What you are experiencing is paddle wheel effect or propwalk - similar to torque steer on a car.
heres all about it as its too long for me to explain...
Two engines sort your problem out - as does duoprops (I think invented as it is now by Volvopenta - not 100% sure tho) - but it was originally invented to keep tordepos straight so they hit their target in the water!! - Basically a set of contra-rotating props on one outdrive shaft counteract the prop kick and keep the boat straight...
heres about duprops...
Any boat with one engine and one prop will propwalk - sometimes (in mooring up) you use it to your advantage - other times, when trying to get the stern in its a right bas**rd!
Hope this helps!