Outboard Design

To check that were not overloading the hull it is necessary to take a stab at the drive system weight, after all were not building a submarine! To do this I use some stock components and masses working around some basic assumptions on power. The guesstimate uses a Lynch motor based drive package as its very simple, about as light as drives come and at this stage costs nothing!



Drive motor (Lynch type motor)

12 kg

Speed controller and housing (Curtis basic unit)

14 kg

Contractor and throttle pot

2 kg

Battery charger (NG1 Zivan 1 kW unit)

5 kg

Batteries low cost lead acid type 48 volt pack 110 Ah

100 kg

Cables connectors and misc items


Total Guesstimated Mass

153 kg

So the drive system estimate comes to around the same weight as 2 adult passengers, if there around my size anyway. The displacement hull will easily cope with this and more so we can open the design up to different drives and battery packages.

As an engineer I now look to the architectural design and documentation, ADD, and the detailed design documentation, DDD, and this is where we start to drill down in to the project detail. This may seem like a lot of paperwork but this is the stage of the project that is only costing you time not your hard earned money. Its easy to rush out and buy lots of toys but what you really dont want is to be 1/2 way through the project and then ordering a widget because the one you have wont fit and worse still having a hand full of spare widgets come the end.


At the ADD stage the system overview is produced and it is posable to start detailing what components will be in the final system. This starts with a ‘what must be in it’ look to define our drive.

outboard overview

We use our system requirements to check that we have all the components we will need and can then where required break the modules down further.

As this is a simple drive system it could be argued that we are close enough for a architectural design and that were ready to move in to the detailed design document stage.

Keen eyes may have noticed that in our basic system there’s no allowance of any ancillary equipment and we could go back and add this as a requirement if we felt it necessary but as this is intended to be a basic project we’ll not be adding this. What we will add however is a battery charge status monitor, or fuel gauge. So our requirements now read;

  • 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 system will incorporate an energy status indicator
  • The project will be fun (always worth adding as a requirement!)

Next the DDD!


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