Resistance factors, planing hull at low speed

[Alik]

Originally Posted by Leo Lazauskas

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?

[Ad Hoc]

Originally Posted by Alik

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.

[antiqueuniflite]

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?

[Mr Efficiency]

It's all in the vibe, I think ?

[Joakim]

Originally Posted by Alik

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.

[Alik]

Originally Posted by Joakim

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...

[Joakim]

Originally Posted by Alik

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?

[Perm Stress]

:
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.

[Alik]

Originally Posted by Joakim

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...

[Alik]

Originally Posted by Perm Stress

:
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.

[Leo Lazauskas]

Originally Posted by Alik

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.

Originally Posted by Alik

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.

[Leo Lazauskas]

Originally Posted by Perm Stress

:
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.

[Alik]

Originally Posted by Leo Lazauskas

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

[daiquiri]

Originally Posted by Leo Lazauskas

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?

[Joakim]

Originally Posted by daiquiri

So it follows that with or without turbulators the test data are all hopelessly inaccurate for displacement speeds?

Why only for displacement speeds?