Bismarck Speed

[foeth]

Very slender (multihull) ships don't have a lot of wave making resistance so that makes it easy to pass the hump speed. So, cruisers with an 1:10 beam-to-length ratio have less difficulty going to higher speeds than tankers. Fast catamarans are always very slender (double) hulls. Once over their hump speed, the required propulsive power reduces, so you need to know the power requirements over the entire speed range. Same story for flying boats and water planes; end resistance for take off is not the worst condition!

I don't think that planing depends only on the bottom shape, but planing vessels are often hard-chine vessels. But I'm not really a hull guy, I'm more into propulsion. Fortunately we have several contributing authors (and hence copies) of Principles of Naval Architecture in my office

[marcelo_malara]

So the hull speed depends on lenght as well as on beam to lenght ratio? Are there any tables about this?

[foeth]

The hull speed is caused by either wave interaction of the hull wave components. If the wave is very slender, you have a small wave from the bow and stern, and you reach the hump speed when the wave from the bow reaches the length of the ship (Froude = 0.5). For some ships, waves from the forward and aft 'shoulders' interact badly at low Froude (~0.2). This means that the hull speed is obtained at a low Froude numbers. A very slender hull can also have this unfavourable interaction but as the waves coming from the shoulder are so small, it's barely noticeable as small humps in the resistance curve. Depending on the hull shape, 'humps' can be present at any speed (Froude-wise) and most resistance curves show many of these interaction hot spots.

So, it roughly (and I mean roughly) depends on beam-to-length ratio, but the hull shape is very important. Although we use these ratio's and prismatic coefficients and so forth, be prefer to calculate the wave pattern and optimize toward a minimum resistance (within certain parameters, naturally). We've had ferries optimized with wave resistance only 10% of the total resistance at 30 knots, barely disturbing the water surface. But not doing so well at lower speeds For ships, more so than for any other vehicle, the design speed has a major influence on the hull shape. The difference between 15 and 16 knots can be very important.

I don't there are tables. Perhaps per type of ship, who are comparable, but not a universal table?

[lwd]

...As far as I can tell, the Atlanta's can clock around 33-34 kts

I do remember sources crediting them with higher speeds. However that could be due to someone stating thier speeds in mph rather than knots and being sloppy about it. I should clarify a bit I'm far from convinced that PE could have gone that fast. I just don't know enough to dismiss it out of hand. There's also the question of whether it would be prudent to do so even if she could.

[tommy303]

The planning hability depends just on the bottom shape? Extremely slender hulls, say a multihull sail boat, has the same Froude number limitations as bigger ships?

Planing hulls can be chined, flat bottom or even rounded bilged, but having at least one chined surface helps. Sailboats, if they had the power, would be similarly affected by the Froude limitation numbers except for two points. One, is that they do not generally have the power to reach the point where planing will start, and secondly the wind force in the sails pushes down on the hull and acts to counter the phenomenon whereby draft decreases as speed increases.

[marcelo_malara]

One, is that they do not generally have the power to reach the point where planing will start, and secondly the wind force in the sails pushes down on the hull and acts to counter the phenomenon whereby draft decreases as speed increases.

Well, modern high speed monohulls reach 30 kt, putting them effectively in the planning regime. You can see at videos shot from above as the "bow" wave forms under the mast.

[tommy303]

These newer sail boats are a whole new breed from the older traditional displacement hulled boats. In essence they ride on the surface, more or less, with a large, deep running, teardrop-shaped bulb on a movable strut. It is this bulb and strut that gives the hull stability to overcome wind pressure on the sails. Together with a narrow entry form for the bow and a flat stern section to prevent stern squatting, they do plane and this gives them a speed advantage because the hull is not limited by a wave length created by a displacement hull. Essentially these fast sailboats act a lot like a windsurfer's board, and comparing them to a displacement hulled vessel like a battleship or ocean liner is a bit like comparing apples to oranges.

[dunmunro]

...As far as I can tell, the Atlanta's can clock around 33-34 kts

I do remember sources crediting them with higher speeds. However that could be due to someone stating thier speeds in mph rather than knots and being sloppy about it. I should clarify a bit I'm far from convinced that PE could have gone that fast. I just don't know enough to dismiss it out of hand. There's also the question of whether it would be prudent to do so even if she could.

USS San Juan (CL54), and USS Oakland (CL95):
USN estimate = 31.4±.3 knots with 75000SHP at 8200 tons
USN estimate = 33.1±.4 knots with 75000SHP at 6900 tons

http://funsite.unc.edu/hyperwar/USN/ref ... el-CL.html

Displacement is a critical piece of information when trying to sort out ship speeds.

[lwd]

There's also the question of whether or not to count "emergency" speeds. How fast can such a ship go if it's lightly loaded and runs the engines in an overload condition? And then there's the question of what the significance of this is.

The more I look at it the more likely I think the high numbers I read are due to someone translating the speed in knots to mph and subsequent sloppiness on the part of someone.

Very interesting url by the way. Thanks for posting it.

[RF]

I find the concept of an ''emergency speed'' curious. Surely there is only one maximum speed a ship can attain for a given weight loading and sea conditions?

[foeth]

The more I look at it the more likely I think the high numbers I read are due to someone translating the speed in knots to mph and subsequent sloppiness on the part of someone.

This is well possible. A nautical mile is around one arc minute of the world's supposed circumference of 40,000 kilometers, or about 40,000/360/60=1852m (fixed at 1852.0). The 'land mile' is only 1609m. Inland ships usually give their speed in kph, I guess this is also true for US inland/great lake shipping?

And then there's the difference between speed through the water and speed over ground. Depending on the current, you can add a few knots. This is why ships sail back and forth during the measured mile in a region with a known stable (or absent) current.

[Antonio Bonomi]

Ciao all,

about " emergency speed ".

One day I was asking a very expert and skilled warship commander about that and he explained to me the following concept.

On a warship you have the steam and your Officer in control of the overall machinery.
He knows how to give you for a short defined slot of time some more speed based on your emergency needs.
You cannot count on that for a lot of time, so it is something you leverage when you are really in trouble.

The example he gave me was the formula one car and how they manage the engines lately during the race.
You can use the overboost for 1 or 2 turns, or to make a surpass, but no more, and you do that only when you need it and you cannot count on that for a large period of time, otherwise your engine will explode.

I have also heard about an addittional boiler added on SH and GU just for that reason, to produce addittional steam for emergency needs, but never found many confirmations about it.

Bye Antonio

[Olaf]

Going back to the hull shape, I've read that the hulls of SH and GU were that oddly shaped that at higher speeds the water displaced by the (forward part?) of the hull actually lifts the stern a bit, hence they always looked 'bow down' at higher speeds, rendering them pretty wet on the f'c'sle. I don't know for BS/TP but if you have a look at a Hipper class cruiser running at high speed, the swell along the hull, in terms of an amplitude, is not as evident as one would expect. Maybe this is because of the better length-width ratio. All the water before the hull's greatest beam needs to be displaced, no matter how long the hull is after that point (which I have read is the principle of 'length runs'...). Ships with a greater beam simply need to deliver more power in order to overcome the greater water resistance caused by the higher amount of water in front of the hull's widest part.

Of course, all of this is dangerous half-knowledge on my part...

Happy guessing ~ Olaf!

[lwd]

The more I look at it the more likely I think the high numbers I read are due to someone translating the speed in knots to mph and subsequent sloppiness on the part of someone.

This is well possible.

Especially since the numbers then match up pretty well with the ones I remember hearing. Of course it could also just be bad numbers.

.... Inland ships usually give their speed in kph, I guess this is also true for US inland/great lake shipping?
....

Here on the US side of the Great Lakes I usually hear ships speeds listed in knots although mph is mentioned occasionally. Not sure about the Canadian side. Officially it's probably kph but ...

[foeth]

At least it's less confusing than for most navies, who give their speed in rpm. This rpm has nothing to do with the actual rpm, but it translates as a speed to them. Or so I've heard