Porposing Four Winns

[regdunlops14]

This summer my family purchased an older 16ft, Four Winns 160 Freedom bowrider with a 90HP Evinrude for use at our summer camp. The engine runs great however at 3/4 throttle and above the boat porpoises severly. Also on turns you can hear the prop come out of or almost come out of the water. This happens with the motor all the way down. The people we purchased the boat from said it always has porpoised since they bought it. There is one hole left on the motor mount bracket so I could drop the engine further into the water. Are there any solutions to this problem?

[Rick Willoughby]

Porpoising usually results from having the boat trimmed bow up. It is more likely with beamy hulls. Tilting the outboard so the prop is closer to the transom should lift the stern and reduce the tendency to porpoise.

Have you got a lot of weight in the stern?

How much does the boat weigh?

Where does the boat ballance on the trailer?

Have you got a photo at rest to determine where the CoB is?

Any chance floatation could be waterlogged particularly at the aft end?

Does the bottom of the hull run fair to the transom or has it been bowed?

Do you notice it more pronounced when crew sit aft?

Rick W

[Willallison]

It would seem unlikely that a large builder like Four Winns would produce a boat with rocker in the bottom, but, without further info, that would seem the most obvious cause (as Rick eludes to in his 2nd last question). Run a straight edge along the keel (for and aft) and do the same part way up each side of the bottom (again, fore and aft) if the there is any convexity in the bottom, then that is most likely the culprit.
If it is, the cure can be relatively simple. Adding trim tabs or wedges at the transom ought to provide you with a fix.
Of course, all that is based on the assumption that nothing else is drstically out of whack... so a bit more info and a pic or two would indeed be useful

[Jimboat]

regdunlops14 - What happens if you trim engine in when onset of porpoising is experienced? If you have no more trim, perhaps trying wedges is a simple and inexpensive solution. Running with less trim is usually best solution to porpoising problems.

[ABoatGuy]

Forward CG? Does it improve with weight moved aft?

[Jimboat]

ABoatGuy - not usually. moving Dynamic CG foreward will reduce susceptibility to porpoising and delay onset of porpoising.

[ABoatGuy]

A good article on dynamic instability can be found in ProBoat #31 and also the SNAME Small Craft CD. See attached. Both indicate as LCG moves forward relative to the centroid of the water plane instability is encountered. This has been my experience as well.

[Jimboat]

When describing Porpoising specifically, the instabilities are interesting. In these cases, the dynamic instabilities are a function of trim and Lift coeff (CL), affected by deadrise angle. Savitsky showed that for a given deadrise angle, there was a specific relationship between trim angle and lift coefficient which defines the inception of porpoising.

It is interesting to see that as the lift coefficient is decreased, indicating a lightly loaded hull and/or a high planing speed, the trim limit for stability is decreased. Further, the effect of increasing deadrise is to increase the trim angle before the inception of porpoising. This relationship is counter-intuitive at first review, but shown to be factual by the Savitsky paper.

In any case, if a boat is porpoising at a given speed and load, the solution is shown to be reduce the trim angle to avoid porpoising. The 1ower trim angle can be acliicved in several ways - one is to move the longitudilial center of gravity forward.

Our research has shown that any method that causes reduced operating trim angle, will have the positive result of altering the onset of porpoising regime.

[regdunlops14]

Sorry for the slow response as I had the boat out this past weekend experimenting a little. The problem does get better with more weight in the bow however it does not go away. Also the original owner stated that the boat has porpoised since they boat it new so I do not believe it is a problem with way the boat is trailered or stored. With the engine all the way down it still porpoises as well. As originally stated the prop seems to almost come out of the water on turns and the boat loses speed. Could it be that the motor is not low enough in the water?

[JackD]

My class C racing runabout porpoises at about 30 mph which goes away at much higher speeds.At 60 or so,not a trace. It is still very annoying.I believe it is a torsional function,and as such it is governed by a moment of inertia and a torsional spring rate (T) (torque per unit angle). The system rotates about the boats CG.The moment of inertia (I) is not the weight per se but the way this weight is distributed about the CG. My understanding of planing is that the center of the wetted area is directly under the CG so it seems to me movement of the CG will not significantly improve porpoising. To affect porpoising, the resonant frequency must be lowered and/or damping must be increased. To lower the frequency one must increase I or lower T. I cant see how to do the latter.Adding a concentrated weight as far forward as possible could do the trick. Since I is mr squared. while affect on CG is linear with r(r being the distance from the CG) a relatively small weight might work. Seems an easy thing to try. Next spring I will give it a try.

[Joakim]

Porpoising comes from too high trim angle (the angle between keel and water level). The maximum stable trim angle depends on speed, weight and hull form. A deeper V allows higher trim angle and is thus used in racing boats even when waves are not an issue. At higher speeds the maximum stable trim angle gets lower, but at the same time the trim angle lowers naturally as well. Depending on the boat porpoising can be a problem at higher speeds, lower speeds, always or never.

I have also a boat that porpoises at lower speeds, but gets stable at higher speeds. This can not be cured by changing the motor trim.

Ideally a planning boat should be on an edge of porpoising, since higher trim angle means less wetted area and thus less resistance. If a boat is well designed, you should be able to control porpoising with motor trim (both on and off!).

The trim angle at a specific speed depends primarily on LCG (longitudal center of gravity) and secondarily on the propeller shaft location and angle, which is adjusted by trimming the motor. Also the propeller type can have an influence.

There must be a momentum balance between all the forces. The propeller force typically tries to lift the bow. Gravity and friction typically try to lower the bow. When the trim angle gets higher the wetted surface moves back and thus the moment caused by gravity increases. Above the maximum stable trim angle this moment changes so rapidly that there is no balance. A bit like trying to stand on a ball. At just one spot there is a balance, but minimal disturbance gets you out of balance.

Since LCG is clearly more important than motor trim, a bad LCG can not be compensated. In this case LCG is too far back. This could be due to a too heavy engine or other installation or just a design error.

[Jimboat]

Porpoising onset is a function of planing surface design (width, deadrise, etc), angle of attack (trim), velocity and location of Dynamic CofG. Since the location of Dynamic CofG is changing throughout the velocity range, the calculation of porpoising onset is a complex one, but predictable by analysis, nonetheless.

While the easiest adjustment to minimize porpoising is to reduce trim angle, there are indeed other design and setup actions that can be taken to change the velocity at which porpoising onset occurs, and to mitigate the effects of porpoising.

[JackD]

When you guys talk about changing the velocity of porpoising onset, I hope you do not mean making it higher. For my boat, this would put the onset closer or even into racing speeds, I think a dangerous situation. Specifically, does reducing the trim angle raise or lower the onset velocity?

[Joakim]

When you guys talk about changing the velocity of porpoising onset, I hope you do not mean making it higher. For my boat, this would put the onset closer or even into racing speeds, I think a dangerous situation. Specifically, does reducing the trim angle raise or lower the onset velocity?

For every boat there is a curve for the maximum stable trim angle and another curve for the trim angle that the boat actually has at different velocities. The maximum stable angle gets smaller and smaller as the velocity increases. The actual trim angle has a maximum and then decreases with velocity. Depending on these curves a boat can be porpoising at low, high, all or none planning velocities.

When you lower the trim angle (by changing motor trim or LCG) the whole actual trim angle curve is lowered. This means that the velocity span for porpoising gets smaller or even vanishes.

My small (3.8 m) "racing" boat porpoises quite badly from 15 to 30 knots and then is stable from 30 to 48 knots. You can not use it in the porpoising region, you just have to accelerate through it, which luckily doesn't take many seconds. With two passenger (who must sit way back) and a driver it porpoises at all planning speeds, thus it can not be used.

[baeckmo]

As this problem is a resonance phenomenon, it is a question of two systems of forces. Both are mainly linear in characteristics; the hull resistance increasing with speed and area (=low trim), and the propeller force increasing with reduced speed.

Physically, this boils down to the following process, (which may be described mathematically as has been noted by others here):

The boat is reaching a speed, where trim is reducing with increasing speed, causing increase in resistance. Simultaneously, the propeller thrust is going down, the speed is reduced enough to cause a trim increase (=drag reduction) and to increase thrust. Speed is now increasing again, and the process repeats.

If you imagine a diagram showing forces (drag and thrust) over speed, the slope of the respective characteristics is critical. If both are flat, small disturbances in thrust/drag will result in big speed variations, easily setting up a swinging couple, and vice versa. Instead of increasing drag/reducing trim by moving the center of mass, you can introduce a factor with a non-linear characteristis; ie its force/speed slope is differing drastically from the others. Such a measure can be:

1/ an increase in width of the low spray strakes up front,
2/ a pad up front,
3/ small transverse step fwd of LCM,
4/ cutting out a triangular surface aft,

A particular boat may even react if an originally sharp strake has been rounded or damaged over years of bumping and bruising; check that the strakes are intact and have sharp edges, their "horizontal" surface preferably angled down 5 to 10 degrees in transverse view.

Typically, the propeller characteristics is comparatively flat in this situation as well, often working with some ventilation that causes a reduction in the slope of the thrust coefficient (even becoming negative if ventilating/cavitating). A change to a prop with more cup/less geometrical pitch (better "bite" in foaming water) will often restore a steeper thrust characteristics.

Longitudinal change of mass does not always give a good result, as planing area and moment of inertia will be changed as well. A vertical shift might be more effective, but not practical from operational and safety perspectives.

[Jimboat]

JackD - your class C racing runabout is a hull design that benefits from Lift (and drag) from hydrodynamic planing forces, motor lower unit & propeller forces (lift, drag and thrust) and aerodynamic forces (lift and drag). The balance and location of these forces is ever-changing throughout the operating velocity range. To change the onset of porpoising, the dynamic CofG (acting forces) needs to be changed so that the full curve of porpoising onset (versus velocity) is changed. The design objective is to change the design and dynamic balance of the operating hull such that operating trim curve is always below the 'porpoising onset' curve. There are both design and setup alterations that can help achieve this.

[JackD]

The boat I am speaking of is about 3.4 meters long but has a relatively wide transom,about .8 meters. The bottom is dead flat for 1.25meters along the keel forward of the transom. It has a double chine. The chine coming off the bottom is a shallow angle to prevent tripping as the boat "skids" around high speed turns. The keel curves upward toward the bow with a shallow
"V" bottom, increasing in dead rise starting at the flat. There are no strakes any where on the bottom. This boat reaches 60 mph (96kph) with only 33 hp. To remain competitive,I would like to not degrade it's performance.
It seems to me that for porpoising to happen,a boat must derive almost all of it's lift from planning (little buoancy) As such, if the trim angle were increased to lower the onset velocity to a point where sufficient buoancy is encountered it would be damped out.
I am considering reducing the width to about 1.23 meters by reworking the bottom. I believe this will increase the trim but I am hoping that it will lower the onset speed of porpoising to where it encounters damping from the shallow angle chines.
Does this make any sense?

[Rick Willoughby]

The boat I am speaking of is about 3.4 meters long but has a relatively wide transom,about .8 meters. The bottom is dead flat for 1.25meters along the keel forward of the transom. It has a double chine. The chine coming off the bottom is a shallow angle to prevent tripping as the boat "skids" around high speed turns. The keel curves upward toward the bow with a shallow
"V" bottom, increasing in dead rise starting at the flat. There are no strakes any where on the bottom. This boat reaches 60 mph (96kph) with only 33 hp. To remain competitive,I would like to not degrade it's performance.
It seems to me that for porpoising to happen,a boat must derive almost all of it's lift from planning (little buoancy) As such, if the trim angle were increased to lower the onset velocity to a point where sufficient buoancy is encountered it would be damped out.
I am considering reducing the width to about 1.23 meters by reworking the bottom. I believe this will increase the trim but I am hoping that it will lower the onset speed of porpoising to where it encounters damping from the shallow angle chines.
Does this make any sense?

You say your transom is relatively wide at .8m but then you say you are intending to reduce beam to 1.23m. I do not understand this.

There is a Savitsky calculator set up here:
http://illustrations.marin.ntnu.no/hydrodynamics/resistance/planing/index.html
It allows you to set up your boat parameters and it will estimate the power, trim and potential to porpoise based on Savitsky test data on planing hulls.

I have attached the porpoising chart with parameters that might be close to your hull although I am confused by the data you give. You will see that at trim angles lower than 3 degrees this hull is close to porpoising but when trim is higher, the margin increase a bit.

You must have a light hull if it can get to 60mph with 33HP. There may be some aerodynamic lift as well at that speed so Savitsky could yield a pessimistic result.

Rick W

[Rick Willoughby]

I gave the reason poising occurs at post #2 here:
http://www.boatdesign.net/forums/powerboats/porpoising-27337.html

Rick W

[Joakim]

"V" bottom, increasing in dead rise starting at the flat. There are no strakes any where on the bottom. This boat reaches 60 mph (96kph) with only 33 hp. To remain competitive,I would like to not degrade it's performance.
It seems to me that for porpoising to happen,a boat must derive almost all of it's lift from planning (little buoancy) As such, if the trim angle were increased to lower the onset velocity to a point where sufficient buoancy is encountered it would be damped out.

I'm quite certain, that a flat bottom is not the fastest setup for this kind of boat. Is it an outboard? Here in Scandinavia we have a racing class T400, which uses (almost) standard 25 hp outboards and reach a top speed of 54 knots (99.3 km/h) despite being rather heavy (240 kg with driver). Here is one example: http://www.winrace.no/index_eng.php?side=hoved_eng

Does the rules limit the hull form (not in T400, except for minimum size)?

The problem of porpoising does not come from buoyancy issues, since porpoising only exists at planning region, in which buoyancy forces are very small compared to dynamic lift. When the trim angle increases the angle of attack between water and the lifting surface (hull) increases. This increases the lift coefficient and thus the same amount of lift can be achieved with less wetted area. But at the same the the lift force moves back, since the area is reduced from forward. As the trim angle increases the lift force creates an increasing moment, which pushes the bow down. Without porpoising there will be a balance, but at the porpoising region this balance is "too hard to find".

Porpoising is very hard to calculate from physics. The momentum balance is easily calculated from the Savitsky method, but porpoising is only derived from experimental data by comparing the calculated trim to curves defining the stable region. At least I haven't seen any accurate model for porpoising.

[JackD]

I am a mechanical engineer but not in the boating business. My interest in boats and racing is purely a hobby. A lot of this stuff is new to me and I am very appreciative of the help and advice you guys have given me. I have been struggling with trying to calculate the porpoising function and I think I have a reasonably good method for finding the resonant frequency based upon the physical parameters of the boat when planning but I have a problem with the speed related forcing function. I was "glad" to hear that this is also very hard for every one else.
Rick, I appologize for the typo relative to the width. The new transom will be .62 meters(not 1.23 tri) In the next week I will try to apply the material you have provided to on my problem. I will be gone on a business trip for a week so it may take a while. I will post as soon as I have something.
Joakim, to answer some questions both you and Rick have, the boat,engine and driver weigh 216 Kg.(minimum by class rules) The outboard engine is a stock Yamoto engine used for professional boat racing in Japan. The class rules require the aft bottom to be dead flat.but I will look at the Web site you have provided (thanks) I have estimated the lift derived from Wig airlift and find it to be negligible at porpoising onset,and still small at top speed.

[Joakim]

JackD, I think aerodynamic lift is the reason for your boat NOT porpoising at higher speeds. According to my Savitsky program it should porpoise at all positive trim angles at top speed. Aerodynamic lift may not be important for carrying the weight of the boat, but it may cause a moment big enough to stabilize porpoising. Do you have any idea of the actual trim angle at top speed?

What do the class rules actually say about the bottom? If you can make it as narrow as you like, you could actually make a V bottom with a flat "ski". This type of bottom has also been succesfully used.

Here is a video of a Finnish boat showing the influence of aerodynamic lift. http://www.youtube.com/watch?v=efIj7UvFIRE

Here is anotherone with some porpoising as well: http://www.youtube.com/watch?v=b8qzQleHinI

It's not a racing boat although they have races for this boat type. They can reach well over 40 knots with a standard 40 hp outboard.

The original Savitsky method is not very good for racing boats due to many limitations. It does not take into account whisker spray drag, aerodynamic drag and the drag of the drive, all of which are very important for racing boats. Also it assumes that the chine allways touches water, which is far from true for light racing boats with V bottom. I have included all these to my Savitsky program, but not the aerodynamic effect on porpoising or lift. I have no idea how to implement that, but I'm quite convinced, that it is an important effect for racing boats.

[Yellowjacket]

[QUOTE=Rick Willoughby;313282]I have attached the porpoising chart with parameters that might be close to your hull although I am confused by the data you give. You will see that at trim angles lower than 3 degrees this hull is close to porpoising but when trim is higher, the margin increase a bit./QUOTE]

Actually if you load parameters of a "C" class runabout you get a slightly different result and it also indicates that the hull will porpoise at higher trim angles, and as the trim angle comes down the hull will become stable. The weight limit for the class is close to 200kg, the CG is about .8 m ahead of the trailing edge of the planing surface. The prop is a surfacing prop so the thrust line is higher than what you assumed. If you load beam widths of about .8m as well as the planned .63m (2ft) and the stability improves slightly with decreasing beam width. Also note that with this model the stability is increasing with reduced trim angle, which appears to accurately model what JackD has seen in his boat. Also remember that driver position has a significant effect on the longitudinal CG location, so by leaning forward the driver can move the CG forward enough to stabilize the hull or, if he moves aft he can likely get it to porpoise. Bottom line is that decreasing the planing surface width does appear to slightly improve the stability as well as reduces the drag somewhat, although that remains to be seen since the maximum speed here is above the range where Satvisky is accurately predicting drag.

[HJS]

Joakim
here comes a report on whisker spray drag to add to your savitsky program


hjs
www.sassdesign.net[/url]

[Joakim]

Joakim
here comes a report on whisker spray drag to add to your savitsky program


Thank You, but I have already done that based on that paper about two years ago. What I don't have is the aerodynamic lift including moment and it's effect on porpoising.

The porpoising curves of http://illustrations.marin.ntnu.no/hydrodynamics/resistance/planing/index.html seem to be incorrect. The beta=0 curve is close to the real beta=20 curve from the Savitsky 1964 paper.

This paper suggests even lower angles, but only beta range 15-25 was studied: http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADB240994&Location=U2&doc=GetTRDoc.pdf

[Joakim]

Actually if you load parameters of a "C" class runabout

You both have beta=20, which should be 0 for flat bottom. Also note that the porpoising limits are incorrect.

[Yellowjacket]

The loaded deadrise was zero.. I noticed that it appeared that Rick and I both loaded a deadrise of 20 degrees, but that is not the case. If you run the program the output mirror is showing a deadrise of 20 degrees if you load in zero. I used betas of 1, 5 and 10 degrees and increasing deadrise showed an decrease in stability as one would expect. If you put in a zero deadrise it shows as 20 degrees, but if you put in 1 degree is shows 1 degree.

Edit..

Oddly enough, if you put in small deadrise values the program seems to work correctly but you can't put in a deadrise of zero and get anything but 20 degrees in the output, so Jokim is correct in that it did appear to load a deadrise of 20 degrees if you put in zero. I have rerun the analysis for a .1 degree deadrise and have edited the previous post. I guess a word of caution is appropriate for using that web site since the curves appear to be offset and inputs of zero deadrise come out at 20 degrees.

[Joakim]

I used betas of 1, 5 and 10 degrees and increasing deadrise showed an decrease in stability as one would expect.

No, I would not expect that. Flat bottom is most prone to porpoising and deep V is least prone. That just shows, that the porpoising curves are incorrect. Take a look at the 1964 Savitsky paper: http://www.boatdesign.net/forums/boat-design/anyone-have-copy-1964-1976-savitsky-papers-10970.html

At 45 kn the predicted trim angle is about 1 degrees. The sqrt(Cl/2) is then about 0.08. These are OK, but according to Savitsky paper the limit of porpoising is already at sqrt(Cl/2) of 0.13 at about 1 degrees and still decreasing. The curve fitting I use yields -0.35 degrees for 0.08.

The www calculator claims, that porpoising limit would be 2 degrees at 0.08 and 3.5 degrees at 0.13.

According to measurements in this: http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADB240994&Location=U2&doc=GetTRDoc.pdf the beta=20 model porpoised already at 0.16 with trim angle of 3.5 dergrees, but the www calculator claims limit of over 6 degrees. Savitsky 1964 gives 4.2.

So the limits seem to be 2-3 degrees too high at this region.

[Rick Willoughby]

T.....

Oddly enough, if you put in small deadrise values the program seems to work correctly but you can't put in a deadrise of zero and get anything but 20 degrees in the output, so Jokim is correct in that it did appear to load a deadrise of 20 degrees if you put in zero. I have rerun the analysis for a .1 degree deadrise and have edited the previous post. I guess a word of caution is appropriate for using that web site since the curves appear to be offset and inputs of zero deadrise come out at 20 degrees.

Yes. I use 1 degree for flat surface as I have found strange things happen when it is set to zero.

I have never bothered to work through the Savitsky formulas so do not know if it is a formula problem or the MARIN version of Savitsky's work.

There are some good results being achieved for CFD code on planing hulls. Maybe there will be better models available in the future.

I expect any 200kg boat doing 60mph will have aerodynamic forces of significance. Savitsky should give a guide to what is happening but the whole picture, including aerodynamics, is going to be more complex.

Rick W

[Frosty]

You can work all this out if you like but what are you going to do when you find a shallow V might help,--Change it? perhaps a longer boat?

Only thing you can do to THIS boat is change -re adjust weight, trim tabs, move fuel tanks, batteries, etc.

[Joakim]

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.

[Rick Willoughby]

...
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

[Joakim]

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.


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).

[Ad Hoc]

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

[baeckmo]

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!!!

[Frosty]

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.

[Joakim]

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).

[baeckmo]

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...

[Rick Willoughby]

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).

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