FYI: meaning of the notation of the draft measurements during Bismarck’s trial runs, fall 1940

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Re: FYI: meaning of the notation of the draft measurements during Bismarck’s trial runs, fall 1940

Post by wadinga » Sat Aug 24, 2019 1:37 am

Fellow Contributors,
I assume because of the density of the water.
Exactly. As was raised but largely ignored before, the Baltic is nearly freshwater, less dense than oceanic water, so while Bismarck may have been immersed as if she were at operational displacement
47,900 m3 = 49,100 mt in seawater (1.025 relative density).
she may well have actually been much lighter. The Baltic density is only 1.0086. When she operated in a more supportive fluid she would have floated higher but may have carried more load. I expect one might argue that the wetted area is the same whether one is light but in less dense water or carrying more load but in more dense water. Naval architects will know.

Several experienced marinators have pointed out that performance on trials is a poor indication of service performance. Bursts of speed over a measured mile in the flat calm Baltic are nothing like operating in an Atlantic seaway.

If the photographs at the beginning of the thread are in freshwater, the amount of exposed boot-topping is puzzling when the draft is 9.25m.

All the best

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Re: FYI: meaning of the notation of the draft measurements during Bismarck’s trial runs, fall 1940

Post by Alberto Virtuani » Sat Aug 24, 2019 6:58 am

Hello everybody,
Marcelo Malara wrote: "shallow waters (meaning about 10-20 m) allows more speed, I think that 65 m is out of the effect."
Thanks Marcelo, I was however actually thinking that shallow waters should have played against speed, due to the viscosity of the water layers in contact with the hull and with the bottom of the sea... but I can be wrong, not being an expert at all.

You are right that 65 meters are possibly enough anyway not to have any significative influence on final speed: Northern Adriatic Sea (around the site where the RN Roma was built) is less than 20 meters deep and in that case it was considered impossible to run meaningful speed trials over there.


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Re: FYI: meaning of the notation of the draft measurements during Bismarck’s trial runs, fall 1940

Post by Herr Nilsson » Sat Aug 24, 2019 8:38 am

wadinga wrote:
Sat Aug 24, 2019 1:37 am
Fellow Contributors,
I assume because of the density of the water.
Exactly. As was raised but largely ignored before, the Baltic is nearly freshwater, less dense than oceanic water, so while Bismarck may have been immersed as if she were at operational displacement
47,900 m3 = 49,100 mt in seawater (1.025 relative density).
she may well have actually been much lighter. The Baltic density is only 1.0086. When she operated in a more supportive fluid she would have floated higher but may have carried more load. I expect one might argue that the wetted area is the same whether one is light but in less dense water or carrying more load but in more dense water. Naval architects will know.

Several experienced marinators have pointed out that performance on trials is a poor indication of service performance. Bursts of speed over a measured mile in the flat calm Baltic are nothing like operating in an Atlantic seaway.

If the photographs at the beginning of the thread are in freshwater, the amount of exposed boot-topping is puzzling when the draft is 9.25m.

All the best

wadinga
AFAIK the average density of the baltic water is 1.015 and that‘s why all German displacement/weight calculations are based on this value.
Regards

Marc

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Re: FYI: meaning of the notation of the draft measurements during Bismarck’s trial runs, fall 1940

Post by wadinga » Sat Aug 24, 2019 9:27 am

Fellow Contributors,

I used the comparative figures from an old article:
Thus the specific gravity of the waters of the North Atlantic is said to be 1.0266, and that of the South Atlantic, 1.0267, showing a difference of .0001 only ; that of the North Pacific, 1.0254, of the South Pacific, 1.0265, a difference of only .0011 ; that of the Baltic, 1.0086, of the Red Sea, 1.0286, a difference of .02 ; the Mediterranean, 1.0289 ; and the Indian Ocean, 1.0263. So that the average difference in the specific gravities of the Atlantic and Pacific Oceans is .0007, or, in other words, the Atlantic is slightly salter than the Pacific.
It would be strange to base all German displacement/weight calculations on a brackish sea.

From Proceedings of the 17th International Ship Stability Workshop, 10-12 June 2019, Helsinki, Finland:
Ships designed to be operated solely in the Baltic Sea traffic, like the cruise ferries between Finland and Sweden, have their stability documentation done for sea water with a density of 1.005 t/m3, and are not allowed to submerge the summer load line.
But as I pointed out maybe the wetted area is the most significant factor, no matter the density.

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Re: FYI: meaning of the notation of the draft measurements during Bismarck’s trial runs, fall 1940

Post by Herr Nilsson » Sat Aug 24, 2019 5:35 pm

According to Evers‘ „Kriegsschiffbau“, 1943: He says the German navy used the term „Deplacement“ as a volume or weight. If it‘s a weight, it’s calculated by using a density of 1,015. He also says it‘s the average density of the Baltic.
The same density is mentioned for the Baltic in „Leitfaden für den Schiffbau“, 1908 and „Johows Hilfsbuch für den Schiffbau“, 1928.
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Marc

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Re: FYI: meaning of the notation of the draft measurements during Bismarck’s trial runs, fall 1940

Post by Bill Jurens » Sat Aug 24, 2019 10:41 pm

One thing being ignored here, or at least minimized, is the rather dramatic effect of propeller efficiency. Especially if one is running close to the cavitation limit, which Bismarck would almost certainly have been during high-speed trials, even a small defect in blade geometry can affect the speed vs horsepower relationship quite a bit. This is especially true if one propeller begins cavitating before its neighbors, which is usually the case. In addition to generating the horsepower, one must actually be able to get it in to the water.

Wetted area is a geometric issue related to draft rather than displacement. One can easily get bogged down in trivialities, e.g. what precisely might have been the temperature and viscosity of the water on the day the trials were run. So, for example, the speed 'published' may represent the actual speed over the ground, or the 'corrected speed' which has been adjusted to account for differences between the physical conditions existing during the test(s) vs the theoretical conditions that may be specified in the contract documents. Nuances like these usually don't survive translation into the secondary literature.

The bottom line is that although one can read the 'bottom line' as horsepower vs speed, the two or three figures obtained really represent a summary -- a very highly compressed summary -- of what often amounts to hundreds of pages of raw data. That doesn't mean horsepower/speed relationships are not useful, but they represent, at best, only a guideline figure usually required to satisfy contractual requirements, a bit like the often highly-regulated and somewhat artificial methodology required to determine representative mileage figures for automobiles. As the old saying goes "your mileage may vary..."

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Re: FYI: meaning of the notation of the draft measurements during Bismarck’s trial runs, fall 1940

Post by marcelo_malara » Sat Aug 24, 2019 11:01 pm

The difference in density from 1.025 to 1.008 traduces in 13 cm in the draught, don´t know it would make much difference in speed.

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Re: FYI: meaning of the notation of the draft measurements during Bismarck’s trial runs, fall 1940

Post by wadinga » Sun Aug 25, 2019 1:29 am

Fellow Contributors,

Wunderbar news for Teutonic-techno fans! Since all German ships are apparently measured in less-dense water, where their draft is deeper, that means they are faster everywhere else, because for a given loading their wetted area is smaller! Like in the Atlantic for instance.

Or not, as Bill sagely observes, because drag is one thing and horsepower translation via screws is another. At the top of the speed range there is a massive and unquantifiable "diminishing returns" effect.

Germany may have attempted to disguise their flouting of the Washington Agreement by an arcane measurement system, but extrapolated performance/displacement figures are just that, ie unreliable.

All the best

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Re: FYI: meaning of the notation of the draft measurements during Bismarck’s trial runs, fall 1940

Post by marcelo_malara » Sun Aug 25, 2019 2:15 am

Guys, I made some rough and very simplified calculations.

Length...Beam..Diagonal...Water plane
250...…..36......126,29......4500

……...t...…….Density....Volume dspl…..Draught...……..Wetted area....Resistance
SW...40000...1,025....39024,39024.....8,672086721...8880,791328....9102,811
FW...40000...1,008....39682,53968.....8,818342152...8954,673721....9026,311

SW/FW....….1,016865079...……………...0,983414634....0,991749.......1,008475
FW/SW.……. 0,983414634

I take a simplified ship 250 m long, 36 m beam, with a prismatic coefficient of 0.50, this means a cuadrangular midship section, located exactly at mid lenght, and with straight sides right to vertical stem and sternpost. The ship weights 40000 metric t, that gives two different volumes of water that are needed at different densities to float the ship, the draught been 8.67 in sea water and 8.81 in fresh water. With both draught I calculate the two wetted areas, adding the bottom to the sides. The relation SW/FW is the same as the respective draughts, but the wetted area is much closer because of the bottom. As the formulas of resistance contains both wetted area and density, I multiply them, and apparently the resistance would be about 1% higher for the ship in sea water.

Then comes the thrust of the propeller. That depends on density, the sea water with greater density produces more thrust, may be that would cancel the higher resistance.

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Re: FYI: meaning of the notation of the draft measurements during Bismarck’s trial runs, fall 1940

Post by Herr Nilsson » Mon Aug 26, 2019 7:57 am

Bill Jurens wrote:
Sat Aug 24, 2019 10:41 pm
One thing being ignored here, or at least minimized, is the rather dramatic effect of propeller efficiency. Especially if one is running close to the cavitation limit, which Bismarck would almost certainly have been during high-speed trials, even a small defect in blade geometry can affect the speed vs horsepower relationship quite a bit. This is especially true if one propeller begins cavitating before its neighbors, which is usually the case. In addition to generating the horsepower, one must actually be able to get it in to the water.

Wetted area is a geometric issue related to draft rather than displacement. One can easily get bogged down in trivialities, e.g. what precisely might have been the temperature and viscosity of the water on the day the trials were run. So, for example, the speed 'published' may represent the actual speed over the ground, or the 'corrected speed' which has been adjusted to account for differences between the physical conditions existing during the test(s) vs the theoretical conditions that may be specified in the contract documents. Nuances like these usually don't survive translation into the secondary literature.

The bottom line is that although one can read the 'bottom line' as horsepower vs speed, the two or three figures obtained really represent a summary -- a very highly compressed summary -- of what often amounts to hundreds of pages of raw data. That doesn't mean horsepower/speed relationships are not useful, but they represent, at best, only a guideline figure usually required to satisfy contractual requirements, a bit like the often highly-regulated and somewhat artificial methodology required to determine representative mileage figures for automobiles. As the old saying goes "your mileage may vary..."

Bill Jurens
In the end all those trials aren't for their own sake. They were done to calibrate and create an approximate scale for daily use.
The first top speed trials were made to find out the maximum power a ship without damaging the machinery. It was forbidden to exceed this maximum power in later top speed trials (based on fuel consumption per heating surface as indicator). These later trials were made once a year to inspect the current capability of machinery and crew, before the crew was partially changed.
Much more of interest were fuel-consumption/cruising-range trials. The first trials of this kind were done to get just a rough guide of fuel consumption. During day-to-day operations the ship's commandant was obliged to use appropriate cruises as often as possible to improve/update the cruising range tables.

IMHO opinion the comparability between results of trials of different navies is very limited, because of the different scales that were used. The Germans tend to have a built-in safety margin such as using always a very low lower heating value for fuel consumption to prevent unpleasant surprises.
However, cruising at 8.6 m draft and cruising at 9.8 m at a certain speed is quite a difference even in the German measurement framework.
Regards

Marc

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Re: FYI: meaning of the notation of the draft measurements during Bismarck’s trial runs, fall 1940

Post by Thorsten Wahl » Mon Aug 26, 2019 4:16 pm

Bill Jurens wrote:
Sat Aug 24, 2019 10:41 pm
One thing being ignored here, or at least minimized, is the rather dramatic effect of propeller efficiency. Especially if one is running close to the cavitation limit, which Bismarck would almost certainly have been during high-speed trials, even a small defect in blade geometry can affect the speed vs horsepower relationship quite a bit. This is especially true if one propeller begins cavitating before its neighbors, which is usually the case. In addition to generating the horsepower, one must actually be able to get it in to the water.Bill Jurens
The data from Bismarcks and Tirpitz Fahrtabellen in connection with the trial RPM ( 270RPM for 30,05 resp 30,15 kn) seem to indicate, that Iowa class has a very simililar propellerefficiency at 30 kn comparing RPM for 15 kn and 30 kn for both ship types.

Bismarck/Tirpitz at 30kn require +~228 per cent RPM compared to 15 kn (118 RPM vs 270 RPM at 30,05/30,15kn)
and Iowa class 225% based on average datapoint 87 RPM vs 196 RPM from FTP 218.
this difference appears negligible to me considering uncertain displacement, draft, hull cleanliness and so on,
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Re: FYI: meaning of the notation of the draft measurements during Bismarck’s trial runs, fall 1940

Post by dunmunro » Thu Sep 19, 2019 7:55 pm

I wonder if anyone can present more information regarding the fuel oil quality used during Bismarck's trials?

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Re: FYI: meaning of the notation of the draft measurements during Bismarck’s trial runs, fall 1940

Post by Bill Jurens » Thu Sep 19, 2019 8:14 pm

That's an interesting question. In practical terms, I don't think that fuel oil quality would have affected trial results at all, or at least not in any meaningful fashion, as -- provided one is willing to equate 'quality' with 'efficiency' -- all that would happen would be that poorer quality fuel would result in poorer overall mileage rather than overall top speed, i.e. more fuel expended for a given output of power. My guess -- but it's only a guess -- is that the sprayer plates in the furnaces could have supplied -- and burned -- more than enough fuel to produce enough energy to run the propellers past the cavitation points.

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Re: FYI: meaning of the notation of the draft measurements during Bismarck’s trial runs, fall 1940

Post by dunmunro » Thu Sep 19, 2019 10:43 pm

Bill Jurens wrote:
Thu Sep 19, 2019 8:14 pm
That's an interesting question. In practical terms, I don't think that fuel oil quality would have affected trial results at all, or at least not in any meaningful fashion, as -- provided one is willing to equate 'quality' with 'efficiency' -- all that would happen would be that poorer quality fuel would result in poorer overall mileage rather than overall top speed, i.e. more fuel expended for a given output of power. My guess -- but it's only a guess -- is that the sprayer plates in the furnaces could have supplied -- and burned -- more than enough fuel to produce enough energy to run the propellers past the cavitation points.

Bill Jurens
The boiler sprayers have a finite capacity, so if the fuel was poor enough it would reduce maximum achievable power. However the specific fuel consumption is a key parameter for measuring power plant efficiency. IIRC, in the past the maximum fuel flow per boiler sprayer on Bismarck was stated. I'll have to dig around to find it.

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