20 is not a 20hp engine
. It rated 16hp continuous and 18hp for one hour.
If it is correctly propped, it should get you to hull speed
(about 6.9 knots) pretty easily, but will struggle to exceed that in almost all conditions.
By comparison, the 2GM
(13hp) in my 28' semi-long keel
will get up to about 6.5 knots in calm conditions with clean prop and hull
. At that point it will be using about 1.5 litres per hour. At 5 knots, it will use around 1 litre per hour. It runs well abut is around 30 years old, so probably not putting out 100%.
Pushing into wind
and/or waves, consumption
anyone's guess, but certainly will use more for a given speed.
Hope this helps.