At South Cerney sailing club we have a 3 digit countdown timer that we use for starting  pursuit races. It works well and can be seen in bright sunlight from over 100 metres. The digits are about 300mm high and 150mm wide and are seven segment displays using LED strips for each segment.

The segments are driven by a bespoke program  on a laptop which uses a USB multi channel relay board and a PSU to power the segments.

Most displays will require a separate PSU as laptops and PC’s can’t supply much power.

We have an outdoor brightness RGB LED starting system at Worser Bay Boating Club , Wellington , New Zealand. 
We are a dinghy sailing club with up to 70 boats starting in 6 fleets each club day.

We use an RGB  LED Matrix Panel mounted in our start box.
This 96x96 matrix ( about 1m x 1m ) is driven by the Race Starting App written by Igor Leonenko of Auckland , New Zealand.

https://starttimerapp.com/

This starting app is frankly supurb. Highly recommended even without the Automations we have added. 
It can be used standalone in an Iphone , Ipad (IOS) or Android, were ia nice voice calls the instructions on what flags and signals to display.

However, we use it on an IPAD in the start box. The screen is bigger for old eyes and a simple stylus gives total onscreen control.

The latest versions of IOS App drives both the Visual Signals (LED via HDMI ) ) and the Sound Signals. ( 12v via Bluetooth )
The LED system is driven by the APP and outputs an HDMI signal to a selectable matrix size. eg  Outdoor Brightness, P10 96x96

The Horn is driven via a BlueTooth signal to a Bluetooth Switch which switches the 12v Horns. 

Our whole starting sequence of 6 starts per race is almost entirely automated meaning only one resource required in start box for starting.
The only non automated task is selecting recalls, and the subesquent restart is then automated.
We use no flags , or other signals in the start box.
The panel is divided into four quadrants.  2 for Class Flags, 1x for Prep ( P,U,Z) and 1x for Recalls.
The "flags" in each quadrant are real presentation of the flags in full colour. they are no symbolic.
The quadrants are equvalent to the 4 flag poles normally used for flag based starts.

The panel glows Orange prior to first warninng and after the last start untill the start line closes. All automated via the APP.

If we want to run a Mark Foy Start ( Pursuit Race ) we simply display a Digital Clock on the LED screen from a PC. 
The software is LEDVISION from ColourLight. ( No IOS equivaents yet ). This is also the configuation software.
The Minute Digits count UP and the boats start when their number changes.
 
The LED Panel is about 1m x1m and hangs inside the start box window and is clearly visible to boats starting out to about 100m.
It was assembled from parts bought from AliExpress.
We also have a small "slave" RGB LED P2.5 96x96 panel inside so that the start box can see what is being displayed on the larger P10 96x96 panel.
The BlueTooth horn switch is availble via the APP website above.

It was built with only very basic expertise. But the kicker was the discovery of Igor Leonenko and his APP.
When we described what we wanted , he had a trial version running within 24 hours.
I estimate the cost was around NZ$1,500. ( ~USD 900 )  
It was built as a "folly" rather than a serious project but it has been working for two seasons and has started hundreds of multistart races with only trivial issues mostly operator error..

If anyone wants a Schematic and a Bill of Materials. Please ask.
Or if you are in Wellington NZ come and have a look.

The system described by Andrew Paterson is exactly what I am looking for, for my sailing club, Norfolk broads yacht club. How can I get hold of your schematic an bill of materials please

My project was very DIY which may make this solution unatrractive. But there is now an almost Plug & Play solution.
Here is a suggested BOM.
1. iPad , with stylus and protective case that will stand it up.
2. HDMI cable iPAD to Sender Box with Charging loop in  the cable
3.  "Sender" Box. Complete with Power Supply.  With compatible Sender Card installed eg "Colorlight S2"
4.  Ethernet Cable 1Gig * Cat 6 long enough to go from iPAD to main LED Display
5.  Stadium Outdoor Waterproof LED Screen Cabinet 96x96 complete with  
18x RGB LED Modules  16x32 pixels  160x320mm  Outdoor Brighness RGB  P10 ( 10mm     2S  ( 1/2Scan) 
and 3x 300W power suppies and a Receiver Card  ( eg ColorLight 5A-75B but others that match the Sender may be fine )