Why do BIG ships move faster than 'small' ships?

I was watching a thing on battleships the other night and as they progressed through their history it became apparent that the bigger the battleship ship the faster it went. I also have a vague memory that one of the motivations for the Iowa class of battleship was that it was the only ship that could keep up with the US carriers near their top speeds.

This seems to fly in the face of common sense. Heavier ship = more water displaced = more resistance to movement. Granted you can put correspondingly larger engines on a bigger ship but you’d think there’d be a fall-off in performance (i.e. a law of diminishing returns should be in effect). We can get speed boats moving at 50 MPH or faster so clearly you can put enough power on small boats to move them quite quickly so why can’t frigates and destroyers keep up with the considerably larger carrier or battleship? One would think they would if they could (reasonably) do this but they didn’t.

What gives?

You may be interested in the concept of “hull speed”. From this page, http://sailing.about.com/gi/dynamic/offsite.htm?site=http://www.marinerkayaks.com/mkhtml/HULLSPD.html%23What

The formula for calculating a vessel’s hull speed is given in the next article on that page as “1.34 times the square root of the wavelength (in feet) equals the speed (in knots), is often used, by substituting the crafts waterline length for wave length, to calculate theoretical ‘hull speed’.”

It’s not really so much a much a question of the larger ships moving faster, it’s the newer ships moving faster. One of the big limiting factors, in addition to the ones you mentioned, is a phenomenon called cavitation. When the props spin too fast, the water can’t flow in behind them fast enough, so bubbles of what amouts to vacuum form, robbing the energy and limiting the speed of the vessel. It was found that the ways to decrease the effects of cavitation were to make the props larger, and use more of them. Some of the larger battleships used multiple prop shafts each with multiple props.

In a nutshell: larger ships are “displacement hulls” where the aforementioned hull speed is a factor. They move through the water. Speed boats and the like are “planing hulls”, which get up out of the water and actually ride atop of it after a certain speed is reached, and are pretty much limited by (mainly) it’s method of propulsion.

I never thought I’d have to ask this: QED can you give me a cite? And I’m thinking about the “each with multiple props” part. Counter-rotating? What gives?

Making a prop larger increases tip speed for a given rpm so that isn’t going to help you avoid cavitation. Most of the advances in recent years in reducing cavitation noise were from increasing blade numbers or changing blade profile. There are some Dopers who know that we had a little difficulty with our Japanese allies about this very information.

The number of props hasn’t changed much. Big ass ships have four props. Smaller ships have two. ( I know there are exceptions but I’m excusing increasing prop number to decrease sound , here).

And FFG’s for goodness sake! One prop on a hot-rod!

Of course I could be wrong…

I’ll try to find a cite. I saw this on a PBS program on the history of battleships a couple weeks ago. They showed a model with the multiple shafts with 3 or 4 props on each shaft.

Actually, the length of the boat is also important. There’s a dimensionless number in fluid dynamics known as the Froude number–if you google it, you’ll find some mumbojumbo. The upshot is this: When a boat is moving through the water, it is making waves. Those waves carry energy away from the boat, and are thus a power sink. The Froude number tells us that, at a given velocity, the power radiated away from the boat in the form of surface waves scales inversely with the length of the boat. Thus, longer boats lose less energy to waves. It’s not quite that simple, but that is the basic idea.

AFAIK, this is actually the dominant effect in determining the “cruising speed” of a ship, as that’s quite concerned with energy consumption.

To actually make a ship of a given size reach its cruising speed, however, requires a method to actually push the ship–this is where cavitation becomes important for large ships.

Here’s a sort of a cite. Best I can do at the moment, I’m afraid. I’ll see what I can find tomorrow.

Earler threads on hull speed include:

Ship question - Nonplaning hull - What happens if it exceeds it’s hull velocity


Could an oil tanker rescue someone at sea?

Cavitation is not going to explain anything regarding this OP. Big ships can move faster (up to a point) for a number of reasons which add up. “Hull speed” is just one of them but consider the following:

Drag caused by friction is proportional to hull surface are which grows with the square of the linear dimensions whereas the ship’s capacity increases with the cube of the linear dimensions.

Example: a 600’ LWL ship is 20 times longer than a 30’boat but it has 400 times the hull area and 8000 times the volume. The ratio volume/hull surface is 20 times larger. You only need an engine 20 times larger to overcome friction but you have space for an engine 400 times larger.

This is the same reason whales can have a slow metabolism and live in frigid waters and hummingbirds have an incredibly fast metabolism and can only live in warm climates.

I believe the point is that if you make the prop larger, you can spin it slower and get the same thrust. This allows you to balance prop size and rotational speed to avoid cavitation. Of course blade design has been the big factor in reducing cavitation. Cheap props with poor design cavitate easily. A blade with a good airfoil design can be tuned to its operating condition to avoid cavitation, but may cavitate badly when run in off-design conditions.

Cavitation occurs when the low pressure on the blade of the prop is low enough to allow vapor bubbles to form. It is not vacuum or air, but simply a pressure low enough to cause some of the water to form vapor bubbles. The blades of a prop are an airfoil (their thrust being exactly equivalent to a wing’s lift), so you have a region of low pressure on the “top” side just like you have on a wing.

And, as sailor says, this has almost nothing to do with the OP. Cavitation affects the thrust and the thrust certainly affects the top speed, but assuming the ship is designed to have adequate power available, it’s hull speed that’s the dominant factor. The point being that it’s relatively easy to add more power, but you reach a point of diminishing returns when this no longer allows you to increase speed for a given hull design.

Dang…I actually knew of hull speed (at least its existence if not the particulars) but in my head it was applied to sail boats and for some stupid reason I did not make the very reasonable and logical connection to any boat. Chalk that one up to a brain fart.

I read the link provided on what happens when a ship exceeds hull speed but the response didn’t quite say. I gather that once a hull speed is reached the power requirement to push the ship faster increases dramatically.

Still, assuming you had a big enough engine on, say, a destroyer what would happen when it reached its hull speed and tried to push past it? Could it push past it at all or does physics conspire to make that speed a brick wall to the ship’s maximum speed? If they can push past it can modern naval vessels reach that speed and go faster still if for some reason they really wanted to or even if they could is it a Bad Idea[sup]tm[/sup] to go there (i.e. the ship becomes unstable, shakes apart, etc.)?

If you want to run this experiment, I think the vessel to use would be a tugboat. They have gobs of power in a short little displacement hull.

Any tugboat captains around?

I’ll venture to say that if you have a bigger ship, you can also have bigger/more engines.

This article addresses exactly that scenario:

For some photos of what happens when huge amounts of horsepower run up against the limitations of hull speed, take a look at this page, http://www.clcookphoto.com/tugs.htm.

Racing tugboats. I kid you not.

sailor can explain this better than I, but as I understand it, hull speed is not a wall never to be overcome, it’s just one factor to deal with. Consider this: A modern aircraft carrier should have a hull speed of about 40kts. As I’ve been told by folks who would know, they can make better than that easily. (No cite, so take that for what it’s worth)

While not the skipper, I did spend 3 years on a YTL (Yard tug, little) and drove it quite a bit. It was 65 feet, and the figured hull speed works out to 10.6kts. Our max speed standing on the throttle: about 10kts. I would guess that if you exceeded that speed by 5-10 kts or so, you would just end up burying the bow into its own wake, and it would require even more power to get over it. Too much power, and perhaps you run the risk of “porpoising” your boat; diving nose first too far into the bow wake. At any rate, tug boats aren’t built with speed in mind.

Best job I ever had, that old tug. Here’s a pic of us leaving the Penobscot River, note the bow wake she puts out.

Sure you can keep adding power to exceed hull speed. Problem is, the different components of drag (wave-making, form, friction, etc.) keep adding up and start to look like a quadratic as a function of speed. Eventually you’ll just require engines that are so huge and suck down so much fuel that the ship can’t carry it all. That, and the ship will be prohibitively expensive.

A few things that people are missing here, the OP had seen a special specifically about battleships and was making assumptions about all ships from that show. Battleships are not “normal” ships so alot of things that seem like common sense doesn’t apply to them.

A couple of reasons for this, first if all other factors are equal the side with the faster ships will have more options in combat, even the option of not fighting if the conditions are not favorable. Having higher top speeds was just as big as factor as having bigger guns or thicker armor, so of course you want your next generation of battleships to be superior to those that your enemy has. Second there were technological advances that increased ship speed. Early ships had steam powered recipicating engines. Next ships were steam turbine powered, but still coal fired. Later they were turbine powered but where oil fired. Each of these advances let you get more power from the same space/weight of machinery.

Yes the Iowa class were designed to keep up with the aircraft carriers of the day, but there were not designed to fight the Japanese superbattleships. (It has been theorized that they could hold their own due to better fire control and damage control but that’s a different debate). The United States was building the Montana class to carry out that fight, they had more armor and more guns but they were about six knots slower. So in this case the later Montana class ships were slower. (Four of these were ordered but where cancelled in various stages of construction once we realized that aircraft carriers would win the pacific war and not battleships)
One other big factor was the Washington naval disarmament treaty that was signed between the world wars. It limited number of battleships and ship sizes. It forced designers to really push the limit of getting the most power out of the smallest space/weight.

Interesting! I’ve never heard of the Montana class battleships. Althought they were never built I still have never even heard of them being proposed designs much less getting to the point of having their keels laid down. Do you have any cites for these things? (I believe you but I am interested in more info.)

As to battleship vs. battleship I was considerig another thread on that topic alone but maybe I’ll see if I can hijack my own thread here.

It is my understanding that battleships very rarely engaged in ship vs ship combat much less battleship vs battleship combat. In the show I watched they mention that in WWI only one such encounter occured in the whole war and nothing was sunk.

Why didn’t battleships enage in more…well…battle? They were supposed to be the bad girls of the sea in their day. Granted the carrier usurped that role but still there must have been occasion to use them. I know they were used for shore bombardments but why not naval (ship vs. ship) actions? Were they just too valuable to risk near the front lines? Just dumb luck they never got in those positions? It seems silly to create such a bad assed behemoth and not use it.