Definition of Planing

It is only possible to talk about planing when the hull is in contact with water. So the planing ceases each time the hull becomes airborne. That's simple and obvious.
When did it start to plane - well, that's a matter of our definitions of planing and is all this inconclusive discussion is about.

When does the planing stop during the slow-down phase, well, here you've made an interesting question.
Since we all agree that planing is influenced by external forces and moments (prop, sails, tabs or other), and assuming that we are able to conventionally define a speed when vessel's motion can be called "planing" (which will depend on trim, weight, etc. etc.), it means that a boat will generally not get on and off plane at the same speed.
That's because during the acceleration phase the boat's attitude will be influenced by the thrust force (with the relative moment), while during the deceleration the thrust will be much smaller, or will even become a drag.
Speaking in mathematical terms, it implies that the vessel's drag curve will show a hysteresis (I know you're not interested in that part, so it's intended for other readers).

Thanks for bringing out this issue on the table. Now we have even more complications to think about.

 

Looks like a mathimatical nightmare and only a computer could do all the calculations i would say . im interested i all things even if they are way over my head !!!!

 

Looks to me like we are slowly but surley getting towards the answer that we had in the first page . This is normal for round here though thrashing the life out of mathamatical formula to come up with the answer.


Planing has no definitive point.

 

Problems like this one are not so much solved as trampled to death.

 

I was not trying to be facetious, but rather to point out that all the discussion was focused on the lower limit only. If the definition of planing includes the hull touching the water, it changes the parameters to certain extent. Hydrodynamic lift is provided by the propeller and appendages so it should be either included or subtracted depending on the definition. Also, if a boat that goes airborne is not planing, then skipping is not planing, only when touching the water is planing. Seems to me that we need to narrow the definition to either exclude or include extreme and particular cases.

 

I still say that the quarter beam buttock angle decides if a boat is capable of being a planing craft or not.

 

But not the only decisive.. And there you miss again an excact point (or angle) of transition from displacement to planing.

 

There is no exact point. Well, the exact point will be when there is no displacement, but vessel still touches water. This is possible only in no gravity condition or with assistance of other lift, like air wings. But in such case dynamic component also will go to zero.
It is interesting. 100% displacement exists. But 100% planing cannot exist, there always will be some portion of displacement.
I do not think angle should have anything to do with a definition of planing. Though angle itself can depend on planing/displacement ratio and even can be an indicator of planing.

 

Wasn't talking about the point of planning haha! Meant the point of buttock angle which doesn't tell much anything without knowing the thrust and many other points. Like many things in boat design everything affects to performance but none of them alone..

 

A boat with a quarter buttock angle of 45 degrees but a flat run, will plane with a bow pointing very high, but will plane.

 

It seems to me that the high end of NORMAL planing occurs when Savitsky calculations start to fall apart. (obviously I'm just talking about prismatic hulls that it applies to in the first place, but bear with me for a minute) That is, when the results you get using his analysis methods start to fall apart at the high speed end.

If you look at Crouch’s coefficient there are some boats that have big numbers at high speeds. Think about boats like APBA racing runabouts that have Coefficients on the order 270, where conventional boats are more like 170 and seldom hit 200. Clearly there is something going on here that isn’t explained by typical planing theory, power required for these hulls isn't going up as a square of speed at the high end. I know there is some aerodynamic lift present, but it isn’t like a hydro that has a lifting surface under the whole hull, so why should these boats be so darn fast.

Jim Martenoff mentioned this as far back as 1971 (see the March 1971 issue of Popular Mechanics) where he had a nomograph for calculating planing hull speeds, and he mentioned that at high speeds the calculations didn't give good results since the boat was "breaking out" and drag was diminished at these higher speeds.

For boats with totally flat planing surfaces there might be an explanation, and I’m just laying out my own theory on this but I think it’s plausible.

When a lightly loaded flat planing surface is going very fast, the trim angle gets really small AND the depth of planing surface gets very small. The surface friction predominates creates most of the drag.

But at these very low trim angles and low planing surface depths you are getting close to the height of the local surface ripples. Racers of these boats know that they go faster when the water isn’t glassy smooth, they’re considerably faster when there are ripples on the surface.

What I think happens is that as the depth of the planing surface gets to the wave height the effective density of the fluid essentially gets lower. That is, there is simply less water to change the momentum of, so the effective density gets lower. Think of it as the boat is knocking the tops off of the waves and pushing them into the troughs.

So once the depth is equal to the wave height (trough to peak), the effective density gets lower, and because of that, the trim angle gets higher, to where it essentially doubles the trim angle since the density is halved. This is much more efficient for planing, and therefore drag is reduced.

As the normal calculated depth becomes equal to ½ the wave height, now there isn’t enough weight to push the wave tops into the troughs, and now, air can travel entirely under the hull, since now you are essentially riding on the tops of the ripples, and this lower density fluid has less drag than if the planing surface was completely wetted. Now it is riding on a mixed density surface with lower overall drag created, which explains why they can get to such a high performance coefficient and there is nothing in the hull that is different from the same hull running with lower power and speed.

Obviously this is a special case, and V hulls or rounded planing hulls aren’t going to ever experience this phenomenon, but it’s interesting to look at this special case, where now the hull isn’t operating as normal planing hull and isn’t going to obey Savitsky calculations any more. At this point I don't know what you would call the mode, because it's still planing, that is, it's creating hydrodynamic lift by pushing down on the water, but it isn't normal planing as experienced by conventional hulls at lower speeds.

 

Im a little bit crazy !!

Interesting the way its written and how you see this top end of planning!
Its easer to visulize whats said than looking at rows of figures and seemingly meaningless graph's !
At some point in the upper planning mode aerodynamics takes over from hydrodynamics and the boat no longer is supported by the hydrodynamic lift of the hull touching the surface of the water but is now supported entirely by possitive air pressure under and on top and around the whole boat !!
Surface effect under the hull as the air presses against the hull and holds it up but also the air flow under the hull is momentarilly squeezed against the waters surface so if it was possible there could be a pressure foot print on the waters surface or an indentation in the waters surface like a shadow !!, this could be an interesting phenomenum to look for !!!
bit like the boat travelling quickly in shallow water and the squeeze or surface effect but in H2o between the hull and the bottom !!
Ok in all this we have only looked at the hull bottom but it is a know fact hi speed air flow over the top deck surfaces also has a contributing effect to all thats going on here !!. its why decks of hi speed boats need to be seriously taken into consideration during the design stage .

 

I had decided that I was not going to make any more comments on this thread due to the divisive nature of many of the posts. Some claim they were trying to be funny (really?), others were just arrogant ("Wow, nobody here is even remotely close."). I don't claim to have any more insight than anyone else, but my comments do come from the perspective of 30 years in developing predictive models for these types of hydrodynamic problems.

And it is from this perspective that I say that predictive models are exactly the wrong way to think about answering the question of the inception of planing. In other words, it is inappropriate to attach things like exceeding a Froude number or to use some measure against a Savitsky prediction or Crouch number to answer this question.

Let me see if I can describe an analogy that gives you a better idea about what I mean. The opening post was to come up with a "definition of planing", or put another way "when is a boat planing".

So, consider the question of a "definition of rain" or "when is it raining"...

Rain is the state of liquid precipitation. Now, there is humidity at all times and there is correlation between humidity and rain, but it is not always consistent due to the complex nature of weather (much like hydrodynamics is complex). So, we "define" the state of raining by when liquid starts to fall.

Using a predictive model (forecasting) tells us when we think it might be raining, but not the true physical state of raining. We have all experienced forecasts of rain (based on predictive models) that were flat out wrong. So the predictive models do not define rain. It is only by measuring liquid that we can actually say "it is raining". How much liquid to we have to recover to say "it is raining"? I do not know that there is one precise definition, but I don't think its exact line of demarcation is important. Once it starts raining, we know it.

Predictive models of planing tell us when we think it might be planing, but not the state of planing. We have to look for physical evidence of planing (which would be analogous to measuring liquid to define rain). This is why we use the VCG and trim measures defined in post #2. These are observable measures of performance that indicate the physical transition from predominantly buoyant supported operation to predominantly dynamic lift supported operation (i.e., the rise in CG and the reduction of angle of attack to maintain lift, a.k.a. trim). Like rain, I do not see that an exact line of demarcation is all that important, but the measures themselves are important.

So, I respectfully submit that we can only define the state of planing by physical observation (VCG and trim, and perhaps actual lift forces if we could conveniently and consistently measure these), but never by a measure against some predictive model - whether that be exceeding an arbitrary speed coefficient or when some criteria is met within a prediction calculation.

(I hope that this thread stays sane, and does not degrade to its previous state.)

Don MacPherson
HydroComp, Inc.

 

thanks - very sane comments
Adding my 5c
Must the definition and measure be the same?
As discussed much earlier in this thread, the rise of mean CG due to hydrodynamic forces is a simple and somewhat unarguable fact. Leo has asked for a definition, maybe it only needs one sentence in the end. There are countless scientific phenomenon that can be defined in principle, but not easily measured or calculated. Theorists will continue to strive for mathmatical answers beacase this is one avenue for engineering to improve any system or product. wrt planing a whole bunch of empirical theory will converge on the answer, eg Froud number, but given the state of current knowledge I see here, each case will have limitations and caveats.

 

Of course not, and they generally are not.
I agree with your post, and it goes along with what has been said in previous pages. We have a definition of the phenomena on one side, and the indicators (or meters) on the other. Just the way it works for many other physical quantities - temperature and pressure, for example.

Definition (for example):
- a vessel is said to be planing if more than XX% of the vertical force acting on it's submerged volume is due to a hydrodynamic lift.

Indicators (for example):
- rise in VCG
- change in trim
- change of slope of the resistance curve
- etc.

Cheers