"WRH" wrote
You don't need, in fact you shouldn't open the
valve all the way open when running it from a hose.

Which brings up the question, "how much is enough?" I test ran my boat a
week ago and noticed the temp was a few degrees higher than expected. I
opened the valve a little more and the temp gauge settled down to it's
accustomed place. (I use a cheesy little plastic in-line ball valve between
the end of the hose and the earmuff to fine tune.)