People don't power sailboats with purely green energy either. It's possible though difficult to put on enough solar to not need to use the engine for power generation (navigation equipment, refrigeration, lights, etc.). You're not going to get enough power for propulsion in port or low wind. You can't coat the decks in solar panels, even if they were more rugged, since you need good traction on deck and an arch can only hold a few panels. Plus the sails cast large shadows. Wind generators don't do much below 15 knots of wind. Tow generators supposedly work well but only while you're moving and they reduce your speed. Even if you could generate enough power, you'd need a monstrous battery bank to be able to accept it. Boats may be more efficient per mile than planes but they're not totally clean.