Scale speed

[melvin hallParticipant @melvinhall89458]

Hi,

I was wondering if there was a way to work out the scale speed of a model ship?

Melvin.

[melvin hallParticipant @melvinhall89458]

how to work out the scale speed of a model

[Colin BishopModerator @colinbishop34627]

Melvin,

The usual method is to apply the formula that true scale speed equals the square root of S x full size speed where S is the scale.

For example a 1:100 scale model frigate where the full size ship could do 30 knots would be:

1/10 x 30 = 3 knots (around brisk walking pace.)

In practice many people like their models to go a bit faster though!

Colin

[Dodgy Geezer 1Participant @dodgygeezer1]

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.

[Colin BishopModerator @colinbishop34627]

Yes, hence my comment about people preferring to go a bit faster – it looks better. Plus, at a nominal scale speed a small scale warship or similar will struggle to make headway in choppy conditions which, if scaled up, equate to mountainous seas.

Colin

[Paul FreshneyParticipant @paulfreshney24971]

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 hallParticipant @melvinhall89458]

Hi,

Thanks for the advice, i will try and work out if my model is as fast as it should be.

I recently installed slow revving direct drive motors as replacements for the reduction geared motors, one of which the gearbox failed. The model is nice and quiet but a bit slow, the shafts are 5mm so am planning to try fitting thrust bearings to try and get the speed up.

Its not really a problem but as has been mentioned when there's wind or when the props get fouled with weeds its good to have a bit extra speed when needed.

Melvin.

[Paul FreshneyParticipant @paulfreshney24971]

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.

[ashley needhamParticipant @ashleyneedham69188]

Melvin. I would have thought that almost any model boat is much too fast compared to the original, and I agree with DG that the scale speed is the speed on the water at which the boat looks its best.

​BUT even then, it is not actually correct as the foam from the water breaking across the bow will be missing….much the same as the churned up bubbly wake of the mighty propellers pushing heroic quantities of water out the back to power the vessel along will be missing.

This is a bit of a contentious issue, much like the lubrication (or not and by what means) of prop shafts.

My view is that you need more than "just enough" power to cater for emergencies and non-scale adverse weather (as stated). We have modern ESC units to control the speed, you do not have to go everywhere with the stick turned up to 11.

Ashley

(sorry musicians joke, guitar amplifiers have volume control knobs marked 1-10, if you want it REALLY loud you turn it to 11)

[Dodgy Geezer 1Participant @dodgygeezer1]

Posted by ashley needham on 29/09/2016 09:01:58:

….
​BUT even then, it is not actually correct as the foam from the water breaking across the bow will be missing….much the same as the churned up bubbly wake of the mighty propellers pushing heroic quantities of water out the back to power the vessel along will be missing.
…..

Ashley

 

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

[ashley needhamParticipant @ashleyneedham69188]

I have often wondered what it would take to create the right froth at the front and rear. perhaps some air blown out of a row of holes?

I recon detergent would give a great result for 5 mins or so after which the lake would covered in froth…?

Ashley

[Paul TParticipant @pault84577]

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

[Malcolm FraryParticipant @malcolmfrary95515]

When it looks right, (realistic shape of wake waves) it is. If the numbers are then worked it is surprising how often the results agree with Colin's formula.

Any displacement hull has its own maximum speed – exceeding this drains power much faster, just like the real thing which is what should be expected from a scale model.

The water bubbles are water bubble sized, governed by the surface tension of the water. If a proper creamy wake is wanted, "you gonna need a bigger boat". The bubbles will be the same size, just more of them and smaller in relation to the boat.

[harry smith 1Participant @harrysmith1]

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

[Author]

Posts