Large-Scale Propulsion

Discussion in 'Propulsion' started by Abraxas, Sep 30, 2009.

  1. Abraxas
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    Abraxas Junior Member

    I'm new to the site, but am currently in school for Naval Architecture. I'm designing my own ship and was hoping I could get some help on how to power it. I'm currently in a Resistance and Propulsion class, but I wanna get a jump start on my work.

    Since my goal is to some day work for a contractor of the US Navy (Lockheed Martin or General Dynamics) or one of their sub-contractors (Gibbs & Cox, etc.), I'm focussing on a design that will replace the current Littoral Combat Ships (Freedom and Independance).

    Currently, it's a smidge over 4,000 long tons at 433.7' long, 55.5' beam and 12.7' draft (about the size of an Oliver Hazard Perry). To get an idea of the hull shape, my block coefficient is 0.448 and my prismatic coefficient is 0.55.

    According to a hull speed program we've got (aptly named Hullspeed :rolleyes: ), I'm gonna need over 77,000 hp to reach 40 knots (optimal top speed). I want it to have a break-neck speed considerably higher then that, but I'll worry about it later.

    If I use 2 azipods for propulsion, how large will they need to be and what kind of Diesels will I need to power them?

    I'm not sure if this forum accompdates ship design questions involving ships this large, but if you can help me, I'd appreciate it!
  2. Yellowjacket
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    Yellowjacket Senior Member

    For a modern ship of that size class and speed you aren't going to be even thinking about diesels here. You are going to go directly to gas turbines.

    Not only for the power, but the weight of the engines will be roughly 1/10th that of diesels, and the space claim in the ship will be much smaller.

    The LM2500 is capable of 39,000 hp with a thermal efficiency of 39%, so you aren't giving up much in efficiency at high power to any high speed diesel, and if you are in a semi-planing hull the weight saved will more than make up for the difference in fuel burn, so the gas turbine will actually be more efficient in terms of specific range (miles traveled per gallon of fuel burned).

    I also don't think that Azipods make any sense in something this fast. The drag from the pods would be huge for a pod that big. Moreover, the turbine doesn't need the torque multiplication that the pod gives you, they have high torque at lower speed so there is no need for the generator/electric motor weight or complexity.
  3. Abraxas
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    Abraxas Junior Member

    Well, I was considering 2 diesels and 2 gas turbines. The former were for propulsion and the latter were for power generation. Of course, it's liable to change in light of new information.

    But in terms of complexity and in the event of failure or damage, I figured diesels offered the benefit of simply being easier to repair. My experience in marine propulsion is strictly with steam turbines, so I don't even know if I'm correct in my thought proccess.

    Electric propulsion removes the shaft, which in the event of attack, may become mis-alligned and make the ship inoperable (see the USS Cole). My hope is that electric propulsion may still allow the ship to move under its own power (provided structrual integrity allows for it, of course).

    I considered "water jets", but personally don't know enough yet to really make heads or tails of them. I'm trying to keep the ship relatively cheap and simple to maintain and assumed water jets were both prohibitively expensive and complicated.

    Thanks for your help.
  4. Yellowjacket
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    Yellowjacket Senior Member

    Most of the high speed ships in this size class are using a CODAG system with both turbines and diesels, and these are driving large water jets. There effectively is no shaft (or a very short one) and the shafts can use universal joints to take out any misalignment. The water jets are much less susceptible to battle damage or damage from running over things like logs or other debris at high speed. Both of the current LCS contractors as well as many fast ferries and fast semi-planing craft like Destriero are all using water jets. I am not a big water jet lover, but they seem to do the job well and are reliable.

    With regards to CODAG, it only makes sense to have a diesel big enough for very low speed, say up to about 4 or 5 knots, above that the turbines should provide most of the power. In the past the theory was to put in big diesels anyway and that the turbines would provide "boost" power for high speed use only. The end result was that the diesel system became so heavy that the turbines had to be a lot larger to get a useable speed increase. It is essentially the worst of both worlds. A much smaller diesel for low speed work and using the turbine for most of the time results in a much lighter and more effective package.

    A huge diesel isn't going to be any more maintainable than a turbine. A 40,000 hp diesel is so huge that you will need massive equipment to take anything apart and that just isn't practical at sea. The turbine is light enough that if you have a problem you can pull it out and replace it quickly. Besides, turbines have reliability that is an order of magnitude better than diesels, and modern condition monitoring can tell you if there are any problems coming up in the near future.

    Onboard power generation is also an area where smaller turbines will find a home in the very near future. The Cole survived only because the on board power wasn't compromised, and it was able to pump itself out. If the on board power had been located in the engine room, the ship would have gone down. Smaller gas turbines distributed around the ship would have a huge advantage in that on board power would not be compromised by local battle damage. The LCS-1 has four 1 MW diesels for on board power. The problem with diesels for on board power is that they are run at low speed. High speed generators are light, low speed generators are heavy. If you couple a shaft driven generator to a diesel you end up with a very heavy system. This is true for both on board generation and for electric propulsion. Look on line and find a 1mw diesel generator and look up the weight, it is massive.

    There is work being done within the Navy to define more fuel efficient small turbines in the 1mw class that could be used for on board power in place of diesels, and, because of the smaller footprint of a gas turbine these can be located essentially in a locker and spread around the ship.

    As you go through this exercise you will find that weight is going to be your enemy. Heavy engines and generators will rob you of the ability to carry the MEP's that you need to get your job done, or will result in a death spiral of increasing weight. The more you look the more you will endeavor to find the lightest weight systems that are effective and reasonably efficient.
  5. baeckmo
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    baeckmo Hydrodynamics

    Good overview on powering there, Yellowjacket! And the weight issue can not be overstressed when elevated speeds are discussed. In fact, in most commercial applications every "unnessessary" kg of equipment or structure is equivalent to a substantial loss of transport profit! Wether weaponry is to be regarded profitable, may depend on other factors....

    However, in Abraxas' project description I feel a definition of vessel duty, including analysis of operational cycle histogram is missing. Until you have a correct idea of the number of hours the vessel will spend in patrol loitering, transfer cruising, bursts of max sprint speeding a.s.o., as well as hotel load in the various modes of operation, any discussion on powering technicalities is just a waste of time.

    As most of us are "technical nerds" (why am I sitting here on a saturday evening......??), we tend to focus on technical issues far too early in most projects. It is frustrating to see how often the basic operational analysis is dusted away in a hurry, only to find the future operators involved in vivid technical discussions on subjects in which they have very little professional knowledge. If they could only focus on what they want the vessel to do, and leave the design to the designers, we would have better boats, lower taxes and fewer accidents.

    One example: If the low speed operating envelope is a certain proportion of the total sea time, the fuel consumption of this cycle may cause the design spiral to reverse. In this case, the use of waterjets for the loitering mode has to be rejected due to the jet's very low efficiency at speeds below some 25 knots. Instead some kind of retractable propeller arrangement has to be found.
  6. Abraxas
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    Abraxas Junior Member

    I wish I knew more about them - and I'm not talking about a PDF from the company that makes them.

    I figured I would have several small emergency generators, which operate the systems I need to start an auxilory generator (compressors, salt water service, fuel oil pumps, etc.) was pretty standard.

    As I start to flesh out my electrical requirments, I'll put a more concrete figure into focus.

    Of course.

    I'm already finding dispacement and hull steel to be one of these "death spirals". I can't increase displacement without increasing steel weight... and I can't increase displacement without increasing steel weight.

    Aluminium is an option... but frightning. Not to mention, the ABS rules for Aluminium construction are complex enough, so Navy standards are probably worse.

    I'm thinking I need to cut down on length to make it "lighter"...

    It's funny you mention that. There's a fantastic report in the "Naval Engineers Journal" about how new design strategies helped make the LPD 17 (San Antonio class) such an efficient combatant. Yes, it had its mechanical issues and came into service later then expected and over budget, but it was the ship the Navy and Marine Corps wanted.

    At the moment, I can't concentrate on the overly specific because I simply don't know enough. Beyond that, it's never expected that a single person completely designs a ship of this size. Afterall, this isn't just a scaled up version of your pleasure craft.

    So while I'd like to be as precise as possible and maybe put this in with some sort of portfolio, it may never develop to a point where that's feasable, which is why I'm being rather inprecise when it comes to this. It's just a fun little thing I wanted to do.

    That's not to say I haven't thought critically about my project. I do have a rough idea of where this will opearte, its common opponents and a vague idea of its mission profile. What I can't find are specific details like how much an Oliver Hazard Perry uses her engines... or the power needed for a 127/64 Oto-Malera lightweight compact naval gun... or how large of a hanger I need to service 2 SH-60S Seahawks. This isn't data I can find with a Google search.

    Basically, my desire to be more precise is beaten down by the reality of the situation. I can't be precise because this data either that doesn't concern a whole lot of people, so there's no where to find it, or it's so precise, governments and corporations don't stupidly post for people to see.

    So I'm simultaneously trapped by how uninteresting my career field is to others... and how cool it is :p .
  7. Yellowjacket
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    Yellowjacket Senior Member

    Baeckmo is correct in suggesting that you get more specific as to the mission requirements, in particular the cruise speed and range as well as the short term speeds as this will basically define your propulsion system and could potentially shift you to a larger or smaller diesel vs turbine trade off as well as impact your overall range capability.

    As you have noted, no person does it all, but, one person needs to be the lead and have the overall vision and knowledge to guide the program. This is likely why you have been given this assignment. If you are simply relying on the people below you to make the key design trades, how will you know if you are being driven off course by on particular requirement that may not be that important after all? YOU need to be smart enough in a number of areas to understand the implications of your decisions and how the requirements effect the overall system. This is an area where I have personally seen a few "fair haired boys" who were annointed as future leaders screw up because they didn't have a grasp of the basics and made bad decisions simply because they weren't seasoned enough.

    You don't have to be an expert in propulsion, but you need to know and understand basics like the efficiency trades of props vs waterjets at different speeds and know that props make sense at lower speeds and at what speeds waterjets become more desirable. You don't have to know a lot about turbines, but you need to understand that turbines don't have as good a fuel consumption at lower power levels and at what point they pay off in terms of power to weight for a given mission length and speed cruising.

    Before you start hacking at length and size I strongly suggest that you put together a short requirements document, listing the key mission parameters (like the ability to carry 2 x S-92's and how many RIB's or small craft you want to carry, range at cruise, max speed, ect) and start adding up the weight of the MEP's that you want to put in as well as looking at current ships (like the LCS-1 and LCS-2) that are out there doing similar missions. If you are trying to do more with less weight, you had best have a game plan as to how you are going to do it.

    If you don't know something, make an assumption, list it and then if things change, you will at least be grounded and know where you are and why. Also a good thing to list the unknowns, that way you know what you don't know and can start to find things out that you need to know...
  8. Yellowjacket
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    Yellowjacket Senior Member

    Also, the LCS-1 has a steel hull with an aluminum superstructure, and the LCS-2 is all aluminum..... Precedence for both..

    Either, or, neither, .... Titanium is good stuff too!!! All it takes is money!!! In fact, you are lucky, you can always buy your way out of any problem if you are willing to spend enough. In my area of expertise, we can't buy our way out, the laws of physics are hard limits and there are no work-arounds...
  9. Abraxas
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    Abraxas Junior Member

    This isn't actually an assignment. This is something I'm doing on my own time because I wanted to learn more about what it is I may be doing after I graduate.

    It's a labor of love. :)

    I'd love to read comprhensive analysis of this stuff, but all I find are PDF's that glorify the technology - seeing as the "source" is usually the company that produces it.

    Size: ~3,500 tons with dimensions = FFG or LCS

    Cruising speed: 15 – 18 knots
    Top speed: 35 – 45 knots

    Range: 5000nm @ 15 knots

    Propulsion Details: Direct drive dual-shaft system or electric drive. Consider Diesels for simplicity. Find endurance profiles of similar ships. Separate propulsion systems into two functioning cells so ship may still operate after damage to one.

    Power: Separate ship power bus from propulsion power bus. Total may be around 30MW.

    Location of operation: Off the east coast of Africa and/or east coast of China. These locations are high-traffic areas with potential for conflict from small, (potentially suicidal) high-speed craft and the increasing threat of Chinese submarines. Chinese submarines may be especially difficult to detect in high-traffic and therefore “noisy” areas.

    Capabilities: Anti-ship capabilities (with focus on small craft defense), anti-air capabilities (vs. 3rd and 4th generation aircraft) and particular attention to anti-sub capabilities (with focus on brown water tactics). Include means to support and deploy small assault force (2 RIB’s) and helicopters (2 SH-60S’s) as well as provide shore-bombardment capabilities.

    ESW: Towed sonar plus bow-mounted high-frequency sonar array. Air/space radar as well as a target-tracking and interceptor guidance system.

    ECW: Reduced radar cross-section design. Include missile defense hardware like balloon/rocket/mortar launched radio and radar broadcasters. Include laser refractors if cost effective.

    Proposed weapon systems:
    - 1x large caliber gun
    - 1-2x medium caliber gun(s)
    - Small VLS (ASW and AAW) system
    - 2x small caliber weapons
    - 1-2x RAM system

    Structure: Make a 3 compartment ship. Put sheer strakes inside the hull. Find midship section for other warships (see MEKO frigate, Prinz Eugen, Graf Zepplin, Jutland and Tirpitz).

    This is what I've been working from. Range was a consideration based off the assumption these ships would operate from allied bases in Australia.

    Are there even any ABS rules governing titanium construction?

    Either way, it's prohibitively expensive. Unlike some people in our defense department, I'm a little conservative when it comes to cost.
  10. Abraxas
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    Abraxas Junior Member

    Sorry for the double post.
  11. Yellowjacket
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    Yellowjacket Senior Member

    Seems like this is very close to what the current LCS-1 is trying to do, you are looking for some more range and have an allowance for some more weight but you are very close to LCS in terms of speed and mission equipment.

    In light of that I would try to learn as much as I could about the LCS and find out what went wrong and what was right. While some in the press malign the Freedom as being overweight and not making all of its performance marks, it is a pretty impressive ship.

    In order to support the increased weight, you are going to need to be bigger and that is going to snowball on you unless you can break the spirial. That will mean looking at key areas where you can reduce weight, the biggest and baddest actors here are the hull and the propulsion system.

    The Freedom already has an aluminum superstructure. Does it make sense to bring aluminum down into the hull some, or even make the whole hull out of aluminum?

    As far as propulsion systems go, the Colt diesels in the Freedom each weigh 34 tons without accessories. The combined power is about 17,000 hp and is likely plenty to push the ship to hull speed. I don't have SFC numbers on them so I don't have a firm idea as to how much fuel they burn. Coupling a moose like that to an electric generator is going to cost you literally tons (probably in excess of 25 tons). The problem being that the generator is only running at 1050 rpm, and then you are going to have a similar weight electric motor. As you can see, going electric instead of direct driving a water jet thru a marine box would likely cost you close to 100 tons more for the two drives compared to driving the waterjets thru a marine gearbox. For a displacement hull, who cares. For a semi-planing hull you would be hosed.

    You need to determine a ROM power requirement for your cruise point and back out a fuel burn for the cruise part of the mission. The hit that you will take in sfc for a turbine needs to be compared with the overall propulsion system weight (fuel + engine + gearbox+ accessories) for the diesel. The SFC for an engine like the LM1600 is going to be .376 lb/hp-hr and the diesel should be down around .34 lb/hp-hr, so the diesel will likely get about 10% better fuel consumption for the same power at cruise. The turbine will be worse at lower power where the diesel will be relatively constant. The LM 1600 has an output speed of 7000 rpm so the alternator will likely weight about 1/5 that of a diesel alternator. This is why gas turbines make a lot more sense for electric drive in applications where weight is an issue.

    Another thing that you could look at is a steam bottoming cycle on smaller (like LM-1600) turbine that provides your cruise power. This would give you a 20% increase in power from the cruise turbine and likely would provide all of your shipboard power and save you a lot of fuel burn for the mission that you are considering.

    As an example, the LM-1600 has an exhaust gas temp of 950 F and flows 104 pounds of air per SECOND. That's a lot of heat, and if you put in a decent size steam unit you could likely get about 4,000 hp out of it. You could then drive a generator with the steam unit and dump some power into the drive system if you had more than you needed for shipboard power. Either way, you have a more efficient power system than any diesel, lower weight, and a lower cruise fuel burn. One LM-1600 (20,000 hp) weights 12 tons dressed and ready for installation, the two diesels in the Freedom weigh over 68 tons bare. If you take a swag at the fuel burn for the Freedom at cruise you are looking at somewhere near 5,000 pounds per hour so if you could save even a few hundred pounds per hour that would save you 66,000 pounds over the 5,000 miles at 15 knots.

    You are going to have to do something unique like an all aluminum hull or a propulsion system like that described above in order to break the weight spirial. Otherwise you are just going to end up with a bigger LCS and that's probably not going to be any better than what is out there now.

    One thing I have learned in 35 years of engineering is that, given the same costraints, groups of independent engineers will very often come up with the same solution to a given problem. The only way you get something different is to cheat, and move the edges of the constraint envelope so that you get a different outcome.
  12. Ad Hoc
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    Ad Hoc Naval Architect

    Seems most of the bases have already been covered. However a few pointers.
    "..I want it to have a break-neck speed considerably higher then that, but I'll worry about it later..."

    That is where most designs fail, even before they start!

    Since you are 'discussing the design' per se, you need to keep your initial focus on the whole design, not the details. Details come later.

    Therefore you need to address this by doing:
    1) Write a spec and/or an SOR (statement of requirements)
    2) Draw a General arrangement
    3) Do a very good weight estimate.

    Since all of your concerns will be dealt with by first addressing the above 3 issues.

    For example.
    Proposed weapon system...this takes up "space", interms of its own seatings also how it can be deployed, this creates locations where it will or wont work (hence dead-space) and also weights of the eqpt, the seating and all the ammunition and its storage etc. One can circumvent this by establishing "blocks" of space and also rough 'blocks' of weights., for drawing out a GA.
    So, how many crew?....what level of chain of command, and hence the Capt. will have a larger accommdation than his 1st offcier, and will have a larger than than his Mate and so on...all this takes up space, where to put it? also takes up huge amounts of weight in the outfit and the aux system to support them...not to mention the mess, galley, storage of food...again, blocks of space.

    This is just the basic tip you need to address.

    Once you have ascertained these "blocks" can then start to position to see what will drive the design and ultimately the hull form. Since do you need lots of open deck space??..or lots of internal volume, for example...again dictated by the location and size of the 'blocks'.

    Hence, the hull form will be selected to suit the space envelope and also the displacment it is to carry (from the weight estiamte). Then you can see how much "room" is available for the ER and the fuel, for the range, is there enough?

    Once you have slowly gone aroud this design spiral, then and only then can you determine what speed she may do. Also add margins on weight and speed, always!

    As for aluminium, nowt wrong with it, if you are finding steel is causing your design to "not work". That is part of the itteration of the design spiral, the what if's, so to speak.

    ABS rules are very very simple actually. BUT way way conservative and you'll end up with an over heavy structure, typical over engineering by US.

    Go for DNV or LR. Both have naval rules attached, and, are far more comprehensive. LR, if you can get the SSC software, is then even easier, saves doing lots of hand calc's.

    A vessel that size will be driven by the SWBM's for long.t bending. Hence drawing up a very simple midship section, just to get a feel for thicknesses of decks and sides, which will then give you a rough hull modulus. You can then check this for rough compliance against the Class minimum. However on HSC vessels local slamming loads may well drive up the local structure requirements above that required for the min allowable for SWBMs.

    One final point about HSC vessel structure. The deeper the draft the higher the scantlings...sounds obvious, but a wide shallow hull can carry the same as a narrow deep hull!...that is generally independent of speed too.
  13. Abraxas
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    Abraxas Junior Member

    Precisely. In effect, I'm designing an LCS without modularity. It doesn't mean I want a "Jack of All Trades but Master of None". It just means I don't agree that modularity equals force multiplication and don't want this ship's combat role to be determined by this (IMO, flawed) concept.

    I had read a report a few years ago about the state of naval ship yards. I think it was by Raytheon, and in it they blamed cost overruns and slow construction on the complexity of new designs. More specialized (i.e. better paid) engineers are in greater demand, but unfortunately, because of this trend, building commercial ships in the US has become prohibitively expensive for the private sector. Furthermore, it's becoming more commonplace to have foreign consultation in the ship design and construction proccess - like AustalUSA's involvement with the LCS-2.

    Personally, if we can't build our own ships... what's the freakn' point? :confused:

    Either way, I know Aluminium requires alternative welding techniques and may prove to be a source of concternation. I'll double-check my ABS rules on the subject and assume the navy has similar... if not tighter... constraints.

    I'll get back to you after I do some research.

    Research is proving to me that gas turbines are considerably lighter, so I guess you're right that diesels were a poor choice.

    You make a very complete and convincing argument for COGAS, which seems like a great idea! I'm a little surprised it's only now becoming popular in warship design, but efficiency, no matter how easily obtained, seemed to be far and away from the minds of past naval architects. For the US, only the LPD-17 was designed with an effort towards greater efficiency.

    I've got some data sheets on GE gas turbines and Rolls-Royce water jets and will assemble a propulsion and power scheme as soon as I can get to a computer with Hullspeed.

    "I mean, by the engineering method, the strategy for causing the best change in a poorly understood situation within the available resources."
    - Professor B.V. Koen

    I've done #1 (see this earlier post by me), am constantly doing #2 in AutoCAD (I just haven't gotten a recent pic to share yet) and completed #3 in Hydromax (with some estimations for internal steel weights).

    Already done - though it's difficult to get accurate data on weapon systems for security reasons, I suppose. "Jane's Guide to Naval Weapons" and have been my primary source for information, plus various technical PDFs I find.

    As it stands, my weapon loadout (including ammunition) comes to ~130 LT. Each weapon and it's weight are then virtually placed on my hull to determine my new equilibrium. I added proposed hull steel weight across it's length and got a few graphs showing me locations of high sheer and moment stresses. Now that I'm making some headway on power and propulsion systems, I can begin to see how these items change my stability condition. I'll start figuring in superstructure steel as soon as I figure out how much I need and exactly where it goes.

    I figure berthings and the size of the captain's office are kinda at the bottom of my list of concerns. Sure, they take up space, but I figure optimal placement of an emergency power generator is more pressing. And yes, furnishings and beds have weight, but this pales in comparison to the pair of 22,000 lb SH-60's that need to be stowed in the helo shelter.

    On the contrary. It is my belief that ship requirments govern its hull form - not the other way around.

    I'll look into them.

    By chance, do you know which classification society the US Navy borrowed from the most?

    I have an AutoCAD drawing with the beginnings of a midship section. It needs work, but I don't want to start placing longitudinal bounderies till I've decided where large pieces of equipment like turbines and generators will go.

    Yeah, I pushed the beam out to decrease my draft under that concept.

    Thanks for everyone's help. Hopefully I'll have a pic soon, plus a power and propulsion proposal (alliteration = :cool: ) for y'all to judge.
  14. Ad Hoc
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    Ad Hoc Naval Architect


    "..I had read a report a few years ago about the state of naval ship yards. I think it was by Raytheon, and in it they blamed cost overruns and slow construction on the complexity of new designs.."

    Having designed for the UK MoD in the past, the reasons are the same, UK, US etc. Bureaucracy and "old style" unionised yards. This 'system' is not efficient - having 5 poeple doing the job of one (in the commercial yards), - it is certainly not commercially efficient.

    Take any normal commercial yard, there is a contract and a budget time and price. That is it. Any time over runs are paid for by the yard. Any budget overruns are paid for by the yard (there are exceptions to this rule...but clear and obvious). "Military" yards.....overruns in time...they just ask for more money or they wont finish it....over spending..opss....sorry spent too much by wasting time and money of eqpt that helps my mates, can we have more money please. (Look at the LCS, over runs of more than $500m, on just 3 boats...crazy!!). Then the "jobs worth". People who on do their little bit of work and will not deviate one bit...and if the paper work is not there and correct, it is not processed, slows the whole system down. And then it drags unions in, because said person is now being pressured to get up of their *** to do real work, rather than nowt....and on it goes.

    The UK now has SMART procurement, which is even worse than the system it replaced.....such wasting of money!!

    "..I figure berthings and the size of the captain's office are kinda at the bottom of my list of concerns. Sure, they take up space, but I figure optimal placement of an emergency power generator is more pressing...

    Berths/cabins etc, on a vessel of this size takes up a significant amount of space and weight and will dictate how your layout is arranged to be ergonomic and efficient in the aux system and other supporting procedures. You may also find that once you locate places for said, the hull is not wide enough, or deep enough or long enough. This then has the inevitable knock on effects.

    A good example is that on a commercial vessel, if the heads (or toilets) are not working, the boat doesn't/can't go to sea. Hence she doesn't earn money. So simple trivial things are very important in the operation of the vessel. This is what you must focus on, the Operation of the vessel. One small break in this link, and it all falls down very quickly.

    "..By chance, do you know which classification society the US Navy borrowed from the most?...

    If the US is/was like the UK, they will ahve done endless research over decades/centuries themselves. As such have their own rules. The UK has NES (Naval Engineering Standards), and also many "classified" add on, like 'shock loading' for example. However today, the situation has changed, well, has in the UK.

    All the research and data was done by British Admiralty, which changed to BSRA and BMT and QniteiQ etc etc....each time being slowly split and privatised, in an attempt to be more efficient and hence 'commercial' and no longer a burden on the tax payer. All this has done is "subcontract" the knowledge to the point that the UK MoD no longer has control of the data nor expertise to control it.

    LR in conjunction with what was left of the "old guard" formulated LRs Naval Rules. This was also to bring naval vessels to be the equivalent safety as commercial vessels. (EU directives and all that bollocks). Same with DNV, they ahve scarfed in naval rules too.

    So, if you use LR or DNVs naval design rules, you'll get a decent result. As I noted before, ABS, are heavier. ABS has its naval rules now too. A few years back i did a 51m patrol boat, using ABS. Boat was steel. The frame thickness had a minimum of 6mm!!!....crazy..when old VT vessels had 3, 3.5 and 4mm plate on same size or larger. Been running about for 20~30 years no problems. Yet ABS insistent on 6mm. I told them it was rubbish, battled, took about 4months, but they relented and i got my 4mm.

    ABS is like the comments i made above..bureaucratic and slow and part of the "old system". Hopeless.

  15. Abraxas
    Joined: Sep 2009
    Posts: 10
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Delaware or NYC

    Abraxas Junior Member

    Alright, this is my CAD drawing thus far.

    Try not to laugh...


    After some consideration, I've decided again to include diesels.

    If this ship is loitering off the coast of Africa or near the Philippines, a gas turbine will be overly expensive to operate. It simply provides more power then I need for low-speed propulsion. My speed vs. power analysis (using the Holtrop and Fung methods) reveal I'll only need ~2,000 kW of power to get up to 15 knots (and I'm leaving a 50% margin of error to compensate for the efficiency of the generator and water jet).

    A pair of diesels (FM/ALCO 251F 18V 1100RPM) would provide enough power at low speeds and run most ship service systems (salt water service, compressed air, etc.). I then use 3 LM2500 gas turbines, which will provide enough power to get up to 45 knots (and beyond if there is need).

    As for weight - the diesels come in at 38.4 long tons each, while the turbines are a hefty 88.4 LT. Yes, the turbines provide more power for their weight... but that's half the problem, so I think diesels make a good compromise.

    Of course, the problem now is that I have 2 types of fuel (one of which I need to keep heated). The Type 45D HMS Daring uses a system like this, so I know it can be done. I just need to decide how much I designate space for HFO and whatever the turbines use.

    If space and weight permit, I may still include a COGAS system, but it's more of a bonus, at this point. I still have other, more important things to include.
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