Mystery - Periodic "slippage" at cruise

I just had my stbd trim tab removed - it broke at the hinge and i wanted to be able to use the boat while i'm waiting ~90 days for a new one.

On the way back to my marina from the haul out facility - every 15 minutes or so while running on plane, the boat would suddenly veer about 5 degrees to one side or the other - the way a car feels when when hitting a patch of ice. This also happened on autopilot - it took 5 sec or so for the autopilot to dapen it out and get back to running.

I checked the aft bilge and found one of the nuts had come loose from the threaded rod connecting the two rudders. The rod itself was not loose though. I tightened the nut back up, but then had the issue happen a couple more times.

Any idea what this could be? I was thinking maybe a bubble in the hydraulic steering - but would it have happened both on autopilot and manual steering if so? Could a rudder be slipping? is that a thing?

Odd that it happened right after the haul out and trim tab removal - but i'm thinking coincidence.

steering fluid level hasn't changed in a couple dozen hours of running

i thought about maybe a synchronizer issue, because i did have the sync on. But, i did not hear a change in RPM or see the tachs flutter.

The remaining operational tab was fully stowed (I thought of this too and checked after the first instance)

No active stabilization, exhaust is in the transom.

The tab actuator arms are slightly recessed into a pocket in the transom, but the tabs themselves are mounted flat on the hull. The net effect is that there is slightly less wetted surface on the side with the missing tab - maybe 20” x 1”-2” (pic attached)

I suppose it’s possible that there is some sort of instability created by this, but it seems quite small in relation to the overall wetted surface.

This was cruising on the Indian river (central fl icw) with winds less than 10 kts and no swell or chop - almost lake-like conditions

I generally use the tabs to move the bow down slightly, netting me about 1-1.5 kts at cruise. I generally don't deploy them until i'm up on plane.

Engines are 1200 hp mans. Synchronizer is a mechanical glendinning (don't know the model - circa 1998). Gears are BW165.

Because you guys are asking about the trannies - i do have a gear oil leak on the port transmisssion. i originally thought it was the shaft cover over the bearing housing. But i've had that replaced and resealed. The last people i had out said they noticed the gear oil cooler, which is mounted above, is a non-OEM replacement and leaking onto the cover below. I'm waiting on a quote from them to have that replaced.

So...knowing what you guys are thinking...is it possible i'm temporarily losing gear oil pressure? I don't see how the pressure would collapse like that and suddenly recover.

Also - this happend three times in a 1 hour ride - and in none of the cases (even when i was paying close attention) did i see the tachs flutter or hear the engines "hiccup"

The two things that seemed to be "amiss" are the missing trim tab and nut that was loose on the rudder connector rod. but it happend another time after i tightened that nut.

I’m 99% sure it was the missing trim tab on the stbd side creating an instability in lift at the back of the boat while on plane. Took 6 mo to get it replaced (thanks Bennett) - splashing the boat in about a week and will be able to verify.

Verified - the problem was the missing trim tab all along. With both tabs installed again, the boat runs beautifully, autopilot holds course as it should, no weird squirrelyness