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Saildrive Raw Water Flow Causing Flooded Engine.


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I heard recently of an issue with SD units on Leopard Catamarans, I thought some might find it interesting. The story goes that  in certain conditions water pressure impinging on the Sail Drive raw water intake can cause flow through the system and into the exhaust without the engine running. There is a blog on this in the cruising forums somewhere and one person even disassembled the plumbing and witnessed it while sailing. Apparently If the raw water plumbing does not have an anti siphon loop the exhaust can fill with water and flood the engine, if the boat is sufficiently heeled, has anyone else heard of this or experienced it ?

 

 

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1 hour ago, Frank said:

I heard recently of an issue with SD units on Leopard Catamarans, I thought some might find it interesting. The story goes that  in certain conditions water pressure impinging on the Sail Drive raw water intake can cause flow through the system and into the exhaust without the engine running. There is a blog on this in the cruising forums somewhere and one person even disassembled the plumbing and witnessed it while sailing. Apparently If the raw water plumbing does not have an anti siphon loop the exhaust can fill with water and flood the engine, if the boat is sufficiently heeled, has anyone else heard of this or experienced it ?

For our Volvopenta D1-30 it was in documented in multiple places through the installation manual.

We ended up making a custom stainless exhaust riser to reduce the siphoning risk because we couldn't get the anti-siphon loop high enough due to the cockpit floor.  Most photos/videos I have seen of that Engine have an anti-siphon loop in the intake.

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only seen this on a multi that went regularly over 30 knots and his impeller was worn out, not sure how an anti siphon loop is meant to help that situation though. I have seen on a new big Leopard a terrible exhaust installation where as soon as she was sailing (or motoring ) the exhaust was underwater and without a proper riser had sunk the engines whilst under warranty, in this case exhaust anti syphon loops would have helped but she needed a full redesign of the exhaust to stop it happening again, which I guarantee has happened as he was heading back to Aussie. I tried to explain this to the owner but he was hell bent on chasing Leopard rather than actually fixing his yacht.

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Certainly need the anti siphon loop and need to make sure the duckbill is not clogged up. This should really be checked every oil change. Overall the engineering quality on leopards seems or have taken quite a nose dive in the newer models.

 

other issue isn’t likely cruising load as I’m sure at boatshow loading the exhaust elbow I well above the waterline and often a vented loop is not fitted when the assumption is it will always be above the WL. Some of the older Yanmar owners manuals have some very good exhaust schematics 

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Anti siphon loop won't stop it. We have electric shut offs between the saildrive & the raw water pump that close & stop raw water flow when the key is off. 

It's not siphoning, its pressure pushing the water past the impeller, and filling the engine. Slow cruising cats won't do it, but it seems that somewhere above 12-15 knots sailing there's just too much water pressure and you end up hydraulicing engines. We did it several times before we fitted the shut offs, luckily Yanmars seem to love salt water. Not very pleasant though in a seaway removing injectors, cranking things over, raw diesel in your face etc. etc.

 

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8 minutes ago, Bad Kitty said:

Anti siphon loop won't stop it. We have electric shut offs between the saildrive & the raw water pump that close & stop raw water flow when the key is off. 

An exhaust riser also solves it. 

Are your shutoffs wired to be closed when power is present? I would be worried about these closing unintentionally due to a power failure. 

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5 minutes ago, CarpeDiem said:

An exhaust riser also solves it. 

Are your shutoffs wired to be closed when power is present? I would be worried about these closing unintentionally due to a power failure. 

I'm more worried about engines full of salt water every time we sail over 15 knots to be honest. Which is a lot.

In 15 years & 30,000miles we've had exactly zero power failures to the ignition circuit. Which doesn't mean it won't happen, but you can only have so many redundancies & fail safes built in?

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4 minutes ago, Bad Kitty said:

I don't think an exhaust riser will stop it. Why will an exhaust riser stop water pressure filling the exhaust manifold & flowing into the engine?

It's coming in the raw water side, not the exhaust side.

Gravity stops the water going up the riser.  The water is pushed out the exhaust pipe. Because the exhaust riser is above the exhaust and inlet system. 

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