Definition of Planing

In Russian sources, planing is often defined as volumetric Froude number 3.0 and higher; assuming hull shape is suitable for planing. Mode between 1.0 and 3.0 is semi-planing, mode below 1.0 is displacement. This is quite handy and I often use it...

 

As someone who has written contracts specs for a living, I would comment that no requirement is valid unless it is testable, and the first thing any wise designer does is ask what the client's test is!

Simon

 

We used to build boats to a minimum target speed. That is easily quantifiable. However, if you buy a boat the builder or designer claims will plane with a 90 HP outboard, that can turn into a legal battle.

 

To me it looks pretty obvious that two essential conditions which have to be met in order to declare that the boat is in a planing regime are:

1) A clear line of separation of the water flow at the stern has to be observable. It can be accomplished via straight aft buttocks and transom stern (dry transom), but also with the use of horizontal appendages like the ones shown here:
http://www.boatdesign.net/forums/boat-design/semi-displacement-33509-2.html#post378957
http://www.boatdesign.net/forums/powerboats/plane-double-ender-33183-2.html#post375533

2) The speed should be sufficiently high to generate a given (conventionally agreed) hydrodynamic/hydrostatic lift ratio. It can be assessed by measuring the rise of the VCG relative to the purely hydrostatic case, and will strongly depend on the hull geometry and displacement.

Cheers

 

Maybe its too early for me to think clearly, but if you can measure the rise in CG, what else is needed to know the 50% dynamic lift point?

 

Ok, had my coffee now. Some other factors are needed but can't they be had from design calculations?

The forum edit function is giving trouble so had to post a new message.

 

The way I see it, when the vessel is moving at a non-zero speed, there will be a non-zero hydrodynamic force acting on it. Its influence on the vessel's trim will depend on the point where its vector is applied. Usually it is applied forward of the CG, so will tend to trim the vessel up. Even a rather small hydrodynamic force (much less than 50% of the vessel's weight) can give a significant trim if the lever arm is long enough.
So it can happen that the CG will either sink or rise due to this trim, depending on how will the hull settle down when the equilibrium of moments given by the 3 forces (weight, buoyancy, hydrodynamic lift) is attained. Someone please correct me if this reasoning is wrong.
Cheers

 

Trouble is, as noted here:
http://www.boatdesign.net/forums/hy...cs/definition-planing-45248-3.html#post591991
not valid in shallow water, to be considered "planing" that is.

 

I see. Looks like the definition of planing will have to be restricted to deep-water operations. The influence of shallow waters will have to be stated as "to be evaluated for each case separately".

What do you think about the simple two-step definition in the post #49?

 

Planning planing boats

As I see it, a planing boat have to go faster than FnL 1.0 -1.1 and FnV more than 2,7-3.0 and have a suitable form. To my assistance I use the attached diagram in the initial stage of planning.

js

 

"Suitable form" is where you have put all the uncertainties - it has to be defined better, imho. The way you have defined it, it brings us back to the starting point of the discussion.

 

Not really. The Fn is a very good starting point.

Far better than any nonsense about a completely arbitrary 50% of the weight being supported by lift.

The larger and heavier the system is the higher percentage of the hull weight that must be supported by lift to achieve planing. For a small, light and short boat it's very different than a very large, long and heavy ship. Look at Destriero, it's considered a "semi-planing" ship even when traveling at more than 50 knots, and it's probably got 50% of its mass supported by hydrodynamic lift at that point, but it isn't planing.

True planing is achieved when the hull goes past the point of powering "over the hump" and, as speed increases over hump speed, the trim angle will decrease with increasing speed. For each design hull, that can be a very different speed, and for some semi-planing hulls it might never happen.

For instance, a hull with a large amount of deadrise won't plane at the same speed as a flat bottomed hull with the same beam and length. And of course bottom loading enters into the equation. A narrow hull will have a different drag curve than a fatter hull.

What actually defines planing is the fact that if the speed increases the trim angle will decrease. If you aren't planing, increasing speed in the sub planing regime results in an increase in trim angle, for a planing hull. Again, for a semi-planing hull that might not happen at all, but for a true planing hull, the speed where it attains planing status is the point where the trim angle decreases as the speed increases.

 

If the vessel is supported more by buoyancy than hydrodynamic
forces, why is it considered to be "planing"? The 50% criterion
is just one way to specify when hydrodynamics dominates over
hydrostatics.

I agree that the trim angle begins to decrease once planing has
set in. The attached graph shows the angle required to maintain
fixed lift equal to weight, as a function of Froude number based
on the location of the centre of pressure of a simple flat plate.
Below FrX = 0.95 the required trim angle is greater than the
initial trim angle. As FrX increases, and the vessel begins to
plane, the angle drops smoothly and quite quickly, which accords
with your criterion.

There are similar graphs that show rapid changes at or around FrX = 1,
for example the centre of pressure moves rapidly towards the front of the
boat (the 1/4 chord in 2D planing) as the pressure distribution becomes more
and more like one half of an airfoil, splash drag dominates wave drag etc etc.

 

Yes Leo good post, I think we can all agree that, when looking at this curve of Fn and trim angle it becomes pretty clear where planing starts.

As we all realize this becomes different for every hull, and for every load condition, every thrust angle, and for every offset of the thrust vector from the line of the planing surface, as well as things like prop rake. It's possible to significantly change the onset of planing by modifying those variables.

When all things are considered, it is far more complex than a simple % of lift. For lightly loaded short, wide hulls, the change in trim angle happens much more quickly with increasing speed. For much larger hulls with heavier bottom loading, the curve will likely be more of a gentle hump, but the reality is, when you are past the peak of the trim angle curve the hull is now in the planing regime. Since there a any number of variables that can push the actual speed at which the trim angle starts to decrease it's really not as simple as even a Fn (although that's not a bad thing to use for a rule of thumb).

 

How do long skinny hulls figure within these definitions?