Think of it this way a real radar turns 400 times in a nautical mile. So a scale radar should turn 400 times in a scale nautical mile. You make the adjustment in the speed the ship travels so further adjustment of the rotation rate is unnecessary. The guns are the same.

Sorry for my delay in getting back to you on this - but I've been thinking. While planking. Possibly not the best combination.

Clearly we've no problem with scaling linear dimensions. All models do this. Lengths, area, volumes (and thereby masses, since we can keep densities the same) can be reduced to make a model that reflects the original vessel.

Problems arise when we want to make a working model, since we can't scale one important effect:

**time**.

If I climbed the original Dreadnought's foretop and dropped a hammer over the edge of it, the hammer would fall 4.9 metres in one second.

On the model, I'd expect to see the (tiny) hammer fall 6.8 centimetres (=490/72) in the same period. Now the time taken for

*this* fall under "full size" gravity is 0.118 seconds - 8.49 times quicker than I'd expect. So (the "video effect") if I ran a video of the model 8.49 times slower than "real life" I'd see what appeared to be realistic effects of gravity on the model.

8.49 is the square root of 72 (what a surprise!) and it's this

*time alteration* which brings us to the scale speed: not surprisingly, since wave formation is tied into gravity.

We know that the scale speed is the one where the model hull generates an accurate representation of the wake of the original. For a 1/72nd scale model, where the original could steam at 21 knots, that means:

Scale speed = 21 / SQRT(72) = 2.47 knots.

The original vessel could do a nautical mile in 171 seconds.

The model's nautical mile is 1/72 times shorter, and at 2.47 knots, it takes only 20.2 seconds to cover. No surprises here, the model is 8.49 times faster.

So, to be strictly accurate, a radar (assuming the Dreadnought had one) rotating at 400 revs per 171 seconds - about 140 rpm - should be sped up to 1190 rpm.

The turrets, which could train at 4 degrees a second, should be sped up to 34 degrees per second on the model.

But clearly this is nonsense. We don't tend to view our boats via video; and whizzing radar, along with turrets moving like dingbats, doesn't read "true" to us. But that's ok, since we can't scale the viscosity of water or surface tension either - and we've all seen older films which included model boats where it's the non-scaling ability of water (big droplets, no "spray") which make us yearn for cgi to be invented.

So the answer must be: follow the "rules" for scale wakes, and apply the "art" for good looking radars and turret movement. I can live with that.

Incidentally, cgi - computer generated graphics - can be fantastic. I'd recommend watching "Ratatouille" for not only a great piece of entertainment, but for a computer generated world which rings absolutely "true". They spent, for example, an enormous amount of time to recreate the physics of liquids during the cooking sequences, and it all pays off.

(Spot the computer programming geek enjoying films.)

Best wishes,

Andy, wondering how I went from physics to rats in one post.