Definition of Planing

I liked your post though I'm not sure that they gave up. I suggest replacing your notion of "inconsistent" with a limited ability to predict and evaluate the performance of certain vessels based on multiple factors.

For myself, this thread has improved my ability over just marketing hype and performance claims.
Doesn't that have value?

 

That's the problem Gonzo.

Real useful definitions are not arbitrary. Like the Froude observation that drag piles up at a very specific speed.

Physics is not arbitrary.

The problem with describing planing is because the conventional definition is flawed.

You tell me what planing is specifically without referring to arbitrary percentages of weight carried or arbitrary speed ranges.

That's the proof that the attempted definitions are flawed.

As an example ... in engineering you have tolerances. Every part has a tolerance. So a bearing might have a workable tolerance of plus or minus 0.005mm.

What is the test of this tolerance range ... it is because when you move just outside the range something specific happens! Failure or shortened life or interference between parts.

But using specific ranges for either the percentage of weight carried by the "planing force" or a specific speed range is just fuzzy - because a knot in boatspeed either way doesn't make any observable difference despite moving outside the specified range

Planing as a boat behaviour is arbitrary usage - but using to describe a force is specific, measurable and repeatable.

Best wishes
Michael Storer

 

YES .. if say semi planing is defined as an arbitrary speed range - say for a particular boat we say "semi planing" is 6 to 10 knots... what happens if you increase power and can get to 10.001 knots.

Same for any defined displacement range, same for any defined planing range.

All these terms around planing are rough descriptions and don't support a scientific viewpoint.

Displacement, semi planing and planing are useful to describe to a client how the boat will go and what the powering should be in approximate terms ... but dig a bit and they fall down as definitions.

Michael Storer

 

Hi Tomas,

Discussion itself has huge value!

I'm just putting it a different way because I think the existing way is a mess.

When speed prediction it is worked out accurately it will apply to all hullforms in a consistent way.

At the moment we are talking about an EMPIRICAL approach. So it is doing the best with what you have using a system of approximations.

This does have a value.

But when you DEFINE something then it is something that is consistent applicable and repeatable.

"Planing" as a force has this type of consistency.

But Planing as a specific phenomenon in boat behaviour does not apply specifically to any specific condition. So the definition falls over when used in this way because of a lack of precision.

Best wishes
Michael Storer

 

Okay, boatmik, let us suppose the following.

1. That boats are supported locally on the surface, regardless of what they are doing. Going back to the tank example a few pages back, this would mean if you plop a boat in a tank and trace the waterline around the perimeter, the waterline will always be at that height at the perimeter. If planing or sinkage results in a change in the draft of the hull, then the corresponding change in the free surface which is needed to make the average of the two surfaces constant is confined to a smallish closed area about the boat.

That would give us a way of looking at the problem in a completely mathematical way for any arbitrary shape. We need only to note the change in the vessel's draft and the description of the surface in the effected area near the boat. There are already formal ways of identifying and handling local support problems mathematically. Planing could be defined as some function of displacement, basic hull parameters, and the bounded area of support.

Of couse, I haven't a clue as whether or not support really is local. I haven't found anything on this yet searching google. There are implications for energy balance and entropy concerns with a local support model. Basically, it would seem to require some additional force that is not currently in the picture- derived from drag I suppose.

 

How about the local effects of the aerodynamic field around a planing vessel?

A hovercraft comes to my mind, as an example of an entirely local pressure field. When hovering, it is fully supported by the aerodynamic pressure inside the air cavity, so it's displacement is zero, right?

Yet, the pressure field inside the air cavity bounded by the skirt acts both ways:
- upwards, holding the craft above water, and
- downwards, pushing on the water surface.
Globaly, the lift force acting upwards on the craft must equal the downwards force pushing on the water. In other words, a hovercraft displaces water though it is not touching it. In fact, a hovercraft even has it's own wave train and the associated wave drag, just like any displacement vessel.

So, if you repeat the proposed water-tank experiment with a hovercraft, you would see a rise in water level on the perimeter of the tank, and the weight of the displaced water would equal the weight of the hovercraft...

Now that's an extreme example of a non-displacement vessel (as we use to intend that term) which displaces water through aerodynamic forces. I am throwing it into the discussion as an example of the practical difficulty to even determine the exact amount of purely displacement force acting on a vessel.

A planing boat is not a hovercraft, of course, but at very high speeds it is partially lifted by aerodynamic force too. It means that an aerodynamic pressure field will exist around the hull, and at least part of it will push against the water surface. Look at the tunnel hulls, for example. How much of this pressure "bubble" enclosing the hull will push against water, and thus spoil our efforts to precisely calculate the water displaced by the submerged volume of the hull, that's a question...

Cheers

 

Definitions are arbitrary, that is the point of them. We decide the limits or scope of something and give it a name. Otherwise, we would have no way of understanding what we are talking about. For example, "sailing vessel". The particulars of propulsion by sails is dictated by laws of physics, the scope of the definition is arbitrary.

 

Hi Gonzo,

There are two types of arbitrariness.

One is what name you give to something. I declare this to be "rkfktt".

For the description part a definition can apply to something real or unreal.

"Gravity" is the mutual attraction of two bodies etc
"Rkfktt" is the light that bodies emanate when gravity puts them into motion.

Planing as a force is similar to my first example
Planing as a behaviour is like the second. Not quite so bad, but when examined it falls down.

You can define something arbitrarily in these two ways ... but for it to be true it needs to stand up to examination and observation

Best wishes
Michael Storer

 

Planing is not a force, it is the behavior of a vessel. What is "rkfktt"? Using the word "gravity" to describe the force that attracts to bodies is arbitrary. There are other forces, like magnetism, that also attract two bodies.

 

the water-tank experiment was suposed to start with a swimming objekt, regardless of the type of the objekt (glider, hovercraft, helicopter, hydroplane, sea plane, or seagull...)

a helicopter is IMO bit easyer to understand than a hovercraft, because first he can start vertical, and second he can stand still, so we can eliminate the horizontal move in the experiment

but both will truly create a hole in the watersurface after they start, a hole with the same volume as before when they swam on the pool

and the level on the perimeter will not rise! (he will fall if the seagull or copter fly outside the perimeter wall)

the difference between a swimming objekt and a hovering objekt is only the form of the hole in the water, for a displacement swimmer the form of the underwater-hole is equal to the form of the underwater-part-of-the-objekt, and both are on the same place at the same moment

for a hovering objekt the form of the hole on the watersurface is equal to the pressure field and this hole is not on the same place as the objekt in the same moment

and a gliding boat must be the same as a starting hover, the hole in the water must be on the same place as the presure field was in the moment as it hold/lifts the boat...... and the volume of the hole must be constant like for the hovercraft

why is it so tricky? we need a rotating glider, at least for a mental picture, a glider who glides on the surface without a horizontal move, glides on the water like a still standing helicopter stands still in the air, a still gliding water-surfase-copter,

and which form of the hole on the watersurface would this rotating surface-objekt,with his presure field, create ??? who can discribe the pressure field in this case

yes, why is it so tricky

 

Yes, right. I was referring to a hypothetical situation where you have a tank, measure it's water level, then put a hovercraft on it and measure again the water level. There would be a rise, and the amount of it would be the same as if the hovercraft was floating on the water (pure displacement mode) with it's fans turned off.

Cheers

 

I like the style this ski boat is planning

 

Some keep posting this argument but I don't believe it is strictly true. If the copter or hovercraft were supported by pressure only that made the depression in the water, then yes it would be true. But the flying machine is also supported by thrust (acceleration of the air from the props or rotors plus any lift over the upper surfaces of the same). If there were no leakage around the skirt or around the props or rotor, then your analogy might work but not otherwise.

What this has to do with a definition of planing escapes me.

 

I am not an expert in hovercrafts. All I know about them is what I can read around. And this is what I have learned so far:

The air escapes the skirt laterally, thus having no vertical thrust component due to acceleration of air mass. And besides that, the spillage is so small that it would give very small contribution to the lift even if it was directed downwards. In fact, I have noticed that the calculations of lift force acting on a hovercraft take unly account of the relative pressure (relative to the atmospheric pressure) inside the air cavity, and disregard the contributions of the spillage under the skirt.

And yes, I agree, it is not directly related to planing boats. But it could be somewhat (perhaps just marginally, don't know) related to boats which are partially sustained by aerodynamic forces - see the final part of my post #576. The point is - sometimes it is difficult to distinguish the displacement component of the lifting force which is due to submerged volume from the one due to aerodynamic pressure pushing on the water.

Cheers

 

What this has to do with a definition of planing escapes me.

I totally agree its just changing to something else because the planning things has been completely going no where and is a load of old smelly sand shoes !!