Definition of Planing

Nice effort and a very nice visualization, but there's an error over there. Namely, seems that you have kept the wetted surface length constant, which is not really the case. As the speed goes up, the wetted area will also vary because the mean wetted length (Lambda, the mid-value between the chine and keel wetted lengths)) will get smaller, unless the hull is shaped in some specific way to prevent is. At very high speeds, some boats will barely touch water.
Cheers

 

By SLR do you mean what you previously called LDR?

Bluejacket 24 has an L/B of around 3.5, and Bluejacket 28 has an L/B of around 3.8.

I don't think LDR by itself is a good measure of bottom area available for planing. A high LDR hull may be narrow and deep and not have much bottom area, or it may be wide and shallow and have considerable bottom area, or it may be in between.

 

Leo started this thread with a quote from the ITTC Wiki Dictionary which can be found at http://www.ittcwiki.org/doku.php/proposal:dictionary:planing

There is another ITTC dictionary available online; the ITTC Dictionary of Hydromechanics http://ittc.sname.org/CD 2011/Dictionary/ITTC Alphabet Dictionary 2011.pdf

It may be of interest while a number of entries in the ITTC Dictionary of Hydromechanics refer to features of "planing craft" there is no entry for "planing". The closest to a definition of planing which I could find appear in the entry for "Speed hump" on p113. "Speed hump" is defined as "The speed at which the resistance reaches a maximum before a planning craft enters the planing phase, or a hydrofoil craft enters the foilborne phase".

 

Odd M. Faltinsen in Hydrodynamics of High-Speed Marine Vehicles has one paragraph dicussing what planing is (p342) out of a 48 page chapter on Planing Vessels:

A vessel is planing when the length Froude number Fn > 1.2 (Savitsky 1992). However, Fn = 1.0 is also used as lower limit for planing; that is, we cannot see a clear line of demarcation between planing and non planing conditions just by refering to the Froude number. During planing, the weight of the vessel is mainly supported by hydrodynamic pressure loads, with buoyancy having less importance. The hydrodynamic pressure both lifts the vessel and affects the trim angle.

Interesting that he starts up using Froude number and referencing Savitsky, then says that Froude number isn't sufficient, and winds up the a general statement about hydrodynamic pressures compared to buoyancy.

 

Yes, sorry for creating confusion. Mikko has made a question where he used SLR, and I have replied in the same manner, assuming that SLR stands for slenderness ratio, which is the same as length/ displacement ratio (LDR) in my previous posts.

It is probably true that LDR is not a universally applicable tool, and you have given us a good example of it's limits. However, in the case of the previously mentioned graphs, we also have a family of hull forms (series 62, if I recall correctly) which those graphs are related to. The shape of the parent hull form, together with the relative resistance data given by the said graphs, gives us the information we need to start making some conclusions. Of course, all within the boundaries of applicability of the said resistance data.

For the rest, the fact that the term planing is not clearly defined in the ITTC dictionary either, as you have noted, is probably yet another proof of how problematic it's formal definition actually is.

Cheers

 

The first chapter of Peter Du Cane's High Speed Small Craft (Fourth edition 1974) is titled "Definition". With regard to planing he explains:

It is when the speed becomes sufficient to drive the boat towards the surface that the borrom form of the hull acts as aplane in some measure. Is when thes conditions prevail that a boat is said to be "planing".

There are, of course, many degrees of planing, ranging from the stage where the water ceases to close in behind the stern until some feet astern to the stage where almost nothing of the boat is in the water as it streaks over the surface.

Planing can perhaps be defined as that stage at which dynamic forces do the motion of the hull though the water begin to make their influence felt.

Du Cane goes on to discuss the speed/length ratio and planing, and then the reduction of wave-making resistance.

 

Clement and Blount, and others have used a loading coefficient in characterising models used in planing craft tests. The bottom loading coefficent is defined as:

Cbl = Ap / Del^2/3

Cbl is the bottom loading coefficient (not sure this is the proper abreviation)
Ap is projected planing bottom area and usually close to the static waterplane area.
Del is the static displaced volume​

For "similar" hull shapes

Ap is proportional to L * B​

A parameter proportional to bottom loading coefficient would be

C1 = (L * B) / Del^2/3
Cbl = C1 * (Ap / (B*L))
C1 is used here for convience and has no deeper meaning.​

LDR is the non-dimensional Length Displacement Ratio

LDR = L / Del^2/3
​

Using the length/beam ratio L/B

C1 = (L/B) / LDR^2
Cbl = (L/B) / LDR^2 * ((Ap / (B*L))​

So the bottom loading coefficienct is proprotional to L/B and inversely proportional to LDR squared.

 

For the prismatic hull, I did not include the root spray line and used only the intersection of the waterline to keep the presentation simple. Likewise I used a swim bow for the flat bottom hull. I added a bow rake to accentuate the shortening/lengthening of waterline length.

What is easily noticeable here is that the hull with deadrise angle narrows down during lift resulting in a high length to beam ratio leading to a great reduction of Clo due to loss of stagnation pressure in the leading edge. See attached graph from SNAME. This is similar to what AH has posted.

LR has a definition of planing, which to me is inadequate, hence, I am following this thread. The rule states "Planing and semi planing craft are crafts with Taylor's Quotient, as defined in Ch 2.2.1.16 is greater than or equal to 3.0".

 

What is written here might help defining planing:

http://www.sassdesign.net/InterceptorpresentationengJGK JS.pdf

pogo

 

We haven't discussed the upper limit of planing. With enough power and the right thrust angle, a vessel can be riding on the propeller blades only. Also, when the vessel jumps off the water it is airborne and not planing. The aerodynamic forces will become prevalent at higher speeds and can't be dismissed.

 

Sail Rocket

Interesting point, Gonzo, well proven by the fastest sailboat on the planet. The picture below shows the forward "pontoon" clear of the water at 65 knots. That means that the only parts in the water were the hydrofoil(holding the thing down) and the forward rudder!

click on picture for larger image:

 

Gonzo and Doug, your interjection of reality (re VSR) may NOT be want the punters are wanting to pontificate upon here.

 

This is what you wanted !! Am dying to see the answers to this one !!

 

What answers do you guys expect? The hull in tunnels' pic appears to be sustained by the aerodynamic lift and by the prop forces and moments only. No planing there because it is not touching the water, so in the moment the pic was taken it is out of the scope of this discussion.

As for Doug's pic, I did not follow the SR discussion, so don't know all the stuff involved, but the pic looks like it was taken when the forward planing surface was momentarily out of water between two wave crests. In this video: http://youtu.be/sZVIj5TUSKE at 1:55 you can watch it in action. Can't see the point there either.

I fail to see a point of bringing out these examples. We can talk about planing if the hull is in water, otherwise it is either flying or is being sustained by other external forces and moments. What else can be said?

Cheers

 

this will be interesting !!!

aaaahh but at some stage it was planning !!
so when did it start to plane ??
and at what stage was it not planning ??
and at what stage will it planning again ??
before it stops planning ??
and finally comes to rest !!!