Delft Series

Discussion in 'Sailboats' started by DavidG, Jan 10, 2002.

  1. DavidG
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    DavidG Junior Member

    Has anyone tried setting up a resistance prediction spreadsheet using the Delft algorithms proposed in either The Modern Yacht Conference 1998 or Chesapeake Symposium 1999.

    The advantage of this research is a seamless transition from displacement to planing mode to reflect Volvo and Open 60 type hulls.

    I have tried both, and find that resistance reduces as LCF is moved toward the transom, I appreciate that LCF separation is desirable but would expect a limit on how far aft this can go.

    I E-mailed Dr Keuning but got no reply.

    Any thoughts? Does anyone no of any more recent code?
     
  2. Steve Hollister
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    Steve Hollister Junior Member

    You can download PCSAIL, which is an Excel-based VPP program (freeware) at

    www.engin.umich.edu/class/eng100/400

    Actually, this is a class site at the Univ. of Michigan for a class called

    Introduction to Engineering 100
    Engineering Aspects of Sailing Yacht Design
    Prof. Robert Beck

    This site includes many other resources, like airfoil design and weight studies information.

    PCSAIL was published in the Chesapeake Sailing Yacht Symposium (CSYS) in 2001 under a title called: A Velocity Prediction Program for the Home Computer"

    This class site is interesting because it gives you the all of the information about the course, which is used as a basic engineering course. Bob Beck was my advisor at Michigan (years ago!) when I got my masters degree in hydrodynamics and it is interesting to see the homework assigned and the tests given - classic Beck! When you got the tests, you would look through the pages to find some easy question to start with - there weren't any! I remember one question he gave on a test where he said that two fishermen, sitting on the shore, were timing the period of the long waves or swells. The question was to determine how far away the storm or hurricane was.

    Keep in mind that the VPP theory comes in two parts - The Lines Processing Program (LPP) and the Velocity Prediction Program (VPP). The LPP uses the actual 3D hull shape to calculate things like displacement, wetted surface, keel draft, righting moments, and funny length values called LSMs. The purpose of the LPP is to reduce the 3D shape of the boat into a small group of dimensions that can be used by the VPP. The IMS rule uses the LPP to calculate the funny LSM values so that the length value used in the VPP is not sensitive to single or point measurements, which was the big problem in the older measurement rules.

    Because the VPP is based on these simple measurements, like length, beam, keel span, wetted surface, righting moments, there is a definite limit to the accuracy that can be achieved by the program. Even the Delft tank data used for residual resistance calculations is reduced to a (relatively) simple equation that is based on just a few dimensions. This doesn't mean that the VPP is not useful. It is just not in the same league as full, nonlinear CFD calculations. Therefore, I tell people that the VPP is good for doing what if questions or parametric variations to see how sensitive the performance is for changes in each variable. If a VPP says that a boat should go 6.52 knots at a particular wind speed and angle, don't expect to see that exactly on the boat. It might be off by 10 or 20%. The basic reason for this is that you cannot accurately reduce the infinite shape variations of hulls into the few variables used by the VPP. This means that the VPP is good for analyzing the performance variations based on changes of the principal dimensions, but don't expect much accuracy if you are making subtle changes, like changing the forward sections from U to V shapes.

    As far as stretching the VPP into the planing region, I haven't kept up with the latest research papers on sailboats so I can only guess. I thought that the people in charge of the IMS rule were trying to add factors that would account for light weight boat acceleration and surging and surfing. I didn't hear anything about full planing calculations for sailboats.

    I know that Savitsky tried this in a Marine Technology paper in 1976 for planing power boats. In the paper, he came up with a regression analysis for the displacement resistance for power boats up through the hump region. Then, he tried to show how it matched up with the resistance curve from his full planing theory. I programmed both of those techniques and it does seem to match up in many cases, but there were many other cases where the displacement mode resistance above the hump completely missed the planing resistance curve.

    The LCF shift that you mention is interesting because it sounds like what I see for planing powerboats. As a boat goes faster, the optimum position of LCG shifts towards the stern. The problem is that there are practical limits to how far the LCG can shift back, the gain in speed is very minimal, and the optimum position is different for each speed. I suspect that the same is true for LCF.

    Final notes:

    For an up-to-date discussion/tutorial of the field of computational fluid dynamics(CFD) for boats and ships, see the article called

    "Modern Computational Methods for Ships in a Seaway", by Beck and Reed in the references section in the web site above.

    I also have a big collection of technical links at

    www.newavesys.com/links.htm

    in the "Research-Journals-Symposia" section
     
  3. DavidG
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    DavidG Junior Member

    Thanks for your thoughts, I have previously looked at HullDrag-32 and PCSail also looks interesting. In fact PCSail looks very similar to the Wolfson VPP but is free!

    Obviously as engineers we must always consider the validity of the "black box".

    However neither seem to seemlessly cover Froude numbers 0.1 through to 0.6 which is the attraction of the Keuning's 1998/1999 papers, what I am interested in knowing is whether other users have found the algorithms/polynomials to be stable, for instace PCSail uses other factors from the 14th Chesapeake paper, but chooses to use the 1993 Hull drag algorithms!

    Your thoughts on LCF moving toward the transom on planing powerboats could be correct, it may be that for a low resistance sailboat hull, we should seek to maximise the LCF/LCB separation, within the constraints of a normal sailing yacht hull form (we should also consider that the aftmost LCF position of the parent hulls was -6.5%), I believe Farr places some importance on LCF separation.

    I have used the 1993 Delft series to optimise CP and LCB location for development class dinghies, where upright resistance is perfectly adequate, I would now like to look at LCF location.
     
  4. Steve Hollister
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    Steve Hollister Junior Member

    Now where are those papers? I know they are in one of these piles!?! These are the piles of technical papers I always mean to read. Keuning's papers are among them, so I really cannot comment on his extensions. However, I would give Keuning's work high credibility.

    A while back I tried to trace the history of development of the Delft series and the residuary resistance formulas that are created. Delft I, II, III, and variations. On top of these are some errors or inconsistencies I found in the original and newer formulas. Also, someone invloved with the IMS rule said that their version of the formula has reverted back to a variation of the Delft II formula. There is also a change(correction?) in the Larsson/Eliasson book from the first to the second edition. These are minor differences, but without the original Delft data, there is no way for anyone to analyze the regression analysis that is done. There is even no written (that I know of) document that describes all of the formulas of the official IMS/LPP+VPP programs. As far as I know, the only way to determine the formulas is to buy a copy of the VPP program from US Sailing and reverse-engineer out the equations. I had to do this for my own VPP program a while back. At that time it was a legacy of Fortran "spaghetti" code. BTW, you can't get the source code of the LPP portion of the software. That is where they analyze the 3D hull shape and do all of the appendage stripping. The LPP also includes some code which really should be part of the VPP.

    For those getting started, the Larsson/Eliasson book is the best source. For extensions/research, the best bet is to track the development of the VPP theory by getting all of the Chesapeake Sailing Yacht Symposia papers. (Does anyone provide corrections or errata sheets?) I am now paranoid about all formulas that I cannot verify.
     
  5. Stephen Ditmore
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    Stephen Ditmore Senior Member

    How it relates to this discussion I'm not sure, but in an old file somewhere I have an interesting pre-IMS article by the late Hugo Meyers on VPPs.

    Anyway, a few years back a client of Jim Taylor's from Nova Scotia had a model of the third "NUMBERS" built and tank tested, I think by IMD and supervised by Jim Teeters, hoping this would be the first of a new series. I think the results were made public. Do you know how those can be obtained, and if anything more came of this effort?

    This article might be of interest:
    http://www.naval-architects.org/page/newsart1

    Also, I think the VPP available from the Wolfson Unit in Southampton incorporates the work of the guy, whose name escapes me at the moment, who did the analysis work for Team New Zealand. I said I think - can someone confirm it?
     
  6. Stephen Ditmore
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    Stephen Ditmore Senior Member

  7. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Clay Oliver is the guy I'm thinking of (sorry for the multiple steam-of-conciousness postings). My question: how much America's Cup research has found its way into public domain VPPs, and is the one from Wolfson the most up-to-date in this respect?
     
  8. DavidG
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    DavidG Junior Member

    Interesting to see this thread come alive again. My understanding is that the Delft Series forms the basis for most VPP's including Wolfson. The question is which one!

    The beauty of the Delft research is that much of it is put into the public domain. I believe that it was originally intended as an initial design tool for use in the first loop of the design spiral.

    Your comment about America's cup research is interesting, maybe other users of the site might be able to identify any other useful public domain information useful to designers.

    I can't help but think that designers working with well funded AC syndicates can't help but use this information for their own designs, for instance what are the basis for different design offices Section Area Curves?

    It is interesting to look at a Farr SAC (lifted off a Beneteau lines plan) and compare it with the S&S curve in Kinney.

    On this subject, do any of you gentlemen remember how to "swing" a SAC to move the LCB or adjust for Cp (I lost my notes on this one).
     
  9. Stephen Ditmore
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    Stephen Ditmore Senior Member

    A paper was published in 1988 by John Marshall and perhaps Carl Scragg & John Letcher, published by SNAME and I think presented at Chesapeake, with section area curves used for 1987 candidate AC designs, showing how to vary them in the way you describe. I don't know whether the paper contained formulae for reproducing the curves, and I think the authors were still keeping mum about the research they did into bulbous bows in connection with the second of three boats they built.

    I, too, would be interested in what has been published since, especially as I suspect Laurie Davidson and Team New Zealand have been doing something just slightly different from the other groups where the SAC is concerned. I'd also be interested in the 1987 bulbous bow research, though the boat with this feature was not successful, and bulbous bows are not legal in the IACC class.
     
  10. DavidG
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    DavidG Junior Member

    Stephen, I had a look on the SNAME site and the Cheasapeake site for the paper you refered to, the only ones that I could find were Stars & Stripes papers by Carl Scragg or John Letcher, I couldn't find any John Marshall papers. I wonder if you have any more detail on the paper?

    I am sure that if the LCB, LCF, Cp and section shape are right, the other important factor is the SAC, if you can get all these right, the expense of tank testing or CFD become rather unecessary (except for appendage design) i.e. let the AC Syndicates pay for it!
     
  11. Stephen Ditmore
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    Stephen Ditmore Senior Member

    I have it in a closet at home, and I'll try to remember to get it out so I can cite it accurately. As I remember there were two Stars & Stripes papers, one on hull design and one on keel design. It's the one on hull design I'm thinking of.
     
  12. Stephen Ditmore
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    Stephen Ditmore Senior Member

    I failed to put my hands on the article quickly, but whether or not I have the authors right it's like I said, the Stars & Stripes paper presented at Chesapeake in '88, the one on hull design, not the one on keels. Put that together with the info on where LCB should be, and what Cp should be, in Larsson, and you should be able to get these things pretty close to right.

    That's not to say there's not room for improvement, though, and I expect that the boats in the coming America's cup will achieve yet another level of refinement given how good the winning boat appeared to be last time 'round.
     
  13. Steve Hollister
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    Steve Hollister Junior Member

    Swing SAC

    DavidG asked

    "On this subject, do any of you gentlemen remember how to "swing" a SAC to move the LCB or adjust for Cp"

    Principles of Naval Architecture (PNA) Vol I, page 19 talks about geometrical modification to lines, including a method known as "swinging stations".

    It's interesting how this "old" method is not generally known these days. I have known about these methods for years, especially the method described by Lackenby, H. "On the Systematic Geometrical Variation of Ship Forms", 1950, Transactions, RINA. (see also Computer Program Documentation: Hull Form Derived From Parent, Maxine Botting, DTMB, February, 1967, which implements the Lackenby technique) This method allows you to take a parent hull form and modify the LCB, CP, and parallel middle body of a vessel. This is done by performing a quadratic shift of the stations of the boat without changing the shape of the stations. You can specify what values you want and the formulas will tell you how to shift the stations. The only limitation is that you cannot "distort" a parent hull too much without generating odd results.

    Customers of mine will note that our software has contained a Lackenby hull variation module for years. Also, in 1996, I wrote and presented a SNAME paper called "Automatic Hull Variation and Optimization". (see www.newavesys.com/articles.htm ). I extended the Lackenby technique so that you could take a parent hull and specify what values of LWL, BWL, depth, draft, displacement, Cp, LCB, parallel middle body and Cm that you wanted. You could do this using a fixed displacement or a fixed draft constraint. A program would take the parent hull and automatically modify it to achieve the desired target values. The optimization part of the program would then evaluate the hull shape to obtain values like wetted surface and half angle of entrance of the waterplane to input into a resistance calculation program. You could then plot contours of resistance by varying input values such as LCB and Cp, while fixing values like displacement and LWL.

    There is also another approach to generating hull shapes from scratch based on form parameters or from the shape of an input sectional area curve. If you really want to get into this stuff, then you should pick up a copy of a book by editors: Nowacki, Bloor, and Oleksiewicz called "Computational Geometry for Ships", World Scientific", 1995. Horst Nowacki was a professor of mine at U of Michigan (now at TU Berlin) who has done a lot of work over the years related to hull shape generation and variation. See also the references in the back of my SNAME paper.

    These hull variation techniques are nice for the conceptual design stage because they are based on an existing parent hull shape and a few basic form factors. They also match up well with empirically-based resistance techniques, like the VPP, because these resistance techniques are also based on many of these same basic form factors. You won't be able to distinguish the subtle differences between slightly 'U' or 'V' shaped sections, but you will be able to evaluate and optimize the main shape parameters of the design.
     
  14. DavidG
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    DavidG Junior Member

    Thanks Stephen & Steve, I will follow up your references.

    Some credit should go to Stuart Roy at Southampton Institute who took some time out to show us these techniques (a few years ago) unfortunately somehow I lost the notes, or it seemed so obvious at the time, that I didn't need to take them!

    I think Maxsurf also has a parametric modeling tool, you put your desired Cp or LCB is, and it does the rest.
     

  15. ErikG
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    ErikG Senior Member

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