Ive seen loads of small ones on youtube etc but mine has to carry a cup of sand across so it has to be more powerful. I was thinking maybe using a thicker peice of rubber or even multple rudders…..any tips gratefull thanks.
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Use multiple rubber bands spinning multiple props
The hull must be stiff, rigid and very strong to take the torsion of winding up the bands. Biggest problem will be the time it takes to unwind as that will restrict the length of the run. To slow down the unwind use a very big propeller as the water resistance will come into play.
Trial and error is the only way in the end.