I don't think I've ever seen a vertical one on a sailboat. All the ones I've ever had were horizontal. And they've all worked fine. I am not aware of any functional difference.
The one we have now is actually amazingly good. It's so good that it creates a problem with fresh water consumption
-- it creates the illusion that you are on land and can take a land-type shower
. It only holds 50 liters, but due to very high temperature of the water and very good insulation
of the calorifier
, it seems like much more, enough for four or five people to take hot showers in the morning. The near-boiling water is mixed with cold water to make the "hot water" which circulates in the boat, which extends the volume of hot water compared to the volume of the tank.
Besides the electrical heating
element, it has two separate coils in it, one for the engine
and one for the Webasto hydronic heater. That's a feature I would definitely recommend if you have a hydronic heating system.