flat sterns cause squat ?

Wally, I wandered off to watch some Olympics for a bit any you got in a post. I think you have it correct. Either that or I have completely screwed up everyone's head.

 

So, just to be clear here Tom... my take on your statement is that the lift generated aft is less than that at the stagnation point, but is still positive in the overall sense? That would certainly be in line with my own thinking on the matter.

The Gerr explanation that you refer to... is that wher you drill a hole in the bottom of the boat. The fact that water is forced up through the hole, rather than being sucked out, proves that the forces are upwards. Can you please explain why this fails to prove the point? (I'm not a 1st year engineering student )

 

Of course, the fact that the water is forced up through the hole would prove that the net force under the hull is upward at that point, except that there may be other forces like Bernoulli that cause some negative "suction" there.
What is apparently being missed is that the force observed in any situation is rarely the only force there. Except in closely controlled cases, there are almost always several contributing forces but only one net observed force.

Therefore, there can be a lot of negative force being generated but still have a net positive force to push the water up through the hole. This is what Gerr is missing in his tests.

 

Hmmm... ok, so we have the possibility of conflicting 'forces' acting upon the hull. But it must be true to say that the net force is positive, otherwise the water would not be forced through the hole... It must also be true, therefore, that the original proposition - that suction pulls the stern down, effectively presenting the bottom at an angle of attack, is also incorrect.

 

No, no. I don't understand why this is so hard. Say we have the two forces and the negative lift is 100# and the positive lift is 500#. The net is 400# and water will be still forced up through any hole.

Now assume we can eliminate the 100# negative lift. The net positive force is now 500# and the stern will be trimmed higher than before. Would you still say that there had been no suction pulling the stern down?

Big racing boats go through a process of "blueprinting" the hull water contact surface to eliminate all little non-fair surfaces. Its expensive and it purpose is to get rid of these negative or undesired forces that not only cause loss of speed but can cause instability. I don't know what all this process entails but that is the gist of it.

 

Mat C started this thread questioning the statement "flat sterns cause squat". Then he goes on to talk about "flat aft sections". Flat would presumably mean a full planing hull w straight cross sectional lines and a straight run aft so what's all this talk about rockered bottoms, suction, and drilling holes in boats to see if water comes through the hole???. Surely if you have a rockered bottom and run it fast the bow's going to go sky high and the stern will be sucked down .. if you go fast enough.

Easy

 

One of the issues on the hole-drilling experiment is that the act of observing purturbs the observable. The hole would eliminate the suction, leaving only the upward force of the water.

~ Michael

 

Tom - yes sorry, one jump too far...
I've been getting some excellent advice on the topic via PM. Once I can digest it, I'm sure I'll have the answer...

Easy - this has nothing to do with planing hulls or otherwise... but you are correct, I should have simply said that the water was acting on the aft sections (flat or otherwise) in my original post.

Michael - ergo we should all drill holes in the aft sections to increase lift...

 

LOL,... well, following that line of thought, speed boats use steps to elimination of the suction in a similarly way to what a hole might. Notice effective steps have a ridge that raises above the waterline. The step introduces air into the area which has suction.

Another thought experiment might be to drill a hole in the hull. In this hole a tube is inserted and sealed with a rubber membrane attached to the other end. One might tightly tie a balloon over the tube. Now, will the membrane poke out are depress inward? ... Or better yet, just attach a monometer to thru the bottom of the hull. neither of these methods completely reproduce the smooth hull, nor measure the unpurturbed presures, but either would be closer than an open hole. These methods would prevent the introduction of an unlimited amount of air in to the area where we are trying to measure the presure.

 

Mat-C,
When you feel you have a handle on it I would be most appreciative if you could share what ever insight you may have gathered through your PM. Not trying to pry into your communications, just might curious about the whole subject.

 

Tom; Thanks for the reply. Ah Yes, WIndmills. I sailed "mills competitvely for several years. Surprisingly good performance from such a basic slab boat. A toast to Clark Mills for having done that design.

We can have fun with this thread and learn some stuff too. Here, we are involved with some principles of physics that involve more than one variable. Velocity has to be one of the components too. Let us agree that the area of maximum lift occurs at the point of impact. Lift per unit of area will diminish as we move downstream. If the angle of incidence is some number of degrees (radians if you prefer) then the angle of reflection is equal. We could draw a vector diagram for a single particle and see that the particle will be farther and farther from the support surface as it moves down stream, or the support surface moves upstream. It follows that the flatter planing angle (incidence angle) will produce less negative pressure for a given downstream location. Whereas a large planing angle will probably create more negative pressure at a similar location. Planing angle will be a function of the supported weight, surface area, and velocity. Velocity being an exponential function will be more influential. So we have variables a plenty. Generally speaking, the faster we go the flatter we can run. When we go slower there must be an increase in wetted or pressurized area to support the load because the unit pressures will have decreased. Given all the variables and the ramifications it may be that there is some obscure combination of factors that could cause suction. I have never been conscious of such a set of conditions however. Does any of this make sense?

 

The holes give false readings. They would create a lower pressure because of the ventury effect. Several theories were formulated based on wrong data.

 

Gonzo, I would readily agree that the holes would give unreliable readings. I was only making comments in support of Tom Lanthorp’s explanation of why holes would not work as Dave Gerr suggested. ... There are a number of forces that come into play once a hole is cut in a hull.

However, I have not read studies you are referring to on venturi effects. I have wondered about the usefulness of multiple rows of very small holes for multiple manometers in obtaining differential pressures along a hull, if one is attempting to create turbulent flow on the hull anyway. It seems the venturi effect could be minimized once the hole size approached the scale of the turbulant layer. Do you have a reference to studies showing some of the pitfalls with multiple manometers, that would be interesting.

~ Michael

 

Sorry guys, I don't quite get it. What do you intend by "Venturi effect" when talking about placing manometer holes in the hull? And I also don't understand the role of turbulence in all this reasoning...

 

Daiquiri, I have not read the paper, so I'm only speculating, but I thought that it was refering to the fluid entering and exiting the same hole, resulting in a presure difference inside the hole. Definantly not a classical example of venturi. That is why I was suggesting that if there is turbulants on the same order of magnitude as the size of the hole, the effect would not be very significant.