Working hard as an aux or on a small tender? I don't think they reach anywhere near max revs when they are pushing a rib along as an aux.
Not exactly what you wanted but I was motoring on my Merc 4hp aux for a while the other day (I use about 3/4 throttle as the rest just seems to add noise rather than speed) and took a very rough observation of fuel use for future reference, it used about 1/2 of the approx 1L tank in about 20 min doing about 4 knots. I reckoned a full tank would give roughly 45min endurance pushing the Vipermax.
Must carry my 12L portable tank more often...