Water Sports - 10 min read

What Size Fuel Line To Use For An Outboard, 5/16″ or 3/8″?

Town and Tourist

Town and Tourist, Updated November 5, 2022

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Every component of a boat is critical to its longevity and performance. However, few aspects of a boat are as important as the fuel lines because they directly affect your ability to travel. Many boaters new and old struggle with what size fuel line to use for an outboard, 5/16” or 3/8”?

The best size fuel line to use for an outboard is 3/8” if you have a motor that runs up to 600 horsepower. Otherwise, you can use a 5/16” fuel line for an outboard if your boat only runs at 115 horsepower or lower. It costs up to $500 or more to replace or upgrade your outboard fuel lines.

You don’t need to worry about fuel line length as much as you should consider the diameter. Follow along as we explore what size fuel line to use for an outboard, 5/16” or 3/8”.

3/8” Vs. 5/16” Fuel Line

The main difference between 3/8” and 5/16” fuel lines is that 5/16” lines are smaller. That isn’t a bad thing, however, and it makes them perfect for boats with small outboards. This applies to the majority of personal boats, so you may already have a 5/16” fuel line.

Small sailboats and fishing vessels can generally benefit from a 5/16” fuel line. They are easy to connect and rig which makes them ideal if you have limited boating experience. Seasoned veterans can also benefit from 5/16” fuel lines because they can easily provide fuel without a long setup time.

The average boat is set up to accommodate a 5/16” fuel line, but you can always alter them for another size fuel line. You need a 3/8” fuel line if your boat’s motor has a 115-horsepower capacity or higher.

Is 3/8 Fuel Line Enough?

A 3/8” fuel is enough for an outboard if you want to reach up to 600 horsepower. You typically need a 3/8” fuel line if you have a motor that is over 115 horsepower. Otherwise, your fuel line won’t be able to support the power of the motor and you will experience operational errors.

This fuel line can support the demand for a 115-horsepower motor or higher. Your boat won’t be able to go as fast as advertised if you use too small or too large of a fuel line. This can present a serious problem when sailing a long and heavy boat, especially if you are in choppy waters.

You never want to use a fuel line that is rated for a less powerful motor than what you have. This can result in operational errors and even motor damage in serious cases.

Can I Use 5/16 For a Fuel Line?

A 5/16” fuel line is enough as long as your boat’s motor is rated for it. You can use a 5/16” fuel line for a boat with a 115-horsepower motor or lower. However, you have to use a 3/8” fuel line once you exceed 115 horsepower.

Your boat must be 30R9-rated if you use a 5/16” fuel line. Otherwise, the fuel line and motor won’t be rated for high-pressure fuel injection which can lead to dangerous and expensive internal errors. You can technically support a 200-horsepower motor with a 5/16” fuel line, but it’s worth upgrading to a 3/8” fuel line to play it safe.

Outboard Fuel Line Length

Outboard fuel line length is less important than the diameter. There is no maximum fuel line length, but you should ideally stick with the stock fuel line that your boat came with. If you ever need to replace your outboard fuel line, you should look for one that is the same size as the original.

Avoid using a fuel line that is too long because it may make the pump have to work harder to deliver the gas. A 16-foot long boat typically requires a 12-14 foot long fuel line. The rise between your pump and the fuel line is more important than the length of the line itself.

Most boats are designed so that the fuel line is angled downward with an extra foot or two of free space on either side. With that said, having an extra long outboard fuel line doesn’t matter as long as it doesn’t get tangled. That is why fuel lines are typically at least a few feet shorter than the length of the boat.

Cost To Replace Boat Fuel Lines

It can cost up to $500 to replace the fuel lines on a boat. Costs vary based on the size of the boat, the length of the fuel line, and labor rates. Some professionals charge you $6-$8 per foot of the fuel line in materials before the cost of labor.

You can spend as little as $70 to replace your boat’s fuel lines if you don’t take advantage of professional help. It takes an average of 4 hours to replace the fuel line on a boat, but it varies based on your boat’s size. Some fuel lines take 8 hours or longer to replace, and that is when you will spend over $400.

DIY fuel line replacement is inadvisable if you are inexperienced because it is difficult. It can also be dangerous to replace a fuel line because of the risk of combustion and permanent damage.

Does Diameter of Fuel Line Matter?

Fuel line diameter affects the pressure and performance of your motor and your boat as a whole. Everything from the fuel source to the horsepower of your motor affects what size fuel line you need. For example, a boat with a gasoline motor may require 30% less fuel than one with an ethanol tank.

Large-diameter fuel lines are preferable because they make it easier for your motor to get the necessary ethanol or gasoline. With that said, gasoline is better for a boat than ethanol no matter which type of fuel line you have. Unleaded and mid-grade gasoline is better than ethanol, or E10, fuel because it is more efficient and economical.

Does Length of Fuel Line Matter?

The length of a fuel line matters because it affects the pressure. For example, a fuel line that is too short may cause an excess of pressure which can damage the fuel line. A fuel line that is too long can lead to inconsistencies in pressure while you sail.

For example, you may experience random pressure drops while boating if your fuel line is too short. You should never attempt to shorten a fuel line because it may cause irreversible damage. Instead, you should use the stock fuel line that your boat came with. Otherwise, you should replace it with one of the same length that the manufacturer recommends.

How Long Do Marine Fuel Lines Last?

Marine fuel lines can last for up to 10 years before you replace them. However, a PWC fuel line may only last for as long as 5 years. It is recommended that you inspect your boat’s fuel lines each year to make sure that there are no leaks or clogs.

Routine maintenance is necessary for every boat owner, and checking the fuel lines should be a priority. Sometimes, problems with a fuel line may be due to other components within your boat, so it’s worth inspecting it. Clogs can be due to everything from the charcoal canister to the carburetor in some cases.

When Should You Check Fuel Lines on a Boat?

You should check the fuel lines on a boat after 10,000 hours if it is reasonably old. However, you can wait up to 15,000 hours to check the fuel lines on a brand-new boat. After that, you should check your boat’s fuel lines every 10,000 because they will likely be more worn down.

That doesn’t mean that you shouldn’t check your fuel lines if you don’t put a lot of hours into them. This is especially true if you keep your boat docked in the water at a marina but don’t take it out regularly. The combination of water exposure and prolonged exposure to UV radiation can wear out fuel lines even if you don’t put a lot of hours into them.

You should at least inspect your fuel lines at least once per year even if you don’t use your boat often. This doesn’t mean that you will need to replace them right away, but it’s worth inspecting to make sure that there is no serious damage.

Is Marine Fuel Line Different?

Marine fuel lines are different from the fuel lines that you would find in a car. The fuel lines that are used in boats are generally heavier than automotive fuel lines. Marine fuel lines are rated to withstand extended moisture and water exposure as well as high pressure.

Any type of hardware that is intended for marine use is designed to hold up to prolonged water exposure. Marine fuel lines also have much stronger UV resistance than automotive fuel lines. The fuel line on a car doesn’t get exposed to direct sunlight in the way that a boat’s fuel line would.

Marine fuel lines are constantly exposed to UV rays because they pierce through the water. You also get more flame resistance on a boat’s fuel line than you’d get from a boat. Boat fuel lines are tested based on a 2 ½ minute burn rating meaning that they won’t combust immediately.

Can I Use Rubber Hose For Fuel Lines?

You should never use a rubber hose for a boat’s fuel line. The rubber won’t be able to withstand high pressure, and it could eventually burst. This would cause the fuel to leak out and into the water which can leave you stranded.

Not to mention, this is also quite bad for the environment. Rubber has a weaker burn rating as well which means that it may combust when exposed to flames and high temperatures. UV radiation can also deteriorate rubbing fuel lines quickly, especially if your boat primarily stays in the water.

Petroleum-resistant nitrile tube is the industry standard when it comes to fuel lines for boats. It is inadvisable to use anything other than that.

Can Increasing The Size of My Fuel Line Cause Problems?

Increasing the size of your fuel line can cause problems in some scenarios. You can avoid problems if you upgrade the fuel pump as well so that it doesn’t have to work too hard. Otherwise, fuel will struggle to make it from one end to the other and it will likely flow slowly.

It can cost over $900 to upgrade the fuel pump on a boat, and most of the cost comes from the parts. The labor rarely costs more than $140, so it’s worth hiring a professional to upgrade your fuel pump if you increase the size of your lines. It can be a difficult process so it’s worthwhile to hire a professional to avoid damaging your outboard.

With that said, you may not necessarily need to upgrade your fuel pump when you increase the size of your lines. This is only necessary if your fuel pump cannot keep up with the flow relative to the length of the lines. Unfortunately, you may have to troubleshoot when you increase the size of your lines to see if your existing pump can keep up with them.

So, What Size Fuel Line To Use For An Outboard, 5/16” or 3/8”?

Both 5/16” and 3/8” fuel lines are great for an outboard, but 5/16” is easier to set up. Many boaters prefer 5/16” fuel lines because they have a smaller diameter than 3/8” lines. Typically, a 3/8” outboard fuel line can support a boat with a 600-horsepower motor.

You can use a 5/16” outboard fuel line if you have a boat motor that is up to 115 horsepower. Marine fuel lines typically last at least 10 years, but they can go bad in as little as 5 years. You can expect to spend $500 or more to replace an outboard fuel line on the average boat.

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