I am considering adding an engine mount to the transom of one of my catamaran's hulls so I can move the electric outboard that is on my tender and get added propulsion when trying to sail to windward (above 90 degrees from the wind direction). Due to the nature of the design of my catamaran (and pretty much all catamarans), its performance is poor when trying to make my way to a windward (upwind) destination under sail, particularly out in the ocean where the waves are also pushing the boat away from where I'm trying to go. I generally run one of my diesel engines at around 2200 rpm (providing roughly 10 hp of propulsion). I obviously could continue to do that, but have upgraded the boat with a considerable number of solar panels (4 kW) and a large lithium battery bank (45 kWh) (to enable me to have air conditioning without having to run a generator) and it occurred to me that my 10 hp electric outboard might be used to provide the added propulsion that my diesel was providing. This would be preferable to me because the diesel noise and exhaust is annoying and I'd like to minimize visits to filling stations in foreign ports where the fuel quality is questionable.
Before I drill holes in the transom of my catamaran to add the engine mount, it occurred to me that I should ask someone knowledgeable about physics and electric engines whether this will work or whether I'll be wasting a lot of precious energy created by my solar panels for little or no added propulsion. I'm guessing if there is no wind I'll get the full benefit of what 10 hp can do to move the boat (very little, maybe 3 knots of speed). I also know from experience sailing a fast trimaran that if the boat's gas outboard is pushing the boat and then the sails take over with the engine still running, the engine revs much higher due to the reduced load and doesn't do anything to increase the speed of the boat. But that's on a fast trimaran (much lighter and MUCH faster than my catamaran). My catamaran will sail close hauled (45-55 degrees from the angle of the wind) at about 5 knots in 12-15 knots of wind. The trimaran, however, will sail that same angle to wind direction at roughly double that speed under those same conditions. The last piece of information I'll put out there that may or may not be relevant is that when I'm running my catamaran's diesel at 2200 rpms (again, providing roughly 10 hp), it does provide an extra 1-1.5 knots of speed and doesn't rev higher. I'm guessing that may be the answer to my question, but that's a considerably larger engine with a much, much larger prop, so it's not exactly apples to apples. Any input whether putting my electric outboard on a mount on my catamaran's transom will provide roughly that 1-1.5 extra knots of speed that running a diesel engine provides when sailing along at roughly 5 knots would be greatly appreciated. If not, will it do anything in terms of added propulsion or will it just rev higher, like the outboard on the trimaran, doing nothing but wasting energy?