Software for flow analysis of moving boat

[Richard Woods]

As Alik said, flow analysis is complicated. Even naval architects can get into trouble using it.

So my thinking was really that going that deeply into the design of the boat was too much for someone who wasn't a naval architect student. After all, when Ad Hoc and I were students non one used flow analysis, yet boats designed back then still met their SOR

So in fact the OP might have been better to add the word "Do" and said "Do I need to perform flow analysis on a moving hull profile?"

Richard Woods of Woods Designs

www.sailingcatamarans.com

[Ad Hoc]

Originally Posted by Richard Woods

. After all, when Ad Hoc and I were students non one used flow analysis, yet boats designed back then still met their SOR

Exactly.

The proliferation of fancy coloured plots from software, which is generally not verified, just fools everyone into "believing it".

The only flow analysis we do, is via tank testing with dye or tail tails and video/photographing the flow result. Beyond that..unless you are doing a PhD or wish to learn programming/advanced mathematics, it is of no real use in everyday design.

Unless of course you buy one and test it validate it yourself from many sources to provide "accurate" results. But this takes a long time in gathering the data and verification. Also, even then when used in anger it is just a tool to provide sensitivity analysis, again based upon the data (limits) you have previously used to validate and verify the program.

Hydrodynamics is just one small part of the endless disciplines that are required to satisfy the SOR. Thus, don't focus on the details, look at the bigger picture...trends, not absolutes.

[Leo Lazauskas]

Originally Posted by Ad Hoc

Hydrodynamics is just one small part of the endless disciplines that are required to satisfy the SOR. Thus, don't focus on the details, look at the bigger picture...trends, not absolutes.

That is sage advice, particularly when you consider that tank testing is also subject to a great deal of uncertainty.
Sadly, I think your collective advice will have absolutely no effect on the majority of new ship "designers" who will
continue to use hydro codes and/or tank tests as absolute truths.

Over the years, I have tried to emphasise the great insights that can be gleaned from examining the free wave spectrum
of hulls. (In his famous book, J. N. Newman also points out the utility to naval architects of the function).
Note, I mean "insights" and trends, not absolute achievable goals.
In 10 years only two people have emailed me with comments or questions.
The trouble is that it takes time to work through the process and, for example, to try different bow shapes, hull lines,
and demihull spacing to see how the free wave spectrum (and hence wave resistance and wave-making) is affected.
I doubt that working naval architects have the time to try out such investigations.

For your mild amusement...
I attached a design spiral to a short report on SES for a Swedish group just so they knew where my hydro calculations
sat in the process. I suspect that you won't argue with my suggestion.
Apparently the Swedes were very much amused that an Australian used an anti-clockwise spiral.

[DCockey]

Originally Posted by Ad Hoc

Agreed.

However, there is a big difference between a student who comes on says:

I have done some research...i found that X states Y and another that A states that B is correct...which would you advise or suggest is applicable.

The inference here is that the student has already done some research and is asking for a wider opinion to assist.

That is a far cry from those that pop up asking for detailed technical information, which, as you're both implying, is outside their course work and constantly ask follow up questions because it is not their field and are thus still confused. If they did the research to begin with, they would know. Isnt that what you're suggesting that they should be doing to begin with?

Yes, though research today usually starts on the internet with Google or another search engine, and Boat Design Forum posts frequently show up. Other reasonable questions are a simple request for a place to pursue the request such as "what's a good book for learning about xxxx?", or "I've done some research and I'm still confused; can someone help?" The latter is frequently the result of an initial misunderstanding or bad assumption.

Thus it appears that many are wishing to get a short circuit to a "click here for answer" button. Helping students along the way is fine as it is our prerogative to train up the next generation of naval architects. (I have been doing this for a long time now all ages, and also occasionally teaching post-grad students). But when part of the training is simply giving them the answer, as the student is unwilling to do the leg work themselves to begin with .. is another matter entirely.

Unfortunately too many students are looking for a short cut, and finding the "answer" on the internet is a favourite one. Perhaps it can be more constructive if possible to respond with a polite explaination about why there isn't a shortcut, though it can be tempting to not be so polite, particularly when it appears the student hasn't been paying attention in their studies.

[Leo Lazauskas]

Originally Posted by DCockey

Unfortunately too many students are looking for a short cut, and finding the "answer" on the internet is a favourite one. Perhaps it can be more constructive if possible to respond with a polite explaination about why there isn't a shortcut, though it can be tempting to not be so polite, particularly when it appears the student hasn't been paying attention in their studies.

It might also be helpful if there was kind of compendium of design "disasters" where someone has blindly used results from CFD, Michlet, or poor quality tank tests.
Students might be interested to know the dollar value of how much their poor judgement could cost them, or the organisation they work for.

I have no idea of the penalties for under-powering, over-powering or poor off-design performance.
I can only give people the tools to make the mistakes.

[Ad Hoc]

Originally Posted by DCockey

Perhaps it can be more constructive if possible to respond with a polite explaination about why there isn't a shortcut, though it can be tempting to not be so polite, particularly when it appears the student hasn't been paying attention in their studies.

Just about everyone does this from the outset, taking a neutral stance with a brief explanation. Unfortunately it does not take into account the receivers mental attitude nor their bias nor the perceived time frames.

It strikes me as odd, when someone is ill, they go to the local GP (Doctor) for a diagnoses and “cure”. Or when some kind of legal transaction is being conducted, one seeks a lawyer, or even when the car is miss firing smoking and making endless random noises, it is taken to the garage to be fixed.

Thus, this process is putting their faith and trust into professionals whom have not just studied, read and done research, but also practice said studying from the theory to the practical day to day usages. Over a period of time we simply call this... experience. I cannot perform heart surgery just because I have read a book on it!

I don’t know about you, but I don’t constantly argue with my GP saying I don’t agree with his advice because it is not what I wish to hear, or the car mechanic saying, I’ve read about this on the internet and you’re telling me the opposite I don’t believe you know what you’re saying, and so on…on it goes time and time again.

Yet that is exactly what occurs here on this website. Posters come on with “a problem” and they seek advice. So are they seeking advice from professionals and very experience amateurs who are just as knowledgeable, (who are not charging any money for giving such advice) or do they just want to find someone that agrees with their thinking? ..or gives them the answer NOW!! damm it NOW!!!

It seems lately many come on here seeking someone to agree with their own findings, because they don’t want to hear the truth. The more acquiesces they find the more they feel vindicated in their initial assumptions prior to seeking advice. The truth being defined as, advice from any one of the professionals, inter alia, on this website with the experience to tell them the truth about their situation.

These types of posters you cannot help. They are the type that argues with their GPs or car mechanic or dentist etc, because “they know better”. Which prompts the obvious, why come on here and seek advice in the first place then??

As for me, they can take the advice or leave it, doesn’t bother me one bit. I think perhaps, for me anyway, my lack of “buying into” their reasoning upsets them and sends them off into rants. Engineering is factual based, not emotive based. A + B= C is not emotive, it is a simple factual statement, go figure!

Originally Posted by Leo Lazauskas

I have no idea of the penalties for under-powering, over-powering or poor off-design performance.

It can be significant!

One vessel we designed many years ago had fuel consumption as part of the penalties. The penalty for each litre per hour over that which was agreed, was €10,000, yes €10,000 per litre! This was in 1995…so in today’s money probably double that! The vessel in question was carry 500 passengers.

Another typical contractual clause, if the boat is more than 2 knots under speed, rejection.

So, finding the trends and balance is terribly important!

[Ad Hoc]

Originally Posted by Leo Lazauskas

For your mild amusement...
I attached a design spiral to ....

I have them going both directions...i pick and choose one that keeps the client happy

[Leo Lazauskas]

Originally Posted by Ad Hoc

It can be significant!

One vessel we designed many years ago had fuel consumption as part of the penalties. The penalty for each litre per hour over that which was agreed, was €10,000, yes €10,000 per litre! This was in 1995…so in today’s money probably double that! The vessel in question was carry 500 passengers.

Another typical contractual clause, if the boat is more than 2 knots under speed, rejection.

That's a good number to have in the back pocket.

It also explains why Grigson was moved to examine skin-friction
lines and form factors after he noticed a rash of vessels that
were under-powered by about 5% back in the 1990's.
He put it down to deficiencies in the ITTC 1957 line.

Of course, naval architects can include "margins" to help them
stay within design tolerances, but there is something fishy about
the whole extrapolation and form factor approach, IMO.

As you know, the NPL series experiments of Molland et al, recommnend
form factors of the order of 1+k = 1.4 or higher.
Tony Armstrong suggested that they were too high because the drag
of the transom should be scaled differently. His form factors are
more reasonable, e.g. about 1+k = 0.96 to 1.05 for the same series.
Yes, that implies negative form drag for the most slender NPL hulls,
but that's just a consequence of using the ITTC line.

I am working on something similar now, and that's why I've been posting
lots of nonsense while I wait for computer runs to finish

I'll try to put something together soon to show different estimates at
full-scale that result from using different assumptions for skin-friction,
form drag, and scaling. I suspect that there will differences of about
2% to 7% depending on which tank tests you believe and how much faith one
places in the various friction lines and scaling procedures.

Leo.

[Ad Hoc]

Originally Posted by Leo Lazauskas

As you know, the NPL series experiments of Molland et al, recommnend
form factors of the order of 1+k = 1.4 or higher.
Tony Armstrong suggested that they were too high because the drag
of the transom should be scaled differently. His form factors are
more reasonable, e.g. about 1+k = 0.96 to 1.05 for the same series.

I only have one of Tony’s papers in my library, but the form factors he uses/quotes, I can’t find any data for the hull. It is also a single hull tested i believe too. Where as Molland et al, have tested a large range of hulls with a wide range of various L/B and L/D ratios.

Bottom line is, the form factors at various hull configurations at different Fns are different, not a single absolute as such. Molland does provide a trend for various hull types…see, there we go again, trends. Good enough for me

[daiquiri]

Originally Posted by Leo Lazauskas

Apparently the Swedes were very much amused that an Australian used an anti-clockwise spiral.

Hey Leo, why am I not surprised by that?

[Leo Lazauskas]

Originally Posted by Ad Hoc

I only have one of Tony’s papers in my library, but the form factors he uses/quotes, I can’t find any data for the hull. It is also a single hull tested i believe too. Where as Molland et al, have tested a large range of hulls with a wide range of various L/B and L/D ratios.

Bottom line is, the form factors at various hull configurations at different Fns are different, not a single absolute as such. Molland does provide a trend for various hull types…see, there we go again, trends. Good enough for me

Sure. And as the latest ITTC noted, form factors using the ITTC line show a Reynolds number dependence, which is contrary to how they are supposed to work. Form factors using Grigson's line (and others?) do not show such a dependence.

Armstrong's equation for NPL series form factors apply to the entire series
and can be found in:
"Practical evaluation of resistance of high-speed catamaran hull forms—Part I",
Prasanta K Sahoo, Marcos Salas and Adam Schwetz,
Ships and Offshore Structures, 2007, Taylor and Francis.
Apparently a copy will be in your email box soon.

Leo.