Have you made 100% sure the alignment with the engine
is correct? Use feeler gauges. Being out of alignment can screw a lot of stuff up.
Have you checked the motor
mounts? Is there any vibration from the engine
? Even if you have no alignment problems "normally" a faulty motor
mount can cause the engine to buck and move as you run it and the boat
. These can temporarily throw you out of alignment.
Have you checked to make sure nothing is on the propshaft? Have you checked the cutless bearing? Is the bearing worn to the point the shaft can wobble? How does your prop look? Nothing wrapped around it?
Put a long stick on the shaft while its turning and hold it while having someone idle, move the boat around in forward and reverse and different speeds. Do any sort of unusual vibrations telegraph through the stick to you? Look for anything that could be causing the alignment to go off.
When you shift the transmission
do you shift quickly and firmly into gear
or do you generally use a slow and gradual movement? I've heard the ZF/HBW gearboxes need to be shifted quickly and firmly, not a gradual movement. You want a firm quick "THUNK" into gear
. Also check the linkages and make sure you have them set correctly.
How do you leave the transmission while under sail? Do you shaft lock it or do you let it freewheel? Do you leave it in neutral or reverse? Probably not an issue, but I'd check manufactorer specs and do what they recommend, or perhaps lock the shaft even if it isn't recommended just to see if that makes a difference.
To your original question... The fluid should be clear generally and most ATF is reddish, but perhaps others colors exist. Cloudy usually means it was overheated and burned. Put a magnet into the fluid and stir it around for a while, does it come up with a lot of metallic particulate? I'd also recommend you send it off for analysis as previous poster noted.
My suspicion is that you have an alignment or wobble issue that is taking its toll on your transmissions. I'd hunt for it.