FLEX shaft all the way...in R/C boats you need change for conditions...moving the prop angle is impossible on straight drive to the prop. Flex allows you to be messy on the alignment part too, but I recomend you not be...it stills has to go around. No real loss in power to prop if you get a good flex shaft.....properly match to a good shaft tube...no flop inside a tube that is to big...loss of power then...

Happy Racing....ZooM

OH YA!!! drive shaft U-Joints are for cars not boats.... Dont put car parts in your R/C...powerboat