Use this Speed Curve to get the children involved and interested. You can work out your speed in knots by timing an object for a known distance such as your boat length.
You can either wait until you sail past a suitable object such as a patch of seaweed, or toss something like an apple core ahead of the bow.
As with timing lights at night, I find that ‘I reckon that’s One Thousand, I reckon that’s two thousand etc…’ pretty much as fast as you can say it works well. Get your watch out and give it a try!
Below is a Free pdf for you to download your own copy.