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[GG]

Like many members of Mayhem I've been filling my spare time with some hobby work.  It's been quite an interesting exercise to see what I've had stashed in my stock of model stuff and I have to admit a fair amount had been forgotten about until rediscovered!


On the completion of the latest model, being unable to nip out to the usual sailing water, it was tested in the garden pond.  Well, it floats, it's stable and can be shunted around OK is all that can be said until better times return.  But, one pleasing thing was that during the design process, the models likely operating weight is estimated.  In this case the expected model weight proved to be spot on at a shade over 4 pounds (approx 2 Kg).


The method used to estimate my models weight is simple, more complex methods could be used but I'm quite happy with an Engineers "Ball park" figure.  This is usually enough to show if my plans are practicable or perhaps nudging into the impossible region and need modifying.


The volume of a block of water needed to float the model in is calculated by multiplying the waterline length, by the beam and draught of the model as shown in the figure.  Now, unless you build your model with a hull shaped like this block, the real hull will not occupy the whole volume of this block.  The block's volume needs multiplying by a suitable fraction less than one, this is termed the "Block Coefficient".  If the resulting volume of the submerged part of the hull is then multiplied by the density of water then you have the mass of water the hull displaces which corresponds to the weight of the model.


It might seem a little complex but can be simplified.  With the sort of models I usually built it's as follows;


Model weight (ounces) = overall length x max beam x draught (all in inches) x 3/8


For example something like a destroyer type of model, 32 x 4 x 1.25 x 0.375 = 60 ounces


The constant 3/8 (or 0.375) was found by looking at lots of my models and proved to be good enough for the combination of Block Coefficient and the density of water to give a result in ounces.  Using other units for lengths and density would need a different constant.


Like I said before, but am obliged to repeat, the constant 3/8 or 0.375 works with the type of models I usually build.  For other types you will probably need a different value.  If building say a model based on a tug, just find the a few similar shaped hulls and use their sizes and weights to work out the appropriate constants value.


All this is not essential when designing a new model, but I'd rather have the reassurance that my plans are in the "Doable" area, maybe nudging towards the "Questionable" zone but definitely not well and truly in the "Impossible" region!


Glynn Guest

[Mark T]

Hi Glynn


We had this same subject discussed about 2 years ago on here and its amazing just how accurate this.  There were a few of us guesstimating the weights of our current builds and mine was very close to what I estimated.  Definitely a worthwhile exercise when building a model boat as it really helped me.


Mark

[Colin Bishop]

Of course if you are building an accurate scratch built model you can work out the model displacement by scaling down from the original.

Colin

[GG]

Ah.... Colin,
               Using the full size data assumes you know which "tonnage" to use?  Warships are not too hard with just two values usually given, light and full load,  but merchant vessels.......
Perhaps some expert could enlighten we confused mortals as what would be best?
Glynn Guest

[Colin Bishop]

If you do a bit of digging around it is often possible to come up with displacement tonnage for merchant ships.

The commonly quoted ‘size’ Comparison for passenger ships is gross tonnage which is actually a measurement of internal space. Queen Mary was 81,000 tons while QM2 is no less than 151,000 tons, almost twice as much. However, when it comes to displacement tonnage, which reflects the actual weight of the vessel, the two ships are much the same and in fact the earlier ship is quoted as 80,000 tons as opposed to QM2’s 76,000 tons. (The big American Nimitz class aircraft carriers displace over 100,000 tons!). Queen Mary was 1019 feet long (311m) with a beam of 118 feet (36m). QM2 is 1,132 feet long (345m) with a beam of 135 feet (41m).  But Queen Mary had a deeper draught of 39 feet (11.9m) compared with QM2’s 33 feet (10.1m) and a rather fuller underwater hull form. Basically what this means is that QM2 offers a huge amount of extra usable space on a similar displacement compared to Queen Mary which reflects shipbuilding progress over the last 70 years.

Colin

[Buccaneer]

I too use the same basic method as Glyn, but work in metric. Length x Beam x Draft (all in centimetres) x a Block Coefficient gives weight in Grams. Divide by 1000 for Kilograms. From the records of my boats a good Coefficient for Merchant Ships is about 0.6 and for Tugs 0.4.

If you are starting from scratch with a kit that has a pre-formed Hull then try the following:

Mark the waterline on the outside of the Hull. Transfer it to the inside of the Hull. Fill the Hull with measured amounts of water up to the inside line, keeping a note of the amount of water used. When complete 1 Litre of water used will need a final all up weight, i.e. displacement, of 1 Kilogram. You can progress further from here by weighing all fittings  etc. and thus working out the amount of ballast you need to fit.
John

[roycv]

Hi I like the constant of 0.375 from GG I will remember that.  I will test it on a couple of my models to see if it suits.

As a quick mental arithmetic estimate I asssume the bow and the stern sections are roughly equal triangles and use underwater length minus the bow section and treat it as a block.  Then estimate the B.C.

The very first computer programme I wrote for myself back in 1966 was a printed (fanfold stationery!) list of prismatic and block coefficients for the size model boats and yachts I liked. The programme was compliled and then punched onto 80 column cards, long gone but useful until I got used to doing the maths in my head.  I find using fractions for mental arithmetic is so much easier than decimal, much simpler to manipulate the numbers.

If you do need extra displacement for a model it is amazing how much an extra 3mm of planking below the waterline can give you.On scale sailing models i have in the past made the keel much wider without any detrimental effect as well.

It brings a small incident to mind involving Vic Smeed.  I can do simple calculations in my head quite quickly and usually do not speak (boring) and wait for someone else to answer.  We were having a Christmas social and Vic was our guest at the club evening, a question needed an answer and Vic easily beat me to it and he was close to 90 at the time.  Must have had an agile mind right to the end.  Still as he suvived flying Spitfires (1943 onwards), you have to be alert and do calculations without thinking too much about them.

regards
Roy

[Shipmate60]

I do the sams as buccaneer but use dry weights that can be weighed on kitchen scales to give the actual weight.
I dont use calculated weights as some ships can be quite tender unless ballasted deep.


Bob

[john44]

I do similar using 1 or 2 litre plastic bottles depending on the size of the boat.
I usually use 3 bottles, forward middle and aft written on them.
I fill the bottles to near the desired waterline, then Finnish with a funnel filling
To the waterle the bottles are then marked on their fill line with a permenent marker.
They will not be the same measure usually.
All I do then is empty out the water bottles, when I get home I refill too the marks
and weigh each one so I know what weight is needed forward middle and aft.
Works for me.


John