planing theory

Geez....this "suction" idea could reasonably easily have been put to rest by pressure sensors in a hull bottom sheathing, you'd think.

 

The sea of beans theory is not correct for incompressible flows (like water flow around boat hulls). But the negative gauge pressure below planing hulls have actually been measured. See the page 5, figure 10 of this work, for example: http://publications.iot.nrc.ca/documents/IR/IR-2001-43.pdf . Or this one, page 3, Fig. 2: http://www.hydralab.eu/hydralabIII/proceedings/NTNU-11_Garme.pdf


As a side note, Newton's model of lift and drag forces generated by bouncing particles (which is the same as Mr. Lathorpe's "sea of beans") gives results which agree well with empirical lif data for hypersonic flows (which have nothing to with boats, of course). See pages 11.3 - 11.4 of this paper, for example, where some equations and graphs are give.: http://www.dept.aoe.vt.edu/~mason/Mason_f/ConfigAeroHypersonics.pdf
It is interesting to note the comment to the figure 11-4:

"Here the lower surface pressure is taken equal to the stagnation value, and the upper surface is taken equal to the vacuum value. We can see that at low speeds the lift is generated on the upper surface, while at high speed the lift is almost completely generated on the lower surface."​

By reading that explanation, one could infer that a planing hull behaves like a mix of the two worlds: we have a nearly constant atmospheric pressure on the top surface, and a highly variable gauge pressures (from high positive in the fwd portions of the hull to negative aft) under the bottom. I know it is not what physically happens there, consider it just a little provocation . I'll try to make few calculations and see what happens when the bean theory (I like the name ) math is applied to a flat plate hull.

More info about the derivation and application of Newtonian theory for hypersonic aerodynamics here: http://www.engr.sjsu.edu/nikos/courses/ae264/pdf/Hypersonic.pdf (from the page 15 - chapter 4)

 

The first layer of molecules sticks to the surface, doesn't bounce off. That's represented by setting the velocity equal to zero at the surface, a necessary boundary condition for the Navier-Stokes equations. the successive layers are dragged by the first, the second less than the first, the thired less than the second, until you get out of the boundary layer and can apply the free stream (viscosity-free) eqns. This describes skin friction near the surface plus form drag. Form drag is more important in planing, that comes from the boundary layer and wake. Skin friction can vary like the speed V, form drag goes like V^2. When a boat runs on its tail like a race boat the skin friction is negligible. That the gearcase (outboard or stern drive) is a significant factor in causing drag is seen by changing the same motor from a V-bottom to a tunnel (catamaran). The motor can be run with the gearcase hub above the water in the latter case and the speed increase is dramatic. In every case there is a 'bow wave' even if it's near the transom, and that's form drag. When a boat plows and makes a big wake that's also mainly form drag.

There's no equilibrium, there is simply force balance if the boat's velocity is assumed to be constant (constant speed in a straight line, no porpoising, no waves to cause slamming, etc).

 

You can also have slip conditions, and partial slip-conditions, too. Here is a recent paper on partial slip:
http://144.206.159.178/FT/158/72638/1241532.pdf

 

You might apply a polymer to the surface to try to generate 'slip', but then the polymer sticks to the surface. A stretching surface, mentioned in your ref., doesn't describe a planing hull. A no-slip boundary condition is mathematically inconsistent with the viscosity term in the Navier-Stokes eqns.

 

It was meant to be simple and not go into obtuse formulas that are alien to the layman. Apparently that upsets some here who regard themselves as experts. Too bad, it was not meant for your eyes in the first place. Neither is there an argument between Bernoulli and Newton in the piece. Both are valid in their applicable spheres.

I stand by what I wrote. If that offends anyones scientific predilections, then they may easily ignore it. If it had not helped many unprofessional boat enthusiasts to understand the actions of a planing boat in the water better, I would have removed the article from the website. The article was never intended for someone to use in designing a planing boat but to gain a better understanding of one. That idea is also apparently alien to some here.

That many experts here continue to think that the suction idea can be disproved by simple pressure measurements or water squirting up through holes in a boat bottom shows that there is some lack of understanding of basic engineering principles. Superposition, for instance.

"If criticism or critiquing like this irks you, then you should reconsider entering into such a field" I see nothing but generalizations in the criticisms you offer. I have tried to limit my posts to things that I am familiar with and avoid general pronouncemnents and guesses. The boats I design are very specific in their intended uses and have proven themselves by enthusiastic acceptance by those who have built them. I do not consider that I am competent to step very far outside that area and I refuse to get into a pissing contest with you or anyone.

 

As expected, nothing happens. It was enough to compare the dCl/dAlpha coming from the Newtonian theory and the values returned by the method from this work of Leo on a planing 2D plate: http://www.boatdesign.net/forums/design-software/p2d-simple-2d-planing-program-41896.html, to see that there is an order of magnitude difference between the results.

So while it remains that the "sea of beans" explanation is a nice and simple way to explain the concept of planing to (as Tad says) laymen, it is also true that it has no real connection with the physical reality. But he also said that it was not an attempt to get into hard math and physics, just wanted to give a rough visual rappresentation about what's going on down below the hull. I think it's ok for that purpose, so let's not argue and get too deep into this folks...

Cheers all

 

Even lashing out at me.

How emotional.

-Tom

 

I agree that science is about observed facts that can be replicated by anyone in the same conditions. Anything else is not science. Many people get emotionally attached to theories even when they are proven wrong. Usually, they refuse to defend them on grounds that "no one understands them" or " the wrong people are reading it". Either way, it is not science. Also, in science it is necessary to have definitions to narrow the discussion. Unless we define planing first, this whole thread makes no sense because we can't know what we are discussing about.

 

Hi sandhammaren,
I wasn't trying to describe a planing hull: it was just a reference to an
example of a part-slip condition because I thought you weren't aware of them.

 

By 'suction' is meant a reduction in pressure region moving aft, as in the Bernoulli effect in the free stream when the streamlines bend as a result of the trim angle. The integrated pressure over the wetted area is positive or else the boat would not plane.

But here's a good example of what you mean by 'suction': a boat with hooked bottom will be 'sucked' down
onto the water, will not plane aft as it should but will tend to wet roughly the entire length of hooked bottom.
That causes high drag and low speed.

 

That's not always true, it depends on how much the boat bottom disturbs the flow into the prop. I'm not sure you can take a flat bottom of the same weight, accelerate from zero, and stay with a pad v-bottom design. Same for competing with a tunnel. With today's surfacing props, set up correctly, there's no enormous slip when you hit the throttle from rest, and then finally the prop 'catches' and the boat takes off. There's just a nice, continuous acceleration to top speed with pad-Vs and tunnels. I'm thinking here of high performance outboards or stern drives.

 

It does give good predictions for rarefied flows, but IMO it's best to use the theory appropriate to the problem ,and there is no reason to use the wrong one in standard water problems.

BTW, just for interest, it has been said that Newton's "mistake" might have delayed aviation by 50 years or more. See:
"Wrong for the right reasons",
eds. Buchwald, J.Z and Franklin, A.
Springer, 2005

I prefer "sea of peas" to "sea of beans".
"mare di piselli" or "mare di fagioli"? Both sound a bit odd to English speakers

 

Really ? Would those planes have been steam powered ? If Newton had never lived, arguably a whole lot may have been delayed substantially, so I guess it is a mixed grill.

 

A hooked bottom just shifts the centre of dynamic lift further aft, though, changing the trim angle, similar to trim tabs would. The concentration of lift at the leading edge of a surface with a constant angle of attack is altogether different to a surface with an increasing angle of attack as is evident in a hook.