I think there are more than one. This is what my 6LY 370 had:
Two on the main heat exchanger on the front end and another one on the back.
One (but sometimes none as this is a US sourced exchanger and it varies with the supplier) on the tranny cooler.
One on after cooler.
One on the lube oil cooler underneath and forward.
I have had two boats with that engine, one a 2006 MS 34T and the other a 2003 MS Pilot 34. Both engines were virtually identical except that the tranny cooler on the Pilot 34 didn't have a zinc.
I have attached a pic showing the two zincs on the front of the main heat exchanger, just above the horizontal V.
My engine is a 2010 and I had one Yanmar mechanic insist there’s only 1 on the whole rig located on the main heat exchanger. Thank you for the info !!!
I have a 2008 W Yan 6LYA and agree with David's info.
My tranny cooler has an anode.
I find the oil cooler anode erodes faster than others and have started to use a longer anode rod there which seems to help.
I an in fresh water and switched to all alum anodes.
My engine is a 2010 and I had one Yanmar mechanic insist there’s only 1 on the whole rig located on the main heat exchanger. Thank you for the info !!!
John
Check your Yanmar engine manual. If you don't have one you should be able to find it online. If unable to locate one check BoatDiesel.Com.
In order to download you will need to join but a search to see what is available can be done w/o a membership.
I have a 2001 model. Mine has 3 in the heat exchanger. Two forward as in the above pic and one towards the back of it. Than one for the oil cooler and one for the fuel cooler.
Total of 5 just for the engine. Than my transmission/twin disk has two.
I looked up the Yanmar 6LY3-UTP- https://www.yanmar.com/marine/product/engines/6ly3-utp/ and not surprisingly the port side of the engine where the main heat exchanger is located doesn't look anything like the 6LY 370 we have been talking about.
The starboard side looks similar so I have uploaded a photo that shows the three locations that may have zincs: tranny cooler, after cooler and lube oil cooler that are shown in the starboard side pic.
To find the locations (if any) of the zincs on the main heat exchanger, I would just get a wrench and unscrew any caps that look like they might be zincs and check them out.
Your mechanic may actually be right about one zinc and I bet it’s the one on the lube oil cooler.
Looking carefully at the stbd side pic, the arrow points to a blank boss where on the 6LY 370 is where a zinc cap is. No zinc on the tranny cooler and there may be none on the main heat exchanger.
Yanmar may have redesigned the RW system to let it all drain down when the engine stops except for the lube oil cooler. Leaving RW in 24/7 causes most of the corrosion and with a redesigned system, they decided to live with any corrosion while running.