america's cup yacht

[kreg]

i'm doing some experimet with hydromax about the stability of an america's cup yacht. I would like to ask you where VCG is located from waterline in a yacht with a draft about 4 meters.
Thanks

[Andrew Mason]

roughly 2 metres is a good starting point.

Also be aware that under the revised AC rule, draft is now 4.1 (not 4.0)metres and displacement max is now 24,000kg (not 25,000)

[kreg]

thank you For your attention.
Could ask you another thing?
24,000 kg is just the weight of the yacht hull except the sail etc.?
So, do I have to test the stability with total weight?
I have read the rules version 5 but i did not understand where freeboard shall be measured.
do you think i have to get the measure from MWL plane or LBG plane?
Thanks very much for your help.
You are very helpfully.

[Andrew Mason]

It depends why you want to measure the stability, if it is to determine righting moment for a VPP then obviously you want to measure the sailing condition with sails on board rather than the measurement condition.

Freeboard are taken from the MWL plane, but at the position of the intersection of the LBG plane with the fore and aft overhangs of the hull. The LBG plane is 200mm above the MWL plane.

If that explanation isn't clear let me know and I will post a diagram.

regards

Andrew

[kreg]

it's very clear.
I would like ask you the last ones.
1) Why are there limits (penality) on freeboard and which are the advantages that you can have with lower freeboards?
2) If i understood well this type of yacht entrusts its stability to the weight indeed to the shape. Is it right?
Do you think that a lower beam is better for wave resistence?
Thanks million

[Andrew Mason]

This is really where you should be asking someone who has been involved as a designer of AC boats, however my uninformed guesses would be -

1. I assume that lower freeboard reduces weight windage and seaworthiness, so my guess is that the freeboard limits were imposed to make sure that the boats did not start trading off seaworthiness and safety for speed.

2. Stability on an AC yacht is dominated by the huge bulb and large draft, and beams have reduced progressively over the past 10 years as a result. I guess most designers would argue that beams have reduced to a practical minimum, both from the point of view of the trade off between wavemaking resistance and form stability, as well as limitations on shroud base and sheeting angles imposed by narrower beams. Now that the AC rule has effectively locked length, displacement, draft and sail area, I think a lot of designers will be looking closely at beam.

[kreg]

ok thanks i understood...
i have the last question....
which methods do i have to use for resistence in hullspeed for this type of yacht?
thanks again for your time.

[Andrew Mason]

Only Holtrop, Delft series, or the Slender Body method are suitable.

[kreg]

ok thanks million

[redcoopers]

Kreg,

About your second question, slender ship (Michell) theory predicts that beam is proportional to wave resistance squared. But this is for upright resistance only, therefore there are many caveats:

First, downwind, a large beam aft will help a boat get on a plane and will dramatically reduce resistance. Slender ship theory cannot predict this (since most slender-ship theories do not account for trim and heave).

Secondly, upwind, the more beam you have, the more form stability you have. This means you can carry more sail area to produce drive. Remember that the moment created by the sails has to be equal to the righting moment from the keel and hull.

Therefore, it's a huge compromise. The initial IACC yachts were quite beamy compared to the last generation. They slowly have been getting more narrow, but I think they have reached a point of negative returns. I would expect the new IACC yachts to have just a little more beam than the last round (but I'm not working for any team, so what do I know!)

-Jon

[kreg]

i have another little question...
Are there limitations on aft and forward hoverongs?

[Mario]

Only Holtrop, Delft series, or the Slender Body method are suitable.
In Hullspeed Help is said that algorithm of Holtrop method is designed for predicting the resistance of tankers, general cargo ships, fishing vessels, tugs, container ships and frigates. The results of Holtrop and Delft series are pretty different for a sample of the AC yacht. Only at cca 11 knt the resistance is the same.
What is the diferent between Delft series I, II and III? Are there exactly shapes of the hull assigned to either Delft series I,II or Delft series III?

[Andrew Mason]

Kreg

My understanding is that the length of the overhangs is not limited by the AC rule.

Mario

The Holtrop work was done on fast ship forms, the Delft series on yachts only, for more details see -
http://www.rina.org.uk/rfiles/HISWA/1996/The%20bare%20hull%20resistance.pdf

[kreg]

thanks

[Leo Lazauskas]

Just a couple of minor points:

1. Slender ship theory and Michell's thin-ship theory are not the same.
2. Thin-ship theory can be used to predict trim and sinkage. E.g. see
SWPE: Speed-up and Squat
www.cyberiad.net/library/pdf/tsl01a.pdf

Cheers,
Leo.


Kreg,

About your second question, slender ship (Michell) theory predicts that beam is proportional to wave resistance squared. But this is for upright resistance only, therefore there are many caveats:

First, downwind, a large beam aft will help a boat get on a plane and will dramatically reduce resistance. Slender ship theory cannot predict this (since most slender-ship theories do not account for trim and heave).

Secondly, upwind, the more beam you have, the more form stability you have. This means you can carry more sail area to produce drive. Remember that the moment created by the sails has to be equal to the righting moment from the keel and hull.

Therefore, it's a huge compromise. The initial IACC yachts were quite beamy compared to the last generation. They slowly have been getting more narrow, but I think they have reached a point of negative returns. I would expect the new IACC yachts to have just a little more beam than the last round (but I'm not working for any team, so what do I know!)

-Jon

[redcoopers]

Yes, you are exactly right.

Slender ship is used by Michell's eqn, but Michell's eqn is not the totality of slender ship. However, to make a velocity prediction on a sailboat, I would do the following steps:

1) Slender ship, Michell's eqn, or Delft parametric for initial design (I prefer Delft series because it may help judge what sort of lift we need. The downside of any code which uses slender ship is that lift cannot be predicted... if I'm wrong, please tell me how I can incorporate lift with slender ship theory, I'd like to know!)
2) SPLASH or another potential flow free surface code (boundary integral: for an example see Faltinsen) for wave-making resistance with the incorporation of the Kutta condition.
3) RANS (or another viscous code) for appendages or blunt body separation affects.

I tend to put all sorts of slender ship theories into the first step for yachts. Although many improvements can be made, I still think that these are only the first methods for sailboat design because of the absence of lift prediction on the keel and rudder. Michell's eqn can predict trim and heave moments and forces (see Yeung 197?). Moreover, I think that for most modern yachts, an improvement could be made to switch from slender ship to shallow ship theory. This would need a redesign of the calculations (anyone remember Green's functions... PDE's ?), and may be more applicable to yachts. However, I think that the basic necessity of yachts needing the Kutta condition prevents this technique from becoming predominate.

In the long run, what I think yacht design needs is a potential flow code which quickly calculates the wave-making resistance which incorporates the kutta condition. Boundary integral theories are quick, but an integral equation solver can be more accurate. Finally, having to make a wake-cut to satisfy Kelvin's theorem means that the solution is only as accurate as the cut which is made. I truly belive that this is the big problem to yacht hydrodynamics. Does anyone have suggestions?

[redcoopers]

Oh Leo, by the way,

Your SWPE5.0 code is very impressive. The wave prediction profiles on the Wigley hull are awesome (I would presonally almost kill for those results with my codes :) ). I'm just wondering, however, if you know what happens when the L/B ratio goes to ~ 4 or 3?

-Jon

[Leo Lazauskas]

I'm not sure what you mean by "lift" in some of your statements. Do you mean the lift due to leeway, or the dynamic lift (and trim), i.e. squat? Or both?

I'm also not quite sure what you mean by the Kutta condition. Do you mean it in the sense of a smooth detachment condition for a yawed hull? Or are you suggesting the Kutta condition is necessary for vessels with transom sterns?

Good luck with the RANS solvers. I hope you have a lot of available computer muscle and time!

You mention that lift on the keels and rufdders is an issue for you. I don't have a hydro code that includes lift and induced drag in ship calculations, but I can include them via other codes I have written. Unfortunately, the Vortex Lattice Methods (VLM) used by many workers are not accurate enough for my purposes. They work very well for lifting surfaces with planforms comprised of straight-line segments (e.g. rectangles, trapezoids, delta wings etc) but they are pretty poor and inconsistent for small aspect ratio curved keels, fins, and rudders.

I have put up some benchmark solutions for wings with circular planforms (for which there is an analytical solution) on my WWW pages. Lifting line theory is hopeless for this case, as you would expect. I'd be very interested to see how commercial codes perform on elliptical planforms (either flat or cambered), but I suspect that many commercial solvers can't handle infinitely thin surfaces.

I think that some aspects of "shallow ship theory" are relatively easy, but other are very tough. See, for example,
"Wave patterns and minimum wave resistance for high-speed vessels"
www.cyberiad.net/library/tsl02b.pdf

My first suggestion for improving resistance estimates has nothing to do with potential flow, but rather with the estimation of viscous drag. I think that one of the best improvements you can make is to dump the ITTC 1957 line. It is not physics-based and there are far better alternatives IMO, e.g. Grigson's algorithm. See:
www.cyberiad.net/bl/grigson/grigson1.htm

But a caveat - you still have to estimate the form drag which is not easy! There are many who still claim that the ITTC line is not a skin friction formula, but rather a correlation line and thus includes some allowance for 3D effects. I am guilty of thoughtlessly chanting that same mantra, but it is just not true. Since the 1978(?) ITTC, the formula is used as nothing more that a simple skin-friction estimator.

Regards,
Leo.

Yes, you are exactly right.

Slender ship is used by Michell's eqn, but Michell's eqn is not the totality of slender ship. However, to make a velocity prediction on a sailboat, I would do the following steps:

1) Slender ship, Michell's eqn, or Delft parametric for initial design (I prefer Delft series because it may help judge what sort of lift we need. The downside of any code which uses slender ship is that lift cannot be predicted... if I'm wrong, please tell me how I can incorporate lift with slender ship theory, I'd like to know!)
2) SPLASH or another potential flow free surface code (boundary integral: for an example see Faltinsen) for wave-making resistance with the incorporation of the Kutta condition.
3) RANS (or another viscous code) for appendages or blunt body separation affects.

I tend to put all sorts of slender ship theories into the first step for yachts. Although many improvements can be made, I still think that these are only the first methods for sailboat design because of the absence of lift prediction on the keel and rudder. Michell's eqn can predict trim and heave moments and forces (see Yeung 197?). Moreover, I think that for most modern yachts, an improvement could be made to switch from slender ship to shallow ship theory. This would need a redesign of the calculations (anyone remember Green's functions... PDE's ?), and may be more applicable to yachts. However, I think that the basic necessity of yachts needing the Kutta condition prevents this technique from becoming predominate.

In the long run, what I think yacht design needs is a potential flow code which quickly calculates the wave-making resistance which incorporates the kutta condition. Boundary integral theories are quick, but an integral equation solver can be more accurate. Finally, having to make a wake-cut to satisfy Kelvin's theorem means that the solution is only as accurate as the cut which is made. I truly belive that this is the big problem to yacht hydrodynamics. Does anyone have suggestions?

[Leo Lazauskas]

Thanks for the kind comments, Jon, but I don't agree that the Wigley wave profiles in SWPE 5.0 are all that good. I have rewritten huge slabs of old code since that release and I can do much better now. I have also nearly finished a program called "Flotsm" that combines the methods of the U.S. Navy's standard ship motion program (SMP) with my total resistance and squat calculations. I'm trying to keep the input and output identical to SMP I/O so that the learning curve for users is
not too steep. I hope to release a demo version in the next few weeks, but chances are that it won't happen until mid-Olympics, when I can get some quiet productive hours in the middle of the night :-)

As regards accuracy for L/B ~ 3 or 4, all I can say is that it depends on the type of hull. Prof. L.J. Doctors uses Michell's methods for much of his work and he noticed that for some hulls with large transoms, predictions are sometimes better for L/B ~ 4 than for L/B > 10, which seems counter-intuitive.

Incidentally - while we are in mutual admiration mode - I thought your report (i.e. using SPLASH) was excellent. It certainly made me chuckle to see that SPLASH took several weeks to get up and running properly, even with assistance from the author, but that it was out-performed in some aspects by Noblessse's much simpler and faster slender body code. Your experiences seem to coincide with other criticisms I have heard of SPLASH (at least of
the pre-2000 versions) in that it sometimes needs to be tweaked differently for different hulls and for different sizes of the (wave-field) domain. On the other hand, the graphics are excellent despite its somewhat uncertain performance when used blindly.

On a final note, you should be a little wary of using the Wigley hull as an example for the performance of some codes. Despite its simplicity and smoothness, some derivatives are not smooth and this can cause numerical problems (and possible difficulties in producing acceptable experimental results). For example, SPLASH predictions (on the SPLASH WWW page) for the Wigley hull are no better or worse than simpler codes, as someone else on this forum pointed out a few months ago.

All the best,
Leo.


Oh Leo, by the way,

Your SWPE5.0 code is very impressive. The wave prediction profiles on the Wigley hull are awesome (I would presonally almost kill for those results with my codes :) ). I'm just wondering, however, if you know what happens when the L/B ratio goes to ~ 4 or 3?

-Jon

[redcoopers]

Hi Leo. It's nice to find someone online who also speaks hydrodynamics! First, maybe, we should start a new thread? But on to the content:


I'm not sure what you mean by "lift" in some of your statements. Do you mean the lift due to leeway, or the dynamic lift (and trim), i.e. squat? Or both?
If I say lift, I mean hydrodynamic sideforce. Anything vertical, I refer to as sinkage or heave. This leaves trim as coinciding with a pitching moment.

I'm also not quite sure what you mean by the Kutta condition. Do you mean it in the sense of a smooth detachment condition for a yawed hull? Or are you suggesting the Kutta condition is necessary for vessels with transom sterns?
My main interest is in sailboats. To begin to predict lift, I need smooth detachment off the trailing edge of the foils (Kutta). But with transom sterns:
1) I don't care too much about boats with these sterns (my hobby is only sailboats... I don't want to do "ships" as a past-time)
2) there is so much turbulence / separation / eddymaking that it doesn't make any sense to enforce a potential flow condition here (but I know that there are a variety of corrections which can be made to "force" a better wavemaking solution).
3) I use the Kutta condition only to help predict lift. If we are not interested in net sideforce for a ship, then let's not complicate things further!

A smooth sailboat (let's say an IMS sled-type) does not have a transom stern, but it does have very smooth and wide aft sections. Moreover, the canoe body creates sideforce itself. While there is still a tremendous amount of turbulence and separation back here on these boats, I think potential flow could be at least more applicable. In theory, a wake cut could be made to satisfy Kelvin's theorem around the canoe body. The big question is: does anyone know how this cut should be made, where it should be placed, or how it interacts with the free surface?

One thing I've been toying around with is to use a simple boundary layer calculation to find out where flow separates. At this point, the outer separation layer is used as the "hull" for potential flow, while a simple friction and momentum calculation is used inside the layer. For slender ship theory, this method can work, since at some point, the longitudinal derivative of the separated surface will go to zero (hence resistance -> 0). However, if I use a boundary element code, there is not a real good way of doing this unless I take my separated surface and just carry it to the edge of my domain (but again, how do I treat the free surface?). Finally, the inner calculation with separated/turbulent flow remains very important, and a good method is needed to handle this.

But this brings me to boundary layers: But a caveat - you still have to estimate the form drag which is not easy! There are many who still claim that the ITTC line is not a skin friction formula, but rather a correlation line and thus includes some allowance for 3D effects. I am guilty of thoughtlessly chanting that same mantra, but it is just not true. Since the 1978(?) ITTC, the formula is used as nothing more that a simple skin-friction estimator.

I remember that in my undergraduate resistance class (Prof Bhattacharya) the ITTC line is only one part of the model-ship correlation. What the 1978 ITTC conference said was that a method (such as Prohaska's) is necessary for calculating 'k'. Without including this form factor (hopefully from experimental data), the ITTC line is meaningless. Altogether, the semi-empirical method has had good results.

But I am very interested in Grigson's algorithm. I've been browsing your web-pages, and I was just wondering: what is your "present method" of your extension to Grigson's algorithm? The 2002-ITTC conference gave a "less-than favorable" view of Grigson's algorithm because of scaling effects. Myself, I have not worked with Grigson's method at all and do not know how it works. Because I am far too lazy to go browsing through RINA, do you think you could give a synopsis of the calculations?


Good luck with the RANS solvers. I hope you have a lot of available computer muscle and time!
Thanks! I'm working on a model right now which is viscous, but I haven't included turbulence yet. The best I can describe it right now is a particle-based lagrangian method. It handles vorticity inherently, and furthermore, because it is lagrangian, the convective acceleration terms vanish. But, because it is gridless and works based on time-stepping, it is very slow right now. My computer's been doing overtime...

-Jon

[Leo Lazauskas]

Jon:


A smooth sailboat (let's say an IMS sled-type) does not have a transom stern, but it does have very smooth and wide aft sections. Moreover, the canoe body creates sideforce itself. While there is still a tremendous amount of turbulence and separation back here on these boats, I think potential flow could be at least more applicable. In theory, a wake cut could be made to satisfy Kelvin's theorem around the canoe body. The big question is: does anyone know how this cut should be made, where it should be placed, or how it interacts with the free surface?


This sounds a bit like the infamous "line integral". Is that what you are alluding to?


...One thing I've been toying around with is to use a simple boundary layer calculation to find out where flow separates.


Are you using something akin to Stratford's separation criterion?


But this brings me to boundary layers:
I remember that in my undergraduate resistance class (Prof Bhattacharya) the ITTC line is only one part of the model-ship correlation. What the 1978 ITTC conference said was that a method (such as Prohaska's) is necessary for calculating 'k'. Without including this form factor (hopefully from experimental data), the ITTC line is meaningless. Altogether, the semi-empirical method has had good results.


I agree that it gives reasonable results, but it could be better. I'm just uncomfortable using a formula that was originally based on an incorrect assumption and poor correlations, tweaked in order to keep a group of ship-builders happy in 1957, and then retained ever since because it is convenient for naval architects. I don't know what that is, but it isn't Science!

The ITTC criticised Grigson's 1993 recalculation of the Lucy Ashton data, but I don't think it had anything else to say about his algorithm. I do think that Grigson's algorithm needs further validation at very high Rn (> 10^9), though.


But I am very interested in Grigson's algorithm. I've been browsing your web-pages, and I was just wondering: what is your "present method" of your extension to Grigson's algorithm? The 2002-ITTC conference gave a "less-than favorable" view of Grigson's algorithm because of scaling effects.


My "extension" is really only a hack to make the algorithm work for smaller Reynolds numbers than Grigson intended. The "Wake Parameter" graph shows the assumption I made to get the results.


Myself, I have not worked with Grigson's method at all and do not know how it works. Because I am far too lazy to go browsing through RINA, do you think you could give a synopsis of the calculations?


Unfortunately it is too complicated to summarise here, and I'm going interstate for a few days. I'll expand the note on my WWW pages when I get back and find some spare time.

Cheers,
Leo.

[SailDesign]

I agree that it gives reasonable results, but it could be better. <snippage> I don't know what that is, but it isn't Science!

Speaking as a naval architect, Yes it is convenient, so "pppppppffffffffffffffffffttttttttttt"

:)

Yeah, it ain't deadnuts accurate, but what is when you start adding waves and things? It is, as they say, good enough for ship work.

Steve

[Leo Lazauskas]

Speaking as a naval architect, Yes it is convenient, so "pppppppffffffffffffffffffttttttttttt"

:)

Yeah, it ain't deadnuts accurate, but what is when you start adding waves and things? It is, as they say, good enough for ship work.

Steve

No, it is good enough for *boat* work. For the Reynolds numbers of most sailboats the ITTC line is almost identical to more sophisticated methods.

BTW, it sounds like your inflatable friend has sprung a leak :-)

[Leo Lazauskas]

I remember that in my undergraduate resistance class (Prof Bhattacharya) the ITTC line is only one part of the model-ship correlation. What the 1978 ITTC conference said was that a method (such as Prohaska's) is necessary for calculating 'k'. Without including this form factor (hopefully from experimental data), the ITTC line is meaningless. Altogether, the semi-empirical method has had good results.


But what this is really saying is that the ITTC line is being used as a skin-friction estimator.

See you in a few days,
Leo.

[SailDesign]

No, it is good enough for *boat* work.

You shippies..... :)

BTW, it sounds like your inflatable friend has sprung a leak :-)

Dang! Now, where did I put that patch kit? :)

Steve

[jonny_IRL]

sorry to butt in, but it looks like some of you might know some good sites on yacht design icould look at for an essay on resistance in racing yachts. i'm in the very early stages of marine engineering so something not too advanced would be good.
thanks