Originally Posted by Nasher
I presume the further you move the boat on the trailer the more accurate the calculation will be.
Basically yes. Actually I think it is really related to how precisely you measure everything in the first place. If you move it 30 cm but only meaure is to the nearest cm you will introduce a bigger error (3%) than if you move 10cm but meaure it (accurately) to the nearest mm (1%). Similarly the more accurately and precisely you can measure the axle - jockey wheel distance the better (should be possible to get this error to be <1% I think). Household scales are probably where most of the error comes... ...logically these should be designed to be most accurate in the 40-100 kg range as that is roughly what most people weigh. Bear in mind that they are not usually very accurate and probably contribute a few percent error. If you move the boat too far back then the weight on the jockey wheel may become very low and out of the optimal range for the scales.
I think you should be able to measure the weight of the boat to something like 5% error or better.
I guess once you start its pretty easy to make a few measurements at different boat positions. And this will give you a good idea of how repeatable it is.