Engine overheats in reverse

"We have a relatively new &#40

"We have a relatively new (120 hours) Volvo D1-20 diesel engine that has worked perfectly since installed.

This weekend on two different occasions when we put the engine into reverse to slow down to grab a mooring pennant the engine overheated within minutes.

Any ideas on why putting the engine briefly into reverse, then back into neutral, would cause the engine to overheat?"

"G'day Phil,
So

"G'day Phil,
Sounds like aeration of raw water when going astern combine this with a u/s pump impeller and the extra heat produce by the engine at low revs after along run.suggest taking vessel for run until all temps & pressures are at normal positions then mimic mooring pickup without going astern, this test should help eliminate one possible problem.
Cheers Shakey."

Thank you for the feedback.

Thank you for the feedback.

Being a novice mechanic I have a few clarifying questions.

1. Are you saying that it is more work for the engine to work at low revs than normal higher cruising revs? This seems counter intuitive.

2. We frequently pickup up a mooring and without going in reverse and have not ever encountered a heat issue.

3. I am not familiar with "u/s" what does this mean?

I appreciate your assistance.

"G'day Phil,
Whe

"G'day Phil,
When an engine is returned to low revs after high load all coolant flows are greatly reduced causing a dramatic increase in work load for the poor old cooling system on some larger engines ancillary pumps over come this problem.if you do not have the problem unless going astern it would appear that water aeration the most likely culprit this can be checked by disconnecting raw water pump outlet and running flow into a bucket whilst checking for air.
u/s =unserviceable.[part knacked]
Also check the water inlet plumbing circuit there may some thing loose or another devil.Cheers Shakey."

"Is this a sail-drive or strai

"Is this a sail-drive or straight shaft.
If a sail-drive, perhaps time to take a dive and check the water inlet on the drive.
When dropping to idle, the 'mini' pump on this engine does not deliver much. Any air or blockage may be enough to cause trouble."