Designing and Building Model RC Warships (eBook)

Chapter Two

Size and Scale

Having decided which vessel they plan to base their model upon, an inexperienced person might then rush into designing and building it. This can lead to problems, which might not become apparent until much later, possibly just as the model is being prepared for its first sailing.

SCALE MATTERS

Scale in this sense simply refers to the ratio of the model’s size compared with the full-size item. It is usually quoted as a fraction, for example 1/100 means all items on the model have been reduced to a linear one-hundredth of the full size.

You could build a model to any size you fancy, but there are some scales that have become popular with modellers and manufacturers. Even when building your own original creation, it can sometimes be handy to use suitable commercial items. Table 1 lists a range of popular scales and the modelling areas in which they are commonly found; this should not stop you ‘borrowing’ them for your model.

Table 1 Common scales used in models.

Plastic aircraft kits of the right type and scale have saved me lots of anguish when outfitting the flight decks of aircraft-carrier models. The thought of making from scratch such numerous and very obvious things that have to look identical does not bear thinking about.

Even a little bit of lateral thought can be handy, such as fashion jewellery, which can make anchor chains, not perfect maybe but close enough and painless. There are also many small businesses that can supply scale details and fittings for this hobby, usually at these common scales. Therefore, choosing a scale for your model, even if it is an original design, that matches or is close to one of the common commercial scales, does make sense.

These scales might seem to have ‘odd’ numbers like 1/96 rather than the neater looking 1/100. This is due to their origins, which might have a practical rather than a coldly logical basis. The multiples of twelve are related to the imperial units of twelve inches to the foot. Thus, a scale of 1/48 can be expressed as one inch equals four feet or 48 inches. To be honest, you can usually mix near scales without causing visual offence. A personal dislike is making ladders – the model railroad scales can provide suitable items. N gauge railway ladders (1/160) have often adorned my 1/144 scale models and no one has yet noticed this scale mismatch!

A further consideration could be if you plan to build more than one type of vessel to a common scale. Coming into this hobby via model aircraft made me familiar with building models using balsa sheets. It seemed sensible to continue with this material and so early models based on destroyers were sized to fit the standard balsa sheet lengths. This worked out to be around the imperial scale of one model inch equalling twelve full-size feet or 1/144. This produced practical models that performed well, not too expensive and, as found out after building quite a few, easy to store. Only later did it become apparent that this scale would allow me to build a range of warship models.

Table 2 shows the effect of scale on three common types of warship. It ought to be clear that the 1/96 scale makes a practical destroyer model, the cruiser is becoming a handful, but the battleship would be quite a challenge to transport, let alone getting it in and out of the water. At scale 1/192, things are turned around with a manageable battleship model, the cruiser is fine but the destroyer has become a featherweight – possible, but quite a challenge, unless you accept sailing in only calm conditions. When built in 1/144 scale, the destroyer is a useful size and you do not need to be a weightlifter to cope with the battleship. This admittedly fortuitous discovery has been proven with models based on cruisers and aircraft-carriers built in this scale, but I have yet to build a battleship model.

STABILITY (OR HOW TO REMAIN UPRIGHT)

It is worth spending a little time on how boats, model and full-size, manage to float and to know which way up they should be floating. Not essential knowledge, until you have problems.

The model’s operating weight (often-termed displacement) is perhaps the best thing to calculate first. Boats, model and full-size, float due to Archimedes’ principle, which is, as the hull descends into water, it ‘pushes’ (displaces) water out of the way. The water pushes back with force equal to the weight of water displaced (up-thrust). Hopefully, the weight of water displaced will match the model’s weight before it submerges. This is a stable position: push the model down and the up-thrust exceeds the model’s weight, so it rises back to the equilibrium position. This explains why models sink lower in the water as more weight is added to them.

It also shows why models are stable, even if there is apparently more of the model above than below the waterline around the hull. Two ‘centres’ are needed to explain this. The first is the centre of gravity, the point inside the model around which the total weight acts. The centre of buoyancy is the point inside the hull where the up-thrust force acts. It is the position of the displaced water’s own centre of gravity. When the model is trimmed to float level, then the upwards’ and downwards’ forces are directly in line and in equilibrium. This is the same situation as a ‘see-saw’ being balanced on its central pivot.

Should the bows of the model be pushed downwards, then more of the forward section of the hull is immersed. This moves the centre of buoyancy forwards and now the up-thrust and weight, which should not move, act to rotate the model back to the level state.

Transverse stability can be a problem and sometimes you can witness someone’s pride and joy rolling from side to side, as it sails along, possibly with any spectators wickedly watching to see if it turns upside-down. A test that I give to every new model is to roll it by pushing down on the edge of the deck until it is at the level of the water. Upon releasing the model, it ought to spring smartly upright, probably oscillating a few cycles, before ending back upright. This has always been proof that the model has adequate transverse stability and, unless I sail it in stupidly rough conditions, no problems will occur.

Most people will realise that this stability requires the model’s centre of gravity to be low, but are surprised that it can still be above the centre of buoyancy. This sounds like an unstable situation as, when rolled, weight pushes down, while up-thrust pushes up, and it seems like it ought to carry on rotating.

The important thing is that rolling the model immerses more of the hull on the lower side, while reducing it on the raised side. This moves the centre of buoyancy to the lower side and the up-thrust, combined with the weight force, ought to act to restore the model to the upright position.

It is worth mentioning two terms that are important for transverse stability: the metacentre and the metacentric height. The former is the notional point on the vertical centreline of the hull about which the centre of buoyancy appears to swing as the model rolls. The metacentric height is the distance that this metacentre is above the centre of gravity of the hull. This has a positive value if the metacentre is above the centre of gravity, positive being stable and good. If the metacentric is below, a negative value is used because it is unstable. The metacentric height of a model can be found from a simple inclining experiment, but successful operation of a model does not need the knowledge of its value. However, what is desirable is an understanding of its existence, plus ensuring that your model has an adequate positive value.

All of this goes to show that for a given depth of hull immersion, a wider beam of model will be more stable as the centre of buoyancy will move further to the low side for the same angle of roll. However, for all models it will pay...