Tricky math?

marco.bovo · Joined: 18 Aug 2007 Posts: 4

I need some clarification no a table published on the Vespoli Website:

http://www.vespoli.com/content/view/285/182/

In this table they compare displacement and waterline measurements of
the latest Vespoli 8+ design with the one of some of the major
competitors.

I'm trying to understand how they came up with few numbers. I
understand that shorter waterline doesn't correspond with lower
displacement but I don't understand how the with Resolute shorter
water line (16,791 m. vs 16,891 m. for Vespoli), shorter beam (0,573
m. vs 0.586 m.), smaller depth (0.183 m. vs 0.182 m.) end up having a
larger wetted surfice?
Isn't the wetted surface the product of these measurements?
Also how come the Wetted Surface is the same to the Hudson in square
feet but it's different in square meter?
Can someone explain me that?
Marco

Archived from group: rec>sport>rowing

Christopher Anton · Joined: 18 Aug 2007 Posts: 190

wrote in message @d62g2000hsf.googlegroups.com...
>I need some clarification no a table published on the Vespoli Website:
>
> http://www.vespoli.com/content/view/285/182/
>
> In this table they compare displacement and waterline measurements of
> the latest Vespoli 8+ design with the one of some of the major
> competitors.
>
> I'm trying to understand how they came up with few numbers. I
> understand that shorter waterline doesn't correspond with lower
> displacement but I don't understand how the with Resolute shorter
> water line (16,791 m. vs 16,891 m. for Vespoli), shorter beam (0,573
> m. vs 0.586 m.), smaller depth (0.183 m. vs 0.182 m.) end up having a
> larger wetted surfice?
> Isn't the wetted surface the product of these measurements?
> Also how come the Wetted Surface is the same to the Hudson in square
> feet but it's different in square meter?
> Can someone explain me that?
> Marco

There was one at Worcester Head on Saturday - I thought the freeboard
ridiculously short and fortunately the water wasn't very rough.

Edward Fryer · Joined: 17 Aug 2007 Posts: 1

wrote in message@d62g2000hsf.googlegroups.com...

> I'm trying to understand how they came up with few numbers. I understand
that shorter waterline doesn't correspond with lower
> displacement but I don't understand how the with Resolute shorter water
line (16,791 m. vs 16,891 m. for Vespoli), shorter beam (0,573
> m. vs 0.586 m.), smaller depth (0.183 m. vs 0.182 m.) end up having a
larger wetted surfice?

Reflects the different cross sectional shapes of the two boats. Think of two
boats - the same length, depth, width. One has a cross sectional shape
that's a semi circle, the other a rectangle. The first will have a much
lower WSA than the second.

So in this case, it seems as though the Resolute has harder bilges - aka a
sharper turn of bilge than the Hudson, which may well have a more semi
circular cross sectional shape - and as a result, the Resolute may well be
more stable.

There are also the smaller effects of differing prismatic coefficients to
take into account - which describes whether the bow and stern shapes are
shorter and fatter, or narrow and tapering gently.

marco.bovo · Joined: 18 Aug 2007 Posts: 4

On Feb 26, 2:50 pm, "Edward Fryer" wrote:
> wrote in message
>
> @d62g2000hsf.googlegroups.com...
>
> > I'm trying to understand how they came up with few numbers. I understand
>
> that shorter waterline doesn't correspond with lower> displacement but I don't understand how the with Resolute shorter water
>
> line (16,791 m. vs 16,891 m. for Vespoli), shorter beam (0,573> m. vs 0.586 m.), smaller depth (0.183 m. vs 0.182 m.) end up having a
>
> larger wetted surfice?
>
> Reflects the different cross sectional shapes of the two boats. Think of two
> boats - the same length, depth, width. One has a cross sectional shape
> that's a semi circle, the other a rectangle. The first will have a much
> lower WSA than the second.
>
> So in this case, it seems as though the Resolute has harder bilges - aka a
> sharper turn of bilge than the Hudson, which may well have a more semi
> circular cross sectional shape - and as a result, the Resolute may well be
> more stable.
>

That was my thought but they seem to have similar cross sectional
shape (at least by looking it on the rack) and anyway if that was the
case shouldn't the deeper hull increase the wetted surface?

> There are also the smaller effects of differing prismatic coefficients to
> take into account - which describes whether the bow and stern shapes are
> shorter and fatter, or narrow and tapering gently.

Shouldn't a narrow and longer bow give me a shorter waterline instead
of a shorter and fatter one

paul_v_smith · Joined: 18 Aug 2007 Posts: 48

On Feb 26, 10:22 am, marco.b...@gmail.com wrote:
> I need some clarification no a table published on the Vespoli Website:
>
> http://www.vespoli.com/content/view/285/182/
>
> In this table they compare displacement and waterline measurements of
> the latest Vespoli 8+ design with the one of some of the major
> competitors.
>
> I'm trying to understand how they came up with few numbers. I
> understand that shorter waterline doesn't correspond with lower
> displacement but I don't understand how the with Resolute shorter
> water line (16,791 m. vs 16,891 m. for Vespoli), shorter beam (0,573
> m. vs 0.586 m.), smaller depth (0.183 m. vs 0.182 m.) end up having a
> larger wetted surfice?
> Isn't the wetted surface the product of these measurements?
> Also how come the Wetted Surface is the same to the Hudson in square
> feet but it's different in square meter?
> Can someone explain me that?
> Marco

Seems like once we get to a normalized mass displacement we're dealing
with a +0.1 to 0.2 square meter difference in wetted surface, or about
1%.
Not very much, and as has been discussed before, the wavemaking
characteristics will vary with the targetted speed of the hull,
unfortunately the hull speed can vary considerably from the Avg system
speed and that may result in chasing a phantom "optimal shape" for the
desired "Gold Medal Standard" avg speed. At least if efficiency is a
primary concern, which I'd suggest, it is not; at the top levels of
competition.

Even the wetted surface must change during the stroke cycle, at least
as it appears from watching the boats, there seems to be varying
degrees of planing and sinking as the hull speed goes through it's
variations. Or does a "displacement hull" not have any properties of
a planing hull?

Maybe the answer would be to come up with a hull shape that takes into
account the variations in speed to achieve the best balance of small
wetted surface and small wave making, as it could "Appear to the
water" to be morphing fluidly between displacement and planing
conditions.

- Paul Smith

Carl Douglas · Joined: 18 Aug 2007 Posts: 93

marco.bovo@gmail.com wrote:
> On Feb 26, 2:50 pm, "Edward Fryer" wrote:
>
>> wrote in message
>>
>>@d62g2000hsf.googlegroups.com...
>>
>>
>>>I'm trying to understand how they came up with few numbers. I understand
>>
>>that shorter waterline doesn't correspond with lower> displacement but I don't understand how the with Resolute shorter water
>>
>>line (16,791 m. vs 16,891 m. for Vespoli), shorter beam (0,573> m. vs 0.586 m.), smaller depth (0.183 m. vs 0.182 m.) end up having a
>>
>>larger wetted surfice?
>>
>>Reflects the different cross sectional shapes of the two boats. Think of two
>>boats - the same length, depth, width. One has a cross sectional shape
>>that's a semi circle, the other a rectangle. The first will have a much
>>lower WSA than the second.
>>
>>So in this case, it seems as though the Resolute has harder bilges - aka a
>>sharper turn of bilge than the Hudson, which may well have a more semi
>>circular cross sectional shape - and as a result, the Resolute may well be
>>more stable.
>>
>
>
> That was my thought but they seem to have similar cross sectional
> shape (at least by looking it on the rack) and anyway if that was the
> case shouldn't the deeper hull increase the wetted surface?
>
>
>>There are also the smaller effects of differing prismatic coefficients to
>>take into account - which describes whether the bow and stern shapes are
>>shorter and fatter, or narrow and tapering gently.
>
>
> Shouldn't a narrow and longer bow give me a shorter waterline instead
> of a shorter and fatter one

I concur with all that Edward has said.

I do worry about promotional claims which a) denigrate the opposition &
b) are not necessarily based on indisputable fact. We've had that from
other manufacturers of late. (And, like Marco, I do worry a bit when
the easy maths doesn't add up).

It never convinces those who know a bit about hull design when a
manufacturer says their boat has the least wetted surface so it must be
fastest. All shells have very high wetted surface/displacement ratios -
the highest, indeed, of any class of boat known. With a long thin
shell, however, fluid friction is surprisingly insensitive to small
changes in length since the friction factor is so much lower near the
stern than it is near the bow. And, while drag from sources other than
skin friction will indeed be of lower degree, it can be surprisingly
sensitive to shape & speed - enough even to reverse any putative
reductions in friction drag coming from reduced wetted surface. Length
does matter, or shells would be a whole lot shorter. All sorts of
details of shape which do not reduce wetted surface do also matter. In
short, it is rarely that simple.

Nor are shells as well served by conventional analysis as one might
think. There are no other commonly known vessel types which have to
travel with strongly cyclic velocity - as much as +/-25% about the mean.
And a boat which may have the best performance in a tail wind (i.e. at
the higher speeds) may perform less well than another design into a
head-wind. Good at one speed is not necessarily good at all speeds.

So let the product stand on its own 2 feet, which perhaps it does. The
only truly objective measure of shell performance will come from
comparative performance measurements taken in a well-conducted testing
program. Perhaps the claims can then be validated, but can be very
difficult to set up such a test, involving crews with their own
prejudices after all, in a way that does not leave its conclusions open
to doubt. The next best test is: do lots of its users win? But to
demand acceptance of claims of the kind made amounts to a demand, for
the huge majority which lacks sufficient grasp of marine design, that
they take what they're told on trust. That's a big & dubious demand.

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: carl@carldouglas.co.uk Tel: +44(0)1932-570946 Fax: -563682
URLs: www.carldouglas.co.uk (boats) & www.aerowing.co.uk (riggers)

marco.bovo · Joined: 18 Aug 2007 Posts: 4

> The next best test is: do lots of its users win?

Winning it's not a measure of hull efficiency. It a measure of the
strength of the crew most of the time. Fortunately in this sport at
the highest level I hope no one can claim that they won an Olympic
medal because their equipment was superior. I think we can talk about
equipment that doesn't slow you down but not that make you win don't
you think?
mmmmm ... Maybe I shouldn't ask this question to someone that sell
equipment Smile

Ciao,
Marco

> 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: carl@carldouglas.co.uk Tel:+44(0)1932-570946 Fax: -563682
> URLs: www.carldouglas.co.uk(boats) &www.aerowing.co.uk(rigge

Pete · Joined: 18 Aug 2007 Posts: 4

On 26 Feb, 21:51, Carl Douglas wrote:
> It never convinces those who know a bit about hull design when a
> manufacturer says their boat has the least wetted surface so it must be
> fastest. All shells have very high wetted surface/displacement ratios -
> the highest, indeed, of any class of boat known. With a long thin
> shell, however, fluid friction is surprisingly insensitive to small
> changes in length since the friction factor is so much lower near the
> stern than it is near the bow. And, while drag from sources other than
> skin friction will indeed be of lower degree

I think a major part of this is a certain species of theoretician will
automatically run through the equations, pick out the drag force whose
relation to speed is of highest degree, and say 'this is all that
matters'. Which is certainly true if you are always considering high
speeds: you don't care too much about shaft friction on a prop, for
example. But even eights do not go arbitrarily fast - and if you
compare drags v^2 and 20v, then the higher degree drag is relatively
insignificant at lowly speeds like 5m/s. This will not stop that sort
of theoretician ignoring it, and giving results which implicitly
assume you're only interested in eights going at 100m/s.

Pete
not to say dumping the lower order terms isn't a very good idea in a
lot of situations, but you do need to think what you're doing!

Carl Douglas · Joined: 18 Aug 2007 Posts: 93

marco.bovo@gmail.com wrote:
>>The next best test is: do lots of its users win?
>
>
> Winning it's not a measure of hull efficiency. It a measure of the
> strength of the crew most of the time. Fortunately in this sport at
> the highest level I hope no one can claim that they won an Olympic
> medal because their equipment was superior. I think we can talk about
> equipment that doesn't slow you down but not that make you win don't
> you think?
> mmmmm ... Maybe I shouldn't ask this question to someone that sell
> equipment Smile

> Ciao,
> Marco

Why not? And then judge me by what I say & how I say it.

I don't think any manufacturer should claim that a crew won because of
the equipment used. The crew won largely because it was fit, strong &
efficient - in identical cars, Michael Schumacher would always beat me!
However, with crews of identical potential the equipment used is bound
to influence their overall performance because, just as race cars are so
different in performance potential that a good driver in the wrong car
or with the wrong tyres may never win a race, so boats, riggers, oars,
fins, etc. will also affect performance.

Thus, as an extreme instance, you cannot say that riggers with small
parachutes attached will enhance the performance of any boat or crew
going into a headwind in a race against a similarly equipped crew which
lacks the added parachutes. Similarly, if one crew in that race has any
other improvement to its aerodynamics, it gains an advantage over those
which do not.

Again, if the riggers on one boat, although aerodynamically identical,
were very much less rigid, or the pins were off-pitch, the other crew(s)
would have the advantage.

You will note here that, in discussing riggers, I referred to their
aerodynamics & mechanical properties. I did not fall into the popular
trap of focussing on their materials of manufacture. What something is
made of matters only to the extent that it allows improvements in
aerodynamics, stiffness, strength or reliability. If you adopt a
particularly vogueish material of manufacture but with it make a more
bulky rigger or one which is less rigid or less durable, then you have
adversely affected its user's performance.

Similarly, it is impossible to argue against providing the crew with the
optimum means of steering. So I find it odd that rowers are so keen to
go on using the same sort of kit that we all know works badly.

If your training matters, then your equipment matters too. Not all
equipment is equal.

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: carl@carldouglas.co.uk Tel: +44(0)1932-570946 Fax: -563682
URLs: www.carldouglas.co.uk (boats) & www.aerowing.co.uk (riggers)

marco.bovo · Joined: 18 Aug 2007 Posts: 4

> Why not? And then judge me by what I say & how I say it.

In italy we have a say that sound like: "You never ask to the butcher
which is the best meat". I'm not sure if it make any sense in
English Smile

Anyway I might agree with you and that is partially I am always very
careful in the choice of the materials.
Although I would say that probably at this moment I think that the
differences between different type of equipments can not account for
difference in performance of most of the crews.
I think that most of the difference in results can still be accounted
by fitness and technique in first place.

That said I agree with you: why putting parachutes on your boat?
Ciao,
Marco

>
> I don't think any manufacturer should claim that a crew won because of
> the equipment used. The crew won largely because it was fit, strong &
> efficient - in identical cars, Michael Schumacher would always beat me!
> However, with crews of identical potential the equipment used is bound
> to influence their overall performance because, just as race cars are so
> different in performance potential that a good driver in the wrong car
> or with the wrong tyres may never win a race, so boats, riggers, oars,
> fins, etc. will also affect performance.
>
> Thus, as an extreme instance, you cannot say that riggers with small
> parachutes attached will enhance the performance of any boat or crew
> going into a headwind in a race against a similarly equipped crew which
> lacks the added parachutes. Similarly, if one crew in that race has any
> other improvement to its aerodynamics, it gains an advantage over those
> which do not.
>
> Again, if the riggers on one boat, although aerodynamically identical,
> were very much less rigid, or the pins were off-pitch, the other crew(s)
> would have the advantage.
>
> You will note here that, in discussing riggers, I referred to their
> aerodynamics & mechanical properties. I did not fall into the popular
> trap of focussing on their materials of manufacture. What something is
> made of matters only to the extent that it allows improvements in
> aerodynamics, stiffness, strength or reliability. If you adopt a
> particularly vogueish material of manufacture but with it make a more
> bulky rigger or one which is less rigid or less durable, then you have
> adversely affected its user's performance.
>
> Similarly, it is impossible to argue against providing the crew with the
> optimum means of steering. So I find it odd that rowers are so keen to
> go on using the same sort of kit that we all know works badly.
>
> If your training matters, then your equipment matters too. Not all
> equipment is equal.
>
> 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: c...@carldouglas.co.uk Tel:+44(0)1932-570946 Fax: -563682
> URLs: www.carldouglas.co.uk(boats) &www.aerowing.co.uk(riggers)

Here's a tricky one (UK interest) This year 1st September falls on a Saturday and indeed is the day of Hollingworth Lake regatta. The rules on junior categories mention "the first of September preceding the event", so the question is (which relates to an entry I might make): a sculler who

Math for Shortwave Absolutely amazing! Beauty of Math! 1 x 8 + 1 = 9 12 x 8 + 2 = 98 123 x 8 + 3 = 987 1234 x 8 + 4 = 9876 12345 x 8 + 5 = 98765 123456 x 8 + 6 = 987654 1234567 x 8 + 7 = 9876543 12345678 x 8 + 8 = 98765432 123456789 x 8 + 9 = 987654321 1 x 9 + 2 = 11 12 x 9

A Strange Question about Math & RVs Hi Folks, I'm wondering if there are any math conversions currently needed in repair or usage of RVs that I can add to BTW, you might find the mileage calculator that's already there quite interesting. If you have any ideas, please let

Looking for a fishing class in Sydney I'm a very novice fisherman and would like to take a fishing class so I can learn about the basics of fishing and cleaning fish. Anyone know of any such classes?

Ricky wants a video Here ya' go Ricky, a video to watch. This is Shep and his set up from 2004. The 2005 motor runs methanol and spins at 10,500 rpm in the 1/4 WITH THE BALANCE SHAFTS REMOVED!!! Current ET is 7.87 I believe. This is the same motor as in my street car but in