Since you can't scale down the viscosity of the water and various constants like gravity, any scaling process is not a matter of getting a precise ratio, but a matter of getting a boat to 'look right'.

The calculated Scale speed for a 1/100 model of a 300 ft displacement hull (36&quot travelling at 18 knots is about 1/5 mph – which isn't going to work. It will look better at 3-4 mph – but you're really better of building the hull larger to get a decent small bow wave. Don't go too fast, however – here's a classic error:

**LINK**

The same sized 36" model of a planing speedboat would have a scale speed of 3 1/3 mph – at least that will sail, but you'd be better off upping the speed to about 10 mph so that it will plane properly.

And of course things will be different for a different sized hull. You're never going to make a 1 ft long battleship perform like the real thing at any speed. Our cousins who fly model aircraft have also found this out – they have the Large Model Association to produce models of Lancasters and Flying Fortresses that behave more realistically in the air…

So scale calculations don't really give us much help when arranging the speed of a model anything.

Another easy way to work our the true scale speed of a model is to time it over a given distance. For example, if there is a fence along the side of lake, measure a specific distance between two well separated posts and then record the time taken to cover that distance.

So for example, if a 1:24 scale model covers 100 'real' feet in 20 seconds the scale distance covered will be:

100 x 24ft = 2400ft in 20 seconds or to put it another way, a 0.45 mile 1:24 scale distance (5280 divided by 2400)

The model has covered 0.45 mile in 20 seconds.

20 seconds is 180th of an hour.

Distance covered in one hour at this speed would therefore be 180 x 0.45 mile or 81 miles in true terms.

Speed is therefore 81 mph. In other words, generally speaking our models go well over true scale speed. We cannot scale down time unfortunately.

On the other hand as Colin suggests, for example my HDML going at a true scale speed appears to be hardly moving and the sailing session becomes rather boring and hence we tend to run our models at usually well over scale speed, not least because the wind and water pressure is the same whatever the scale.

Paul Freshney

Melvin

Extra speed is also handy when you need to get your model rapidly out of the way of errant other models on the water!

Paul.

I once saw an article on the large models of displacement hulls used for film work, and one had a pair of upward-facing jets mounted underwater at the bow to provide a realistic bow wave….  I wonder if detergent released from the rudder might give a more realistic wake…?

Edited By Dodgy Geezer on 29/09/2016 09:21:46

Ash

1-10? You need to give up blowing that horn and get a proper amp, my vox bass went from 0 to Hypersonic and was always turned up to demolition level.

The belting hot summer of 76

Happy days

Paul

I washed and scrubbed down a NS14 sailing boat with a detergent made from oranges, which cleaned the boat very well.

Also repaired the trailer but that's another story!

The boat was a bit frothy to start with and a lovely bubbly effect for about 5 minutes in salt water.

The boat speed was increased, which made the granddaughter and son in law happy.

They finally got they first top three finish in a race.

Harry