5+ shaft ship designs

[Myasischev m50]

In real life, the vast majority of all warships have, at the most, 4 shafts. However there seem to be quite a few planned or studied ships with 5 or 6 shafts, usually with extreme (300000+ hp) powerplants. Would these ships actually be practical to build?

 

[muttly]

Practical, not really. Bad to the bone and fun to imagine what they would look
like, oh yeah.

 

[ceccherini]

In real life, the vast majority of all warships have, at the most, 4 shafts. However there seem to be quite a few planned or studied ships with 5 or 6 shafts, usually with extreme (300000+ hp) powerplants. Would these ships actually be practical to build?

Among the ones I remember:
-german 1939 battlecruiser projects Kw45 and Kw50, 5 shafts, 300000 hp.
-IJN 1916 battlecruiser study, 6 shafts 245000 hp
-some 1940 Montana preliminaries, 6 shafts, up to 330000 hp
-1947 supercarrier United States preliminaries, 6 shafts
Also the post Littorio battleship study of the Italian navy had 5 or 6 shafts combining steam turbines and diesels through there are no known drawings (note this is not the UP41).
Also there are multiple liner projects with 5 or 6 shafts and up to 400000 hp (malglaive and hardy 1937 proposal, '50 United Nation project, 1937 NDL Amerika project).
As a side note one can mention the only 6 shafts design actually built even if not equipped with a very powerful powerplant: the famed Popovka circular monitor.
In general the more the propellers the less the propulsive efficiency. Still the proposals I mentioned are all realistic enough to have been seriously considered for actual building. So this is not an elegant solution but one reasonably effective to power very large and very fast ships with the technical limitations of the time. I find quite curious that the logical alternative, counter rotating propellers on a single shaft, never received much attention even if largely adopted in torpedos and used by the training ship Cristoforo Colombo in the late '20

 

[Rhinocrates]

Thinking laterally, if you use electric podded motors (without shafts), you have a lot more flexibility. Look at all the proposed eVTOL designs with multiple props/motors. In fact, that goes back to Habakkuk, which was to have motor pods attached like barnacles.

Now, modern liners such as the Queen Mary 2 have these, but only 4 so far. They offer advantages in maneuverability as they can be individually oriented, which reduces the need for tugs. However, I hear that they suffer from reliability and maintenance problems (you can fix a motor inside the hull at sea, but a podded eternal motor is problematic, which is why military designs don't have them). Designing using larger numbers of smaller motors with the assumption that some will fail with enough redundancy to allow for this and enough modularity to use the same pods for different sizes of ships might be considered in the future.

 

[EwenS]

Some of the earlier 1943/44 British Malta class carrier designs had a 5 shaft machinery layout. But the final 1944/45 design used a more compact 4 shaft arrangement. Must have been considered practical enough to consider given the technology of the period.

 

[Myasischev m50]

In real life, the vast majority of all warships have, at the most, 4 shafts. However there seem to be quite a few planned or studied ships with 5 or 6 shafts, usually with extreme (300000+ hp) powerplants. Would these ships actually be practical to build?

Among the ones I remember:
-german 1939 battlecruiser projects Kw45 and Kw50, 5 shafts, 300000 hp.
-IJN 1916 battlecruiser study, 6 shafts 245000 hp
-some 1940 Montana preliminaries, 6 shafts, up to 330000 hp
-1947 supercarrier United States preliminaries, 6 shafts
Also the post Littorio battleship study of the Italian navy had 5 or 6 shafts combining steam turbines and diesels through there are no known drawings (note this is not the UP41).
Also there are multiple liner projects with 5 or 6 shafts and up to 400000 hp (malglaive and hardy 1937 proposal, '50 United Nation project, 1937 NDL Amerika project).
As a side note one can mention the only 6 shafts design actually built even if not equipped with a very powerful powerplant: the famed Popovka circular monitor.
In general the more the propellers the less the propulsive efficiency. Still the proposals I mentioned are all realistic enough to have been seriously considered for actual building. So this is not an elegant solution but one reasonably effective to power very large and very fast ships with the technical limitations of the time. I find quite curious that the logical alternative, counter rotating propellers on a single shaft, never received much attention even if largely adopted in torpedos and used by the training ship Cristoforo Colombo in the late '20

I suppose United States was the only "ultraliner" we got in real life. Seems like long range aircraft smothered most of them in the cradle. Would have been pretty interesting to see what we could have gotten had aircraft not been in the picture.

All of those projects you list seem to be going to 5 or 6 shafts to circumvent the 70000 hp per shaft limit imposed by cavitation. I wonder if contra rotating propellors would actually circumvent that problem. Still, there are ships that have quite a bit more than 70000 hp per shaft these days. Emma Maersk has (nominally) 110000 horsepower through her single shaft. Is that limit really just a technological limitation?

 

[Tzoli]

Propeller size matters too. In ww2 the limit was around 40-45.000shp with iowa went to 50.000+ with larger propellers so does the supercarriers like nimitz

 

[muttly]

I guess it's the same for ships as it is for aircraft. The more power means bigger
propellers or more of them or maybe both.

 

[Dilandu]

Well:

Six shafts, six screws. The famous "Popovka"'s, round coastal-defense battleships, build in 1870s for Black Sea Fleet.

P.S. Albeit I should point out, that after several years in service outermost machines & screws were removed, because trials demonstrated that they added too little to ship's speed.

 

[Tzoli]

5-6 shafts also considered for the very fast very big 1944 Lion designs of the RN

 

[Myasischev m50]

Propeller size matters too. In ww2 the limit was around 40-45.000shp with iowa went to 50.000+ with larger propellers so does the supercarriers like nimitz

Scharnhorst and Bismarck were sitting right around 50000 hp per shaft as well, Tirpitz made 160000 hp so it was around the same hp per shaft as Iowa. Though to be fair, Iowa was designed with a 20 percent overdrive on her machinery for over 60k hp per shaft.

 

[Myasischev m50]

Propeller size matters too. In ww2 the limit was around 40-45.000shp with iowa went to 50.000+ with larger propellers so does the supercarriers like nimitz

I do wonder about the maximum now, the largest military powerplants these days are the US CVNs at 70k hp per shaft. But because of how clandestine the propulsion of modern warships are I do wonder if they could transmit more power to the water. Even the 1949 USS United States' final design had 280000 hp.

 

[Nik]

I've been watching the 'open' ( non-premium) reports over at... https://www.offshore-energy.biz/worldmaritimenews/
...which, IMHO, is a poor shadow of its pre-takeover version...

That aside, there's a serious buzz around new combinations & permutations of fuels and propulsion equipment...

Bubble-mat tech apparently provides ~5% saving on fuel for same speed, even after 'blower' requirements. IIRC, it also seems to mitigate 'fouling' and 'sliming', saving a few more percent by reducing 'scrub' frequency / down-time.

Pods: The early versions were ruddy 'Murphy Bombs', with an eye-watering zoo of unexpected failure modes. They're getting better, MTBF rising, ETR shrinking. Approaching 'plug n play' replacement, dock-side rather than dry-dock. Way to go before MilSpec on support vessels, further before robust enough for warships' shock-damage...

Props: box-ships and VLCCs with one (1) mega-engine and one (1) uber-elegant mega-prop are wondrously efficient at cruising speeds, but you'd better have towing company on speed-dial...

 

[CV12Hornet]

I suppose United States was the only "ultraliner" we got in real life. Seems like long range aircraft smothered most of them in the cradle. Would have been pretty interesting to see what we could have gotten had aircraft not been in the picture.

All of those projects you list seem to be going to 5 or 6 shafts to circumvent the 70000 hp per shaft limit imposed by cavitation. I wonder if contra rotating propellors would actually circumvent that problem. Still, there are ships that have quite a bit more than 70000 hp per shaft these days. Emma Maersk has (nominally) 110000 horsepower through her single shaft. Is that limit really just a technological limitation?

Contra-rotating propellers might work to help circumvent that problem; the classic application of marine contra-rotating propellers is for torpedoes, to achieve maximum thrust on limited diameter. I certainly can't find anything discrediting the idea from a hydrodynamic standpoint. However, contra-rotating propellers are mechanically complex, a problem that's especially bad with large military propellers, both from a raw size point of view and also because azimuth pods aren't an option due to the fact that you can't maintain them while underway, which is no bueno for military applications.

Maersk Emma's listed horsepower is her engine horsepower, as far as I can tell. Based on the usual power loss on propeller shafts she's basically right at the usual 70,000-shp limit of shaft horsepower.

Propeller size has hydrodynamic problems of its own. Bigger blades means the tip moves faster, and also means the leading edge causes more problems while cutting through the water. There are ways to limit both problems, and the low-pressure issue, but they require truly enormous amounts of hydrodynamic testing to optimize the configuration. Little wonder that for decades nobody's moved the needle on the problem.

These days, computer design has made the design issue so much easier and advances in material science mean wilder propeller designs are possible on the construction end. It's entirely possible that the shp limit has been raised with new-design propellers; there have been rumors since the late 90s that the Fords would have such propellers and thus a higher shp output, though obviously nothing is confirmed.

 

[Tzoli]

Maybe some submarines had contra-rotating propellers?
But yes, WW1-WW2 era propellers were typically 3-4 bladed ones with some having 5 blades while modern ones could go up to 6-7 blades as well

https://wonderfulengineering.com/wp-content/uploads/2019/03/Submarine-propeller.jpg

http://www.hisutton.com/images/SIERRA_Prop.jpg

 

[Yellow Palace]

Maersk Emma's listed horsepower is her engine horsepower, as far as I can tell. Based on the usual power loss on propeller shafts she's basically right at the usual 70,000-shp limit of shaft horsepower.

Propeller size has hydrodynamic problems of its own. Bigger blades means the tip moves faster, and also means the leading edge causes more problems while cutting through the water. There are ways to limit both problems, and the low-pressure issue, but they require truly enormous amounts of hydrodynamic testing to optimize the configuration. Little wonder that for decades nobody's moved the needle on the problem.

Remember that all the other ships are being rated based on their engine power, and have the same losses to contend with, so the E-class certainly are running with the highest shaft loadings. I don't think there's anything magical about a 70,000-hp limit; that may have been the state of the art at one point, but a large part of that would be because there wasn't demand for more. A sufficiently large, slow turning propeller - keeping the tip speed down - ought to be able to be designed to absorb any amount of power. The ultimate limit is probably a structural one.

Likewise, there's no great engineering mystery in building a ship with an arbitrary number of shafts. It's undesirable, both because of inefficiency and because of the width of machinery needed, but it can be done easily enough. The main reason for trying would be to get relatively high power on a shallow draught, broad vessel. Because the breadth/draught ratio for most ships doesn't vary that much for ships, it's unusual to get into the range where four slightly bigger propellers won't work better than five or more smaller ones.

 

[Hobbes]

The E-class has a much lower design speed than previous container ships. That means prop speed can drop as well.

 

[Myasischev m50]

I suppose United States was the only "ultraliner" we got in real life. Seems like long range aircraft smothered most of them in the cradle. Would have been pretty interesting to see what we could have gotten had aircraft not been in the picture.

All of those projects you list seem to be going to 5 or 6 shafts to circumvent the 70000 hp per shaft limit imposed by cavitation. I wonder if contra rotating propellors would actually circumvent that problem. Still, there are ships that have quite a bit more than 70000 hp per shaft these days. Emma Maersk has (nominally) 110000 horsepower through her single shaft. Is that limit really just a technological limitation?

Contra-rotating propellers might work to help circumvent that problem; the classic application of marine contra-rotating propellers is for torpedoes, to achieve maximum thrust on limited diameter. I certainly can't find anything discrediting the idea from a hydrodynamic standpoint. However, contra-rotating propellers are mechanically complex, a problem that's especially bad with large military propellers, both from a raw size point of view and also because azimuth pods aren't an option due to the fact that you can't maintain them while underway, which is no bueno for military applications.

Maersk Emma's listed horsepower is her engine horsepower, as far as I can tell. Based on the usual power loss on propeller shafts she's basically right at the usual 70,000-shp limit of shaft horsepower.

Propeller size has hydrodynamic problems of its own. Bigger blades means the tip moves faster, and also means the leading edge causes more problems while cutting through the water. There are ways to limit both problems, and the low-pressure issue, but they require truly enormous amounts of hydrodynamic testing to optimize the configuration. Little wonder that for decades nobody's moved the needle on the problem.

These days, computer design has made the design issue so much easier and advances in material science mean wilder propeller designs are possible on the construction end. It's entirely possible that the shp limit has been raised with new-design propellers; there have been rumors since the late 90s that the Fords would have such propellers and thus a higher shp output, though obviously nothing is confirmed.

The vast majority vessels built these days that might bump up against the 70k hp limit are military, (since the E class is a bit of an outlier, most container ships have 2 shafts afaik) and it would be in their best interest to conceal the ultimate capability of their vessels. The details of most modern warships are classified anyway so its hard to say what advances in shaft horsepower they have made in the last 70 years. Continued ocean liner development past the 1950s might have had some interesting results, since going past 4 shafts seems to have serious diminishing returns.

On that subject, is there a limit to the maximum speed of a displacement hull, irrespective of power? If the speed could continuously increase with the application of more power I imagine 40+ knot crossing speeds would be possible. Now clearly that still doesn't hold a candle to a transatlantic aircraft but could that have prolonged the arms race a little bit? Would be interesting to see what 40 or 45 knot liners would look like.

As far as I can tell, the malglaive and hardy proposal was supposed to achieve 36-38 knots with its 400000 hp powerplant, so thats a start. United States of course made 38 knots but iirc the crossings averaged 30 knots throughout its career.

 

[Tzoli]

Possible but very very very costly I would imagine above 40knots for such a large ship anything below the main deck from nose to aft would be nothing but engines. engine power increase was never linear at that high speed you basically had to double the shp for each extra knot. Or you want kilometer long ships for the better beam to length ratio then again you will not be able to dock anywhere. Steam Turbine power requires 3 kinds of objects: Fuel space, boiler space, turbine space, Gas turbines provides better engine space management as they eliminates the boilers. Just look at the late WW1 era battleships and battlecruiser like QE and Hood or Nevada and Lexington

 

[Avimimus]

So why was that? The surface area to volume ratio goes down - so my initial instinct is that less power should be required for a larger ship. Obviously I'm wrong - but I'd like to know why!

 

[CV12Hornet]

The vast majority vessels built these days that might bump up against the 70k hp limit are military, (since the E class is a bit of an outlier, most container ships have 2 shafts afaik) and it would be in their best interest to conceal the ultimate capability of their vessels. The details of most modern warships are classified anyway so its hard to say what advances in shaft horsepower they have made in the last 70 years. Continued ocean liner development past the 1950s might have had some interesting results, since going past 4 shafts seems to have serious diminishing returns.

Probably the biggest factor for stagnation on the military side is that the Nimitz-class have for the last 50 years or so been the only military vessels bumping up against the limit, and they were limited more by the steam generation of their reactors than their shaft load. The class started out only producing enough steam for 260,000 shp, and it's not until their reactors were recored that they could output 280,000 at all. Why bother going to the trouble of designing new propellers when your reactors can't put out more horsepower anyway?

 

[Myasischev m50]

So why was that? The surface area to volume ratio goes down - so my initial instinct is that less power should be required for a larger ship. Obviously I'm wrong - but I'd like to know why!

You're only partly wrong, for a larger ship the required horsepower per ton goes down but the tonnage increases as volume increases meaning that you would still need more power.

 

[Myasischev m50]

The vast majority vessels built these days that might bump up against the 70k hp limit are military, (since the E class is a bit of an outlier, most container ships have 2 shafts afaik) and it would be in their best interest to conceal the ultimate capability of their vessels. The details of most modern warships are classified anyway so its hard to say what advances in shaft horsepower they have made in the last 70 years. Continued ocean liner development past the 1950s might have had some interesting results, since going past 4 shafts seems to have serious diminishing returns.

Probably the biggest factor for stagnation on the military side is that the Nimitz-class have for the last 50 years or so been the only military vessels bumping up against the limit, and they were limited more by the steam generation of their reactors than their shaft load. The class started out only producing enough steam for 260,000 shp, and it's not until their reactors were recored that they could output 280,000 at all. Why bother going to the trouble of designing new propellers when your reactors can't put out more horsepower anyway?

The kirov class only has two shafts and 140000 hp. So there's another ship that hits the limit anyhow. I wonder if the conservatism shown towards individual shaft horsepower is in the interest of reliability. A "if it ain't broke don't fix it" mentality if you will.

 

[ceccherini]

So why was that? The surface area to volume ratio goes down - so my initial instinct is that less power should be required for a larger ship. Obviously I'm wrong - but I'd like to know why!

You're only partly wrong, for a larger ship the required horsepower per ton goes down but the tonnage increases as volume increases meaning that you would still need more power.

He's absolutely right: power requirement are not directly related to weight but to surface. For every cube increase of volume surface and thus power increase by square. So if you want to go faster without reducing the payload factor the simple solution is going for a bigger ship

 

[Yellow Palace]

E class is a bit of an outlier, most container ships have 2 shafts afaik

The E class are actually a bit of a weird transitional state, designed before the 2008-2009 financial crisis (and rising environmental awareness) led to a reduced emphasis on speed and increased emphasis on efficiency, but still trying to make economies. The single shaft was part of that. Newer container liners return to two, relatively lightly loaded, screws because they can be optimised for minimal fuel consumption at relatively modest speeds.

Older ships - and the preceding Maersk B class are an extreme example - had two shafts for very high speed. In the case of the B class, 34 MW (46,000hp) per shaft for a sustained 29 knots, and ran trials at 34 knots.

He's absolutely right: power requirement are not directly related to weight but to surface.

This only applies to frictional resistance, which is one of two principal components of resistance for a ship. The other is wavemaking resistance, which is not a function of surface area, but of hullform and displacement. At very low speeds, or away from a free surface, wavemaking is negligible, which is why nuclear submarines look the way they do and have atrocious surfaced performance.

Wavemaking does weird things with peaks and troughs of resistance - troughs corresponding to integer numbers of wavelengths along the ship, troughs at the half-integers. Once the wave length equals the ship length at a Froude number of 0.4, wavemaking starts growing at an eyewatering rate, sometimes as quickly as the sixth power of speed. If you really want, you can push on to a Froude number of 0.5 or so, which is the 'hump speed' where the bow is at the crest of a wave and the stern is in the trough. Above that speed, you move into the semi-displacement, and ultimately fully planing, regimes, where fundamentally different hullforms are needed.

Interestingly, though, if you scale up a displacement hull, it gets longer, so the Froude number goes down. This means that a larger ship will usually have less wavemaking resistance per tonne of displacement, often falling off much quicker than frictional resistance. A similar phenomenon is why it's sometimes possible to lengthen a ship, increasing the displacement, and yet get the same or higher speed from the same powerplant.

 

[Myasischev m50]

E class is a bit of an outlier, most container ships have 2 shafts afaik

The E class are actually a bit of a weird transitional state, designed before the 2008-2009 financial crisis (and rising environmental awareness) led to a reduced emphasis on speed and increased emphasis on efficiency, but still trying to make economies. The single shaft was part of that. Newer container liners return to two, relatively lightly loaded, screws because they can be optimised for minimal fuel consumption at relatively modest speeds.

Older ships - and the preceding Maersk B class are an extreme example - had two shafts for very high speed. In the case of the B class, 34 MW (46,000hp) per shaft for a sustained 29 knots, and ran trials at 34 knots.

He's absolutely right: power requirement are not directly related to weight but to surface.

This only applies to frictional resistance, which is one of two principal components of resistance for a ship. The other is wavemaking resistance, which is not a function of surface area, but of hullform and displacement. At very low speeds, or away from a free surface, wavemaking is negligible, which is why nuclear submarines look the way they do and have atrocious surfaced performance.

Wavemaking does weird things with peaks and troughs of resistance - troughs corresponding to integer numbers of wavelengths along the ship, troughs at the half-integers. Once the wave length equals the ship length at a Froude number of 0.4, wavemaking starts growing at an eyewatering rate, sometimes as quickly as the sixth power of speed. If you really want, you can push on to a Froude number of 0.5 or so, which is the 'hump speed' where the bow is at the crest of a wave and the stern is in the trough. Above that speed, you move into the semi-displacement, and ultimately fully planing, regimes, where fundamentally different hullforms are needed.

Interestingly, though, if you scale up a displacement hull, it gets longer, so the Froude number goes down. This means that a larger ship will usually have less wavemaking resistance per tonne of displacement, often falling off much quicker than frictional resistance. A similar phenomenon is why it's sometimes possible to lengthen a ship, increasing the displacement, and yet get the same or higher speed from the same powerplant.

I had no idea that the B class was so quick, I guess the E class seems like a logical middle ground between the B and the EEE in terms of speed.

Do you gain much efficiency by moving to a semi displacement design for a 40+ knot vessel? I found some information about a company called FastShip that was planning for 1400 teu express ships intended to cross at 38 knots with 5 gas turbines on a semi displacement hull. Course that would be insanely inefficient but it would still be groundbreaking. They're one of the fastest proper ships I've seen.

 

[Yellow Palace]

Do you gain much efficiency by moving to a semi displacement design for a 40+ knot vessel?

It depends on the size. A 40-knot, 10-metre vessel has to be a fully planing hull. It just isn't possible any other way.

A 40-knot, 400-metre vessel is perfectly capable of using a displacement hull - it's about as remarkable as a 31-knot, 240-metre vessel.

The FastShip concept comes in at a Froude number of 0.441, which is in the range where a displacement hull is feasible but hugely inefficient. Semi-planing is likely to pay off here - but a good displacement hull might beat a poor semi-planing one. And bad machinery choices will ruin either.