Engine overheats in reverse

[J&P]

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?

[Neil Gordon]

How does the engine know if the transmission is forward or reverse?

Perhaps there's growth on the raw water intake that's acting like a check valve... streaming astern under normal conditions but flapping forward and blocking the intake when the prop reverses.

[Kevin Kaldenbach]

Perhaps the prop is cavitating when in reverse and the raw water intake is drawing in this air? Or maybe the load on the engine when using reverse to slow down the boat is too great?

[Ron M.]

Yes, Neil has a point. Somehow raw water is not getting to your heat exchanger. You may have some growth or blockage. Maybe in reverse the props cavitation is causing too many air bubbles to pass your intake. Do you have a strainer that has the angled scoop on the outside of the hull ?
Did you install a vented loop,(anti siphon) in your intake before the heat exchanger ? perhaps it's plugged or restricting intake.
I have the same engine,( fewer hours) and don't have a problem in reverse. The PYS shaft seal does burp more water from the vent nipple in reverse........but that's another story.
Let us know what you find out.
________
Honda Aircraft Company

[Neil Gordon]

Or maybe the load on the engine when using reverse to slow down the boat is too great?

Pretty much the harder you work them, the happier they are.

I would suggest trying to replicate the conditions that caused the overheating while at the same time watching the exhaust for cooling water. Try it at varying boat speeds, including holding speed at zero. (Don't be surprised if you can't ever make it overheat again.)

[J&P]

Thank you for the feedback.

I have a little more information to share after reading the comments and discussing with my wife who was at the helm this weekend.

We have used the reverse on numerous occasions to help set the anchor without issue. In addition earlier this year I practiced reversing around our marina again without incident. In these circumstances reverse was engaged in a controlled manner.

The two times we encountered overheating the driver used reverse in an attempt to slow us down quickly for fear of over running our mooring.

After hearing that aeration may be the culprit I am thinking that the aeration created by using reverse (at high RPMs) to slow the boat down is different than the aeration created when the boat is already stopped (anchor setting) or moving backwards freely (and likely at lower RPMs).

But I am still surprised that the engine overheating could happen that quickly. Best guess is that reverse was only used for approximately 10 seconds in each case to slow us down.

Note: I did swim under and did not see any growth that would restrict waterflow and am unaware of any loop venting.

[J D]

in just 10 seconds? how do you verify that it is really overheated and not a fluke thing with the alarm?

[Boyd]

I suggest checking the following:

Raw water strainer and intake
Raw water pump impellor
Open up the ends of the heat exchanger and look for obstructions, pieces of zinc, impellor blades, etc.
Verify the thermostat is opening correctly
Coolant level

In reverse the transmission ratio is usually different than in forward. In my case its 2.05 to 1 forward and 1.86 to1 reverse, allowing the engine to rev faster and higher in reverse. This alone may account for quick overheating if your in a marginal overheat situation to start with. I think you just have to diagnose a run of the mill overheating situation, nothing complicated like cavitation.

Harbor freight has those lazer non contact thermometers really cheap. If you monitor the temp at the thermostat fitting on the engine vs the temp of the heat exchanger when under load that will tell you more about what is really happening.

Boyd
s/v Tern
CD30 MkII
Fort Lauderdale, Fla.