Electric Outboard

Ok the first guide to conversion will be what I consider to be the most simple!

Living near a river and having a passion for the water led to the reasonably natural idea of having an electric boat to troll up the local estuaries.

My 1st stop was to a boat building friend to see what type of hull I would need and to get a guide on the power required to push it along. Andy, from Ecocats, offered us the loan of a 4m displacement hull that would easily accommodate the battery load and would push through the water with around 10Hp of effort.

The next task was to decide on the method of drive. As the hull was designed to accommodate an out board, and as it was on loan, we decided not to fit shafts for an inboard but to go with the outboard fitting.

The donor outboard was scavenged from a local marine engineering company who had a selection of outboards that were ‘beyond economical repair’ in the motive department and we acquired 2 drive legs for the princely sum of £25! We selected 2 lags based on original motors of 10Hp and 25Hp to give us some scope for a larger electric drive.

The motors on a conventional outboard drive leg are easily removed from the leg and generally couple to the drive shaft through a spline to a hardened shaft. They come in 2 flavours, 2 stroke and four stroke and generally run at speeds around 1000 rpm to 6000 rpm. the anticipated trolling speeds for the motor will be around 2000 rpm but this is all based on an educated guess and some guidance from Andy.

The next step is to get down some electric drive requirements, getting down your requirements at the start of a project is key and worth some effort, after all if you don’t know what you want how can you achieve it!

Our requirements were;

  • A drive must be capable of producing a nominal 10Hp
  • The drive motor must run at speeds from 0 to 4000 rpm
  • The drive must not run faster than 6000 rpm
  • The drive must couple to a hardened steel shaft
  • The drive must be able to cope with the local environment (ie high humidity and the occasional splash)
  • The drive must be self contained but may be modular
  • The drive must be capable of being installed and removed manually
  • Drive components must not exceed 30kg each
  • The drive voltage will be at a safe, no shock risk, level
  • The drive will be capable of running for a minimum of 2 hours at 25% load
  • The project will be fun (always worth adding as a requirement!)



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