Porposing Four Winns

Rick: There is no problem at beta=0 for the Savitsky model. Only the 2006 introduced whisker spray model has a singularity, since the whisker spray angle formula used can not handle it, thus an if(beta<0.1)... is needed.

I think the Savitsky method with 2006 additions is very good. I have also added a model for hull roughness. The accuracy is much more limited by the input values than the model. E.g. LCG, aerodynamic drag coefficient, hull roughness are seldom well known. For a racing boat all these have a huge effect.

Being a CFD professional I don't think there will be much more to be gained from CFD for truly planning vessel, with a bottom fitting to Savitsky model. Semiplanning might be a different issue.

 

Joakim
This paper shows some of the work I was referring to:
http://nparc.cisti-icist.nrc-cnrc.gc.ca/npsi/ctrl?action=rtdoc&an=8895319&article=0
They have published other papers as well.

The results are very interesting if you are into planing.

I think some of the work Leo Lazauskas is doing with lift and sinkage may be relevant for the semi-planing mode.

Rick W

 

You don't happen to have a link for the CFD-results? Or to the porpoising paper?

I did some tests with my Savitsky program just ignoring the flat part and using beta=5.9 and beam to chine. I could reproduce the results from 3 to 7 m/s within +-7% in resistance and +-0.5 degrees in trim. Mostly much more accurately and all the changes due to changes in LCG or displacement were accurately reproduced. The speed for the porpoising onset was spot on.

Also the 2 and 2.5 m/s results were within +-10% and +-0.7 degrees, althogh Savitsky model is no longer valid there (bow touches water).

 

You can try here for the graph of 'limits' for porpoising (according to Savitsky)
http://www.boatdesign.net/forums/powerboats/porpoising-27337.html
post #4

 

You must not forget the origin of the Savitsky method. It is NOT a stringent mathematical model of a physical phenomenon. It is a set of empirical curve-fitting algorithms describing mean values of a number of test observations from a variety of test facilities using a multitude of hull shapes, sizes and proportions put into a context through application of hydromechanical principles! The bulk of these tests date back to the 50-ies.

Treating the outcome from the playstation as a de facto truth, without realizing the scatter and application limits of the ingoing data, is just nonsense. Most of the tests cover deadrise angles 10 to 20 degrees, and many hulls are not pure constant-deadrise. Going outside this range is in fact unceartain territory. And further, as Joakim correctly has observed, real world surface relative roughness is higher in the common boat sizes discussed here, than in the lab prepared models.

I have previously remarked on the lack of compensation for aspect ratio in the drag calculations as well as in the calc of COP and the resulting trim. It influences the basic drag equation (tan(trim)+surface friction), where there is a 3-d flow generating induced drag connected to the trim term. This has been neglected by most Savitsky users. It becomes obvious once you start checking the various programmes for validation. The general trend is that the Savitsky algorithms underestimate drag and overestimate trim. Introducing a simple aspect compensation à la Joukowsi is a first step that works out fairly well.

This becomes obvious in one of the basic equations that describes the difference (wetted keel length)-(chine length). Savitsky is taking this difference as (b/pi*tan(deadrise)/tan(trim)). Obviously the expression goes to zero with deadrise zero. BUT that requires an infinitely wide plate; a flat boat bottom generates a strong three-dimensional flow in the spray region of a planing, zero-deadrise flat surface. Ie there will be a difference between Lk and Lc in reality.

One manifestation of that phenomenon is a change in COP and trim as if there were a small, but finite deadrise. This is the explanation for Joakims result, setting beta=5.9 degrees, resulting in a "fake" COP and drag that is closer to empirical reality.

So, the Savitsky method is a practical tool, nice in terms of calculating time, but still just a tool with a tolerance band and application limits; to be used with sound engineering judgement!!! And it can still be improved upon!!!

 

Reg, I guess all this is above your head as it is almost every one. This happens sometimes.

To get your boat to a drivable situation, moving stuff you can move forward will help as indeed you have said it does. You may try additional weight such as sand bags on the bow. If this cures the prob then you know how much weight you need to move.

Some trim tabs would definately help. They are small flaps, say 1 foot by 1foot fitted to the transom on the planing water line. This tricks the boat into thinking it is longer. Some are fixed and some are hydraulically adjustable.

This alone may cure the probs.

I had a Black shadow at Windermere, it too was silly. It had a fuel tank up forward and if I did'nt put 40 galls in it it was a pig. It should have been in a circus.

 

Yes, I do know that the Savitsky model is based on experimental data as are almost any other model. There is no pure physics based model for any turbulent 3D flow. And the base measurements are old, just like the base measurements for pipe flows and wing shapes.

The basic model for lift and drag is developed for flat bottom, thus I don't think beta of 0-10 degrees could be unknown territory for the Savitsky model.

Could you be more specific about the Joukowski compensation? If it is just induced drag, why it would affect COP and trim as well?

The surface roughness is not a problem for lab model vs. real boat. They do not need to have the same roughness. The problem is that you need to have a model for friction that takes into account the correct roughness for the real boat. The ITTC model + standard roughness allowance is typically not enough for boat size with clearly lower Reynolds number than ships have. Of course the friction model originally used for towing tank model must be correct as well, but I don't think that is a problem.

The results from my Savitsky program are very close to each other with both 5.9 and 5.1 beta. 5.9 is the real angle of the hull surface and 5.1 is corrected for the flat part in the keel. For this example the predicted trim angles were mostly lower than the measured ones. The predicted total resistance was lower (3-6%) than the measured at 3-4.5 m/s, but at 5-6.5 m/s it was very accurate. Under 3 m/s some parts of the model are not valid, thus I don't compare those.

The trim angle is spot on at 3 m/s, 3.5-5.5 m/s the predicted trim is too low (0.2-0.4 degrees) and then 6-7 m/s spot on.

The comparison above is for LCG 0.53 m, 29.6 kg from http://nparc.cisti-icist.nrc-cnrc.gc.ca/npsi/ctrl?action=rtdoc&an=8895319&article=0

The surface roughness and the accurate location of the towing point were not reported. These can have some effect. Also the aerodynamic drag can have a minor effect. I used a surface roughness of 10 um and set f=Epsilon=0. The aerodynamic drag in my program was 0.3 N at 3 m/s and 1.5 N at 7 m/s. There was no whisker spray drag (only accounted for, if positive).

 

Joakim, we should not hijack this thread with a deeper diskussion on Savitsky's algorithms. You are welcome to mail me directly, and you may also find some of the litterature in the thread "State of art of planing hulls" interesting. The intention of my comments above was to remind some of the readers here not to stretch their conclusions too far into the region of fanatic playstation-ism. Arguing on fractional degrees of trim is just on the edge, and obscures one's perspective on real world engineering, as Frosty aptly noted.

Just a short note on your quests: For a flat planing surface of finite aspect ratio, the lift induced drag coefficient is 2/pi*Cl^2/A in its basic form, where A=(span)^2/area. It is to be added to the wave-making and friction coefficients.

The influence upon trim is seen if you imagine the spray-root (~stagnation line) of your zero-deadrise surface. With reducing A from infinity, the spray-root changes from a straight line into an arch-shape. The center of pressure and the pressure integrated over the surface will change correspondingly. Trace this influence through the chain of algorithms in your program, and you will see the difference. In his 1964 paper, Savitsky is referring to the 2D case when dealing with drag prediction (Fig 13 ibid). I cannot see why he didn't include finite-span correction to the drag issue, when he spent the rest of the paper on the effects of aspect ratio on lift???? Just bloody tired of the s--t, maybe?

"I can lead the horse to water, but he has to do the drinking himself...

 

This is Thornhill CFD data against empirical methods:
http://books.nap.edu/openbook.php?record_id=10834&page=640

There is also another paper here showing good correlation between CFD and empirical:
http://www.icmrt07.unina.it/Proceedings/Papers/B/14.pdf

Porpoising will get into a more complex regime involving time domain modelling. It is another step in complexity over stable planing.

I have time domain modeling for electronic control systems, mechanical instability, machine modeling and, more recently, some biomechanical modeling so I can lead you into the analysis if you wanted to.

Rick W

 

PORPOISING-Four Winns

regdunlops 14
After many posts over several months no one seems to have given you solid advice on how to stop your porpoising problem. I apologize for causing this thread to stray into technical "outer space" Since there are many boats of the same brand that do not porpoise, it seems to not be a design problem.
Therefore it must be something related to the engine or the way it is set up on your boat.All this Savisky stuff is interesting but gives no insight into the mechanism of the oscillation and how to make it better. I believe I know how it comes about and the parameters that make up such a system.I am not a proficient enough mathematition to predict if a boat will porpoise or not(as Savisky procedure does),but I can say with some certainty what will make it better. There are really only two means to reduce amplitude or kill it all together. Increase losses in the oscillating system, or reduce the input energy into the system Whenever the former is greater than the later,oscillations will cease. Losses are pretty much determined by hull parameters that are not easily changed,such as deadrise angle, while input energy comes from the thrust-drag couple. Reducing the distance between these vectors minimizes the feedback that results in oscillation. So, raise your engine up as high as you can without uncovering the water intake . Some prop surface piercing is ok, just as long as the shaft c/l is a bit below the surface.
I hope this helps-----godd luck

 

I don't think porpoising is related to the force vector locations you mention in any other way than which trim angle they produce.

The distance between those two has a (rather minor) influence on the trim angle and thus porpoising. In the lab scale porpoising tests the model is pushed from the towing point close to CG and still it porpoises. Also the first generation seaplanes had catastrophic porpoising problems, which was one of the main reasons to start the research around porpoising and planing hulls. Thus I think porpoising is a natural property of a planing surface and not caused by changes in thrust.

I have plenty of experience about raising the engine up with my small "racing" boat. Not much changed regarding porpoising even when the propeller shaft was finally 5 cm above the keel line (starting from the normal 15 cm below). Raising the engine requires a propeller that can handle ventilation. Standard outboard propellers can't and loose their grip totally.

Putting weight forward did help and I have never seen a case in which it would not help, but at higher speeds weight forward increases resistance considerably. But mostly that is something you have to live with unless you want to make changes in the hull from.

The Four Winns 160 Freedom seems to be most often fitted with a stern drive, which is much heavier and in different longitudinal location than an outboard. Do you know how the other ones with an outboard behave? I would not rule out that this boat just happens to be a bad design for this combination at that speed. I would try to contact other owners and find out do they have similar problems and how they have solved it.

As has been said many times in this thread, the first thing to try is to lower the trim. This can be done by lowering the trim of the outboard and/or by changing the weight distribution forward. It would help, if there were some photos of the boat at different speeds and also some details about the hull form (at least angle of deadrise).

 

Every oscillator,electronic or mechanical needs a positive feed back mechanism to oscillate. It is not the steady state vectors of which I refer to. I agree with you that the steady state couple does little to influence the trim angle. It is the fluctuations in these vectors that create the sinisoidal feedback into the oscillating system. When the bow rises the transom drops,pivoting about the CG.This is the torsional system of which I speak. The momentary increase in angle of attack in turn causes a momentary increase in drag.which in turn slows the boat. A "fixed" throttle setting increases thrust to counter the increase in drag. The opposite action occurs on the bow down(negative) part of the cycle If you study the geometry of this fluctuating couple you will find it acts in phase with the trim oscillation.(As demanded to cause an oscillation) This is indeed a small effect, but it need not be very forceful. The only thing it must do is to provide enough energy to overcome the losses in the oscillating system. If it can do this, it will continue to oscillate about the "neutral trim angle indefinitely. If you really want to under stand this phenomenon, you must stop trying to explain it with steady state forces, etc. I have been studying torsional systems for several years. This, although complicated,is a classical torsional system. If you can buy into what I have said and you are interested, I can show you how the main elements , moment of inertia and equivilant torsional spring influence the frequency and losses of such a system.

 

There are several points in your explanation, that I do not agree with.

1. The changes in forward momentum during porpoising are very small and thus this has a very small effect on thrust. Take an example of 5 m 500 kg boat porpoising at 1 HZ at full speed (say 70 hp and 35 kn). Calculate the thrust, its ability to accelerate during 0.5 s half cycle and then how much moment the change in thrust would cause. Remember that most of the thrust goes to overcome drag thus it is not available for acceleration.

I have tested on many boats the speed at porpoising and just under that (the only change is the motor trim or your sitting position). Not a big change at all!

2. During porpoising the boat does not pivot around CG. This would mean a big change in draft porpoising. It is a bit like rocking a boat. It does not around CG.

3. It is true, that drag changes during porpoising, but drag does not cause a big momentum since it's vector is quite close to CG.

4. Clearly the biggest change in momentum is caused by the change of effective point of lift. When the bow rises the waterplane shortens and the lift coefficient increases due to higher angle of attack. This will cause the lift vector to move aft and causes a huge moment to push the bow back down.

5. If the system is in labile condition (=at a trim angle in the porpoising region), there will always be some disturbance which will onset porpoising. There is no need for an external oscillating force like thrust.

If you want to understand these moments better, I think you should go through the Savitsky model. Then you could add inertia to it and see what happens if you input a small disturbance in trim and calculate it time dependently. In Savitsky model there is two degrees of freedom (trim angle and vertical position).

I have no idea whether the forces calculated in the Savitsky model are accurate enough to actually model porpoising, but certainly they are in correct order of magnitude.

 

Perhaps you don't even need to consider inertia, since the experimental curves depend only on deadrise, lift coefficient and trim angle. Still they are rather accurate.

You can find some papers by Katayama and also this could be helpfull: http://www.boatdesign.net/forums/boat-design/understanding-porpoising-9509-2.html

 

I will try to answer your comments by number as they appear in your post.
1. In a mechanical oscillatory system,no external forces are needed to accelerate masses.The only need is for external energy to be added to make up for losses in the system. I know it is difficult to visualize what actual forces do cause acceleration. These come solely from within the spring/mass system.The trim angle changes sinisoidaly, about the neutral axis. Energy is continuously exchanging between potential and kinetic energy.At the extremes of the excursion all energy is , potential energy, in the "spring". In passing through the neutral point the potential energy is reduced to zero but the angular velocity reaches a maximum.(kinetic) After some initial disturbance getting it going, this motion would in the absence of losses(damping) continue indefinitely. With real losses the oscillation would die out. At what is known as critical damping, the oscillation will die out very quickly, after a single excursion. What I have just described is "free oscillations" We have "forced" oscillations.Here external energy is supplied to overcome the losses and sustain the oscillation.The energy(not a specific force) comes from the drag/thrust couple as I have previously alluded to.In any event, the 500 Kg weight of the boat in your example does not in itself get accelerated. Only that part that is distant from the pivot,and contributes to the "moment of inertia".
2. I believe that the pivot of the system must be at or below the CG.
3.You are still thinking steady state.
4. I agree with this. What you have described here is the pseudo spring that drives the oscillation.It does indeed try to return the trim to its neutral position.
5.The question is not one of initial disturbance. Instead, why does it not go away long after the disturbance is gone? Porpoising happens on glassy smooth water. I don't believe Savisky had a specific model in mind when he devised his procedure.A prominent parameter is the prop shaft center line to bottom distance.Cg location rel the transom, weight itself and dead rise angle which is clearly a loss mechanism.As I previously pointed out, one can "kill" porpoising by making the losses greater than the input energy. This can be accomplished by decreasing the energy feedback, increasing losses or both. Raising the porpoising frequency, a method I will not tackle for now is a powerful way to get more losses with out adding frictional mechanisms.
Keep an open mind, refresh your memory of your courses in vibrations, by reading some texts on the subject or else consult with someone who is expert in the physics.