gday para h,
i think you need a g/box , due to the drag+size of your hull+load caused by your prop , 75 mm ??
if you have being advised by some-one else who sold you the motor , or has used that motor to drive your prop , ignore me , because i should be wroung .
BUT my experience w motors out here in OZ, and i do experiment w a lot of different motors , tells me that there is a big increase in load w a large prop , motors dont like to be held back , and if they can`t spin up to speed , they burn out . a heater fan motor is only happy w approx. a 50/60 mm prop , direct drive , but w 3:1 g/box , will drive 100mm+ prop .
graupner 900 motor w approx. 2:1 gears , drives only a 65 mm prop in they ZED drive unit , thats w gears+65 mm prop , there are some motors out there ,that only rev slowly , and will turn big props direct , but not many
i think , just by looking at your motor , but without all the facts , motor specs , i would be fitting a t/belt home made g/box+3:1/4:1 ratio
we are very limited on oZ for motors , but im drawing up plans for a 1100mm+ tug , narrow beam ,w 90/100mm prop , and it will be driven by 700 jaycar motor via 4:1 g/box , and it should chew stumps all day !!another mob uses brushless motors , w 2.5:1 reduction , to 90 mm props , notice g/box !!
i hope i havent sound big headed,all confused you more , but thats my humble opinion, if you want more info , just ask -bill s