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Notes & Comments


I added this page as a place where I could provide some thoughts on work in progress and discuss thoughts on different possible approaches that I am considering.

March 13, 2016

Although I have continued to work on some of the stuff behind the scenes for this site, I realized that I hadn't made any updates to the actual site for some time.  As such, I'm going to try and bring most of the text on the website up to date with respect to information that I have collected and information that I have included into my spreadsheets.

Spetember 3, 2014

I made some updates to the Surface Combatant Hullform Definition section to bring it more in line with the format and layout of the Aricraft Carrier Design hullform calcs and output.

November 10, 2013

Started to add information on Aricraft Carrier Design - located here - MNVDET-CV.

August 16, 2010

Currently I am working on updating a number of the figures here and updating the text to match the latest spreadsheets.

November 24, 2007

Back in 2007, I wrote that I had taken some time off from working on this site because I had just started a new job, and also because I wanted to step back for a little bit and reconsider some issues related to propulsion plants and how they relate to fuel consumption calcs, and hence fuel tankage and total ship displacement.

In very simplistic terms, you can make a rough estimate of total fuel tankage based on the estimated resistance of a hull, vessel speed, days endurance, and a notional specific fuel consuption rate for a given type of propulsion plant. However, such an approach doesn't really address whether a given plant could be produced during the time frame in question and may also gloss over some of the details of plant layout that may have a relatively significant impact on fuel consumption and other aspects of total ship performance.  A closer look at the US Navy's FFG 7 design might help illustrate some of these factors.

Data I have on the FFG 7 suggest that for the original design the vessel had;

  • Loa - 135.7 m
  • Lbp - 124.4 m
  • Bwl - 13.78 m
  • Dm - 9.14 m
  • Tm - 4.37 m
  • Disp - 3731 t
  • w400 - 119 t
  • w700 - 96 t
  • V design - 28 to 30 kt
  • Range - 4200 nm @ 20 kt
Plugging some of this data into the relationships and equations derived elsewhere at this website suggests that in order to make a design speed of 28 kts, the ship would require about 30,000 kW (40,000 hp).  If the ship were to have a design speed of 30 kts then the ship would require about 38,000 kW (51,000 hp).  For a 20 kt cruise speed though my estimates are that the ship would require approximately 4700 kW (6300 hp).

In general, right now the way I have been setting up my calculations, I haven't really addressed what type of engine components might be available at any given time, and as such a 38,000 kW COGAG plant is treated more or less as just a larger version of a 30,000 kW plant.  However, in the mid-1970's timeframe there were probably some significant limits on the type and size of different propulsion plants that would be available for such a ship.
Specifically, in the US the LM 2500 gas turbine at the time the ship was designed was capable of about 15,300 kW (20,500 hp) and in Great Britain I believe that both the Olympus (16,250 kW - 21,800 hp) and Tyne (3,170 kW - 4,250 hp) gas turbines were also in naval service, as well as a number of diesel engines of various sizes.  As such, in the mid-1970's options for powerplant configuration for the ship could include;
  • A 30,000 kW (40,000 hp) Nuclear Plant
  • A 30,000 kW (40,000 hp) Steam Plant
  • A twin-shaft Combined Diesel or Gas Turbine (CODOG) Plant with either 1 LM 2500 or 1 Olympus Gas Turbine plus 1 smaller diesel engine of about 2,350 kW (3,150 hp) on each shaft.
  • A twin-shaft Combined Gas Turbine or Gas Turbine (COGOG) Plant with either 1 LM 2500 or 1 Olympus Gas Turbine and 1 smaller Tyne Gas Turbine on each shaft.
  • A Combined Gas Turbine and Gas Turbine (COGAG) Plant with 2 LM 2500 or 2 Olympus Gas Turbines on a single shaft.

In addition to these there are also several other theorectical options such as an all diesel (CODAD) plant and/or a Combined Diesel and Gas Turbine (CODAG) plants however, although large diesels were in service commercially at that time it's not clear that diesels of adequate size existed in naval service at that time for an all diesel plant, or that reduction gear technology would support a CODAG plant at that time.  Specifically, if you look at the Lt Ship Wt Estimating sheet you'll see that the current data I have for diesel plants don't go much above 10,000 to 12,000 kW and I don't believe that I have any CODAG plants shown.

If we were to select a single-shaft COGAG plant then one option would be to use 2 LM 2500 engines which would provide a total installed power of 30,600 kW (41,000 hp).  Alternately, if we were to develop a similar plant based around the Olympus gas turbines, total installed power would be 32,500 kW (43,600 hp).  Either plant would likely be large enough to propel the ship to 28+ kts, though each appears to be too small to provide enough power to propel the ship to 30 kts.

Overall, in the current weight calculations I've been looking at, because I haven't gone into detail on powerplant configuration, as I noted above a 38,000 kW COGAG plant is treated more or less as just a larger version of a 30,000 kW plant though it is not clear how a 38,000 kW (51,000 hp) COGAG plant could be easily configured from the specific engines available in the mid-1970's time frame.  Nor is it clear whether it would be possible to make an all diesel plant in this size range during that time period.  As such, although you could easily run the calcs for a mid-1970's era FFG 7 style ship assuming a 38,000 kW (51,000 hp) COGAG or CODAD (or even CODAG) plant, in reality it might not really be all that realistic a design.

Another issue of concern relates to the use of a notional specific fuel consuption rate for each type of propulsion plant.  Because the fuel efficiency of a gas turbine will typically fall off significantly at partial power ratings, in order to get a realistic estimate of fuel consumption at cruise speed some detail of the configuration of the power plant would be useful.  Specifically, for the LM 2500 alternate ~ 30,000 kW COGAG plants discussed above, it appears that the ship may only need about 30 to 35% of the total power output rating of one of the two engines to propel the ship at 20 kts, which can be a significant issue in determining the total fuel consumption requirements of the ship, and hence the total displacement of the vessel,

Because of these concerns I've been giving some thought to trying to put together a table of engine types and power output ratings that were historically available (with options to allow the user to input his/her own data as well) so the user can define his notional power plant configuration early in the design, and then see how that notional configuration effects the rest of the design.  Unfortunately this will take some time to put together, so I'm currently struggling to decide if I should put in some temporary placeholder method (where plant sizes and configurations are unrestrained) and then replace this method later with a more realistic one, or if I should take the time to try and address these issues now.

Since that tiime I had a chance to review some theses on ship synthesis and power plant sizing and have adoped some of the information contained in those references into the spreadsheets that I have been working on.  As of mid-August 2009 I have developed some preliminary spreadsheets for:

  • Doing preliminary sizing estimates based on the definition of the primary mission payload of the vessel/design [eg a Pre-Processor]
  • Doing the main weight iteration calcs for a vessel/design [eg the main core of the calcs - referred to as Mn-MNVDET]
  • Developing a preliminary hullform for a design
  • Estimating the required enclosed volume for a design

In the near term I hope to continue to update and refine these existing spreadsheets as well as develop additional spreadsheets to:

  • Assist in devloping internal arrangement layouts for a design
  • Estimate centers of gravity anf initial stability issues
  • Perform some preliminary seakeeping calcs
  • Estimate rough costs
  • Review manning requirements