flat sterns cause squat ?

Mat-C and Messabout, there are many forces that come into play when trying to understand any real life fluid dynamics problem. As with any science, empirical data as well as theory is expected to agree to within the range that the models are considered valid. As I understand, the Computational Fluid Dynamic (CFD) models and tank test do not completely agree on the finer details of fluid models for hulls. So, there is not a consensus of understanding, once you get into fine measurements or details of pressures for a model of flow around a 3-dimensional hull.

Messabout, I don’t think you will get a consensus of agreement on your first point:

On the other hand, there are basic principles of fluid dynamics which are useful and readily accepted. The fact that forces are vectors, and are additive, is widely accepted, as Tom explains above. Bernoulli’s equation is very applicable and models the 2-dimensional foil well. A great deal of experiment and modeling has been done of boundary layers add turbulent flow layers. Real world situations beyond these are very complex, very difficult to control, and very difficult to measure.

I’ve been studying various aspects of fluid dynamics for many years. One of the more rewarding areas of study is the foil. Here are a couple of references that were given to me, I believe on this forum: Theory of Wing Sections, by Abbott and Von Doenhoff (Dover); Aerodynamics of Wings and Bodies, by Ashely and Langahl, (Dover).

Also often recommended here is Marchaj’s and/or Fossati’s books on Aero-hydrodynamics because these discuss the topics which are directly of interest to boat design. Due to the complexity of Aero-hydrodynamics, both a mathematical model as well as the physical or conceptual model is usually required. So for a mathematical background, I recommend a good book on fluid dynamics titled: Fluid Mechanics by Robert A. Granger, published by Dover. Reading through it couple of times should be good background to understand Marchaj’s and/or Fossati’s books on Aero-hydrodynamics.

 

I think you guys might be over-analysing Gerr's assertion. If my memory serves me correctly (it doesn't always!) he was merely pointing out that water being forced up through the holes, rather than being sucked out of it, is evidence that the pressure - in the overall sense - is positive. Whethe there are positive and negative forces at play is beside the point.

 

Ok, now I get what you meant. It is a rather broad usage of the term "Venturi effect"...
As for errors in static pressure readings with probes inside holes, it can be reduced to an acceptable level by reducing the hole diameter. It is akin to the problem of determining the optimum geometry of pitot-static tubes. I'm enclosing two readings on the subject. In particular, see the page 651 (pdf-page 10) of the first paper for some hole geometries of interest.
The part about turbulence is still not clear to me, but it might be because it's 1.45 a.m. here and Morpheus is calling me. I better go to sleep now. Cheers

 

Will,

If that were his position I can't imagine that there would be any controversy. Unfortunately, a look at Gerr's book on page 139 asserts that Lord was mistaken about there being suction present under the hull. No mistaking the meaning as he then goes on to debunk the existence of this mysterious suction. I have and regularly use his books so I have no bone to pick other than that this is too important an issue to pass over.

 

hmmm.... leave it with me.....

 

Messabout,
"Let us agree that the area of maximum lift occurs at the point of impact."
What makes you think that? It would seem to me that the pressure on the bottom of the hull may/should increase as we go aft. The water is being pushed downward further and further as we go aft and there is more water around the hull preventing it's escape to the sides. As one comes over the hump the bow lowers itself more and more as the boat goes faster. Is it because the bow loses pressure or lift .. I think not. It is because as one goes faster the water under the after plane has less time to escape past the chines creating more pressure under the after plane raising the stern. All this is assuming a straight run aft. There may be less pressure aft on a rockered hull but no suction if the boat is operated anywhere near a safe speed.
By the way I have a boat w a hole in the bottom about 5" fwd of the transom. It's a drain hole and there is a fairing just in front of it. As one can imagine when I remove the plug there is a very strong suction but I'm sure the suction would disappear if I were to remove the fairing and the boat would fill w water pronto. The boat is a Crestliner aluminum skiff and the drain hole is a standard feature. No I will not remove the fairing. I will submit at this point that there is no suction under flat after sections of boats designed to be operated in a planing mode.

Easy

 

Oh... go on.... please take pics....

 

MCollins and Easy I am here to learn. I have been lead to believe in the first point of impact notion but I swear that I have an open mind (well most of the time) Please illucidate.

 

Messabout,
here is Tom's link to a relevant diagram given earlier in the thread. It presents a sketch which is more along the lines of what I would visualize.

Notice that in this diagram at the front of the region of impact, the fluid is pushed forward. There is some theoretical stagnation point, the point where the flow does not go forward nor backward. Aft of the stagnation point the fluid flows aft. This idea of a stagnation point is also commonly used with Foils, and is a very successful model.

http://www.bluejacketboats.com/planing_boat_theory1.htm

So, it would be nice if we could calculate the mass and the velocity of the flow at all the locations of concern. If we know velocity distributions, we could then account for some of the relative presures on the hull. However, there is a variety of other forces. For example, hydrostatic presure. The deeper the hull sits in the water, the greater the presure due to deepth of the water. There are also presures resulting from the viscosity of the fluid. Losely we could think of viscosity as frictional type forces resulting due to fluid rubbing against itself as well as rubbing against the hull. Lucky for us, because of the superposition concept, if we can caculate these different types of presure we can add them up to get the total presure. But as you might imagine, it gets fairly complex, fairly quickly.

I hope this helps get a general idea about some of the problems.

~ Michael

P.S. The stagnation point is a point of max presure.

 

Holed Hull

Easy,
I think this is a unique and important observation.

From this observation, I would draw a conclusion that there is a suction. It seems to me that you directly observed the suction.

Why do you think the fairing invalidates the conclusion that a suction in produced? If the shape of the fairing can produce a suction, would not the shape of the hull produce a suction on a larger scale? I don't think the fairing is creating suction as much as it is preventing other forces from interfering with the flow through the hole.

As far as the suction disappering, ... if you introduce an unlimited amount of air into a vacuum, you no longer have a vacuum. The amount of sucction is expected to be relatively small. As was said be fore, we know the boat is held on top of the water, thus the over whelming force is upward.

~ Michael

 

Michael,
I think you've drawn the wrong conclusion from Easy's comments. Removing the fairing may not eliminate any suction that could be present, it will however, ensure that the the overwhelming force where the hole is is now positive - ensuring a brief & wet ride

 

so, what role do you think the fairing plays in terms of producing forces? Is the faring producing the suction?

 

Just returning to the original post in this thread... I'd like to add my 2c to try to describe what's going on. The suggestion is that flat sterns initially create suction which causes squat. We are talking about hulls from their initial movement until such time as dynamic forces become predominatory (is that a word? )
It's not really clear whether Rick means flat sections or a flat transom, but either way, a simple understanding of wave-patterns is necessary to understand what is going on.
As a vessel begins its forward movement it creates a wave-system. Initially, the vessel will appear to sink a little as the waves will reduce the amount of buoyancy that is available to support the vessel as they are larger at the ends than they are in the middle. In most boats, this will result in the boat actually trimming down by the bow as the finer nature of the pointy end is less able to support the weight than the beamier stern. (the boundary layer comes into play here too, but we'll ignore that for the sake of the argument.
As the speed increases, so too does the wavelength. The centre of buoyancy moves aft and so the vessel begins to trim down by the stern.
As speed increases further, the wavelength exceeds the length of the vessel, exposing an ever increasing portion of the aft 'body' of the vessel, which in turn reduces the amount of buoyancy that is 'available' to support the vessel, so it trims further down by the stern.
By my reasoning, there is no 'suction' here - simply a loss of supportive buoyancy owing to the vessels own wave system.
Of course, as speed increases, dynamic forces come into play... those which involve the hole in the bottom etc... but they would be outside the field of the original question.

 

Sorry Michael, was typing when you replied... but simple answer...yes. Otherwise what would be the point of having a fairing at all. And if suction were the overwhelming force at play then the boat would sink - hole or no hole

 

I'd have to concur with Mat... simple answer yes. That's how a venturii works

Nice description of the forces at play BTW Mat...