Re: possibiliy of an electric outboard???
1 HP is about 700 watts/hour. So your boat with 115HP outboard is 700Watts/HP*115HP = 80.5KW/Hour. A fully charged standard series 24 battery is about 75 Amp/hr. So, the wattage of this battery is 75 Amp/Hour * 12V = 900 watts(close to 1 KW).
So you will discharge more than 80.5 standard batteries per hour running at full power. Let's see a 24 series battery is about 30 pounds, you will need to carry 80 * 30 = 2400 pounds of batteries for an hour's run time.
Of course LI batteries are lighter per watt than lead/acid batteries, but you get an idea of the wattage necessary to power a boat or car. Todays batteries have come along way from the simple car batteries of my example, but the current beleif is that battery technology is not going to provide much more improvement in weight per watt.