PLASTIC BAG OVERHEAT SYNDROME IS A TERM USED TO DESCRIBE ENGINE OR POWERHEAD DAMAGE ALLEGED TO HAVE BEEN THE RESULT OF A FLOTSAM OBSTRUCTION OF THE SEA WATER COOLING PASSAGE. THE FLOTSAM IN THIS INSTANCE BEING A PLASTIC BAG.

THE OCCURRENCE DESCRIPTIONS VARY CONSIDERABLY IN THE PARTICULARS OF THE UNDERWAY SPEED AND OR SIZE OF THE BAG FOUND. IN COMPARISON TO OTHER REPORTED DAMAGE OCCURRENCES IN THE YEARS 1990 - 1997, THESE REPORTS ARE RARE, ABOUT 1%. THE DOLLAR DAMAGE REPORTEDLY DUE TO THIS SYNDROME EXCEEDS $810,000.00.

AN EXPERIMENT WAS COMPLETED UTILIZING A DEEP V STYLE RUNABOUT WITH AN INBOARD OUTBOARD DRIVE. DIFFERENT SIZED PLASTIC BAGS WERE DEPLOYED UPON THE WATER AND INTENTIONALLY RUN OVER AT DIFFERENT SPEEDS. THE RESULTS WERE RECORDED ON VIDEO TAPE.

THE EXPERIMENT INDICATES THAT ONLY THE SMALL PLASTIC BAG WAS CAUGHT BY THE OUTDRIVE AND THIS OCCURRED ONLY AT IDLE SPEED. ALTHOUGH IT WAS CAUGHT ON THE OUTDRIVE AN OVERHEAT DID NOT OCCUR.

The plastic bag overheat syndrome has been a conundrum in our industry. Our clients often ask us if it is possible to pick up a plastic bag on an outdrive or outboard motor and their policyholder stating that it happened in such and such a way. When the policyholder is asked to produce the corpus delicti, in almost every case it is reported as having been thrown away. These circumstances lead to skepticism and our client has begun asking us if this syndrome is credible, looking upon us as the nirvana of flotsam knowledge.

On first examination, the plausibility of the syndrome comes into question. However the MARPOL treaty specifically refers to the overboard discharge of trash and specifically therein it refers to ‘plastic trash’, with specific procedures for the proper disposal of same. Therefore countries have recognized a problem with plastic trash and as such it is a recognized flotsam problem.

Since plastic bags exist as flotsam we must look at the reports of overheat occurrences to discern if there is a causal link. Our computer records are coded by occurrence type and their accuracy as a statistical model of the real world damage occurrence to boats has been proven.

A sample base consisting of 135 reports within the years 1990 through 1997 was created from our computer records. The total for all occurrences within the sample time frame is 13,500. The reported plastic bag syndrome occurrence is 1% of the total of all reported occurrences. The average repair bill was $6,000.00 per occurrence for a total average reported dollar damage of $810,000.00.

The 135 reports of the policy holders were inspected finding them to indicate that:

In 80% of cases- "I was running along on plane when I saw steam coming from the engine or it stopped running. When I stopped I removed a plastic bag ( all type of sizes) from the outboard, or outdrive.

In 10% of cases- " My engine stopped running and when the marina took it apart they found a piece of plastic in the water pump, or oil cooler, or water passage"

The remaining case synopses were too polarized to group.

In general the policyholder reports described the plastic bags as large ‘garbage or trash’ sized, to medium or small ‘ice bag’ sizes. In a few instances they reported pulling the bags from the water intake or that they were wrapped around the propeller and the drive. In most instances they were operating at planing speeds when the engine quit. A few reported stopping their boat after the engine quit and then seeing a plastic bag floating away from the stern when they went to investigate their problem.

The water intake on an outdrive or an outboard is basically the same. There are a series of holes in the vertical portion of the lower unit housing that may or may not have a screen covering. These holes feed a friction pump driven by a vertical shaft. The pump pushes sea water up to the engine circulation pump on an outdrive or to the powerhead on an outboard. The pump impeller is of rubber construction and this pump degrades rapidly if water is not present as it is lubricated by the water. The vertical portion of the lower unit is aerodynamically shaped to reduce drag in the water and to provide optimal propeller performance.

The plausibility of the syndrome is questionable. Although plastic bag flotsam is a known problem, the occurrence reports indicating high speed operation immediately prior to the overheat are inconsistent with the design criteria of the outdrive housing. The reports of ingestion of the flotsam are also inconsistent with the design criteria of the outdrive housing.

In order to empirically determine the plausibility of the syndrome it was decided to run an experiment using a deep V design runabout (the most popular design in numbers of units) with a Mercruiser Alpha One outdrive installed (the most popular in numbers of units) wherein plastic bags would be intentionally run over in an attempt to induce the plastic bag overheat syndrome.

Since the reports did not define a bag size, nor were there more reports of one size bag than another several size bags would need to be used. The reports did not confirm one speed of operation more than another, except for more reports of the occurrence being discovered at planing speeds. Therefore it was decided to run tests at different speeds. Wave height or direction was not included in the sample data, as it was not reported on by the policyholders. Since it may have an effect, we decided to complete passes over the deployed bags at different compass points in order to obviate its impact, if any. If wave action might have an effect, then operation of other boats and their wake might also, so the test was intentionally run when boat traffic was minimal and sea state was calm.

The test course was a small inland lake’s bay protected to the north by a small island, the east by a peninsula, the south by shore and the west shore with a traffic channel. The seas were flat calm, wind less than 1 mph, haze, temperature 22 C, 72 F. The boat traffic was virtually non-existent, with a few fisherman trolling in the vicinity.

The test course, as defined, was run on an alternating cross pattern of North/South and East/West directions. Each size deployed bag was run over ten (10) times at each operational speed (3), except for the small bag at idle speed which was run over twenty (20) times.

The bags used were retail garbage style bags, as follows:

Large size 2’6" X 2’10" 30 gallon "Trash bags"

Medium 22 ¾ " X 2’ 5 3/8 " 13 gallon "Tall kitchen bags"

Small 22" X 24" 8 gallon "Medium garbage bags"

Each bag was deployed upon the water and then run over at idle speed, medium speed (1100-1500 RPM) and fast (planing) speed (2800-3200 RPM). The small size bags could not be seen at high speed and therefore were not run over at these speeds.

The results, illustrated in table 1, reveal that we were able to catch a small plastic bag at idle speed. The instance did not create an overheat and the bag floated free on its own when the outdrive was lifted to the full tilt up position with the engine off. Since this occurred on the third idle speed pass we increased the number of idle speed passes to twenty (20) in an unsuccessful attempt to duplicate the occurrence.

TABLE 1.

* picked up on third pass, total pass attempts increased to 20 from 10 in an unsuccessful attempt to duplicate

The large garbage bag was sunk on one idle speed pass and had to be replaced with another. The replacement bag suffered cut and scar damage when it was retrieved after the medium speed passes.

The medium bag was shredded during the medium speed passes and most of the remains were retrieved. It was replaced with another bag during this series of tests. The replacement demonstrated no mechanical damage when retrieved after the test. During the medium speed passes it was noted that this size bag had a tendency to fill with water and submerge for a time in the propeller wash. It always floated free after a short time.

The small bag was picked up on the third idle speed pass. It was not located in the propeller wash and after sometime elapsed in searching it was decided to shut down and raise the drive to check for it. During the search interval of approximately four (4) minutes the engine temperature was monitored for fluctuations from its 150 F norm and none were noted. When the drive was lifted the bag floated free and to the starboard side of the vessel. The idle speed pass test was increased in frequency in an attempt to duplicate the event, however the bag was not caught upon the outdrive again.

Because a bag had been caught on the drive a field decision was made to improvise the catching of a bag on the drive in order to determine if changes in operational speed might cause a caught bag to be pulled off the drive.

Since only a small bag had been caught on the drive our test utilized a small bag. Originally we intended to tape the bag to the outdrive but it was decided that the adhesion of the tape to the drive and the bag might obscure the result by artificially holding the bag to the drive. Instead the bag was folded kerchief style (half diamond) and the end of the bag tied in a single knot under the cavitation plate and behind the propeller. In this position the bag covered both sea water intakes completely.

The outdrive was lowered into the running position and the engine started. The boat was idled out from the shallows and brought up to medium speed.

While at idle speed and leaving the shallows the engine temperature began to increase. This occurred within 5 seconds of restarting the engine.

In the process of going from idle speed to medium speed the wake pattern was noted as abnormal, a vibration was noted and an unusually louder engine noise was present, until medium speed rpm was achieved. At this speed all visible and audible abnormalities disappeared.

At this time, 30 seconds into the test, the engine was shut down due to temperatures now at 200 F . The wake and water behind the boat was searched for the bag and it was not found. The drive was lifted, with the engine off, and the bag floated free in a fashion similar to the prior test instance.

The bag was retrieved and found to exhibit mechanical damage to the corners that had been tied in a knot.

The reports of plastic bag overheat syndrome are rare, about 1% of the total reported damage occurrences. The majority of the report descriptions indicated the overheat was discovered while operating at high rates of speed. In other reports pieces of the plastic bag were discovered in the cooling passages or cooling devices.

In the tests the two bags that were damaged did not remain with the drive, get stuck on the drive or propeller, nor did any pieces of the bag enter into the cooling system or any of its parts.

Based upon the tests performed it is concluded that the successful catching of a plastic bag upon an outdrive must include the following factors:

1. Occur at idle speed

2. A plastic bag no larger than 22" X 24"

The idle speed tests with a small bag resulted in a success ratio of 1:20 in catching a bag. However increasing the number of test passes would likely increase the ratio. So it is concluded that in .5% or less of all damage occurrences a plastic bag may be caught upon the drive.

As the caught bag did not create an overheat situation, it cannot be predicated with certainty in how many caught bag instances an overheat may occur.

Since it can be artificially created, as in our supplemental test, a plastic bag overheat syndrome situation can occur. However , as the caught bag occurs at somewhere less than .5% of all damage occurrences, this occurrence percentage is likely less than .025% of all occurrences..

The plausibility of the syndrome has been demonstrated by these experiments. An investigation as to the type and extent of the damage that results from the syndrome’s occurrence is planned. The collection of this data will assist in the diagnosis of this extremely rare failure by adding primary causal failure data to the descriptive factor data already indicated..