The outboard needs cooling water as well as a load on the prop. A OB owner
or mechanic can't properly test or adjust an outboard without running it in
water.

Those clamp on muffs will provide cooling water but leave a hell of a mess
around the test stand. The smaller or dingy outboards don't receive the
muffs very well..

It is also strongly recommended after operation in salt water to flush the
system and stop corrosion.

I keep a plastic barrel at home to run my dingy outboard between uses.

Steve
s/v Good Intentions

The outboard needs cooling water as well as a load on the prop

a prop in a barrel doesn't put a good load on the engine, though it does put
some load on it. There used to be -- and I assume still are -- special test
props to load up an engine in a tank.

But yeah, the engine has to have water to run. Running them out of water for
even a short time can ruin the rubber water pump impeller.