Best Ethanol Fuel Treatment for Boat Engines

None of the five products tested were capable of preventing phase separation, one of the most troublesome problems associated with ethanol gas.


[Updated December 6, 2018]

Since the introduction of ethanol fuels at dockside pumps, weve had a slew of reader inquires about the effects of E-10 on fuel lines, fuel tanks, two-stroke engines, and four-stroke engines. We recently tested some products designed to address these issues, specifically those that claim to combat the problems associated with phase separation in E-10. Practical Sailor tested: E-Zorb from Marine Development Research Corp. (MDR), Sta-bil Marine Formula Ethanol Treatment from Gold Eagle, PRI-G from Power Research Inc., Star Tron from Star brite, and Techron from Chevron. The claims of each product varied, but the test products fell into one of these general categories: conventional fuel storage additives, ethanol fuel storage additives, and restorative gas additives. Our tests looked at whether the additive itself would leave ash deposits, gum deposits, or residue that might cause contaminated lubrication oil; whether an additive had a tendency to emulsify, suspend, or absorb free water; and whether the additives could delay the onset of cooling-induced phase separation.

In the January 2007 issue, Practical Sailor looked at some of the problems caused by the introduction of 10-percent ethanol (E-10) blends at dockside pumps. Two of the most serious concerns were: The fuel acts as a powerful solvent, even dissolving some fiberglass tanks; and phase separation, when water in the fuel separates out of the fuel mix and drops to the bottom of the tank, where it can be sucked into the pickup feed or promote corrosion within the tank.

Not surprisingly, a variety of additives claiming to cure or prevent the E-10 ailments are popping up in marine stores. As with many of the existing fuel additives, the claims of these newer product are often difficult to prove. In fact, Practical Sailor was not able to confirm one of the key claims (either inferred or explicit) in most ethanol additives: They will prevent phase separation. But before we delve into these E-10 “miracle cures” and Practical Sailors recent tests, a bit of background is in order.

Ethanol Gas Additives

Photos by Drew Frye

Phase Separation in Your Boat’s Fuel Tank

For many years, methyl tert-butyl ether (MTBE) was the oxygenate of choice for reformulated gasoline. MTBE was very soluble in gasoline, forming a stable blend with physical properties consistent with conventional gasoline. MTBE was also found to contaminate groundwater. For this reason, it was removed from the nations reformulated gasoline pool.

Switching to E-10 brought some unintended consequences, not the least of which was the process called cooling-induced phase separation, a process by which typically 1 to 3 percent and potentially 8 percent of the fuel can precipitate to the bottom of the tank. Heres how it works:

Ethanol is soluble in gasoline only if the alcohol is very dry. Unfortunately, the mixture is hygroscopic, drawing water from the air. In addition, the blend can become unstable when the temperature drops with the change of seasons. During the day, as the temperature rises, ethanols vapor pressure increases markedly. In fact, E-10 begins to boil at a mere 95 to 105 degrees Fahrenheit. (MTBE gasoline, by comparison has a far more stable vapor pressure.) These rapid changes in vapor pressure effectively act as a pump, drawing more humid air into the tank every night.

In addition, E-10 is an active absorbent. A half-full tank will become moisture saturated within 30 to 90 days, depending upon the temperature range and humidity. (Unless there is a leak, MTBE can collect water only from condensation, which is usually caught by a fuel/water separator.)

In E-10, the solubility of water decreases with temperature-0.6 percent is soluble at 85 degrees, but only 0.4 percent at 32 degrees-and when the solubility drops due to cooling, a portion of the water drops out of suspension, along with six times its volume in ethanol.

In places where daily temperatures do not vary sharply (Florida, for example) this may not be a serious problem, but around the Great Lakes and in the Northeast, a fall cold front can result in phase separation in as little as a week.

A side effect of phase separation is that it lowers octane by removing ethanol. However, because the octane stripping is incomplete, the fuels octane level has been documented to drop only 1-1.5 units. (About 2.8 units of the E-10 octane rating is due to 10-percent ethanol addition.) While not desirable, this octane stripping alone, over the short term, should not lead to any serious running problems in most marine engines. (Some additive makers contend the octane stripping is more complete, and thus its effects are more severe.) Apart from this octane stripping, there is no other effect on combustion. Storage stability with regard to oxidation and any gum buildup is the same as MTBE gasoline.

Ethanol Gas Additives: What We Tested

This Practical Sailor comparison focuses primarily on products claiming to combat the problems associated with phase separation in E-10. Aiming to include a cross section of whats on the market, Practical Sailor tested five products from five manufacturers: E-Zorb from Marine Development Research Corp. (MDR), Sta-bil Marine Formula Ethanol Treatment from Gold Eagle, PRI-G from Power Research Inc., Star Tron from Star brite, and Techron from Chevron. The claims of each product varied, but the tested products fell into one of four general categories:

Conventional fuel storage additives. These classic stability additives are designed to extend storage time beyond what the minimal additive doses in standard gasoline can deliver. They do not explicitly claim to prevent separation. Sta-bil Marine Formula Ethanol and Techron are examples of this type of product.

Ethanol fuel storage additives. These products claim the above, plus limited improvement to phase stability of E-10. They do not claim the ability to restore separated fuel. PRI-G and Star Tron from Star brite are examples, but it should be pointed out that neither product claims to deal with the extreme – though not unrealistic, in our opinion – case of cooling induced phase separation simulated by our test.

Restorative additives. These are not storage additives; they are designed to prevent separation and to restore separated fuel. E-Zorb is the only product we tested that claims to restore separated fuel. (MDR also makes a separate storage additive, X-TEND, which was not tested.)

VALUE GUIDE: Additives for Ethanol Gasoline


E-10 Gasoline Sunoco 87 Octane



Marine Development and Research (MDR)


Power Research Inc.

Sta-Bil Marine Formula Ethanol

Gold Eagle

Star Tron




Price/ treated gallonBase799757/ 6
Ash (undiluted)<.01><.01><.01><.01><.01><.01>
Non-volatiles (undiluted)<.01>83%1.1%15%<.01>25%
Non-volatiles (recommended dose)N/A.03%.01%none0.2%
Non-volatiles (42:1, E-Zorb only)N/A2%N/AN/AN/AN/A
Emulsion Breaking (gasoline/water)30 minutes:Gas 3/ water 1Gas 3/ water 3Gas 5/ water 8Gas 2/ water 1Gas 2/ water 1Gas 3/ water 1
1 hour:Gas 3/ water 1Gas 3/water 2Gas 3/ water 8Gas 1/water 1Gas 2/ water 1Gas 3/ water 1
6 hours:Gas 1/water 1Gas 1/ water 2Gas 1/ water 10Gas 1/water 1Gas 1/ water 1Gas 1/ water 1
Phase Separation (gasoline/water)30 minutes:Gas 5/ water 1Gas 2/ water 1Gas 5/ water 7Gas 3/ water 1Gas 3/ water 1Gas 5/ water 1
1 hour:Gas 4/ water 1Gas 1/ water 1Gas 4/ water 8Gas 2/ water 1Gas 3/ water 1Gas 4/ water 1
6 hours:Gas 1/ water 1Gas 1/ water 1Gas 1/ water 8Gas 1/ water 1Gas 1/ water 1Gas 1/ water 1
CommentsPercentage of samples
initial weight that
remains after reducing
to ash at high temperature.
All samples were
very low, no distinguishable
problems from ash.
Percentage of undiluted
samples initial weight
left after burning. This is
a measure of non-volatile
residue that might deposit
in the intake and lube oil.
Percentage of samples
initial weight (standard
dose) left after burning.
The amount of
residue was very low in
all samples. No issues at
standard dose.
Measure of non-volatile residue left
after burning a concentrated treatment
dose (E-Zorb only). Percentage
of samples initial weight left
after burning. Note that E-Zorbs 2%
residue is the same concentration
(50:1) as two-stroke oil.
Checks samples ability to prevent water
emulsion. Objective is clear water and
clear fuel, no emulsion. In all samples
6.0 mL of water phase-separated and
settled at bottom of test tube.
Measures ability to prevent cooling-induced phase
separation. No samples succeeded. The volume of
water settled was the same for all samples at all times;
1.8 percent phase separated. Phase separation began
at 70 degrees with all samples.

Ethanol Additive Testing Challenges

Considering the range of standard tests available, the Practical Sailor evaluation is limited, focusing principally on ash residue, emulsion breaking, and phase separation stability. A full explanation of the testing appears above, and tables documenting the results appear on the following pages. The protocols were modeled on standard tests recognized by the American Society of Testing and Materials (ASTM).

Some makers worried that the results could be misleading. Star Tron chemists suggested that continuously vibrating the test samples (as a running engine would cause a tank to vibrate) would better demonstrate how the enzymes in Star Tron work. They also contended that our 0.6 percent concentration of water in samples used to simulate overnight phase separation was excessive. Our testers did not believe the protocol was unrealistic, although it is arguably strenuous.

Practical Sailoris researching the possibility of an onboard field test that would produce reliable data. This is a nut that even the fuel specialists have yet to crack. Here are some of the other fuel properties thatPractical Sailor did NOT test:

Gas line freezing. Since phase separated material contains too much ethanol (40 to 85 percent) to freeze, this function is obsolete.

Fuel oxidation and detergency to prevent gum and varnish formation. Unlike lube oil, refined gasoline is inherently unstable and wants to polymerize, so the refiner applies more additives to stabilize gasoline. These additives also add detergency, reducing engine and fuel tank deposits. They are not intended to clean tanks or emulsify water, but can help prevent dirt and water from attaching in the first place.

Many additives at the retail level mimic the same chemistry used by refiners; polyisobytlyene amine and polyether amine are common active ingredients. The Sta-bil, Techron, and PRI-G test samples contained this or related chemistry. Star Tron uses an enzyme chemistry.

Effect on two-stroke oil. Many products claim to improve two-stroke performance. We repeated the emulsion breaking tests with a 50:1 dose of TCW 3 Pennzoil two-stroke oil; emulsion breaking was 20 percent slower, but the difference probably would not be noticed in practice.

Bacteria and algae growth prevention. While micro-organisms can thrive in diesel fuel tanks, theyre not known to grow in gasoline tanks.

Certainly, many more questions need to be answered, but the data collected highlighted the differences among the various products tested, and isolated a few that stood out. Here is a closer look at the individual products and how they fared:


MDR says E-Zorb is the only product designed to react with water and ethanol at the bottom of a fuel tank and absorb it back into fuel. Thick and amber in color, E-Zorb stands out because it is slightly heavier than gas and can seek out the phase-separated water at the bottom of the tank. For this reason, it did not mix as easily with the fuel as the others in our test.

The label suggests one ounce of E-Zorb for every 20 gallons of gasoline at each fill-up to help the fuel better tolerate water absorption. If free water contamination is found in the tank, a 1:1 ratio of E-Zorb-to-
water is recommended to totally emulsify the free water back into the fuel. At the recommended maintenance dose, E-Zorb and all other products tested contributed only insignificant amounts of non-volatile material (material that would not burn during combustion).

However, when E-Zorb is dosed high enough to absorb free water (which it does, just as advertised), the non-volatile contribution
becomes the dominant non-volatile component in the fuel stream. This, in our opinion, could effect lubrication in two-stroke engines over the long haul. This would cause less concern in a four-stroke engine. There is also a chance that salt water could be passed through the fuel system instead of being intercepted by a fuel/water separator.

E-Zorb, like the other tested products, did not measurably inhibit phase separation in our test. E-Zorb is available in 16-ounce bottles.

Bottom line: Practical Sailor does not recommend routinely using the higher dose of E-Zorb to remove phase-separated water, particularly in a two-stroke engine, which could be more vulnerable to any non-volatile residue. Carefully draining or filtering out the water is preferable.


Power Research Inc. develops a range of additives for industrial and marine purposes. PRI-G is a relatively thin, brown liquid that mixes easily with gasoline. It claims only limited protection against phase separation, and is primarily an oxidation stability additive. Users are instructed to mix 1 ounce per 16 gallons of fuel. It comes in 16- or 32-ounce bottles with a built-in measuring dispenser. A 1-gallon bulk jug is also available. PRI-G yielded the worst results of all the additives tested, requiring the longest separation times and yielding cloudy water with an unresolved ragged layer.

Bottom line: Based on the test results,Practical Sailor does not recommend the PRI-G over the other products.

Sta-bil Marine Ethanol

A well-recognized name in the fuel-treatment field, Sta-bil offers a wide range of fuel storage and treatment products. Its Marine Ethanol treatment is a medium-green liquid that mixes easily with gasoline. It recommends 1 ounce per 5 gallons of fuel for storage protection, or 1 ounce per 10 gallons of fuel during the season. The high levels of additives in this product suggest that Sta-bils recommended doses are conservative. The product comes in 16- or 32-ounce bottles with built-in measuring dispensers. This product delivered exceptionally clean fuel/water separation.

Bottom line: It doesn’t prevent phase separation (nor does it claim to), but this economical product performed well in testing and comes with a long-standing reputation. It is our Best Choice.

Star Tron

Marketed by Star brite, maker of a wide range of marine maintenance products, Star Tron stands out because it does not use conventional chemical oxidation inhibitors, relying instead on a proprietary enzyme technology. Because it works differently, the makers say, it is difficult to judge using conventional testing methods. This thin, light blue liquid mixes easily with gasoline. The recommended dose is 1 ounce per 8 gallons of fuel for storage protection, or 1 ounce per 16 gallons during the season. It comes in 16- or 32-ounce bottles with narrow necks for easy pouring. Testers found this product to have a remarkably low non-volatile content and exceptionally clean fuel/water separation.

Bottom line: Economical, Star Tron is a safe choice for those concerned about residue. Contrary to claims, Star Tron, like the others, did not prevent phase separation in our test. It is our Budget Buy.


Developed by energy giant Chevron, this thin, clear liquid mixes easily with gasoline. Chevron recommends 20 ounces per 20 gallons of fuel for system cleanup. The dosages for continuous usage or winter storage are not stated, but they would logically be much lower. Adding 1 ounce per 5-10 gallons would seem correct based on our non-volatile test results and product specs. It comes in a 16-ounce bottle with a narrow neck for easy pouring, but has no dispenser.

Bottom line: Techron gave exceptionally clean fuel/water separation. It is expensive as a cleanup dosage, but at a lower maintenance dosage, it would be economical.

No Apparent Antidote for Separation

As mentioned in the main text, the tests focused on ash residue, emulsification, and the ability to stabilize phase-separation. The results are documented, with comments, in the tables on the following pages. The tests used Sunoco 87 octane E-10 gasoline from a single batch, dyed light blue to aid visibility. All leftover additives, gasoline, and test residues were recycled as fuels by FCC Environmental.

Testing for Non-Volatile Residue

PS wanted to see if the additive itself could leave ash deposits, gum deposits, or residue that might contaminate lubrication oil. A case in point: two-stroke oil for motorcycles contains ash that can leave residue in a cooler-running outboard. TCW-3 two-stroke oil for outboards does not. Similarly, non-volatile additives can accumulate in the crankcase oil or form intake deposits that can harm the engine. Some injector cleaning additives even advise users to change the oil after using them.

First, PS used the standard ASTM 482 test for ash in fuel oils; a 3-gram (0.10 ounces) sample is reduced to ash in an electric furnace and the difference in weight recorded. Then, in three additional tests, non-volatile material was measured by running the ash method at lower temperatures. The test was performed on undiluted samples and at prescribed dilution ratios (E-Zorb was tested at both treatment and maintenance doses). This is a comparative measure of how much of the additive will not be burned during combustion.

None of the additives tested contain significant ash and should be safe at recommended maintenance doses. Be wary of more concentrated treatment doses, though. In the PStest, a dose of 2 percent E-Zorb was needed to re-absorb phase-separated water. Such super-doses used to absorb water might result in non-volatiles contaminating the crankcase oil, in our opinion. Two-stroke engines may be more vulnerable because of their limited lubrication. (MDR says its long record with a similar product, Water Zorb, points to the safety of using these doses.)

Emulsion Breaking

In this test, the tendency of an additive to emulsify, suspend, or absorb free water is tested. At first blush, an additive capable of suspending free or phase-separated water sounds fantastic-the water magically disappears. Of course, it doesn’t. If it is salt water, it can pass through your engine, chlorine, sodium, and all. Oil companies and car manufacturers have generally dismissed the approach.

Onboard commercial and naval vessels, the objective is emulsion breaking- preventing contaminants from being mixed with the fuel. On these ships, chemicals are employed, followed by filters and oil/water separators, and sometimes centrifuges. Emulsifying chemicals and absorbents are strictly forbidden by International Organization for Standards (ISO) marine fuel standards, mostly due to the risk of contamination from sea water.

For this test, we added 1 milliliter of free water and two times the recommended additive dose to 50 milliliters of E-10 and shook vigorously for 20 seconds. Separation was visually observed over time. The desired test result would be no emulsification or absorption of water, with clear gasoline and clear water produced in the minimum time period.

Stabilizing Phase Separation

This test (an adaptation of American Society of Testing and Materials Test D6422, Water Tolerance of Alcohol Blends) checked whether these additives could delay the onset of cooling-induced phase separation. Phase separation begins slowly; the difference in density between the alcohol-water mixture and gasoline is very small; the drop size is on the order of a few molecules, and there is very little density difference to drive separation. Though only an emulsifying or absorbing additive could suspend significant amounts of water when introduced as large drops, some chemists suggest it is possible to stabilize the phase-separating material while the water droplets are still present at molecular dimensions and prevent them from coalescing, without becoming an emulsifier or absorbent.

To simulate a fuel tank that has been saturated by humid air and then cooled, 50 milliliter samples of E-10 were saturated with 0.6 percent water at 85 degrees, and then slowly cooled to 32 degrees in a water bath. Recommended doses of each additive were introduced, and observations were made over six hours. None of the products were effective at preventing phase separation in this test.


In our opinion, the makers and retailers of these additives arent doing a good job telling consumers what these products do and don’t do. None of the additives had any measurable effect on the amount of water that separates due to overnight cooling. It is Practical Sailors view that any products claiming even limited ability to prevent or delay phase separation should pass ASTM test D6422 using 85-degree, saturated E-10 cooled to 32 degrees-a reasonable worst-case scenario.

So, don’t expect miracles. These blends may reduce maintenance, but they can’t fix material problems.

If you are interested only in oxidation stability and detergency, choose from Sta-bil, Star Tron, Techron, or a similar formula from another respected maker. Testers saw no substantial performance differences.

With its fair price and good separation, Sta-bil Marine Formula Ethanol Treatment is our top choice. Avoid alcohol-based treatments; they are not for E-10.

If you have free water in your tank or phase-separated material, remove it.Practical Sailor strongly recommends fuel/water separators on any boat, even outboards with portable tanks.Practical Sailor does not recommend using an emulsifier to re-suspend free water.Although water emulsions have been burned in controlled settings, the fuel tank of a boat is different.

Practical Sailordoes not recommend using E-Zorb to dry a wet tank. (MDR contends years of experience with a similar product, Water Zorb, do not support our testers concerns regarding non-volatile residue.) Drain the free water or use filters and separators instead. This is not a concern at the recommended maintenance doses of 1 ounce to 20 gallons. Testers did not like PRI-Gs tendency to form discolored water emulsions, hazy fuel, and a ragged interface.

Whenever possible, refill your tank after each use, particularly in the latter half of the season when temperatures are dropping.

For winter storage, the logical approach is a full tank and treatment with an oxidation prevention additive; this minimizes evaporation and exposure to oxygen and humidity. A full tank of E-10 should not absorb enough water to phase separate within nine months, nor admit enough oxygen for refinery-treated fuel to oxidize or form gum. If you drain the tank, it must be completely dry. A near-empty tank in a cold climate will surely phase separate.

Some yards have reported success storing partial tanks and then topping off with high test in the spring to compensate for any octane stripping. This approach might work in warm climates with little temperature change, but in cooler climes, the half-full tank could promote phase separation. Either way, take it easy the first day and top-off with a few gallons of high-test if a small loss in octane is a concern.

One final note: If you want to check the stability of your existing fuel, place a small glass jar of the fuel in a bucket of ice water for one hour. Though minor haze is not grounds for rejection, any free water hints of trouble in the spring.

Practical Sailor has been independently testing and reporting on sailboats and sailing gear for more than 45 years. Supported entirely by subscribers, Practical Sailor accepts no advertising. Its independent tests are carried out by experienced sailors and marine industry professionals dedicated to providing objective evaluation and reporting about boats, gear, and the skills required to cross oceans. Practical Sailor is edited by Darrell Nicholson, a long-time liveaboard sailor and trans-Pacific cruiser who has been director of Belvoir Media Group's marine division since 2005. He holds a U.S. Coast Guard 100-ton Master license, has logged tens of thousands of miles in three oceans, and has skippered everything from pilot boats to day charter cats. His weekly blog Inside Practical Sailor offers an inside look at current research and gear tests at Practical Sailor, while his award-winning column,"Rhumb Lines," tracks boating trends and reflects upon the sailing life. He sails a Sparkman & Stephens-designed Yankee 30 out of St. Petersburg, Florida. Darrell is booking speaking engagements in Colorado, Idaho, California, the Pacific Northwest, and British Colombia this summer. You can reach him by email at


  1. I have used this PRI32-G fuel stabilizer for 2 years and enjoyed its easy to use 2.2 pounds container. I will be buying again a container of this PRI fuel stabilizer, preferably online ahead of the next winter season.