Hydraulic Lock - Solutions?

[awol]

I posted about my 3YM20 running on 2 cylinders a while ago seeking advice on whether continued use was wise. Well, after 2 months of apparently trouble-free running - the diesel feed to the zero compression cylinder had been rerouted to the tank return - I lifted the engine and found an explanation!


My assumption for the cause is hydraulic lock due to sea water ingress through an open exhaust valve on No3 cylinder. The problem arose (I think - 'cos I heard nothing) after an enthusiastic spinnaker run from the Ross of Mull and up the Sound of Kerrera to Oban (some 30nm) with a following sea. Why this was a problem on a 42 year old boat that has had the same exhaust system with originally a 2GM and now a 3YM20 and has sailed many thousands of miles I don't fully understand but I certainly want to prevent it happening again.
Solutions -

• a high rise exhaust mixer - there is no room below the cockpit floor
• a larger waterlock - the one fitted is (I'm guessing) about 5 litres - at least the engine is out but the space is tight.
• a flap valve exhaust exit - exit is under the counter - would this work?
• a 50mm plastic ball valve to prevent in-flow but would need discipline to close and open it - would trying to start the engine with the inlet cock open and this closed cause the backfill problem I'm trying to solve?

As to the engine solution - there is some scarring at the bottom of No3 cylinder which I intend to dress, hone and ignore - ring gaps well clear. The crank journal appears sound. Rod and pistons are available for a 3TNV70 engine used on John Deere excavators etc. with the same part numbers as the 3YM20 but considerable cheaper. Liquid metal will sort the ding/crack in the sump. In all about £200 for bits. A full strip down and rebore is considerably more as is a new engine. Thus I'm tempted to go for the simple cheap solution to the engine and won't have lost much if it doesn't last.

I now await your verdict on why it will all go wrong!

 

[Tranona]

Do you not have an anti syphon valve in the raw water system? If you don't that is probably your answer. 5 litres is fine for the waterlock. Standard size for an exhaust system of up to 50mm diameter and "normal" length. Do you have a loop or swan neck just before the transom. If not then it is possible that water could have been forced up the exhaust from following waves, but a loop above the waterline should prevent this. The flap would help, but that is primarily used on large powerboats that have straight through exhaust - that is no waterlock or loop.

youtube.com/watch?v=X-GbaV0IALg This is the Vetus video on exhaust system which tells you all you need to know. Just after 10 minutes is the section on anti syphon and a diagram showing the key dimensions and components for a safe system around 22. You may have to take your antisyphon outside the engine bay perhaps into the quarter berth to get it above the waterline. Alternatively you could have an open vent peeing water into the cockpit. I expect getting a loop of the exhaust above the waterline inside the transom may be a challenge given the skinny nature of the stern of your boat, but sure other owners have found a solution. If the Yanmar proves uneconomic to repair a Beta 20 or 25 are good alternatives. Hopefully, though that won't be necessary.

 

[bikedaft]

I fitted a seacock on the exhaust, within a cockpit locker (for ease of access). I also drain the exhaust (garden hose from bottom of vetus waterlock to an easy to reach spot in the bilge).

We also do not have space for a high riser mixer.

Hydraulic'd twice, more then 10 years ago now, no problems since 3GM30F. Was only the 2nd time that I realised what had happened.