I am considering an electric outboard for my 2.6 m Inflatable tender. Tender weighs around 55 KG with solid fiberglass hull.
I currently use a 3 HP mariner which is getting a bit tired. The 3 HP was a little undersized I think so a 4 would be better. Does anyone have any experience with electric outboards and what size would I need to give the same thrust as a 4 HP 2 stroke. Also what battery size would I need to run the outboard for an average day when moored. In and out to the beach 5 or 6 times.

According to the theory 4HP is equal to 2983W. At 12V you would need 746 amps to get 2983W of power from an electric motor. Because of the difficult technical issues making a small DC motor that runs on 746 amps and heavy cable size the 4HP motors that I have seen run on 48V.

If you did have a 48V motor you could put 4 lead acid batteries in series to get 48V. If they were 150AH batteries you would have a fully charged power capacity of 48V x 150AH which is 7200W (for one hour). With that set up, in theory, you could run the 4HP motor for over 2 hours flat out. (7200 divided by 2983 = 2.41).

If the trip from shore out to boat was say 5 minutes you could do around 12 return trips.

The problem like in electric cars is the mass of batteries to provide the amps. As SO18 mentioned petrol motors have much more grunt (power to weight ratio).

Rant: The manufacturers I just looked at (like Tesla cars) love to say their electric outboards are "emission free" because they conveniently forget about the tons of coal burnt at the power station up the road to generate the 240VAC power to charge the batteries. Like solar panels they aren't emission free the place where the emissions occur is just moved somewhere else. Solar panels transfer emissions from Australia to mostly China. Its silly to see greenies (and I am one) lauding solar panels and complaining about pollution from China when heaps of that pollution came from smelting the materials to make their wonderful solar panels in the first place!

I doubt whether you can compare electric to petrol by Hp ratings. A 4 hp electric motor is a fair size and weight. It's more about torque. Check out the electric motors in the Le mans race cars, they are smaller than the petrol engine in output but have the same performance.

http://www.draysonracingtechnologies.com/projects/B12/project_article_B12.html

As a side note I was reading an article today from the Edinburgh uni where they carried out a study and found 85-90% of the airborne pollution from vehicles was from tyres and brake wear. Electric cars because they are much heavier have just as much tyre and brake wear before you even think about burning coal etc.

Well that is a lot more complicated than i thought.
I think i will stick to a 4 HP four stroke.
I thought one of the electric outboards used for trolling in a tinnie might of done the job, but the hassle of carrying and keeping a battery charged is probably more trouble than it is worth.

I was in BCF today. The electric outboards are far more expensive than 4 hp two strokes, before you even think about a battery.