Definition of Planing

It depends on the bottom loading, and the cg location. High bottom loading hulls with an aft cg will respond like a water skier on a slalom ski. Big hump and a very well defined conversion to planing.

Lightly loaded skinny hulls with a more forward cg will have a transition that is much less defined, but will still have bow rising until they reach a plane and then the bow will drop. If the hull is too skinny or has too forward a cg, it may not plane, it will just plow along until you run out of power.

 

At this point, the question is - what definitions? We have a pot with all sort of vegetables cooking inside...

 

Long and skinny is precisely the border where you have to be extra careful how to choose the speed and hullform.

js

 

So let's try to put some order in this thread and see if we can find a minimum set of requirements, without complicating our lives too much.

Several folks here have proposed the dry transom plus rise in CG criterion. I have expanded the "dry transom" condition to include hulls with no transom, as follows:

Condition 1)
A clear line of separation of the water flow at the stern has to be observable, obtained by sharp corners, chines or lifting plates in the stern area.​

Condition 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.​

Are these two conditions enough, and if not - why not?
Please note that the speed, either dimensional or adimensional (Fn), does not appear in the above conditions, because it's value will strongly depend on the hull geometry and displacement.

Cheers

 

Just as devils advocate: is a lifting plate a hydrofoil with flow on both sides?
If so, what if there was such a lifting plate forward?

 

What difference would it make to the use of the definition if Condition 1 was not included in the definition? What does it contribute? How sharp is sharp?

 

Good question. The idea is to make sure that the rise is VCG is due to hydrodynamic lift and not, for example, due to some odd combinations of trim and heave in displacement speed regime.

 

My personal opinion is that it is planing, on purposely made planing surfaces. I see a fundamental difference with respect to a hydrofoil, in that a hydrofoil (just like a wing) is designed to operate fully immersed in a single phase fluid (water), while lifting plates are designed to operate on the air/water interface.

 

Here is another interesting way of looking at it. Since the basics of planning, visually are well know can we draw any other conclusions or trends from this? Well, we can all agree that there is an increase in frictional resistance which eventually begins to dominate at higher speeds owing to the wetted length. So taking that thought….

Below is a summary of various hulls and their resistance. I have checked around 15 different hulls, but for clarity just given a summary. I have plotted the frictional resistance and the residuary resistance of several hull forms. If you look at the 17m, there is a crossover of frictional resistance at around Fn =1.3. Looking at the tank test pictures and the boat when finished, on sea trials, visually one can easily say the boat was/is “planning”. The residuary resistance is plotted as dashed, frictional as solid lines.



So this crossover of frictional to that of its residuary confirms the hypothesis that when planning there is more frictional resistance than residuary resistance. This was true for all the other planning hulls I checked.

I took a 24m semi-displacement and a 40m semi displacement monohulls. These are also plotted and not once does the frictional resistance dominate. For that to occure a much higher speed is required!

Then took a long slender high L/D ratio catamaran. Not unsurprisingly these high L/D multihulls are dominated by high frictional resistance, so the hypotheses does not seem valid. Unless of course one excludes multihulls? However, when plotting a heavy cat, i.e. a low L/D ratio..it does indeed follow the same trend that the frictional does not cross over the residuary to be in the planning zone.

So for a vessel to be truly planning, the frictional resistance must exceed the residuary….some food for though and can anyone counter the values with other test data?

 

Sure.
For example, surfers move CG forward temporarily to get through the "hump" and into the planing regime.

 

Or "If a vessel is truly planning, the frictional resistance will exceed the residuary reistance."

Boat support primarially by hydrodynamic lift
-and-
Frictional resistance exceeds the residuary resistance

Which is the cause and which is the result? Based on Ad Hoc's study:
Planning always results in frictional resistance exceeds the residuary resistance.
-but-
Frictional resistance exceeds the residuary resistance can occur in some cases without planning.

Seems to me that the simpliest way to define planning is along the lines of the hydrodynamic lift compared to hydrostatic lift (or whatever other word someone prefers to "lift").

 

I think it is time to ask the following question-

Can planing be usefully defined based solely on the vessel's state at a single speed? Or must the trend in behavior at nearby speeds also be considered? Many of the posts here imply the second case, but no one has explicitly said so.

 

To throw a monkey wrench in that very valid observation...several years ago we completed a prototpye propulsion installation for another marine products company on a 38' deep-V mono hull, that added vertical-plane prop thrust vectoring to the craft; automatically trimmed and dynamically controlled. The original idea was to achieve a high degree of stabilization or motion reduction in seas, but...

With the combination of thrust vectoring and trim tabs, that thing would exhibit hardly any positive (bow up) trim change at all as it shot from a dead stop, through hump and on to plane. So remarkable was the difference in the trim v speed profile compared to the original configuration that it quickly became a "feature" in their marketing, even though it was an "accidental discovery" falling out of their motion control intentions. I don't think they managed to capitalize commercially on their invention but it serves to illustrate that a planing boat can be got on plane without the usual trim up and back down again characteristic.

 

Resistance curve

It makes me wonder that the resistance curve has not yet been used. Planing has for me no value in itself, only if the dynamic lift reduces the total resistance of the boat. Attached is the curve of the total resistance coefficient of a DTMB-63. If it were just for the viscous resistance, the "curve" would be proportional to the well known friction line, so almost horizontal.
So why not use the maximum of the curve of the total resistance coefficient as the point to distinguish the regimes. Beyond this point dynamic forces reduce the overall resistance coefficient in such a manner, that it decreases instead of increasing. Left of the maximum the resistance increases stronger than the viscous resistance and right of the maximum the resistance increases not as steep as the viscous resistance.
My 2ct.
Uli

 

Then this definition would deny the semi-planing or semi-displacement concept, since there is a definite transition point.