When I posted my earlier reply, I implied that, for an outboard , it was not a legal requirement but common courtesy to cover the prop. (or common sense for that matter) I have two reasons for this assumption
1. When I did my HGV training, I remember being stunned at how long a load protrusion can be before legal marking is required. somone suggested 1-2 metres ?
2. IMO if it was a legal requirement, the prop marker would be stipulated to be of a certain shape and colour & size to explain the hazard nature, but such a marker does not exist AFAIK. Youve probably seen the stipulated ones for wide loads for example these are red and white slashes in a triangle
Now the tricky bit is .. using some sort of overhang rule,.. .. would this be deemed to be from the end of the boat, or would it be from the end of the trailer ?, thats the one to ponder, but ofcourse if you have number plate extension rails that extend the length of your trailer lights and number plate beyond the engine,... surely this would be deemed to be part of the trailer, then the engine, and boat is a load as 'one' and all inside the trailer length ?
he he... good question