Having two props versus one will definitely have a difference but I don't think it is what you are going to notice in the long run. The factors that really matter are prop diameter, pitch, blade shape, depth
, and positioning relative to the rudder
. Outboards by definition don't do a good job of putting the prop low in the water
. In addition, they can be hard to maneuver under power because of a lack of waterflow over the rudder
unless you actually turn the outboard
which adds a level of complexity. Two outboards means twice the running gear
in the water
which will add to drag some and two props so you will double effects related to the hub, tips, etc.
Having two props means having two engines which means twice the maintenance
and significantly more cost. In addition, you will need to find a way to hook up two sets of controls to your satisfaction. Two engines does give you redundancy (without much thrust) but maintenance will become a chore.
On most sailboat hulls, it will be extremely hard to mount twin engines in a satisfactory manner. Many modern transoms have a swim ladder or something in the center which won't allow an outboard
to be mounted on the centerline. With two engines, you will end up with mounting off of centerline by definition. This will make motorsailing only possible with one engine
at most times because the propeller
on the other engine will constantly clear the water. There was recently a thread on one of the sailing forums
where someone tried building a mount for a larger sailboat that would pull the outboard clear of the water and they gave up due to problems with prop depth
It will certainly work for some situations but it might be a real hassle.