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  #11  
Old 05-19-2009
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MaineSail did the point !! This is exactly why it must be mounted facing aft (just look to Volvo install instructions, gensets, etc). Others not flooding the waterlock were just lucky as their intake hoses/valves/siphon didn't reach to the limit. Same caused by insistant temptatives to crank your engine with an open raw intake. Guys, this is known since a long time, and surprises me too ...
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Last edited by negrini; 05-19-2009 at 12:27 PM.
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Hmm ... does anybody have a link to a volvo publication or something that says the scoop is supposed to be mounted slots facing aft? Could help when I talk to boatyard.
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Old 05-19-2009
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Okay, I am not familiar with the Volvo Motor

that the OP mentions, but exactly how does the water flood the engine past the Raw Water Pump? On my boat the water enters the boat through a thru-hull past the shutoff seacock (when open) through a short hose up about 8 inches to the Raw Water strainer, out of the Raw Water Strainer through a hose the drops about 7 inches and goes into my engine mounted Raw Water pump.

My Raw water pump, as many others here, has an impeller that has 6 rubber fins. These fins block the water from flushing through the engine unless they are turning and they don't turn unless the motor is on. So unless the OP's motor and Raw pump are set up differently, I don't see how surfing down a wave or another way of achieving a speed that "scoops" water in, will force water past the stationary impeller.

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Originally Posted by DrB View Post
that the OP mentions, but exactly how does the water flood the engine past the Raw Water Pump? On my boat the water enters the boat through a thru-hull past the shutoff seacock (when open) through a short hose up about 8 inches to the Raw Water strainer, out of the Raw Water Strainer through a hose the drops about 7 inches and goes into my engine mounted Raw Water pump.

My Raw water pump, as many others here, has an impeller that has 6 rubber fins. These fins block the water from flushing through the engine unless they are turning and they don't turn unless the motor is on. So unless the OP's motor and Raw pump are set up differently, I don't see how surfing down a wave or another way of achieving a speed that "scoops" water in, will force water past the stationary impeller.

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Contrary to popular misbeliefs and misconceptions raw water pumps are NOT positive shut off valves and water can and does leak by them.

Just ask the guy at my boat yard who last fall shut his engine off with the raw water intake hose in a 5 gal bucket in the cockpit. Instead of pulling the feeder hose out of the bucket before shutting down the engine he left it in the bucket. Once the engine stopped it began to siphon because the bucket was higher than the siphon break. It siphoned this water, filled the water lift muffler and then when he went to crank his engine it was seized or water locked with water in the cylinders. All this happened THROUGH the raw water pump..

In a perfect situation a raw water pump can stop water flow but not all pumps are perfect, not all impellers are perfect and not all pump bodies or face plates are free from wear..

Never count on a raw water pump as a valve..
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Last edited by Maine Sail; 05-19-2009 at 07:27 PM.
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Well I found the proof I was looking for ... from Nigel Calder's book "Marine Diesel Engines":

"A less common cause of engine flooding is the installation of a scoop-type water inlet facing forward on the outside of a sailboat hull. Anytime the boat is moving at more than a few knots, this generates pressure in the raw-water system. In normal circumstances, the vanes in the rubber impeller raw-water pump will hold this pressure at bay when the engine is not running. But if the vanes get damaged, water will be driven up through the heat exchanger and into the exhaust, where it will build up and flood the engine. Scoops, if used, should be fitted backward."

So that sucks. Going to have to talk to the yard. Based on what everybody said here, it sounds like the install will most likely work OK which is good to know. But like MaineSail and my book says, there is a chance it will not and at $82/hr they should have known better.
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Good extract from the book (I have it too !!). I have an extract from VOLVO parts book (part 26.1-3, cooling water intake chapter 26). This explicity states the strainer must be faced FWD for powerboats only, while must be faced AFT for sailboats. When I installed my MD2040 with SailDrive, a very nice US VOLVO dealer (I can't remember who, but it was in New York) handed me a copy of installation manual for engines, and that was years ago. Again, I'm very surprised your boatyard is not aware of this well known old specification .... send me your e-mail so I can send you the pdf, then you can print and show to them.
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negrini you got a pm
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The only thru-hulls that should have a forward facing scoop on a sail boat is the intakes for the for HVAC or refrigeration, they assist in the pumps keeping their prime.

The yard that fitted your thru-hull for the engine raw water intake is incompetent, it is a rookie mistake and even if a rookie installed it there should have been a final inspection before launching by an experienced person. The Yard should haul and correct the problem without you incurring any costs!

Mainesail is correct, raw water pumps do not block the flow, there are wear items in a pump, the 'cam' and the 'cover plate', even a new impeller will not seal properly if these are worn, add a worn impeller to the mix......

It does not take much wave action or to force water into a forward facing thru-hull, particularly when the boat is heeling and the thru-hull is closer to the water-line and the relative height of the thru-hull and the engine is reduced due to the hulls lateral rotation as it heels.

If you cannot get satisfaction with the yard, until you get this corrected, I would not sail the boat without the thru-hull being closed to prevent the possibility engine damage..
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Today the yard removed the entire thru-hull and reinstalled, reversing the scoop so it now faces aft. I think all is good ... they had to put in a new thru-hull under the scoop because old was destroyed on removal. It looks a little bit different from the other marelon ones ... hard to tell it's covered by the scoop now. They wouldn't put in a non-marelon plastic thru-hull under the waterline would they? I'm paranoid now. I'll need to verify that I suppose before launch tomorrow.
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Pic of install (it's since been painted). The thru-hull under the scoop is the part I'm hoping is marelon ...
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