Definition of Planing

As has been mentioned previously in this thread, some boats with immersed (at rest) transoms can have "holes in the water" behind the transom, ie separation from the edge of the transom, when the boat is not being supported by dynamic pressure and not planing.

 

You are claiming that a moving plate creates displacement.
If that were right then any boat would increase its displacement by moving.

My point is that displaced and displacement derive from the same word but the concepts are different. You might as well say if a rotating propeller displaces water there is displacement but that is a different meaning.

It would be clearer if we used buoyancy as the force supporting the boat.
The formula :- Weight of boat = buoyancy + planing force might cause less confusion.
Examples: a hydroplane with thin plates at the back edge of the sponsons going fast enough for only the plates to be in the water. Volume of hull under water negligible therefore buoyancy negligible. Weight of boat entirely supported by planing force.

Flat bottom hard chine boat planing, depth of water at sides and transom nil. Where is the buoyancy? Same with a ski.

By using displacement to mean both the hole in the water caused by the hull at rest and the volume of water pushed aside by the hull when moving forward it is hard to analyse what is going on.

I agree in practice a small proportion of a hydroplane is supported by buoyancy.

 

It has nothing to do with what I claim. It is what Naval Architects agree on. I had some issues with that a long time ago but realized that it agrees with basic principles. The "displacement" is not created by the moving plate it is only a part of the at-rest displacement.

If you were thinking of a plate with no weight, the example means nothing. All things planing in the water assume a trim angle that is exactly enough to offset the loss of at-rest displacement resulting from from lift (or vice-versa). No weight means no trim angle and the example becomes nonsense.

 

Dynamic lift can be greater than the displacement of the boat, in which case it goes airborne.

 

Like Galileo and Torricelli`s vacuum and siphon experiments Archimedes (the EUREKA boy) "The science of fluid at rest" and Hydrostatics in general is not well understood after hundreds of years.

 

In light of this thread lulling a bit, I have a design challenge I'd like to present for your consideration.

As a displacement monohull approaches hull speed (~8 knots), it's small, lightly loaded outriggers could begin to plane.

For this they would need a low l/b ratio.

But at low speed, they can be more heavily loaded and forced into discplacement mode where a high l/b ratio is favoured.

How to design these outriggers for both scenerios?

 

Why not start a new thread with your question?

 

change the trim or better use a slender and smaler outrigger.....
simply a 6 feet long outrigger moves with froude<1.0 at 8 kn......



my hope for the future:
we do not longer need a different theorie for every hull type, (heavy, fast, disp,planing, multi etc...)

we just have to understand the difference in the flow typs
-laminar,
-turbulent with decelerated flow [v < sqrt(gh) for free flow]
-turbulent with accelerated flow [v > sqrt(gh) for free flow]

(what is the correct english hydrodynamic term for the last both?)

just so, the hump barriere for a havy displacment hulls should no longer be the normal aproach, this should be the more extraordinaire part of the theorie.....

then planing will be just a flow with so much acceleration and following higher presure that the hull climbs up.

 

Interesting thread, I have minimal experience in naval architecture, but love the science and math. Got to about page 16 before jumping to the end. So forgive me if I have missed something in between.
Now for my 2 cents worth.
As I see it once an object has reached the point when it is planing, if you could remove all surfaces that are not "wet" whilst maintaining mass, centre of gravity, velocity etc, the object would remain on the plane. Should the volume of the water "displaced" due to the motion (including the depression behind the planing surfaces) be included in the "instantaneuos displacement" of the object. This would explain why a stepped hull and hard transom are helpful as they create a more defined depression. It would be interesting to include these volumes into the equation as zero mass/zero friction hulls but include them into the calculation of centre of bouyancy etc. If the centre of bouyancy has actually moved behind the transom that would also result in the "bouncing" of the hull when planing as the hull would be pivoting about the centre of bouyancy.
Thats enough for now, time to do a bit more thinking.

 

hi, within the last 80 posts we discussed the idee of a boat planing in a pool on a scale, to ask how can we hold all vertical forces and weights in a zero-sum and will the outside surfase of the water in the pool rise or fall once the boat starts to plain.... we do not found a clear answer, not eaven once if the volume of the depression behind the boat is equal to the lifted volume of the hull...

but if your view is correct... then the water level would stay constant in the pool....

ok

with your idee to include the depresions volume in the calculation of centre of bouyance then I have the question how can we transfer the lift-forces of this part of the bouyanc trough the the water (or trough air ?) to the hull in front of the depression ???

 

What matters is the WEIGHT the planing surfaces carry.

An angle of about seven degrees produces the highest lift to drag ratio on a planing surface such as a sponson (most lift for the least drag). However, it also produces a violent response to waves, which is why hydroplanes are designed with much lower sponson angles.

 

Lowest drag (best L/D) for a planing surface is on the order of 3 to 3.5 degrees, not 7 degrees.


Also the running depth, and consequently the displacement amount is different for different trim angles and hull configurations. It is possible to generatre large amounts of "downforce" or negative lift at low trim angles and if the CG is too far forward. This can result in significantly more running depth and of course much higher drag, so the displacement amount for a given hull is dependent on CG location and the resulting trim angle for a given speed.

 

Unlimited hydroplane designers don't run 3 to 4 degrees on their sponsons because it's the lowest drag. They run it because it's the best compromise between drag and reaction to waves.

 

I'm not talking about hydroplanes, I'm talking about planing hulls. The point is that the best L/D, or what Savitsky is referring to at the drag to displacement ratio is not at 7 degrees of trim angle.

In Savitsky's paper on high speed planing hulls the following assessment of the influence of a planing hull's trim angle on the effort needed to propel the hull:

"...the performance of a planing hull is very dependent upon the longitudinal location of the center-of-gravity that controls the trim angle of the craft when planning. The trim angle in turn has a major effect upon the resistance/weight ratio. Typically, the resistance of a planing hull is a minimum at trim angles between 3-4 degrees and increases for both higher and lower values of trim.

The link below is to the paper, you can read it yourself.

http://legacy.sname.org/newsletter/Savitskyreport.pdf

 

There's all kinds of planing hulls -- hydroplanes, tunnel hulls, V-hulls, etc.