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Discussion Starter · #1 · (Edited)
My engine was overheating last year, the input strainer thru-hull was corroded and not letting much water in. So I had the boatyard replace said intake thru-hull today, used to be 1/2" scoop (clamshell) bronze, I upped it to 3/4" marelon (also scoop or clamshell). The OLD thru-hull was mounted with the slots facing BACKWARDS. I assumed the yard would install new 3/4" the same way, but of course they put it in with slots facing FORWARD this time. They're telling me this is how it's supposed to be, and there won't be any issues. Is this right?

I'm worried that with a bigger thru-hull that is now facing forward, the scoop will ram more water in than the 'ol volvo can handle (1980 MD7A 13hp by the way) possibly I dunno flooding the engine with saltwater or something ... ?? Dunno.

Oh and they drilled for the scoop holes & bedded it with 5200 so it's going to be a big deal to change.
 

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Given the speed your boat sails at, I seriously doubt that it will "ram" enough water into the through-hull to cause your engine problems. Yes, IIRC, this is pretty typical on slower boats.
 

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Maybe not a big deal

I haven't seen a lot of thru-hull strainers, but most that I have seen are pointed with the grate aft, but I have seen them like your new one is positioned also.

Having the grate in either direction is not the controlling factor in the amount of water that comes through the grate. Basically the weight of the boat pushing down on the water, the % of open area in the grate, and the size of the hole in the thru-hull dictates the flow of water through the hull.

The thing that controls the amount of water flow through the motor is the water pump, not the thru-hull size or it's orientation. The bigger the water pump and/or the faster it turns, the more flow through the motor. So as long as you are using a water pump that is properly sized for your motor, you should have no issues.

DrB
 

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Discussion Starter · #4 ·
Ok thanks dawg and dr. Good to know it's not a big deal so I won't raise a big stink. Just wish the guy would have asked me before he switched the orientation.
 

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Discussion Starter · #7 · (Edited)
MIne way originally pointed aft. When I changed it out a couple of years ago, I pointed it forward. Didn't seem to make any difference.
Very good to know.

Want to add that this is the first time I've had the boatyard work on my boat. Up until now I've done everything myself, and I was surprised how difficult it was for me to watch the work going on ... made me very nervous. Guess I care more about my boat than I realized. Paying $82/hr for the labor (and seeing 3 mechanics standing around looking at my thru-hull) wasn't helping either ...
 

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:) hehe.

FWIW I turned a strainer to face forward on purpose on my boat - this may make it slightly more succeptible to debris, but on the other hand it probably improves the water intake somewhat.
2 years later, over a 1000 miles of motoring - and no issues so far.
 

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The reason for it

The reason it is intended to face aft on sailboats is so not to flood the engine by forcing water through the raw water intake. With a low enough anti-siphon loop and a good turn of speed down a wave it is entirely possible to "scoop" enough water to force it up over the anti-siphon and begin filling the wet exhaust.

I am very surprised your boat yard would 1) install it this way and 2) after messing up still insist it was correct..?

Hardly seems worth the $82.00 per hour.. Personally I am a non-believer in external grates and much prefer a 1" or larger intake that can be snaked from inside the boat if necessary..
 

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The use of 5200 is probably the more disturbing part. They went for the easy way out. You may want to complain to the yard management about improper installation and improper materials being used. Maybe they'll cut you a deal, although it won't be anywhere near enough to cover the cost of repair when you need to remove the thru-hull and 5200.
 

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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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Discussion Starter · #12 ·
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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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.

DrB
 

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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.

DrB
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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Discussion Starter · #15 · (Edited)
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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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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Discussion Starter · #19 · (Edited)
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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