Large-Scale Propulsion

[Abraxas]

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 (http://2.bp.blogspot.com/_MTE3roZy35A/SJLkSltDLiI/AAAAAAAACnc/LLnJmFO2tHQ/s400/USN+USS+Freedom+LCS-1+file.jpg) and Independance (http://defense-update.com/images_new2/lcs_independence.jpg)).

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 (http://www.proprofs.com/flashcards/upload/yuiupload/382115620.jpg)). 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!

[Yellowjacket]

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.

[Abraxas]

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.

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.

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).

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.

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.

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.

[Yellowjacket]

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.

[baeckmo]

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.

[Abraxas]

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.

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

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.

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.

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.

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"...

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.

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 .

[Yellowjacket]

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.

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. After all, this isn't just a scaled up version of your pleasure craft.

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...

[Yellowjacket]

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...

[Abraxas]

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.

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. :)

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.

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.

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...

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.

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...

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.

[Abraxas]

Sorry for the double post.

[Yellowjacket]

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.

[Ad Hoc]

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.
Range....5000NM
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?...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"...you 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.

[Abraxas]

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.

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.

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?

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.

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.

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

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 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.

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.

"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

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.

I've done #1 (see this earlier post by me (http://www.boatdesign.net/forums/propulsion/large-scale-propulsion-29496.html#post304639)), 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).

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.

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 NavWeaps.com 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.

Range....5000NM
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?...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.

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.

Once you have ascertained these "blocks"...you 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!

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

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.

I'll look into them.

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

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.

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.

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.

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.

[Ad Hoc]

Abraxas

"..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 ass 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.

[Abraxas]

Alright, this is my CAD drawing thus far.

Try not to laugh...

http://i120.photobucket.com/albums/o177/abraxas_365/CADPicB10-14.jpg

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 (http://www.fairbanksmorse.com/engine_fm_alco_251.php?return=marine_power.php)) 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.

[Ad Hoc]

Looking good :)

Interesting hull profile...i thought you may want to follow the Series 64 type, with approx 50% draft at the transom. Our own research has shown that at higher Fn's the Stern type you ahve drawn has slightly higher resistance.

Not easy fitting a waterjet with zero immersion at the Stern too. Waterjets operate best with zero head, ie the CL of the jet is coincident with the DWL.

She looks like she might be stern heavy. Not much buoy aft, but a lot of "stuff" aft. Have you done an LCG/LCB check yet?

An LCG chase is essential in a vessel like this too...but not sure if your software program can do this or even give reasonable results to use as 'design'.

One thing to bear in mind with GT's is all the aux stuff required. From our experience the weight savings of GTs are not fully realised owing to the massive G/boxes, the huge air filtration and air intake systems to maintain air purity etc etc.

2 types of fuel....have i missed something?

Philippines etc your assumption is correct. These countries are good for repairs of diesels, all low tech. Anything high tech requires time and specialized support which is few and far between in such countries.

[Abraxas]

Looking good :)

Interesting hull profile...i thought you may want to follow the Series 64 type, with approx 50% draft at the transom. Our own research has shown that at higher Fn's the Stern type you ahve drawn has slightly higher resistance.

In all honesty, this is just a parametric transformation of a hull I found in Maxsurf. I've distorted the ratios of length, beam and height to serve my needs and I flaired the hull a bit to help it better reflect radar.

I wish we had a larger library of designs, but we don't. Do you use Maxsurf, by chance? If so, do you have any hulls for public distribution? I'd totally understand it if you said "no".

Not easy fitting a waterjet with zero immersion at the Stern too. Waterjets operate best with zero head, ie the CL of the jet is coincident with the DWL.

Unexpected, but not surprising.

Hopefully any alterations I might make to the stern won't severely damage it's performance.

She looks like she might be stern heavy. Not much buoy aft, but a lot of "stuff" aft. Have you done an LCG/LCB check yet?

An LCG chase is essential in a vessel like this too...but not sure if your software program can do this or even give reasonable results to use as 'design'.

We use a program called Hydromax, which uses our Maxsurf design for stablility and strength calculations.

http://i120.photobucket.com/albums/o177/abraxas_365/LoadcasePic.jpg

This only includes the hull steel (which I assumed to be 1" throughout to overcompensate) and weapon systems (which were the only things I could get concrete numbers for). It does not include the weight of the superstructure. I have to determine just how big it is, first.

I left almost 140 feet in front of the bridge so I could move it forward some if neccessary. Also, that platform amidships (with the VLS) is 40 feet long, which is bigger then it needs to be, so I can also calibrate my ship's trim that way.

My (very) quick weight estimation puts the ship at 2712.39 LT. This includes the superstructure, deck plating, weapons, power and propulsion systems, 2 RHIBS and 2 SH-60s. I obviously made some assumptions, but always tried to err on the side of caution. For example, I made deck steel area equal to 5 times my DWL waterplane area and 1" thick. I figure the 1411.11 LT it came out to is a little excessive...

My next task is to determine how much fuel I'll need and what it weighs.

One thing to bear in mind with GT's is all the aux stuff required. From our experience the weight savings of GTs are not fully realised owing to the massive G/boxes, the huge air filtration and air intake systems to maintain air purity etc etc.

2 types of fuel....have i missed something?

Gas turbines don't run on HFO... do they?

Philippines etc your assumption is correct. These countries are good for repairs of diesels, all low tech. Anything high tech requires time and specialized support which is few and far between in such countries.

I hadn't actually thought of that, but it's another great reason :D !

[Ad Hoc]

Abraxas

I don't use maxsurf. We've been lucky over the years to have someone do the lines for us from our hand sketches (well, that was their job!). However, I now have paramarine, which is seriously seriously sophisticated for hull modelling. Makes maxsurf etc look like a chisel and stone! (You can get free downloads for a trail version now). So just starting to use this for lines.

But hull shape doesn't really make that much difference. When looking at computer plots etc it seems like there are real differences and advantages to be made. Not so. The length displacement ratio is the key to the hull 'design'.

It would be worth doing a "quick" midship section. Use DNV or LR, or at worse ABS. Establish the SWBMs and do a spot check on worse case, midships, for slamming. You can then derive some scantlings. Since Global and local loads will determine the minimum thickness required.

When you have completed your weights, total weights, add a margin. For now i would recommend 10%, since so much is unknown.

I assumed you would be using marinised GTs!!

BTW
I assume you are using the standard 8 weight groups, for your weight estimate??

[Yellowjacket]

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 (http://www.fairbanksmorse.com/engine_fm_alco_251.php?return=marine_power.php)) 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.

First of all, you don't need two types of fuel. Turbines burn JP8 or diesel just fine. If you are fueling with sludge that's a problem, but the diesels won't burn it either. Anything you can burn in a diesel you can burn in a turbine.

I was thinking that 2mw is a pretty small amount of power for something this large at that speed. I know we are talking about displacement speeds here, but this seems low to me. You are still going to need about 4 mw for shipboard power, so your total power requirement is going to be at least 6 mw. That is too small for a LM2500 for sure. You need to be comparing much smaller turbines to the diesels. Two three megawatt turbines would weigh about 33 long tons less EACH than the diesel. A high speed (15,000 rpm) alternator for 4 mw will weigh less than 1,000 pounds.

The weight you are quoting (88 long tons) is for an LM2500 with an electric generator system. The engine alone weighs 21 long tons. There is no reason to couple the gas turbine to an electric generator that large since you can drive the waterjets directly or use a single reduction gearbox, which won't weigh anything near what you are talking about here. Remember if you generate the electricty, you need a motor of equal size as well as a control system to run it. Figure at least 30 more long tons per big engine to reconvert from electricty to shaft power. You are paying 80 tons per waterjet for nothing by trying to drive them electrically. Turbines have maximum torque at lower speed, so there is no reason to use an electric drive with a turbine, even on prop system. With a diesel you have a problem accelerating the boat since the power available is a function of speed. What that means is that at low speed, even with a diesel throttle open, you still don't have big power until you get up to speed. With a turbine you accelerate the gas generator and the power turbine puts out more torque. You don't need multi-speed gearboxes or electric drive, it just adds more weight.

The diesel system you are looking at is running at 1100 rpm and appears to be an all electric output system. That is why it is so heavy. A gear drive to the waterjets would be a lot lighter. You could still put a smaller generator on the shaft and make electrical power, but any system running at such a low speed is going to be heavy. You would be better off running an 1800 rpm engine of about 2000 hp directly to the waterjets with a shaft speed alternator (of up to one mw) piggybacked into the drive system, and then distributing 2 or 3 one mw generators around the ship to provide additional electrical power. With what you have now, you are going to need an additional 20+ tons of drive motors to drive the waterjets and that is unnecessary.

You are also underassuming what your payload will have to be in terms of what you carry in the weapons system bay. The LCS has room for 5 small craft and they have found out that the payload bays aren't big enough for what they want to do. The small craft are limited to about 11m and 22,000 pounds. The problem is that the craft for minesweeping and ASW are heavier than that, as well as the need for more crew protection in the 11m ribs. This is making life very difficult for the teams developing the MEP's. Room for larger and heavier MEP's is going to be imperative and you need to have room for more like 5 or maybe even 6 small craft. Figure out how to get any one them in and out of the ship without launching any others first. This is why the LCS is limited to 5 small craft.

Finally, I'm not a NA, but I would have thought that you need more lift aft if you are going to push this hull anywhere near 45 knots. I would have thought that it needs to look more like a semi-planing hull or you will hit the wall at 25 knots.

[Ad Hoc]

With respect to the waterjets, the largest diameter waterjet to use is the best and as such the RPM is to be low as possible. Gearboxes are required.
One cannot drive a waterjet directly from a GT, the ratio is out by a factor of 15~20.

The power delivered to the jet follows the cube law. So it matters not if more power is available lower down the RPM range, the jet cannot absorb it anyway.

As for speed, being some 130m long (I don't understand imperial units, sorry), at 25 knots this equates to Fn = 0.35, even at max speed this is a tad over 0.5. This is a displacement hull, not a planning hull.

The Fn is the important figure to understand, not the speed.

Issues such as:
"...The LCS has room for 5 small craft and they have found out that the payload bays aren't big enough for what they want to do..."
sadly are all too common. The initial design will have accounted for them. But during the life cycle of the project, the tenders will have changed, the cost overruns will ahve required modifications to the parent ship, to ensure the main principals are not affected, time for delivery etc etc. All these play a part, in slowly eating away at the original design intent. Not to mention politicians who were elected to bring the "ship to their backyard" for employment, pushing to get the job finished in their tenure....all looks good on their CVs!

[Yellowjacket]

With respect to the waterjets, the largest diameter waterjet to use is the best and as such the RPM is to be low as possible. Gearboxes are required. One cannot drive a waterjet directly from a GT, the ratio is out by a factor of 15~20.

The power delivered to the jet follows the cube law. So it matters not if more power is available lower down the RPM range, the jet cannot absorb it anyway.

As for speed, being some 130m long (I don't understand imperial units, sorry), at 25 knots this equates to Fn = 0.35, even at max speed this is a tad over 0.5. This is a displacement hull, not a planning hull.

I was looking at 1.3 x the sq rt of the waterline length and thinking that hull speed would be about 26kts. Pushing it to 50 kts appeared to be about 2.5 x the square root of the WL lenght and I was thinking that was a bit much, that's all.

I didn't mean to imply that a gas turbine wouldn't require some type of gear reduction. It is just some gas turbine engines already have a built in reduction box, so a separate one isn't going to be needed.

One thing that is changing is that the speeds for water jets that are driven by gas turbines are being increased. The previous designs were optimized around diesel systems and were larger than were necessary because of the low speed of the diesel. The specific speed of water jets is increasing since the resultant jets are smaller, lighter and take up less room in the vessel. If you look at the latest Kamewa product line the flow volume has been increased one step for the same frame size. This trend will continue as designs that are more axial in nature, as compared to the current mixed flow designs, are brought along. I've seen some of these newer designs and the reduction in size is pretty impressive. As a result, while a reduction gear is going to be required, in the future the overall ratio for a large system (Like an LM 2500) is going to be closer to 6 or 7 to one, as opposed to 15:1. Moreover, since the input speed is higher, the overall gearbox is a lot lighter and smaller than if you were coupling a diesel engine to it.

I was only mentioning the low speed torque capability in regard to prop systems, as you noted, most water jets have an issue with cavitation a high power and low speeds.

[baeckmo]

............The specific speed of water jets is increasing since the resultant jets are smaller, lighter and take up less room in the vessel. If you look at the latest Kamewa product line the flow volume has been increased one step for the same frame size. This trend will continue as designs that are more axial in nature, as compared to the current mixed flow designs, are brought along. .......

Unfortunately, the suction specific speed (based on flow, rpm and NPSE at impeller inlet) is difficult to increase, even if the axial pumps are inherently slightly better than the diagonal type. The design challenge is to reduce inlet losses AND obtain an even velocitydistribution into the impeller. This is the real limiting factor for the low speed high power operation of water jets, and will boil down to a limiting relative velocity at the blade inlet tip. One danger with these applications is that there is an increased risk for early cavitation damage before there is a noticeable performance loss.

[Ad Hoc]

baeckmo has hit the nail on the head, with regards to waterjets.

In a nut shell, one selects the largest diameter waterjet one can use, regardless of its weight. There are so many waterjet boats that have serious cavitation issues or have their top speed cut-back owing to incorrect jet size selection in the design and/or vibration problems. More often than not being attracted by the lighter weight and top end sales figures for it speed, but accidentally on purpose omitting the obvious from the sales blurrb.

The velocity of the flow of the water though the duct is to be as close to the velocity of the water at the ship side....in other words, no loss. In reality this is not possible, but it is the 'design intent'. The closer to this 'ideal' the higher the efficiency. One cannot do this with small diameter ducts, regardless what their performance figures look like. Why, the escape velocity of the water is proportionally higher, hence the max efficiency is lower.

As for:
"..I was looking at 1.3 x the sq rt of the waterline length.."

You noted you are not a naval architect, so there is not point me criticising this assumption. This type of "generalisation" formula, some work some don't and most have their "limits", just like any computer program. Hence understanding the 'limits' of generalised statements and formulae is just as important as understand what ones software can or cannot do.

When comparing hulls and what is or is not possible, one only really needs to know the Froude number [V/sq.root(L.g)] and the length displacement ratio. These two ratios can inform a naval architect immediately of the hulls potential, or otherwise. Other form factors, such as L/B ratio also help, but are ostensibly just a derivative of the aforementioned.

[baeckmo]

........When comparing hulls and what is or is not possible, one only really needs to know the Froude number [V/sq.root(L.g)] and the length displacement ratio......

Just a detail, but since naval architecture is so full of "dimensional constants", I would like to wave a flag in favour of the nondimensional slenderness ratio; LWL/(displ.)^(1/3)!

[Ad Hoc]

I see your flag waving from here...jolly nice it is too :D

[Abraxas]

Sorry for my absence. I've had a busy few weeks capped off with the 3-day Annual SNAME Conference.

Anybody here go?

First of all, you don't need two types of fuel. Turbines burn JP8 or diesel just fine. If you are fueling with sludge that's a problem, but the diesels won't burn it either. Anything you can burn in a diesel you can burn in a turbine.

My experience is with boilers exclusively, so that was news to me when I found it out.

I was thinking that 2mw is a pretty small amount of power for something this large at that speed. I know we are talking about displacement speeds here, but this seems low to me. You are still going to need about 4 mw for shipboard power, so your total power requirement is going to be at least 6 mw. That is too small for a LM2500 for sure. You need to be comparing much smaller turbines to the diesels. Two three megawatt turbines would weigh about 33 long tons less EACH than the diesel. A high speed (15,000 rpm) alternator for 4 mw will weigh less than 1,000 pounds.

The weight you are quoting (88 long tons) is for an LM2500 with an electric generator system. The engine alone weighs 21 long tons. There is no reason to couple the gas turbine to an electric generator that large since you can drive the waterjets directly or use a single reduction gearbox, which won't weigh anything near what you are talking about here. Remember if you generate the electricty, you need a motor of equal size as well as a control system to run it. Figure at least 30 more long tons per big engine to reconvert from electricty to shaft power. You are paying 80 tons per waterjet for nothing by trying to drive them electrically. Turbines have maximum torque at lower speed, so there is no reason to use an electric drive with a turbine, even on prop system. With a diesel you have a problem accelerating the boat since the power available is a function of speed. What that means is that at low speed, even with a diesel throttle open, you still don't have big power until you get up to speed. With a turbine you accelerate the gas generator and the power turbine puts out more torque. You don't need multi-speed gearboxes or electric drive, it just adds more weight.

My power scheme has changed slightly:
- 2x FM 251F 16V 1000 RPM Diesel GenSet (2.4 MW each)
- 2x LM2500+ Gas Turbine GenSet (29MW each)
- 1x LM500 Emergency Gas Turbine GenSet (4.2 MW)

I disagree a ship of this size needs 4MW for ship service systems. Our training ship, I admit, is not a warship, but it services all our needs, including laptops, TVs, video game consoles and iPod chargers plus lighting, SW, FW, LO, DO and FO pumps. Throw in FO heaters, air and reefer compressors, LO purifiers and various HVAC fan rooms and we still don't come close to consuming the 1.5MW we have available. I can't remember off the top of my head, but I doubt all this is more then 1MW.

Assuming 2MW for ship service systems and 10MW for ESW, ECW and weapons systems, I figure 2 diesels and an LM2500+ provide enough power (29MW) for standard operations. The other LM2500+ GT GenSet allows for much higher speeds around 40+ knots (even against the exponential rise in the power v. speed diagram).

I mean, the RN Type 45 Destroyer has a max output of 40MW... so I doubt I'll be underpowered.

I admit that right now, my only data is from Hullspeed, which isn't the most accurate program. I'm working on a NavCad Pro file now, which is much better.

The diesel system you are looking at is running at 1100 rpm and appears to be an all electric output system. That is why it is so heavy. A gear drive to the waterjets would be a lot lighter. You could still put a smaller generator on the shaft and make electrical power, but any system running at such a low speed is going to be heavy. You would be better off running an 1800 rpm engine of about 2000 hp directly to the waterjets with a shaft speed alternator (of up to one mw) piggybacked into the drive system, and then distributing 2 or 3 one mw generators around the ship to provide additional electrical power. With what you have now, you are going to need an additional 20+ tons of drive motors to drive the waterjets and that is unnecessary.

I'm reconsidering my propulsion methods in light of some research done with various propulsion combinations for the Joint High Speed Sealift (JHSS) ship. It actually has some pretty interesting data on how more axial-flow waterjets change power requirments at certain speeds when compared to more conventional mixxed-flow waterjets (which have a combination of radial and axial flow).

I'm going to read the report and find out what their conclusion is and report back. Hold tight.

You are also underassuming what your payload will have to be in terms of what you carry in the weapons system bay. The LCS has room for 5 small craft and they have found out that the payload bays aren't big enough for what they want to do. The small craft are limited to about 11m and 22,000 pounds. The problem is that the craft for minesweeping and ASW are heavier than that, as well as the need for more crew protection in the 11m ribs. This is making life very difficult for the teams developing the MEP's. Room for larger and heavier MEP's is going to be imperative and you need to have room for more like 5 or maybe even 6 small craft. Figure out how to get any one them in and out of the ship without launching any others first. This is why the LCS is limited to 5 small craft.

I never viewed the LCS as an assualt ship. That's what the LPD-17 is for and it's silly to try and get a 4,000 LT frigate to even try and compete.

IMO, 2 RHIBs provide enough resources to conduct searches, support commando operations and patrol the vessel in potentially hazardous waters. Fortunately, the 2 rectangles amidships are actually 40' long alcoves for a 36' Zodiac RHIB. I think this is enough for what it needs to do.

Finally, I'm not a NA, but I would have thought that you need more lift aft if you are going to push this hull anywhere near 45 knots. I would have thought that it needs to look more like a semi-planing hull or you will hit the wall at 25 knots.

Correct... but unfortunately I have no data sheets on planing hull dimensions, so I can't accurately design one.

But a displacement hull can still reach high speeds... provided it has enough power. Aircraft carriers can reach speeds over 30 knots (closer to 40 knots) by virtue of its atomic power plant and even the JHSS, which has a relatively standard displacement hull, is expected to reach 39 knots and beyond given 150MW of available power.

"Exponential" does not mean "impossible"... just "frickn' hard" :D .



More to follow soon... but now it's bed time. I have WAY too much sleep to catch up on.