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Old 25-05-2018, 03:57   #1
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What happens if an engine intake sucks in a plastic bag?

What happens if an engine intake sucks in a plastic bag?

My current boat has two Verado outboards, so I guess I would see an overheat condition on the affected engine, or maybe some kind of cooling water warning. In that case can I just tilt up the engine and pull the plastic bag out, or will it be jammed way up in there and I limp to a boatyard on one engine?

I am planning to get a boat with two Volvo Penta stern drives. What happens with the plastic bag on that type of engine? I believe these engines have their intake protected by a wire screen. So, if I get a bag, then I just stop the engines when I get an engine warning, dive into the water, pull the bag off and start going again, or is it more complicated than this?
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Old 25-05-2018, 05:12   #2
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Re: What happens if an engine intake sucks in a plastic bag?

You might not have to jump in the water to pull the bag off. Just a suggestions but you could try this:
  • Close the seacock for that engine
  • Remove the cap from that engine's strainer
  • Carefully open the seacock. If little or no water bubbles up from the strainer, you've got a clog.
  • Close the seacock again
  • Break out a wet/dry vacuum (always good to have on board, anyway), hook the hose to the exhaust side on the vacuum, attach the other end of the hose to the strainer opening (a modified funnel will help)
  • Start the vacuum, which is now blowing out air, and open the seacock. After a few seconds, you should hear air bubbles against your hull.
  • Shut off the vacuum and see if water now flows into the strainer.
  • Close the seacock, button up the strainer, and reopen the seacock.

It only takes a couple of minutes to try and it may be enough to dislodge anything like a plastic bag that got sucked up against the hull. If it doesn't work, you can always dive it.
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Old 25-05-2018, 08:15   #3
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What happens if an engine intake sucks in a plastic bag?

Youíll never get a plastic bag over a Veradoís intake, never.
You can however get sand in the thing from dragging the motor through sand, itís a good idea to connect the hose frequently and flush the motor, you donít need to start it or use a flush kit or anything else, just connect a garden hose and let it run. You can even do this boat in the water.

Guys, a Verado is Mercuryís very sophisticated Supercharged line of outboards.
However in the one in a million chance that you do turn the motor off, 99% percent chance it will come off on its own, or motor off, reverse the other motor and again 99% chance it comes off. But there are several intakes, on both sides of the foot, so covering them all isnít at all likely.
In the one in a Billion chance it doesnít come off, like you say tilt it up and take it off with your hand.
If memory serves the Verado has several water intakes, to include ones way down low so that it will still get water if you tilt it up higher than you should trying for that last knot of speed. If you have a Merc monitor or other device that connects to the Can Bus, you can see water pressure of the engine and know for sure if there is a plug or not.
Also Iím pretty sure the Verado limits power and maybe even shuts down cylinders on a rotating basis in an overheat situation. You have to try to break the thing, about the only way you can hurt one is by ďbarkingĒ it, that means jumping the boat at full throttle, prop comes out of the water, when it renters the water, itís a heck of a shock through the drivetrain. Of course that will tear the boat itself up too, break the seats loose etc.
It is a very sophisticated engine, in a class of its own in my opinion.

Now the green motors I canít speak for, many of us have a bad taste in our mouths from them, from decades ago.
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Old 25-05-2018, 08:54   #4
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Re: What happens if an engine intake sucks in a plastic bag?

It can and does happen more often than you'd think, kelp is by far the more frequent offender on the west coast, but as mentioned above just turn the engine off and/or tilt it up and it will most likely be gone or fall off in the process. The idea of a bag getting sucked into the intake with the force of a jet engine simply does not happen. Once the obstruction starts to enter the intake on an inboard, loss of suction happens rapidly and unless it gets ripped up somehow will very rarely get sucked into the strainer and then usually only in pieces. With outboards the object tends to hang up and wrap around the leg blocking multiple intakes and once the water flow is reduced the suction is almost non-existent. Just make sure you have good access to your raw water strainer in you planned sterndrive and should only take 30 seconds to remove any obstructions and worse case as mentioned above with the shop vac or use a foot pump from your dingy (if you have one) to blow it off (external hull strainers can cause objects to become trapped)
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Old 25-05-2018, 09:51   #5
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What happens if an engine intake sucks in a plastic bag?

Anytime I have gotten anything wrapped around the foot of an outboard, seaweed, kelp etc., it makes the prop cavitate. Usually just a very short distance in reverse and itís gone.
My dinghy for some reason seems way more prone to it than any of the bigger boats I used to have.
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Old 25-05-2018, 10:45   #6
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Re: What happens if an engine intake sucks in a plastic bag?

plastic bags do effectively stop flow of water in to strainer of inboard diesels. we use em to stop ingress of water while repairing thru hull issues. easy to remove.

for outboards, turn off engine, tilt engine, remove bag and see if you still have function. should be ok unless shear pin or replacement these days thereof is sheared.
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Old 26-05-2018, 10:03   #7
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Re: What happens if an engine intake sucks in a plastic bag?

Quote:
Originally Posted by JohnEasley View Post
You might not have to jump in the water to pull the bag off. Just a suggestions but you could try this:
  • Close the seacock for that engine
  • Remove the cap from that engine's strainer
  • Carefully open the seacock. If little or no water bubbles up from the strainer, you've got a clog.
  • Close the seacock again
  • Break out a wet/dry vacuum (always good to have on board, anyway), hook the hose to the exhaust side on the vacuum, attach the other end of the hose to the strainer opening (a modified funnel will help)
  • Start the vacuum, which is now blowing out air, and open the seacock. After a few seconds, you should hear air bubbles against your hull.
  • Shut off the vacuum and see if water now flows into the strainer.
  • Close the seacock, button up the strainer, and reopen the seacock.

It only takes a couple of minutes to try and it may be enough to dislodge anything like a plastic bag that got sucked up against the hull. If it doesn't work, you can always dive it.
After you remove the cap, pull the strainer basket out and dump the contents, debrirs or even part of the bag could ne in the basket.

M
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Old 26-05-2018, 12:10   #8
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Re: What happens if an engine intake sucks in a plastic bag?

I can only address the question of what happens when a plastic bag stops the flow of water to an inboard engine. The seawater that cools the engine is drawn into the boat through a scoop, which has a row of filter bars. From there the seawater flows to a course filter called a strainer and from the strainer to the impeller pump of the engine. In the case of a hydraulic gearbox there will also be an oil cooler between the strainer and the impeller pump, that oil cooler usually being bolted to the side of the engine. It is almost impossible for a plastic bag to pass through the scoop, and completely impossible for it to pass through the strainer, but when sucked onto the outside of the scoop, it will block the flow of water to the engine, which will wreck the impeller and the exhaust system, because the exhaust system is cooled by that same seawater, when having passed through the heat exchanger, it is injected into the red hot exhaust gas at the mixing elbow, which is where the exhaust manifold would be if the same engine block was being used on a land vehicle. With all of that in mind, it is essential to install an exhaust gas temperature sensor, connected to an audible and visual alarm, either on the mixing elbow or on the exhaust hose immediately below the mixing elbow. When the flow of water stops, the exhaust temperature shoots up, the alarm goes, you shut down the engine immediately and you've saved the exhaust system. You then find the cause of the lack of flow of sea water. Sadly, that is usually a failed impeller rather than the legendary plastic bag. Impellers should be changed annually, not when the fail.


I hope that helps,


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Old 26-05-2018, 16:39   #9
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Re: What happens if an engine intake sucks in a plastic bag?

Undoubtedly a blockage can be catastrophic on a standard marine diesel engine and gear box. The general principal on installations is to have the screened hull intake at a low enough level so that air intake on a rolling or heeled vessel is highly unlikely (this problem led to the loss of a yacht a few years ago north of Cairns).
Another consideration is to use two intakes. The likelihood of both being blocked is extremely remote unless you motor through a kelp bed, but that of course means more cost and maintenance. Temperature alarms are also a great idea and can be purchased and installed reasonably easily. The probe does not necessarily have to be in the raw water flow. Mine is attached to the exhaust resonance box surface and was purchased from the local auto shop along with a gauge and alarm.
Most engines will tolerate (hopefully) a very small time at elevated temperatures, so an alarm would seem to be the best preventative. Many standard engine instrument packs will allow for an audible alarm on the fresh water route.
Another source of overheating can be traced to marine growth in the plumbing from the skin fitting to the primary filter. A reasonable program beside hose replacement, is to back flush the hose with fresh water with some detergent added. Regular maintenance.
The advice given by JohnEasily using the air flow from a vacuum cleaner is good, but it can be done carefully just using your lungs if they are in good condition - depending on intake depth.
Regularly replacing the impeller is also great advice. A small trick is to keep a spare impeller attached nearby the intake pump so that kit can be "found" when needed.
These intake pumps also have other wear and sealing issues which should also be regularly checked.
My own learning experience derives from an event on the Swan river in 1978. Plastic bag, no alarm, mangled impeller, overheating. Just as I was approaching my mooring buoy, thankfully. Could have happened elsewhere with awful consequences.
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Old 26-05-2018, 19:10   #10
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Re: What happens if an engine intake sucks in a plastic bag?

This is one of the reasons I encouraged the strict control of plastics in all environments. We have taken a very lazy and accepting attitude toward this material that appears harmless and flimsy. BS! I have seen very expensive engines destroyed because of water flow blockage. Usually it is a small outboard or the sea strainer on a gas or diesel engine and combined with a negligent, sleeping, and or ignorant captain the results can more than ruin the day. Please make attempts to use other materials when you can, and to remove plastic and harmful materials when you are on your outings. Do it in a proud conspicuous manner and others will copy your behavior and your children and grandchildren will thank a few of us for being thoughtful. Use a potato to stuff into a through hull to block water intrusion. It is biodegradable and the fish will appreciate the chow.

Usually the plastic bag gets wrapped around the prop with the plastic monofilament line on outboards. Sometimes bits of degree get past the strainer to the impeller on outboard engines. Need to know the symptoms and how to correct it at sea or on the lake.
Crusty says practice while at the dock. Sxxx happens!
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Old 27-05-2018, 14:52   #11
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Re: What happens if an engine intake sucks in a plastic bag?

A slight drift from the OP's outboard question, but for inboards another cause of blockage can be jellyfish. Years ago when leaving Trinidad my engine overheated in a swarm (?) of small jellyfish just as I exited the channel onto the north coast. Fortunately the sails were ready and I was in the wind and able to shut down before any damage occurred. The jellyfish was gone by the time I got to Grenada. I am a great believer in audible overheat alarms - more than ever.
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Old 28-05-2018, 06:00   #12
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Re: What happens if an engine intake sucks in a plastic bag?

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Originally Posted by jsc7 View Post
What happens if an engine intake sucks in a plastic bag?

I am planning to get a boat with two Volvo Penta stern drives.
Dont ........ that is a huge step back from the Verados .......... Stern drives are really poor design ..... as the leg stays submerged and it will corrode and give you hassles ..... Unless you can hoist the boat out the water or dry store it ..... I just hate those things :
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