This is showing the time remaining (by counting down from 5 minutes) before the start of the race heat.

The clock helps the racers time their crossing of the starting line, between two buoys, in this case.

I remember the old days of watching the hydro races on the Niagara river, they had a black clock without numbers,as the time ticked down, it would change to orange, when the whole clocked turned orange, that signaled the start. I guess digital clocks have replaced it now.

I remember the sweep hand clock, too.

First time seeing this one for me.

You do not want to cross the start line before the clock goes to zero.

I crewed in my first sailboat race recently. They used a series of horn blasts. I can't recall the exact pattern, but something like one w/ 5 mins to go so you could synch your watch, then 1 w/ 1 minute to go and then the final horn. No visual clock. I guess they often use flags as well on a starter boat, but this race was a little under manned...fun though. We had a storm blow up at the end which made things interesting.

Did you have fun?

We were dead last but since I hadn't had a chance to sail in quite a while, and had never participated in a race, yes I had fun! The storm made things exciting at the end too, got to use a little sailing seamanship,...tho fortunately not as exciting as for the kiteboarding fellow in the thread I'm about to post.