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  #136  
Old 12-15-2010, 12:58 AM
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Alik Alik is offline
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Quote:
Originally Posted by Leo Lazauskas View Post
Just a couple of points...

1. Boundary layer thickness was thought to have an influence, but then Doctors et al changed their mind in the light of their more recent experiments.

Form the abstract:
"There is now strong evidence that the transom-draft Reynolds number plays a much lesser role in this phenomenon than previously thought."

A Study of Transom-Stern Ventilation
Doctors, L.J., Macfarlane, Gregor J., and Young, Richard,
International Shipbuilding Progress.

2. I agree that turbulence stimulators are necessary at low speeds on small models, but they introduce quite a lot of uncertainty themselves.
There is a very interesting paper on this topic.

Bertorello, C., Bruzzone, D., Caldarella, S., Cassella, S., Cassella, P. and Zotti, I.,
``From model scale to full size. Investigation on turbulence stimulation
in resistance model tests of high speed craft",
Proc. FAST 2003,
Ischia, Italy, Oct. 2003, pp.\ A1-1--A1-8.

It should send a chill through anyone who places too much faith in experiments on small models
Thanks Leo,

1. Agree, but this effect exists. If model is running with wet transom and full-scale boat is dry transom, there should be some difference. If the flow from propeller effects flow separation at transom, there should be some difference also.

2. 62 series models were 8' long. Do You agree that without turbulence stimulators such tests at FnL=0.3 and lower can not be considered accurate enough?
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  #137  
Old 12-15-2010, 01:39 AM
Ad Hoc Ad Hoc is offline
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Originally Posted by Alik View Post
Do You agree that without turbulence stimulators such tests at FnL=0.3 and lower can not be considered accurate enough?
Everything in physics is based upon the quality of the devices used for measuring an observed event.

The methodology and the accuracy of the measuring instruments being used play a significant part in any “experiment”. One can only answer the question of low Fn experiments by knowing what they measured how they measured it and with what devices and how accurate these devices were; are they calibrated correctly and frequently?.

Theories are based upon real world observation. But the observation requires measurements...and measurements that are qualitative. The more accurate the device the better the quality data, which leads to better theories.

As Leo has noted before, quiet rightly, the procedures/equipment that some ITTC test tanks use in their experimentation are somewhat lacking. Hence providing spurious results and leads to further confusion and hence lack of confidence.
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  #138  
Old 12-15-2010, 02:01 AM
antiqueuniflite antiqueuniflite is offline
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semi displacement hull at displacement speeds?

is this plausible for all weather conditions? 27.3' uniflite is the boat me and my dad are wondering what to re-power it with?
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  #139  
Old 12-15-2010, 02:42 AM
Mr Efficiency Mr Efficiency is offline
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It's all in the vibe, I think ?
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  #140  
Old 12-15-2010, 05:28 AM
Joakim Joakim is offline
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Originally Posted by Alik View Post
My numbers come from own experience and communication with colleagues.
Yes, I mean commercially successful design offices having income exclusively from development of sailboat designs.
You can list those offices here, as well as number of sailboat designs they produce per year
If you specify exclusively on sailboat design, you may be right. But here are some which certainly design several sailboats a year and at least part of the staff is likely working about 100% with sailboats:

http://www.hoekdesign.com/
http://www.judel-vrolijk.com/
http://www.finot-conq.com/
http://www.germanfrers.com/
http://www.farrdesign.com/
http://www.juanyachtdesign.com/
http://www.mills-design.com/
http://www.kerdesign.com/index.html
http://www.botincarkeek.com/
http://www.simonis.com/
http://www.reichel-pugh.com/index.html
http://www.humphreysdesign.com/
http://www.felciyachts.com/ita/
http://www.blankyachtdesign.de/
http://web.me.com/jcboats/CorbyYachts/HOME.html
http://www.seaway.si/

I don't know how many sailboats they designed 2010.

Then there are many local small offices many of which do not even have web sites.

Here is one from Finland:
http://personal.inet.fi/yritys/strahlmann/index.htm
He designed at least three production sailboats for 2010 (one designer office).

Also there are several people designing purely sailboats for just one brand/company, like Jeppesen, but these you don't likely count, since they are not design offices.
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  #141  
Old 12-15-2010, 06:04 AM
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Alik Alik is offline
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Quote:
Originally Posted by Joakim View Post
If you specify exclusively on sailboat design, you may be right. But here are some which certainly design several sailboats a year and at least part of the staff is likely working about 100% with sailboats:

http://www.hoekdesign.com/
http://www.judel-vrolijk.com/
http://www.finot-conq.com/
http://www.germanfrers.com/
http://www.farrdesign.com/
http://www.juanyachtdesign.com/
http://www.mills-design.com/
http://www.kerdesign.com/index.html
http://www.botincarkeek.com/
http://www.simonis.com/
http://www.reichel-pugh.com/index.html
http://www.humphreysdesign.com/
http://www.felciyachts.com/ita/
http://www.blankyachtdesign.de/
http://web.me.com/jcboats/CorbyYachts/HOME.html
http://www.seaway.si/

I don't know how many sailboats they designed 2010.

Then there are many local small offices many of which do not even have web sites.

Here is one from Finland:
http://personal.inet.fi/yritys/strahlmann/index.htm
He designed at least three production sailboats for 2010 (one designer office).

Also there are several people designing purely sailboats for just one brand/company, like Jeppesen, but these you don't likely count, since they are not design offices.
In Your list, just at first glance:
- Judel/Vrolijk design powerboats one of those is very nice Grand Azur 33...
- Mills was asking on this forum questions on powerboat design...
- Humphreys has designed motoryachts one of famous is Ermis...
- Etc. etc. etc.

Dig a bit deeper and You will find a lot of interesting facts
Besides many of designers also provide brokerage services, so they do not make living on design only...
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  #142  
Old 12-15-2010, 06:39 AM
Joakim Joakim is offline
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Quote:
Originally Posted by Alik View Post
In Your list, just at first glance:
- Judel/Vrolijk design powerboats one of those is very nice Grand Azur 33...
- Mills was asking on this forum questions on powerboat design...
- Humphreys has designed motoryachts one of famous is Ermis...
- Etc. etc. etc.
I didn't say these would be exclusively designing sailboats. Are you saying they are designing mainly powerboats like your office?
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  #143  
Old 12-15-2010, 07:43 AM
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Perm Stress Perm Stress is offline
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Originally Posted by Perm Stress View Post
Sorry for being so stubborn, but my question was "why I could not use data, as published by the author?". If author find it possible to publish data starting from Fn=0.25, it is hard for me to see the reason not to use it.
The question is where the data is published and why they are not included in technical paper? Agree that PBM is not scientific publication.

I will ask Donald about this data, really interesting.

It would be interesting to see the answer here.
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  #144  
Old 12-15-2010, 09:17 AM
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Alik Alik is offline
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Originally Posted by Joakim View Post
I didn't say these would be exclusively designing sailboats. Are you saying they are designing mainly powerboats like your office?
I am sure some of them are now...
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  #145  
Old 12-15-2010, 09:20 AM
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Alik Alik is offline
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Originally Posted by Perm Stress View Post
:
It would be interesting to see the answer here.
Yes, for me also.

But the details I found today morning are quite important - if there were no turbulence stimulators so the results are not reliable.
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  #146  
Old 12-15-2010, 10:21 AM
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Leo Lazauskas Leo Lazauskas is offline
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Quote:
Originally Posted by Alik View Post
1. Agree, but this effect exists. If model is running with wet transom and full-scale boat is dry transom, there should be some difference. If the flow from propeller effects flow separation at transom, there should be some difference also.
Yes, there is something going on but, as I said previously, it's a poorly understood flow regime. The experiments of Doctors et al suggest that BL effects are not important, so the differences between small models and full-size could be due to unsteadiness, non-linearities, surface tension, viscosity, wave-breaking, all of those, or something else.

Quote:
Originally Posted by Alik View Post
2. 62 series models were 8' long. Do You agree that without turbulence stimulators such tests at FnL=0.3 and lower can not be considered accurate enough?
Yes, I agree.
But, even if stimulators were used, I would still be suspicious of the data.
And moreso if the ITTC line was then used to extrapolate to full-size.

Leo.
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  #147  
Old 12-15-2010, 10:25 AM
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Leo Lazauskas Leo Lazauskas is offline
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Quote:
Originally Posted by Perm Stress View Post
:
Sorry for being so stubborn, but my question was "why I could not use data, as published by the author?".
1. Because the data was not meant to be used in the manner you propose to use it.
2. Because it would be unprofessional to misuse the data that way.
3. Because one should not have blind faith in results, even if they are published in a reputable journal.

Just a few reasons off the top of my head

Leo.
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  #148  
Old 12-15-2010, 10:32 AM
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Alik Alik is offline
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Originally Posted by Leo Lazauskas View Post
Yes, I agree.
But, even if stimulators were used, I would still be suspicious of the data.
And moreso if the ITTC line was then used to extrapolate to full-size.
I had similar problems tank testing small-scale sailboat hull models, at low Fn. Since then, used to be careful with TT results
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  #149  
Old 12-15-2010, 10:33 AM
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daiquiri daiquiri is online now
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Quote:
Originally Posted by Leo Lazauskas View Post
Quote:
Originally Posted by Alik View Post
2. 62 series models were 8' long. Do You agree that without turbulence stimulators such tests at FnL=0.3 and lower can not be considered accurate enough?
Yes, I agree.
But, even if stimulators were used, I would still be suspicious of the data.
And moreso if the ITTC line was then used to extrapolate to full-size.
So it follows that with or without turbulators the test data are all hopelessly inaccurate for displacement speeds? In other words, no hope for the humanity to get some reliable low-speed data from towing tanks? :\

A logical question then follows: ...and now what?
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  #150  
Old 12-15-2010, 10:50 AM
Joakim Joakim is offline
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Originally Posted by daiquiri View Post
So it follows that with or without turbulators the test data are all hopelessly inaccurate for displacement speeds?
Why only for displacement speeds?
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