Inches or mm?

[Dave_E]

Hi All,

 

Never gave this a thought until I watched many videos and understand the vast majority of builders live somewhere other than the U.S. and use the metric system. Is building using the metric system the norm? I have no problem with that, just have to get some new rulers and scales.

[BenD]

Living in Canada I've gotten used to using both systems. I built my ropewalks and do home repairs using imperial. All of the kits I've been building are in metric. Come to think of it all of my calipers and rulers have both on them, it's kind of like knowing two languages. Metric is very easy to use though. 

 

@Dave_E I'd say just use the one you are used too. 

[allanyed]

Hi Dave

I really don't know about kits as they are more of a modern convention using modern methods and tooling (and I wish we did the same and adopt metrics in the US), but if you are looking for accuracy in scratch building or kit bashing and can find contemporary drawings for English ships up to 1826  they are usually 1/4" = 1 foot in English units.  FWIW The English units were replaced by Imperial units in 1826 by a Weights and Measurements Act.  There are other contemporary drawings I have seen in 1/2" = 1 foot and 1/8" = 1 foot.    The British Establishments, The Shipbuilder's Repository, Steel's Elements and Practices of Naval Architecture and more, also use English units.

 

An interesting thing I found is that the contemporary model of the LN at RMG is built to a scale of 3/8" = 1 Foot but when RMG gives the dimensions of the model in the description they are in metric.    https://www.rmg.co.uk/collections/objects/rmgc-object-66562  

 

Allan

[Jim Lad]

Living in Australia I, like Ben, speak both languages and often use a mix of imperial and metric.  I grew up with imperial and find it very easy, but I generally use metric for very small measurements as I find it more convenient.

 

John

[shipman]

Being of a certain generation I was at school when the UK began the change to metric.

Being dyslexic with numbers as well as letters it was a god-send. I found metric so much easier to get my head round and has since been my go to system when accuracy was required. Strangely, mentally I visualise things in inches, feet and yards in a vague way to estimate the general size of something. But when actually working on something metric is the only way for me.

If all you have are Imperial rulers, callipers, drill bits, then you're stuck in the Roman era (hence Imperial). I would suggest buying a simple dual scale ruler and calliper, then you can see how they compare.

Of course a dimension as an Imperial fraction may not match exactly a metric size. Sometimes that can be useful if you have both sets of drills for example.

Be careful, there was at least one multi billion space mission lost due to a US contractor being given metric drawings who then assumed they were Imperial. Imagine the scratching of heads there.

All modern engineering and science is metric.

[gjdale]

At the end of the day, they are just numbers so it doesn’t really matter which you use. Where it does make a difference is in tooling - for example a mill or lathe, which will have their lead screws calibrated in either one system or the other (your choice on purchase). In that case, you may need to convert from one measurement system to the other, but then again, if you have a digital readout it again just becomes a matter of dialling in the the right number. 

[consitution]

It would be great if there was just one universal measurement say millimetres I would be behind this every inch of the way

[mtaylor]

15 minutes ago, consitution said:

It would be great if there was just one universal measurement say millimetres I would be behind this every inch of the way

And speed should be furlongs per fortnight.

 

On the serious side... either way works for me or even both.   Back in the '70's, I had two cars, one American and one Italian and both for some reason had a mix of hardware.  Some things were "English"/"Imperial" and others bits were metric.

[glbarlow]

We really should go the extra mile to convert to metric. 

[davyboy]

Being an old expat Brit I use Imperial measurements as I build models of 18th Century English warships in 1:48th scale. Metric is not really of much use as far as I'm concerned. My Vernier caliper and Micrometer are imperial as are my Rules. To each their own I guess. I still remember my school Teacher saying in (1948 or 49) forget Metric as it's foreign stuff we don't use it 'sic'. 

 

Dave 

Edited December 3, 2021 by davyboy

[Roger Pellett]

I not only use the Imperial system of measurement, I think it.  If someone gives me a metric dimension, it makes no sense unless I mentally convert it into feet and inches.  If someone mentions a 75mm gun, my brain automatically tells me, “that’s about 3in.”  Like Allen, using original drawings of American vessels measurements are in feet and inches.  I have tried to get into the habit of taking measurements directly using an architect’s scale.  8 ft on a drawing is measured at the correct scale as 8ft on the model.

 

During my working career I worked on some industry standards committees for piping components.  Many American standards have been converted to metric using “Soft Conversions.”  For example 6 in pipe becomes 150 Nominal Diameter Metric Size.  The actual pipe diameter hasn’t changed; it’s still 6-5/8in.  

 

Roger

[IronShips]

Roger, I understand how you are feeling. For me like many others it is the same, just the other way around! ☺️ I have grown up with the metric system. However, during my restoration projects (some of them in scale 1:1), I have found out that it helps to stay “imperial” if the original ship was build “imperial”. We never have all dimensions on our plans and shipbuilders have always been trying to use simple, straight forward measurements and dimensions. Reverse engineering some details of a ship, no matter of the scale, it helps to come closer to the original.

 

[Dr PR]

I am a lot like Roger. The guns we had in the Navy were 3", 5", 6" 8", etc. When I hear "155 mm" I instinctively have no idea how big that is. I have to stop and divide by 25 (25.4)  or 150/25 = 6 to figure that it is a 6" gun.

 

However, even though I am 76 and grew up in  the US I really am not sure how many ounces there are in a pound or gallon! I guess I must have gotten some bad scores on tests in grade school. I think the people who came up with noggins, inches, furlongs, ounces (and Troy ounces),fathoms, pottles, bushels, firkens, kinderkins, pints, gallons, yards, chains, pecks, miles (and nautical miles) ... and all the other ridiculous Imperial measurements were a few cans short of a six pack! None of it makes any sense. No one in his right mind would come up with such a mess, and were are all stuck with it!

 

All of the scientific work I have done was in metric, and that makes perfect sense to me!

 

But it was in electronics design that some sense was worked into the Imperial system. We did everything in decimal fractions of an inch. Not true metric, but much better. To this day I think in 0.25", 0.125", 0.0625", and such units instead of 1/4, 1/8 or 1/16 inch.

 

But even there some weirdness crept in. Transistors and integrated circuits originated in the US along with their packaging. Everything in decimal Imperial units. But when we started using parts from Japan and Europe I really had to scratch my head The units were metric, but the values made no sense. Why would anyone design a new part with a dimension of 1.27 mm? Or 5.08 mm?? Or 1.5875 mm?? Then I realized that the parts were being produced overseas in Imperial units (1.27 mm = 0.05" or 50 thousandths, 5.08 mm = 0.2 inch, 1.5875 mm = 0.0625 inch or 1/16 inch) but the data sheets used metric dimensions. That was really strange! But I got used to it and found it was easy to make part of a design in Imperial units and another part in metric. Whatever. I could have used cubits if necessary. The actual units just don't matter. Things are as big as they are no matter what units you choose to measure in.

 

****

 

But in ship modelling I have a word of advice. Use the measurement system the original ship was designed in. This will avoid a lot of measurement conversions, and that invites errors. Also, sometimes the actual dimensions of a part are unknown, but you can make a close guess. It is far more probable that an old English or American shipwright worked in common fractions of the Imperial inch than in millimeters! Likewise, you wouldn't expect a French or Dutch shipbuilder to use Imperial units.

 

To carry this idea a step further, I do all my design work in CAD.  Over a period of 14 years I created a very detailed CAD model of the exterior of a 610 foot guided missile cruiser, including every part 3/32" (0.09375 inch or 2.38 mm) or larger. Approximately 1/3 of the approximately 1 million parts are rivets, screws, bolts, nuts and washers. Almost everything was based on the original builder's blueprints and drawings in technical manuals. I designed the model 1:1 scale - no problem in a computer where the virtual universe is practically infinite in size. This allowed me to proceed without wasting time converting measurements to a smaller scale. That reduced the chance for error.

 

Of course, not all dimensions were in the paperwork, and I was only able to get a few critical dimensions by visiting the last remaining ship in a museum. But when I had to make "guesstimations" from photographs and sketches, I was confident that the mid 20th century US designers did everything in fractions of an inch or foot - 1/16, 1/8/,1/4 and 1/2. Every dimension in the blueprints and manuals used these measurements. In most cases guesses based upon the closest fraction fit seamlessly into the known dimensions.

 

When I get around to finishing the 1:96 scale model I will just change the scale of the entire drawing to 1:96 and take my model measurements directly from the scaled CAD model. No time wasted with unit conversions and scale calculations.

Edited December 4, 2021 by Dr PR

[grsjax]

I agree that metric is easier to use but that said if you have a set of drawings in Imperial it is generally better to go with that rather than converting everything into metric.

Digressing a bit, I had a 1970 Triumph 500 that had metric, SAE and Whitworth fastenings.  Made for a real nightmare when trying to find the right wrench.

[shipman]

During WW2 Packard took on building RR Merlin engines. First they went through thousands of factory drawings and re-drew them, mostly due to US thread sizes which were incompatible with Brit ones. Though both at that time they shared the Imperial system, each side had evolved different thread forms.

That would have driven the guys crazy, working on them in cold wet fields where the action was.

What a nutty situation.

[Bob Cleek]

For those bent on accuracy, don't forget that a "foot" varies from time to time, nation to nation, and trade to trade, in too many ways to count. I believe that the confusion over various reported measurements of old ships is not infrequently the result of different measurement standards. Even different cities had different "feet." Before the metric standardization, a foot in Belgium could be no less than one of eleven different "feet," depending on the town: Ypers was 273.9 mm, Bruges was 274.3 mm, Brussels was 275.75, Antwerp was 286.8, and so on.  In Venice a foot was 347.73 mm. Twenty-one different "feet" were used in Germany and five in France. Similarly, there were nine different Dutch feet depending on the town and a foot was 296.9 mm in Sweden and 313.75 in Norway.  Adding up the differences over the length of a sizeable vessel can result in significant differences overall between differing foot-standards. There's quite an extensive list of Imperial to metric local and historical equivalents at: Foot (unit) - Wikipedia

 

One of the most maddening exercises I've encountered in marine plans were the set of 1936 English builder's plans for a famous yacht which were drawn to a scale of 1" to 12.5". It took some detective work to unravel why a respected naval architect would use a scale nobody'd  ever heard of before. It turned out the plans were drawn for use by a Swedish yard contracted to build the vessel. By drawing to a 1"=12.5" scale, the yard could use all of its Swedish measuring tools when working with the plans!  

Edited December 4, 2021 by Bob Cleek

[allanyed]

Davyboy

I cannot find what the difference in length is for British units and Imperial units but the British did not use Imperial units until the 19th century (1826)  Honestly I do not know which measurements changed in 1826 when they went from English units to Imperial which of course changed again in 1965 to metric but I would like to see what the differences are if anyone knows.  I did a lot of searching but only found a reference to a pole standard of 6 feet based on outstretched arms of a 6 foot tall person, nothing about how long an inch is in English units versus Imperial units.    Maybe they were the same, but ?????  I suspect for our purposes there is little or no difference, but if anyone can share their knowledge on this that would be great. 

TIA

Allan

[Jaager]

Dave,

 

You sorta have to use both.  a digital caliper of sufficient quality will have 3 readouts:  metric, Imperial digital,  Imperial fractions.  I would advise ignoring the fractions option.

For length  a tick strip can save on interpolation errors.

As has been presented above, the wise choice is to use the units of the original plans and tables of scantlings.  Interpolation and conversion errors are far too easy to make. 

 

@Bob Cleek @allanyed  National differences are obvious to any beginner looking at ANM - with the different scales on every plan - and add to that: the exact standardization  within a country being a bit fuzzy -  I had not put this together until now, but obsessing about +/- 0.01" on model timber stock is a wasteful and unproductive thing to worry about.  A foot or two translated to scale difference in a model hull is nothing to worry about.  The actual vessel would have a significant +/-.  The key factor is to be internally consistence.

[mtaylor]

2 minutes ago, Jaager said:

I had not put this together until now, but obsessing about +/- 0.01" on model timber stock is a wasteful and unproductive thing to worry about.  A foot or two translated to scale difference in a model hull is nothing to worry about.  The actual vessel would have a significant +/-.  The key factor is to be internally consistence.

Very true that.   No power tools just hand tools usually an adze.  Close did count for something back then.

[Bob Cleek]

8 minutes ago, Jaager said:

National differences are obvious to any beginner looking at ANM - with the different scales on every plan - and add to that: the exact standardization  within a country being a bit fuzzy -  I had not put this together until now, but obsessing about +/- 0.01" on model timber stock is a wasteful and unproductive thing to worry about.  A foot or two translated to scale difference in a model hull is nothing to worry about.  The actual vessel would have a significant +/-.  The key factor is to be internally consistence.

 

This is true, however, it seems problems often arise when researching the identity of vessels by comparison of unidentified contemporary models with plans, customs office registrations, and other contemporary documentation. Differences of several feet in overall length, depth, and breadth are quite common.

Edited December 4, 2021 by Bob Cleek

[Bob Cleek]

2 hours ago, allanyed said:

Davyboy

I cannot find what the difference in length is for British units and Imperial units but the British did not use Imperial units until the 19th century (1826)  Honestly I do not know which measurements changed in 1826 when they went from English units to Imperial which of course changed again in 1965 to metric but I would like to see what the differences are if anyone knows.  I did a lot of searching but only found a reference to a pole standard of 6 feet based on outstretched arms of a 6 foot tall person, nothing about how long an inch is in English units versus Imperial units.    Maybe they were the same, but ?????  I suspect for our purposes there is little or no difference, but if anyone can share their knowledge on this that would be great. 

TIA

Allan

 

Yes, the English foot and the Imperial foot are the same length at  304.8 millimeters.Not my knowledge on this, but Wikipedia provides metric equivalent tables for pre-1826 English measurements and post-1826 Imperial measurements at:

 

English units: https://en.wikipedia.org/wiki/English_units

 

Imperial units: https://en.wikipedia.org/wiki/Imperial_units

 

 

 

[shipman]

British units and Imperial units are one and the same. Until now I've never heard of it otherwise.

And I'm a Brit.

[Dr PR]

And there is the problem that the measuring devices used to design the vessels probably weren't exactly the same as the tools used to build them. Look at the yard sticks and measuring tapes used in modern construction and you can see significant differences in length. If you have ever taken measurements in modern house construction you know that actual construction is far from precise!

 

I have five "accurate" rulers sold for drafting and engineering measurements from reputable companies like K&E, Staedtler-Mars, etc., and when I carefully line them up I can see as much as +/- 0.002 inch difference over a length of 12 inches.

 

I'm willing to bet the rulers used in different shipyards a few centuries ago were not calibrated to very exacting standards! Plus, often there were no real plans. It was all in the head of the ship builder and whatever came out of the ways is what you got. No two ships of the same type would be the same dimensions.

 

So don't sweat the small stuff. Good enough is good enough!

[allanyed]

Hi Shipman

The Imperial system is a relatively young system, not appearing until 1826 and is different in some ways than the previous systems of English units.

 

English units were the the units of measurement in England up to 1826  when they were replaced by Imperial units , which evolved as a combination of the Anglo Saxon and Roman systems of units. Prior to the Imperial system various standards have applied to English units at different times, in different places, and for different applications. 

 

The two main sets of English units before the Imperial system was developed were called the Winchester Units, used from 1495 to 1587, as affirmed by Henry the VII, and the Exchequer Standards in use from 1588 to 1825, as defined by Elizabeth I.   These English units were replaced by Imperial units on   January 1, 1826 by a Weights and Measures Act which kept some but not all of the unit names and definitions. 

 

Allan

[GrandpaPhil]

I usually convert to imperial units of measurement.  
 

However, all of my measuring tools are in inches, which definitely plays a part in that.

[shipman]

Allen, thank you for your explanation. Never too late to learn something new!

 

 

Another thought....if one is working direct from plans to wood a good proportional divider compass could save a head full of conflicting numbers, at least on the larger dimensions?

[allanyed]

What  you are saying Shipman makes a lot of sense, but in redrawing plans from RMG over the years I have found that they are often a bit distorted in one dimension or both due to folds and changes from aging.   I always check the scale on the contemporary drawing and the called for length on the gun deck or other given dimension often found on the heading of the drawing, then insert the drawing into my CAD program and adjust the dimensions accordingly  before having them printed or if I am redrawing them.    I have found as much as a half inch differential in length of the drawing compared to the called for dimension on the original at times.  I have no doubt the draftsman had it right when he prepared the plans, but 300 years can have its effect on the physical drawing.

 

Allan 

[Roger Pellett]

Shipbuilders did not work from plans to build the actual hull.  They worked from patterns developed on the mould loft floor.  These patterns were developed from lines laid down on the floor.  These lines, a full sized copy of the Master Shipwright’s draught or Naval Architect’s lines were drawn on the floor from measurements provided by the designer; a digital map of the hull shape.

 

Books on lofting are filled with geometric construction techniques to avoid actual measuring where possible. Straightedges, compasses, and chalk lines were commonly used.

 

As construction progressed, specialists, each with their own tools and measuring devices, applied their trades requiring varying degrees of accuracy.  This stage of construction also had to take into consideration tolerances for work that had been done before.

 

 

[tkay11]

Pieds and pouces, anyone? Converting those from the French to anything is another concern. The Ancre publications refer to both metric and ancien when providing measurements. If you use CAD, though, it's fairly simple to draw the plans at full size in whatever measure, then reduce to whatever scale needed and then take the measurements off those plans using either metric or imperial. CAD doesn't yet do pouces and pieds, though.

 

How about the length of one of the mediaeval kings arms or feet, though? That might prove fun.

 

Tony

[BobG]

I wish the USA would convert to metric but I doubt it will ever happen and, most certainly at my age, I'll never see it. So, I guess it won't matter much in the end since we'll all end up 6 feet or 1.8288 meters under!