Direct Drive Propulsion vs Pod Propulsion

Discussion in 'Boat Design' started by MikeStrasbaugh, Sep 13, 2014.

  1. MikeStrasbaugh
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    MikeStrasbaugh Unique Houseboat Designs

    Hello! I am a novice when it comes to designing ships (my latest triumph is my modern design for a new SS UNITED STATES 2) and I was curious about something when it comes to propulsion...

    Which is better to achieve a service speed of up to 45 knots? Pods or Direct Drives? For my SSUS2, I have designs for both, and I am having a friend make the design into a detailed 3d rendering that will eventually be 3d printed into a 1/350 scale model that will be remote control.

    My design calls for the SSUS2 to be approximately 1,490 ft long, 155 ft wide, 235 ft from keel to top of the funnel light mast

    ANY suggestions and comments are definitely welcome!

    Best Regards,
    Michael Strasbaugh
     
  2. AndySGray
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    AndySGray Senior Member

    Interesting question.

    I know that many of the largest cruise ships are indeed running pods but maybe that is to do with the docking and low speed maneuverability than drive efficiency. I understand one of the most recent cruise ships went the non pod route but had a massive number of bow and stern thrusters, though there were reliability problems on a couple of pods which was cited as the reason.

    Your target 45 knots is going to need every ounce of power and so the propulsion system needs to be the most efficient it can be. Pods do have a potential advantage in that they can be located in cleaner water.

    I note that many of the cruise ships use a non azimuthing (fixed) central pod, if shafts gave a slight advantage, I wonder why a combination would not be used?

    There's going to be a lot of bronze in motion, any momentum calculations 5 pods vs three larger ones?
     
  3. jehardiman
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    jehardiman Senior Member

    Mike,

    There are huge design differences between a liner and a cruise ship; propulsion being a major one.

    Rather than asking which is "better', because neither is an absolute "better" or "worse", rather understand why each is fitted and then select the configuration which is best suited to what you wish to accomplish.

    FWIW, large ships do not run pods for propulsive efficiency, but for other reasons such as maneuverability and hotel vs MPU power trade-offs. With a model, because you have very different load-power-density ratios than full scale, you can effectively toss real world design requirements and do what you would like. If your scale model was scale powered, I'm not sure you would be pleased.
     
  4. MikeStrasbaugh
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    MikeStrasbaugh Unique Houseboat Designs

    My design is definitely a Liner. I hate cruise ships lol. Too boxy and too much like hotels. Ill definitely take your suggestion jehardiman, and I think the direct drive will prove better so far as attaining higher speeds
     
  5. jehardiman
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    jehardiman Senior Member

    If you go the straight shaft route for the model, understand that the wheels and rudders will most likely have to be larger than scale to give adequate maneuvering performance. Just one of those scale effect things.
     
  6. NavalSArtichoke
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    NavalSArtichoke Senior Member

    45 knots is a very high speed for a displacement-type vessel. Most such vessels reach a max. speed limit when the speed-length ratio is about 1.15. Since the speed-length ratio is defined as V/SQRT(L), where V is vessel speed in knots and L is waterline length in feet, your SS US2 would be on the order of (45/1.15)^2 = 1540 feet long minimum, which is quite a jump in size for liners or cruise ships.

    Also, the only difference between liners and cruise ships is that a liner moves people from port A to port B on a schedule, while a cruise ship starts and ends its voyage usually at the same port.

    This article contains a graphic showing some of the largest vessels currently operating by type:

    http://en.wikipedia.org/wiki/Maersk_Triple_E_class

    Remember, when the SS US was operating in the 1950s and 1960s, fuel oil cost about $2/bbl, or about $15/ton. Now, fuel oil goes for about $600-$650/ton, so cruising speeds for large vessels rarely exceed 25 knots. After all, you can't outrun a jet in a bote.
     
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  7. jehardiman
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    jehardiman Senior Member

    1.15 is a little low for the bluewater liners and is more driven by modern economics than alsolute limits. Realistically, for L/B ratios approaching 10 it has a lot more to do with B/T ratio, block coefficient, and deadrise. Modern cruise liners, Pana-maxes, and VLCC's are driven to much larger B/T ratios and flater floors than optimum because of port limits. Bluewater liners, like some warships, were built for speed only and readily operated a speed-length ratios of 1.2-1.3 (SS UNITED STATES herself was 38 knts reported with a LWL of 940 so a S/L ratio of 1.23). For a discussion of high speed monohulls and S/L ratio affects see this paper http://legacy.sname.org/newsletter/Savitskyreport.pdf

    I think you need to do some more studing. Bluewater liners of the early to mid 20th century have little in common with modern cruise liners...they have totally different design criteria. The easy way to tell the difference is to count how many decks high the forepeak is.

    Wasn't cost of fuel that killed the liner, but time. Jets are far less fuel efficient per passenger mile, and they shove you into a tiny seat instead of a 8x8 cabin, but they get you there in 8 hours not 3 days.
     
  8. NavalSArtichoke
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    NavalSArtichoke Senior Member

    While the Big U did make 38 kts on trials and was capable of such a sustained speed, I emphasize that it was done on trials. In service, the Big U did capture the famed Blue Riband operating at an average westbound speed of 34.51 knots and an average eastbound speed of 35.59 knots, but over her career as a liner, this vessel typically averaged 30 knots, which is quite normal for a vessel of her size and installed power.

    I'm not sure if this is an objective criterion for differentiating a 'liner' from a 'cruise vessel'.

    The ocean liners which we associate with the term now were generally designed to operate on the North Atlantic run, between ports in western Europe and the UK and the east coast of the US, principally New York City. This operating route lends itself to regularly having high seas/high winds, thus the vessels operating on this route tend to be rather more closed in design to keep out the elements and to have higher freeboards to minimize the amount of seas which come over the deck.

    Now, cruise ships are all about the benjamins, so you design them like a hotel: give the guests a lot of amenities and diversions, like restaurants and casinos and floor shows, and cram as many of them as you can into each voyage, which typically occur in routes with milder weather than the north Atlantic. But all of this takes a lot of space, which is one reason why modern cruise vessels tend to look a lot like a floating hotel, rather than an ocean liner of yesteryear.

    Like I said, you can't outrun a jet in a bote.

    But still, the economics of operating any high-speed displacement vessel changed after 1973. The high-speed SL-7 container ships were designed with a top speed of 33 knots and it was planned they would operate on a liner service (there's that word again) between western Europe and the US. However, just as they were being delivered, the price of fuel jumped from $2/barrel to $8/barrel, and the economics of these vessels didn't stand a chance. They did go into service, but their operating speeds were kept much lower than what their plants could produce, and these vessels were eventually laid up, although almost brand new. Eventually, the USN stepped in and claimed them for the Military Sealift Command.

    Interestingly, the SL-7s were almost the same size hull as the Big U, and it took them 120,000 SHP to make 33 knots, while the Big U had 240,000 SHP available. This would suggest that the Big U could loaf along at 30 knots using only half its plant, and bring the other half on-line for a high-speed dash.

    Another interesting factoid: although the SL-7s were built in Europe for Sea-Land, both the SL-7 design and the design of the Big U were handled by Gibbs & Cox.
     
  9. AndySGray
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    AndySGray Senior Member

    It's YOUR design so you could push the boat out a little;-

    You know what a RIB looks like, I know what a RIB looks like, Willard knows what a RIB looks like, Juliet Marine knew what a RIB looked like too - So, did they build ANOTHER RIB, or did they take a couple of supercavitating 60' torpedoes, turn them round so the propellors are at the front, add a swath hull that looks eerily close to Darth Vaders Shuttle and a hold with room for 16 navy seals or 90 Nemesis missiles (difficult to decide which payload is more deadly) - definitely NOT a RIB - 4000+ horsepower with 30 day fuel capacity 2" ballistic glass windshield, Oh and it's called Ghost as it has stealth technology...

    That supercavitating technology, sucking air from the surface with a blower, has also been applied to larger hulls (superyachts)(though no liners, yet?) making them about 900 times slipperier through the water, so maybe your 45knots could look more like a cruise speed than a sprint.

    Yes a Jet can do it in 8 hours, but add in 3 hours advance check-in, 2 hours of queuing for customs, baggage claim and immigration (all of which could be handled aboard), then you need a night in a hotel to recover...
    Generous luggage allowance and a proper bed, and edible food - a 2 night transatlantic crossing starts to look saleable.

    Concorde used to do London to New York in 2 Hours but all the ancillary headaches and the extra cost made the 6 hour time saving moot.

    Technology has moved on - the Japanese are trialing massive Carbon Fiber props on cargo ships - We have a better understanding of surface piercing propellers...

    Could the Liner experience a renaissance if the 50 knot barrier were broken? I doubt you can out 'retro' the Australian guy re-doing the Titanic, and you opened the door with 'modern', so decide whats inside and whats outside the box (Nobody told Gregory Sancoff there was even a box and look what happened :eek: see below)
     

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  10. NavalSArtichoke
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    NavalSArtichoke Senior Member

    Before you get too excited, the carbon fiber prop which is currently being trialled is fitted to a small Japanese chemical carrier of 499 GT operating in their domestic trade. The prop itself has a diameter of 2.12 m, a tad under 7 feet.

    http://gcaptain.com/first-carbon-fiber-main-propeller-installed-merchant-ship/

    When they get to this size, 34 feet in diameter, or a little over 10 meters, then there will truly be a revolution:

    http://gizmodo.com/this-five-bladed-behemoth-is-the-worlds-largest-cargo-1546432773

    Still have yet to see any new Titanics lately, retro or otherwise.

    If the 50-knot barrier is broken, it will not be with conventional displacement hulls. The barrier will remain in place with some as-yet undeveloped technology unless the economics can also be made to work. With conventional fuel going for $600/ton and up, that will be quite a hurdle to overcome.

    You will probably see triremes come back into style before any of this happens: Triremes where the passengers sign up to be oarsmen so they can row to stay fit while they are on their vacay.
     
  11. Stumble
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    Stumble Senior Member

    The only way I can see a vessel breaking 50kn Trans-oceanic is if it is either nuclear or 3000' long. I can't imagine either of these entering commercial service however.
     
  12. AndySGray
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    AndySGray Senior Member

    Damn those inscrutable Japanese, they come up with a great idea then go and miniaturize it :D - Of course it would have been the correct size if they were using a modern unit like the inch - Any engineer worth his salt sees 212 on a plan and realizes they must mean 17 foot 8 inches ;)

    I understand part of the inspiration for the Ghost was a Russian Supercavitating rocket powered torpedo capable of over 200mph though there were a few 'ahem' issues with it :eek:
     

  13. AndySGray
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    AndySGray Senior Member

    I hadn't been paying attention - the Yamatai has actually been in service now for several years.


    http://travelships.wordpress.com/2010/08/17/mhi-completes-module-carrier-yamatai/

    but the latest thinking is to combine that technology with a superhydrophobic (based on an unwettable plant - a fern?) coating system which would give a massive reduction in friction and presumably not require anti-fouling either.
     
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