Designing a Ship

By Stuart Slade
Updated 04 July 1999


How does a warship really arrive in the world?  This isnít a simple question; the process is long, involved and full of uncertainties.  Often, the ship that finally enters service proved to be very different from that which the originators of the project had in mind.

Firstly, we start off with a staff requirement.  This basically identifies a geo-political situation, the resulting threats, the corresponding capabilities of the existing fleet and what capabilities are required to make the fleet meet the threat.  They are driven by a thing called ďThreat AnalysisĒ - an easy term that covers an incredibly difficult subject.  Threat analysis looks at the level of a threat by evaluating two factors - the magnitude of the threat (how much damage is this threat likely to cause) and the severity of the threat (how likely is this threat to happen).  This style of analysis means that a threat that is potentially extremely damaging but has a very low probability of actually happening may well be less significant than a less damaging threat that has a high probability of occurrence.  The recent British Strategic Defense Review is a very good example of such a document - read carefully, it is using threat analysis to tell us what the next generation of British surface ships will look like.  The staff requirement has many names but they all mean one thing - this is what the ship is going to have to face.  They are not easy documents to put together.

Now we have to translate that requirement into something approaching reality.  We need some extra information here.  So we go to the Treasury Department and find out how much money is likely to be available for the program.  Having got that bit of information, we start to talk to the operators (the guys who drive the existing fleet around), the technical bureaus (who tell us what we have now and how well it works, what is being developed and how well advanced it is, what the Bad Guys have and how well advanced that is), analysts (who try to come up with realistic threat scenarios) and consultants (people like yours truly who try to look at things from unconventional angles, identify good and bad ideas and suggest what works and what doesn't).  Above all, we try to identify how many ships of the new type are actually going to be required.

This is critical because once we have the money available (from the Treasury) and the number of ships required (from the assessment process), we know how much money we can afford to spend per ship.  This acts as a design discipline that's absolutely crucial.  Now we face CHOP-ONE.  This is - do we really need this ship?  What does it actually give us?  What will we have to sacrifice to get it?  Is there another way to get the same results at lower cost?  Can we fulfill this requirement by modifying existing assets (a process called CILOP - Conversion In Lieu Of Procurement).

Assuming the program passes CHOP-ONE, we can now begin to look at designing a ship.  We now convert the staff requirement into a set of Design Requirements.  In effect this lays down a series of performance targets, driven by the selected roles the ship has to fulfill.  These can be quite startling.  For example, if one of the duties the ship will have to assume is that of a carrier screen, she has to keep up with the carrier.  Assuming a CVN will hold 30 knots and will bomb up every four days, that means her screen have to hold 30 knots for four days - in other words will require enough fuel for 2,880 nautical miles at 30 knots.  Add in a 20 percent safety reserve and we have (roughly) 3,400 nm at 30 knots.  This equates to (equally roughly) 5,000 nm at 20 knots or 8,000 nm at 17 knots.  Already we are looking at a big ship to carry that amount of fuel.  We are also looking at a very fast ship - to run with a carrier doing 30 knots, we need a 20 percent design speed excess - in other words a designed speed of 36 knots (to give a trials speed of 32 -33 knots).

The Design requirements will also begin to determine what sort of weapons and sensor capability will be required.  This is where most of the money gets spent so decisions here are crucial.  Always, that design cost is hovering in the background - giving a ship designed primarily for ASW an Aegis system may well sound a Good Idea but add so much to the cost that the ship can't be built in the numbers needed.  And so it goes.

Now we need another set of outside data.  This is the most critically important document we have Ė itís called the "Instructions To Designers."  This carries a whole mass of information on what is and is not acceptable in ship design.  It includes things like desired stability criteria, design features - even down to how many members of crew per head.  It'll tell us what dockyards the ship will be expected to use - this gives us a draft limitation that will become critical.  The ItD will also give us some new criteria - so we want to keep the crew size down?  How about low radar cross section?  What are the anti-pollution requirements?

So, at last, we have the Design Specifications.  These will give us speed, range, sea-keeping ability, threat orientation, crew size and other bits of data.  In effect, we can design a ship from this basis (in fact the people involved in getting this far usually consider getting to this point "designing a ship" and consider everything that follows merely irritating details).  Note that we haven't even begun to specify the actual weapons fit, layout or anything else yet.

So, the project is up and running, the Government has approved, the Treasury has not disapproved and the decision has been taken.  We'll build a new class of battleships.  For the sake of argument we'll assume that Washington Treaty limitations of 35,000 tons apply.  The first step is to appoint a Chief Designer who will have full authority over the program.  Everybody sits around, not daring to move until somebody remembers that "Old George" is away on vacation and won't be back for ten days.  "Old George" is promptly appointed Chief Designer and the paperwork rushed through before he gets back and can change it.

Next step in the procedure is to start some feasibility designs.  There is a rule of thumb that around 60 percent of a battleship's displacement is allocated to weapons, armor and propulsion.  That gives us roughly 21,000 tons to play with.  So we go to the ordnance people and ask what guns they have under development.  They tell us they have an updated version of the old 12-inch that looks good, a modernized version of the 14 inch and a new 16 inch gun with all the mistakes made with the previous design fixed.

So the design staff do a series of outlines using each gun.  The centerpiece of each series is a "balanced" design, that is the ship's protection is adequate to resist her own guns and the speed is whatever it is.  The 12 inch series immediately hits the problem that 35,000 tons is far too large.  A balanced 12-inch design will still be a weak ship; if the extra weight is used for protection, she'll be able to resist 18-inch gunfire, which is absurd for a 12 inch gunned ship or if its put into power she'll be very fast.  Somebody comes up with a design that uses the extra size for a large number of guns rather than caliber.  This leads to a ship with 20 12-inch guns in 5 quadruple turrets, 32 knots and armored against 6-inch gunfire.  Nobody plans to actually build this design but the question is, what would somebody else use such a ship for and do we have to worry about similar designs?  When its shown to a fleet Captain he mumbles a few things and strides off.  30 years later, when writing his memoirs he claims that he had single-handedly saved the Empire by preventing the ship from being built.  Two days later he's found stabbed to death with a sliderule.

In reality, the 12-inch series is out; a realistic design is implausible unless the Treaty limitation is cut to 25,000 tons.  The 14-inch designs look more promising.  A balanced design seems possible using a 28 knot speed rating.  The 16-inch series is also plausible but a balanced design will only be possible if a 23 knot ship is accepted.  These two alternatives are taken as the basis for the formal requirement.  Now, our preliminary proposals are fed back to the authorities, with the message, yes we can do it, and this is what the new ship will be like.  The authorities get the go-ahead, the design staff gets ready and the Chief Designer's wife starts quietly preparing for a divorce.

Now we are starting serious design work.  The first step is to get the Ship's Cover from the previous design and see what was done and how.  By now, if we're lucky, the Cover will contain comments from the Captain and the refit yards on what went wrong with the design and how it can be fixed.  If we're really lucky, there will be a couple of letters from the Chief's Mess, to the Admiralty telling them what they think of the design.  Such input gets treated with a lot of respect; even though the suggestions may be impractical, they point to problems and/or suggestions for the day to day running of the ship that couldn't come from anywhere else.

With all that background we can start real design.  Firstly, take the previous design and ask, can we get what we want by simply changing the guns?  What impact would it have if we upped the speed to the new spec?  Can we use the old design at all?  How do we start?  If we are going to build a battleship today, we would have to start from first principles - modern practices are so different from those of the WW2 era that we simply canít do now what we did then.

The powers that be come back with a comment that they smile upon faster designs.  So the designs take a series of options on 14-inch armament.  They may take 8 guns in four twins, nine guns in three triples, ten guns in two triples and two twins and twelve guns in four  triples.  Then, they take various levels of protection.  If this is set in the 1930s, they are probably using immune zone theory to determine how the protection is laid out (today we would do things rather differently).  Now we put together a whole series of alternate designs which fulfill the Design Requirements in different ways.  These are the preliminary sketches that start to appear in the press and in trade shows.  Each suggested layout shows what certain assumptions will do to the final product.  Many of these layouts are really intended to show why we can't do certain things.  Others, to show the catastrophic cost implications.  This is a very iterative process.  Ideas are put in, evaluated, discarded or formalized over and over again.  Weapons are added, evaluated then subtracted.  At every point, the cost of the product is assessed and compared with the targets.

Eventually, they come to a range of designs including fast 8-gun ships protected against 14-inch guns up to slow 12 gun ships protected against 16-inch shells.  As part of this process, the outer dimensions of the hull are being estimated and a very crude estimate of the horsepower requirement of the ship made.  The Engineering people are then asked what powerplants they are developing.  They may have 15,000 SHP, 20,000 SHP, 30,000 SHP and 45,000 SHP plant available.  This sets the minimum power configuration at 30,000 SHP (twin shaft) and the maximum at 180,000 SHP (quadruple shafts).  This means the machinery weight for each option can be calculated.  A series of trade-offs will be made for varying combinations - for example, 60,000 SHP could be reached by using either four 15,000 units or two 30,000 units.  That decision will bounce backwards and forwards as the internal arrangements of the ship are discussed.

The 16-inch variants are held on a back burner at this time; implicitly they mean a slow ship so they'll only become interesting if the slow options become acceptable.  This doesn't happen, slowly the idea of a medium-speed 12 gun ship protected against 14 inch gunfire gets to be the front-runner.  At this point, the intelligence people toss a bombshell - there is a strong chance that the threat will go for 16-inch guns.  The authorities demand protection against 16-inch gunfire.  This throws everything into a loop and the Chief Designer is forcibly restrained from hurling himself out of the window.

The design team now has a well-advanced design and an urgent need to work in extra protection without unbalancing the ship.  Frantic work is undertaken in an attempt to juggle the extra protection needed into a ship with four triple turrets.  Suddenly, in the middle of the night, a great light bursts:  The Quadruple Turret!  If we can use quads to pack 12 guns into the space previously needed by three triples, we have a chance.  Now, back in the earlier stages, we had a ship that sacrificed some gunpower for protection from 16-inch projectiles and a little extra speed.  Quite a few people liked it, but the 12 gun designs got the nod.  So, they go back to that discarded fast 9 gun design and replace the triple turrets with quadruples.  We take the chosen powerplant from the previous 12 gun design, add it into the new variant and take a deep breath.

Wonders!  It WORKS.  Itís actually QUITE GOOD.  The design is now converted to a Spring Style (so called after the sketches used for Ladies Fashions) which shows people what the ship will actually look like.  This is a worked out design; she'll trim right and the maths works.  Proudly, the design team shows their labors to a meeting of the Admiralty and representatives of the fleet.  And wait for the applause.

It isn't long in coming.  Tributes to their expertise descend from all quarters "Call that a ship?", "My God what do we pay people like you for?", "My three year old daughter could do better than that", "Your daughter - my dog could do better - and does, on the carpet every morning." "Are these people mad?"  At this point, the Chief Designer's head starts to rotate around on his shoulders, green steam comes out of his ears and, possessed by the spirit of Jackie Fisher he starts screaming "Speed is Armor; Speed is Armor."  Sadly, his deputy carries him away to a darkened room far from the sea.

The meeting now proceeds to serious design input.  Gunnery asks if the quad turrets are really necessary; the reason is explained and he sighs.  Gunnery really doesnít like quad turrets.  His department also questions the AA armament; they have grave reservations about the guns selected for the secondary battery; they would like fewer, heavier weapons.  Itís explained the guns have to be DP or theyíll need two different guns and weight economy wonít take it.  The point is taken.  Engineering queries the propulsion choices; they can offer a slightly more powerful turbine in the same weight and volume - would the designers like some more power?  They could have another 10,000 SHP if they really need it.  The design team accepts gratefully.  Aviation demands the entire ship be decked over and converted to a carrier; they are taken into the car park and beaten into insensibility.  When they recover, they ask for a second aircraft and a cross deck catapult; this is included.  Seamanís division doesn't like the tripod mast aft of the forefunnel, itís likely to suffer from smoke interference.  Thatís noted and the mast reversed.  This has an impact on bridge layout but nothing that canít be handled.  Signals state they have to have a clear masthead position; they can't say why but itís very important.  All this input is fed into the design and modifications made as needed.  Eventually, when all the departments have reviewed the design, itís approved.  This means the ship now faces CHOP-TWO.  Can the design requirements be met with the funds available?  If not, we have various choices (a) reduce the number of ships and increase the cost-per, (b) get more money, (c) reduce the design requirements or (d) find another way of doing things.

Letís assume the program survives CHOP-TWO.  NOW we can start to design the ship.  By this time the Chief Designer has recovered from his breakdown and is back in his office (he thinks he is a potato and that the Admiralty wants to bake him, but thatís another matter).  The Spring Style is taken and split up into a series of vertical slices.  The designers then calculate the weight of each slice.  They can also make another calculation - they know the length of the slice, its draft so the only unknown is the beam at that slice.  They can then calculate the beam for the slice that will result in the below-water portion displacing enough water to support the slice at the desired waterline.  They do this for each slice.  This usually gives a shape like a demented dog-bone.  So they shift things around until they have a series of beams for the slices that give a relatively hull-like appearance.

This then goes to hydrodynamics who provide a set of hull lines that encompass those calculated beams.  Applying this to the hull slices gives a new series of figures.  Each slice will weigh a calculated amount; it will displace a calculated amount.  This gives each slice a calculated waterline.  All the hull slices together will give the hull waterline.  The slices will have positive or negative buoyancy relative to that line which will give a series of stress vectors.  Now the designers shift things around to reduce those vectors as far as possible.  Then they do this over and over again; the slices get smaller and smaller as the stresses in the hull are recalculated with each design change.  By the end of the process, the hull has been split up into tens of thousands of elements, each of which has its stress moments calculated individually.  That's why making apparently minor design changes late in the design process can be a disaster.

Meanwhile, the junior designers do everything from designing bulkheads and bunk spaces to working out where the bilge pumps will be.  The role of the Senior Designers here is to supervise the work and ensure that no idiotic mistakes are made.  The Chief Designer supervises the Senior Designers.  In passing, one bad problem that the RN had in the 1930s was inadequate numbers of Senior Designers.  This meant that work wasn't properly supervised and bad mistakes were made.  The Dido class cruisers were regarded as a particularly bad case of this effect.  There were many features of their design which would not have been allowed had proper supervision been possible.  The final details of weapons, etc., selection takes place during this process and is largely driven by it.  What is the effect of changing the bulkhead spacing?  What sheer is required to the bows (is any sheer a good idea?  It may be that the bow has marginal positive buoyancy and adding extra steel for sheer and flare may eliminate that buoyancy, doing more harm to seakeeping than good).

Detail design can easily take a couple of years.  Eventually, the design blueprints are completed, signed off, and the ship put out to tender.  Usually, at least one shipyard will come back with their own "improved design" that shows what a modern, forward thinking yard can achieve as compared with the hidebound reactionaries in their Admiralty ivory towers.  These designs are invariably completely unrealistic and far inferior to the official product.  However, they do have merit in that they are printed on nice, smooth, soft paper.

The shipyards then have the design job of taking the ship blueprints and converting them into a real hull.  Each individual hull component is drawn out, full scale, in a place called a ďMolding LoftĒ Templates are made from these drawings and used to fabricate each hull member.  Trusted shipyards will have quite a lot of latitude here; others will be very tightly watched.  But, eventually, the ship gets built.  Usually there are some design changes on the way and this adds to the fun.  If the designers are really unlucky, somebody turns up with a new, world-beating idea thatís only been proved to be a failure about 500 times before and they have to convince people of its ultimate idiocy.

At the end comes the great day when the new battleship enters the fleet.  The commissioning ceremony is held, the crew bring the ship to life and commentators pass their judgment on the new class "a sad day for the Empire," "Anyone can see how useless she is," "What were they thinking of?" "One look at a picture in Janeís and I can that tell the German Ships are far better."  "Why don't we fire these idiots and give the jobs to the shipyards?"  "If they can design 200 ton corvettes armed with 18 inch guns for the export market, why can't we have them?"

The Chief Designer ambles away sadly, keeping a sharp lookout for Admiralty officers with potato-baking forks and decides he needs another holiday.  Thatís when the next project gets launched......
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