Ship hull shape question

The other day I was washing a bundt pan in the sink. This particular bundt pan has the central column seen in the link but lacks the hole in the middle of that column. By manipulating the pan in the water, I was able to evacuate the air from this column and fill it with water. Bailing the water out of the rest of the bundt pan resulted in the pan floating upright but lower in the water than you would expect, because the water level in the central column was higher than the general water level in the sink. This same effect would have been achieved by leaving however many cc’s of water directly in the ‘hull’ of the cake pan - it’s just an issue of the weight of the water in the column.

But when I tried to lift the pan from the water, I realized that it had been converted into a very large suction cup. As I see it, the force needed to lift the pan is now related to its weight plus some air pressure x cross-sectional-area-of-the-column-at-the-waterline term. You can push the pan deeper into the water more easily than you can pull it out. Since the column flares at the bottom, it seems to become a bigger suction cup as you lift it. Pitch and roll don’t seem to be affected, but that’s probably due to the geometry of the pan, and some clever hull shape manipulation might make a difference there.

At any rate, is this principle - having a vaccuum under the hull - utilized by naval engineers at all? What about the opposite: a bubble or column of compressed air? I can’t see any immediate advantages to either system, but someone else may have.

How could you use it? Ships are not lifted out of the water in their everyday use.

A vacuum would serve no useful purpose and would degrade performance by increasing drag.

Air or bubbles, OTOH, are quite handy, since air has much lower density than water. Hovercrafts use this as do Ground effect vehicles, along with hydrofoils. In the mid90s I remember reading an article about a company that had built a racing boat (looked much like a cigarette boat, but don’t know if it was in that class) which had inlets on the top of the boat that channeled air through the boat and dumped it under the hull, creating what they called an “aerated foam” which lowered the drag on the hull at high speeds.

Some colossally-large cruise ships are using bubbles to reduce noise and vibration.

The MS Eurodam was launched this past July, and they use an air cushion over the aft azipods to reduce transmission of noise to the dining room. As this is being used specifically to improve the passenger experience, no specs are given as to what effects this has on drag or efficiency.

As I recall, the Russian rocket-powered *Shkval * torpedo cruises along in a bubble it creates. How it makes the bubble is a little unclear to me–something about the nose cone shape and venting of gasses from the engine–but the result is that it’s a fast fast little devil.

By the way, there is no “suction cup effect” in the OP. It is just the plain weight of the water being lifted. If you have a bottle full of water upside down in the water, neck down, totally submerged, it takes exactly the same force to lift it out of the water whether the bottle is closed with a stopper or it’s open to the water. (Until the opening of the bottle is outside the water surface and the bottle begins to empty.) You are just lifting the weight of the water. No atmospheric suction effect.

It’s supercavitation. Totally different thing than described in OP.

Band name.

Nah, its got “ipod” in the name, so Apple would sue you, and you’d be forced to change your name to something like Aft or Aft Azunes. :wink:

Don’t know if this is related to the bundt pan effect or not but the faster a sailboat hull goes, the lower it rides in the water – making the boat more stable. If a sailboat is towed faster than its theoretical hull speed, it will be pulled under the water.

(To the technically minded: Actually, it doesn’t ride lower in the water, it makes waves that rise higher on the hull … and of course, this does not apply to planing hulls.)

This is wrong. A sailboat will rise as it gains speed and if it gains enough speed will rise over the bow wave and is then said to be planing.

http://home.golden.net/~capone1/wsc/calc.htm

Your earlier post is still wrong. A sailboat with a displacement hull, such as mine, will not sink in the water as it gains speed, it will rise. And given enough speed it will plane. The is no such thing as a clear cut distinction between planing and non planing hulls. Planing are those that are light enough to plane easily, like small light sailboats and 100% displacement are those so big they would never plane. But there is no clear-cut separation and it is a matter of degree. A sailboat displacing a couple tons is not going to plane under normal conditions but it will plane going down a big ocean wave and I bet it would plane if towed fast enough (although I have never seen this happen, for obvious reasons).

You assertion

is false. You might want to try searching for some supporting cites.

Oh my … my limited studies of sailboat hull design were over 20 years ago. I was trying not to get too technical, just to state an interesting factoid about sailboats. I could be wrong but I’ll attempt to make my point.

Granted that there is no clear, sharp dividing line between planing and non-planing hulls, I am sure you are aware that sailboat hulls are generally categorized as displacement, semi-displacement or semi-planing, or planing. Also granted that most any sea going vessel might plane down the face of a 100 foot wave, though I suggest that has more to with gravity and the unusual attitude of the vessel than with hull design.

My point is that due to the wineglass-like shape of the standard displacement type sailboat hull, as the waves created by the hull get bigger more of the surface of the hull is in contact with the water, thereby increasing the waterline of the vessel, thereby increasing its speed and stability … heeling the boat has the same effect. Increasing the length of the waterline means the boat is displacing more water and is therefore “riding lower in the water.” (Are we having a semantic argument over that phrase?)

You know what … I think the question of whether a sailboat towed beyond its theoretical hull speed with be pulled under water is perfect Mythbusters material; could provide some great footage with one of the guys riding in the boat being towed. I am submitting it to them.

I do not think this notion that it would be pulled under water is believed widely or even by a minority. I have never heard it and it goes against intuition and against experience so I really can’t see why they would want to debunk something which isn’t bunked in the first place.