Overheating only when making sternway

[Cartersailor]

I have beneteau Oceanis 351 which has a YANMAR 3GM30F diesel engine.

My temperature buzzer has been going off at selective times. She is fine once I start up and make headway. The past couple trips out to a small bay 30 minutes out under motor, the temp buzzer goes off when I put in reverse and attempt to move astern. I had to switch off the engine and let cool for 10-15 min and then restart. There is no overheating once she's making headway. However making sternway causes overheating.

I checked the raw water intake seacock and hose to the water filter and they are both clean and allowing water to flow.

Any suggestions would be much apprciated.

Thanks
Ron

[PorFin]

Originally Posted by Cartersailor

My temperature buzzer has been going off at selective times. She is fine once I start up and make headway. The past couple trips out to a small bay 30 minutes out under motor, the temp buzzer goes off when I put in reverse and attempt to move astern. I had to switch off the engine and let cool for 10-15 min and then restart. There is no overheating once she's making headway. However making sternway causes overheating.

I checked the raw water intake seacock and hose to the water filter and they are both clean and allowing water to flow.

Ron,

Given your description, my first guess is an electrical problem in your warning circuit. Have you confirmed that the engine is in fact overheating and not a malfunctioning switch or sending unit? What's the temp guage reading in reverse? Does the temp guage show a change when you shift between fwd/neutra/rev?

How long are you running in reverse?

[Cartersailor]

The temp guage does not have a meter, just a warning light and buzzer so I can't tell if there's a change in temp.

I first noticed the problem some weeks ago. After motoring for about 45min, I got to a bay and put in reverse in order to lay anchor. She usually takes a while to get sternway, so after being in reverse for about 30+ sec, the buzzer goes off. This has happened a couple of times since. It seems to be triggered after the engine has been under strain, other than when making headway.

[PorFin]

Hmmm.

Have you checked to see if there is water coming out of the exhaust while in reverse?

[Minnewaska]

Any bizarre chance that prop wash in reverse is going over the seawater intake thru hull and interfering with pickup. I can't imagine an entire model would have that flaw, however. Was it ever moved? Does it have an external strainer that has been painted and could be further compromised with the prop flow?

Total WAGs.

[mm2187]

Have you checked the tension sea water pump or the impeller itself. I find that when backing down a boat you dont necessarily get into the higher RPMs, but it does seem that if you only were in reverse temporarily it would be a short amount of time to overheat.

[jrd22]

I would guess that it's a prop wash issue. As Porfin suggested, check to see if you are discharging water with the exhaust when in reverse.

[Tempest]

I would troubleshoot the two things Por-fin and Jrd have mentioned 1st

1.) do you have good water flow/ normal flow coming out of the transom when in reverse.

2.) verify, somehow, that the engine is indeed overheating and it's not some kind of electrical issue...trace the alarm wiring..etc.

[Cartersailor]

Today the over heating progressed to not only in reverse but also a couple times while in neutral. I checked the sea water pump motor belt only to fine that it was very slack - about an inch and a half play. I will also replace the impeller.

Thanks for the assistance.

[wescarroll]

Where is you exhaust outlet when backing? If it is under water and your raw water pump is weak you could possibly simply be overpowering the outflow, but I would go along with the propwash as the mostlikely. But a weak intake pump can excaserbate this problem especially if your intake is positioned poorly or even partially clogged. PO had mine facing stearnward, and I had a bit of a problem till I replaced the cam in the freshwater pump and spun the intake foreward.