Three shafts versus four

Propulsion systems, machinery, turbines, boilers, propellers, fuel consumption, etc.
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Three shafts versus four

Post by ufo » Mon Jul 24, 2006 5:40 pm

foeth wrote:...

Stuart Slade has a few excellent hydro-articles at warships1.com. In on article he comments that Hipper’s ship without the center shaft only gains 1 knot. Slade concludes the center shaft is not very efficient. Although I really like his articles, this conclusion I strongly do not agree with. This is another discussion, but I think center shafts are good, efficiency wise.
Who could say ‘no’ to such bait! :shock:

Beeing aware of Stuart Slades fine articles on warships1 and being aware that the two shaft SMS Prinzregent Luitpold (missing the centre shaft end engine) could easily run in company with her three shafted sisters – a three shaft arrangement looks fairly inefficient to me.

And that is without Stuart’s criticism regarding vibrations in ships with an odd number of screws evens mentioned.

Foeth, would you enlight me on the issue why in your view a three-shaft arrangement might be a fine solution after all? If it is understandable to a simple Physicist, I would be delighted to here you on this topic!
So many Thanks! :D

Oh - and the rest of us - any other ideas on that old issue out there guys?

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Post by Karl Heidenreich » Mon Jul 24, 2006 5:44 pm

:cool: I´ll like to hear about this too, because in a previous thread it never came clear if the three shaft arragement was inferior or not to the four shaft one.
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Ulrich Rudofsky
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Post by Ulrich Rudofsky » Mon Jul 24, 2006 7:44 pm

Whether right or wrong, there is a lot of thinking that went into the choice of 3 screws vs. 4 or 2. In paragraph 149 in "Kriegsschiffbau" 1931 (rev. 1943) Heinrich Evers lists some of the reasons such as turning radius, fuel consumption, hull design, weights, drag, cavitation etc....... I will translate this section when I have time. I even read somewhere that it had something to do with negotiating the Kaiser Wilhelm Kanal - reducing outboard wash distances???. Can anyone go back to what has already been written on this MB and repost it?

http://i18.photobucket.com/albums/b138/ ... spg132.jpg
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Post by foeth » Mon Jul 24, 2006 8:12 pm

Fortunately some of what I typed earlier is still around:

Doubts about three shafts stern of Bismarck.

Nice to see to thought 60/80 m/s was a good tip speed for a propeller. Pretty high! Nice to read that a rudder in the propeller slipstream is recommend, and Biz has a different setup!

But: a propeller at the center line operates almost always more efficient than on an off-center position. Hence, if you can sail well on 2 props and you do not gain much speed by switching on no 3 prop, it's not the propellers fault. As I explained earlier, at high speeds you can double your output and gain only a few knots. If you have a twin screw ship with plenty power, of course it can keep up with a tripple shaft ship if you have more power on two shafts. Doesn't mean your fuel bill is lower. If you can do without a center shaft, well then get rid of it for complexities sake. But if you NEED >2 shafts... then it's time to choose between 3 or 4 and 3 does quite well. As the entire afterbody changes radically, it's difficult to say what one will do better, especially in the dark ages of WWII :D

So, no definite asnwer, as is usually the case, but efficiency wise there is really nothing wrong with tripple shaft. Vibrations are not due to "turbulences" or somesuch what Slade wrote as a default for being a center shaft. Iowa with her twin skegs had more than her fair share with vibrations. Poor propeller flow is the culprit. Having a nice afterbody is the key to solving that particular problem and the center shaft makes that more difficult. Adding skegs of couse isn't a good idea either! You get two sources of vibration. And no, they do not cancel :lol:



Simple case: image a ship as a simple body dragged through the water. It drags water with it in its wake. If you can design a propeller that pushes this water back so that you'd hardly notice a ship has passed, it does quite well! The ship looses less momentum in the flow. This dragged flow is directly behind the hull and that's where the center prop is. The double screw ship also thrusts it's water back, but next to that dragged wash. So when that ship passes, you'll notice a but of drag with the ship -and next to it- to jets of water going in the other direction! That;s a lot of energy lost.

Very simple of course, but basically why double screw ships do a bit worse. You can get a clever idea and think: if I need to restore this "drag" behind the ship and can't do it with 2 props, why not create two drags! Kind of catamaran. This is what's being done so here and there with twin skegged ships. Link! http://www.sspa.se/shipdesign/twinskeg.html

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Post by ufo » Wed Jul 26, 2006 11:25 am

Thank you !

Things to think about in there! And I wasn’t aware that the ‘Nanny’ was also twin shafted. Two propeller tankers are awesome monsters! The Swedes say on their page that: “… and low propeller induced vibrations compared with those of a corresponding single screw ship.” Is that more or less along the lines what Stuart writes about the topic, i.e. cancellation of vibration or is that due to the possibilities to run the propellers under less load?

So – if I get you correctly one main point against Stuarts thrashing of three shaft layouts is that the source of drag is in the centreline of the vessel thus a centre-propeller works most efficiently in countering the drag.
So a four-shaft layout would carry a kind of thrust-thrust-drag-thrust-thrust pattern behind while a three-shafted vessel would trail a thrust-nil-thrust pattern with the centre shaft about cancelling the drag of the ship?

Ciao,
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Post by foeth » Wed Jul 26, 2006 12:14 pm

Well, cancelation of vibartion is not possible, but avoiding it is. Although from an efficiency point it view it wouldn't hurt to have your prop in the ships wake, that does mean the inflow is not homogeneous and that will cause vibrations. As the wake behind the ship is often worse than for twin-screw, vibration problems can occur easier. The twin-gondola ship has to be designed well to avoid a bad wake. But if you can do it and decrease the overall thrust per area, you can reduce vibrations and sound production.

Note that with wide afterbodies of 3 o4 shafted ships, it is difficult to say which one will do better. You need to have different shapes and may work out differently. I'm not saying 3 will do better per se than 4! I suppose you can design a 4-shaft afterbody that will reduce your drag. If you really want to know, we could always go to a good martime research institue. If we can save say $200,000, we can have two series of tests.

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Post by Ulrich Rudofsky » Fri Jul 28, 2006 4:15 pm

As I indicated above, I would put into English what Evers said in 1931 (although it does not seem to add a lot to the discussion):

E. Space Allocation and Configuration of the most important Items

149. Number and Configuration of Screws. The circular screw diameter is limited by consideration of a sufficient clearance of the blade tips from the water surface (air suction of the screw, protection against hits) and from the keel line (protection against contacting bottom). In fast, high performance ships the screw’s turning radius must rotate through below the keel line. The circumferential speed of the blade tips depends on RPMs and screw diameters, and is limited to about 60-80 meters/second, if extensive cavitation (void space formation by air suction into the water) is to be prevented, and thus, this limits the RPMs for a given diameter. In fast warships with the usual RPMs and screw shapes, the screw output can be at most 1.4 kg/cm e2 of the unwound screw surface area. Limitations of screw diameter also limit the output propulsion, and therefore, also the engine output; thus, for ships with moderate to heavy performance, an apportionment to several screws (2,3, or 4) is necessary.

The further subdivision into several independent main powerplants presents advantages by providing better watertight partitioning, lessening of a reduction of performance by failure of a powerplant, better protection due to reduction in height of the engines (lower position of the armored deck), greater stability due to lowering of the center of gravity, and, in three-screw ships with the rudder positioned amidships, there is a reduction in turning radius due to the direct action of the screw stream of the center screw on the rudder blade. Disadvantages of excessive subdivision are generally the relatively greater weight and space requirements, the increase in pipelines, the additional complexity and difficulty in operational controls.

The number and position of the screws strongly influences not only the overall space allocation, but especially the shape and partitioning of the stern portion of the ship. Certain screw diameters and weight and space allocations for the required main engines, often leads to the necessity for driveshaft divergence from the horizontal; in individual cases they may even converge. (see Plates II......VI [a detailed set of plans of the SMS BAYERN; ca. 1:500]). Often, the driveshafts have to be inclined aft from their vertical positions (destroyers and cruisers), and in back-to-back engines this is often of variable magnitude.
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Post by RNfanDan » Mon Jul 31, 2006 5:34 am

The effects of a ship's hull form also play into this discussion. At some point, marine engineers must decide where to place the propellers, that are required to interface the machinery with the sea. A ship's hull form, especially at the stern, must affect to a great extent the shape and flow pattern of the water. For example, a ship employing a skegged hull will have a radically different flow-pattern of water, than will a V-bottomed ship with a recurve hull form.

I'm not confident that the science of hydrodynamics was advanced enough during thé early to mid-20th century, to predict and/or measure water flow patterns, with great precision. If a propeller is located in an area where water flow isn't favorable, it isn't going to be at peak efficiency, regardless of its speed, size, or number of blades.

The inner screws of the South Dakota-class battleships, originally 5-bladed, were changed to 3-bladed ones because of vibration problems. Why was a four-bladed screw not used? For that matter, why did a five-blade prop cause vibrations at all, and why was the problem not foreseen in the first place? Also, what would have been the effect on propulsion of having NO skegs and using open shafts?

I am not an engineer, but I know enough about fluid dynamics to say that the flow patterns in the area of the propellers will vary considerably with ship speed, shape of the hull, and even the type and temperature of the water it moves through!

It would seem relatively easy to design an efficient propeller through trial-and-error, using a few simple instruments and a test-tank; but, put thes same propeller on any two different hulls, and it will not only behave differently from that of their test-tank environment, but differently with each hull, as well.

Am I off-base with this line of thought? :think:
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Post by foeth » Mon Jul 31, 2006 8:34 am

RNFanDan wrote:I'm I off-base with this line of thought?
No, you're actually quite right. I would like to specify that the efficiency of a propeller is usually defined how well it can transform a shaft torque into thrust at a given speed. Even in poor conditions, bad inflow patterns, and so on, the efficiency is not necessarily bad, may be even better than a 100% homogeneous inflow. The inflow doesn't change that much with temperature and speed of the ship. However, different drafts, trims, and ship motions will influence the wake. Steering makes life even worse. A colleague of mine is currently investigating the influence of maneuvering on propeller inflow and engine response in order to allow for a program to choose the most silent combination of torque, pitch, and rpm. (No wonder the USN goes to the Netherlands for their silent propeller programs ;))

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Re: Three shafts versus four

Post by frontkampfer » Sun Jul 04, 2010 3:19 am

My father was a Fire Controlman for the main 16" guns on the USS Missouri in WWII. As part of his job he had to know what the ships speed was at any given time. When she was deployed to the Pacific she made the voyage unescorted with an average speed of 30 plus knots. My father told me at one point in the voyage the ships speed indicator was 35 knots. For what its worth she has four screws. Not meaning to cause any controversy, I have always felt that if Bismarck had had four screws instead of three she could have avoided the fate that befell her-IMHO!
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Re: Three shafts versus four

Post by Bgile » Sun Jul 04, 2010 11:17 am

Interesting. I was told by a New Jersey sailor some time around 1970 that when they came out of the yards after reactivation they achieved 35 knots on one of their trials. It must have been in light condition though, because at Truk Iowa and New Jersey were only able to make 32.5 knots. Maybe at low fuel near the end of the trip?

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Re: Three shafts versus four

Post by Ulrich Rudofsky » Sun Jul 04, 2010 1:52 pm

A Gulf War WISCONSIN veteran who works as a guide on the ship in Norfolk tells visitors that the WISKY could hit 38 kn!!!! :think: :lol:
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Re: Three shafts versus four

Post by frontkampfer » Sun Jul 04, 2010 5:18 pm

I'm sure that she could. Wisconsin was the "youngest" of the Iowas.
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Re: Three shafts versus four

Post by Karl Heidenreich » Sun Jul 04, 2010 5:45 pm

A discussion of the sorts took place some time ago in this very forum. It is important to read foeth's posts in this because he is the authority in this kind of topics.
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Re: Three shafts versus four

Post by lwd » Tue Jul 06, 2010 1:51 pm

My gut feeling is that there is no generic defintive answer to the question of whether 2, 3, or 4 shafts are better. Each will have it's own set of advantages and disadvanteges. When combined with a particular design further sets of advantages and disadvanteges will accrue. Even then the answer may not be definitive. Now if you look at particular instnaces you may be able to say a ships particular desing servered her well or poorly but that doesn't say a whole lot about the design in general unless the particular situation was an extrmely likely one to occur.

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