see astern of a powerboat which is operating
way above what we loosely term "hull velocity".  We lack the power to do
that in a rowing shell - unless our name is Asterix the Gaul.
Simplistic design answers remain just that - they only seem obvious
because one ignores (at one's cost) other important factors which say
"go this way".  The optimum shape is never that which optimises only one
of a range of competing influences.

Quote:
>  I have also thought
>about the recovery of the stroke and if the water did catch up it
>should exert positive movement on the hull as it hits the square stern.

The water does not catch up.  Nor would you want it to, because the
energy penalty of making it accelerate 7 chase you would be intolerable
for the rower & there is no fluid dynamic mechanism that would make it
do so.

Quote:

>I agree with you about the pitch problem, however I think I have a
>solution to that that I am working on.

>I am not an expert on this so I may be way off but appreciate any
>comments.

>Regards,

>Geoff.

We are all of us somewhat inexpert, even the most expert of us.  At a
conference some 12 years ago on shell design I began by telling my
audience that I was there to tell them how very little we all knew, &
then to explain what we did know & where that led us.  Others told the
audience it was all understood and that shell colour was the most
important thing :^)

Anyway, wouldn't life be dull if there was nothing else left to learn?
What does disturb me is those who build them but don't have a clue about
the fluid mechanical design of rowing shells, & don't let that stop them
from pretending that they are expert designers.  And then there are
those who actually swallow that stuff.

Maybe my next lecture should be on the subject of peristalsis?

:)
Carl
--
Carl Douglas Racing Shells        -
    Fine Small-Boats/AeRoWing low-drag Riggers/Advanced Accessories
Write: The Boathouse, Timsway, Chertsey Lane, Staines TW18 3JY, UK

URLs:  www.carldouglas.co.uk (boats) & www.aerowing.co.uk (riggers)

 
 
 

Stern design question?

Post by Carl Dougla » Tue, 17 May 2005 22:46:24


Quote:
>Thanks Kieran and Carl,

>I am still having some issues really understanding this however:

>Kieran, can you please explain your comment "the flow of water over the
>hull may separate from the surface" for me please.  You mention that
>this type of design will cause eddies, however I would have expected
>that this would only occur after the water had passed the hull and was
>converging.  As the hull was no longer in that area it shouldn't have
>an impact, should it (I wouln't have expected it to be like
>aerodynamics)?

[I'm reposting this because, as some may have noticed, my ISP sometimes
disappears postings, as it has done with my initial reply here, only to
send them out weeks later.  No doubt the original of this one will pop
up in due course, when we've all forgotten what it was all about]

Kieran will respond in his own words, but a few points anyway: Water is
a fluid & so is air.  At all times your boat is enveloped by these 2
fluids.  As the boat moves forwards it parts both and as it passes they
both close on it (not behind it) because they remain always in contact.

As a first approximation, where the water is most displaced by the boat
its internal pressure is least.  There being an air/water interface, the
water level everywhere reflects its internal pressure, so the surface
falls under this internal pressure decrease.  And as the boat passes the
water expands to fill the space available (you don't leave a hole!) and
regains pressure, so the surface level rises.

All of that is fine if the flow is very smooth & if all streamlines flow
mutually parallel at all parts near & far from the hull surface.  But
water cannot be compelled to fill the space vacated by the boat as fast
as we might hope - there is a bit of a lag.  If this lag is significant
(whatever that may mean) the local pressure imbalances stir up the flows
around the boat while the main flow streams past rather ignoring the
boat's afterbody shape.  Then you get messy flow (big energy loss) &
much reduced pressure recovery.

Quote:
>If this is the case then wouldn't the reduction in wetted surface area
>of a blunt stern provide a better alternative?

Sometimes, yes.  But the result is the sum of many subtle &
inter-related effects.  As in all compromises, the optimum solution
depends on circumstances.  So with some shapes a transom stern may work
better, especially if it gives a sailboat better stability, or an
effective re-alignment of its underwater hull shape when heeled.  With
others, it does not.

Quote:

>Carl, You mention pressure recovery which makes sense but during race
>conditions I would have expected that the speed of the boat would
>outrun any pressure differential.  For example if the stern was
>straight and square and the skiff was travelling at a speed that left a
>indentation in the water of the hull shape for a small period of time
>before the waters converged then even if the stern was wedged shape
>there wouldn't be any advantage should there?

As I said above, it is a complex compromise.  And you do not leave the
sort of indentation in the water that you appear to describe, which is
the sort of thing you may see astern of a powerboat which is operating
way above what we loosely term "hull velocity".  We lack the power to do
that in a rowing shell - unless our name is Asterix the Gaul. Simplistic
design answers remain just that - they only seem obvious because one
ignores (at one's cost) other important factors which say "go this way".
The optimum shape is never that which optimises only one of a range of
competing influences.

Quote:
>  I have also thought
>about the recovery of the stroke and if the water did catch up it
>should exert positive movement on the hull as it hits the square stern.

The water does not catch up.  Nor would you want it to, because the
energy penalty of making it accelerate & chase you would be intolerable
for the rower & there is no fluid dynamic mechanism that would make it
do so.

Quote:

>I agree with you about the pitch problem, however I think I have a
>solution to that that I am working on.

>I am not an expert on this so I may be way off but appreciate any
>comments.

>Regards,

>Geoff.

We are all of us somewhat inexpert, even the most expert of us.  At a
conference some 12 years ago on shell design I began by telling my
audience that I was there to tell them how very little we all knew, &
then to explain what we did know & where that led us.  Others told the
audience it was all understood and that shell colour was the most
important thing :^)

Anyway, wouldn't life be dull if there was nothing else left to learn?
What does disturb me is those who build them but don't have a clue about
the fluid mechanical design of rowing shells, & don't let that stop them
from pretending that they are expert designers.  And then there are
those who actually swallow that stuff.

Maybe my next lecture should be on the subject of peristalsis?

:)
Carl
--
Carl Douglas Racing Shells        -
     Fine Small-Boats/AeRoWing low-drag Riggers/Advanced Accessories
Write: The Boathouse, Timsway, Chertsey Lane, Staines TW18 3JY, UK

URLs:  www.carldouglas.co.uk (boats) & www.aerowing.co.uk (riggers)