Your pictures say a thousand words. for what ever reason, you have engine versus load problems. Try taking the load off and running the engine .(wack it in neutral) I am sure that you will see as the engine builds up revs that the"smoke " will clear. All of the above
advice may be the problem. At the worst it may even be shaft binding (unlikely) In reverse the load on a lot of props is way less so may not show the symptoms. To much
fuel is a product of to much load. This may be because your engine is stuffed ! The injectors or
pump are the same.....the prop (or your
hull ) is fouled....there is friction in the drive train (have you free wheeled the gear box by hand) If I am right it will happen more at low revs ...call it a chug...the engine is reluctant to come up to full....(at the full range of the govener) . I (being a poor bugger) have lived a life with old diesels. The beauty of them is that you can eliminate the problems one by one.
1. prop....no foul...no damage
2. shaft...easy turn
3.
boat...no excess crap
4.
gearbox can you turn it by hand.
5. Its your engine !! START AGAIN
1. check your fuel...clean and no sludge...how old
2. check your filters air and fuel.
3. does it start easily IN COLD
WEATHER
4. when it starts does it blow smoke on start up , and for how long
5. well there is no 5 ...you are down to engine stuff...
Raw water cooled ? you may have rusted a valve....or a million other possibility's. sorry...