Would you begin a road trip with just a quarter tank of gas? Didn’t think so. It is just as important, if not more so, to check your fuel level for your boating trip before heading out. Understanding how to do so is important because it will help you gauge how long you can be out on the water before needing to head back to re-fuel. A lack of gas can potentially become dangerous if you don’t have a VHF radio or any cell phone reception to call for help.
Planning for a trip in your boat is different than in your car though, as boats consume fuel differently. Boats don’t specifically show you how many miles you can go before you run out of gas. So, how do you figure out how long your current fuel level will last? Luckily, many newer engines come equipped with fuel monitors to make this easy for you. However, if you have or are looking to purchase an older model, you will need to know how to calculate your real fuel consumption.
The formulas below will give you an estimate of your fuel consumption, using averages. It is important to note that they apply only when the engine is at peak horsepower and no outside factors are considered.
First, you need to understand that to calculate your fuel consumption, or the number of gallons you will burn per hour (GPH), you will be measuring the pounds of fuel used per horsepower (HP) developed per hour. Because of this, it is important to note that diesel fuel weighs 7.2 pounds per gallon and gasoline weighs 6.1 pounds per gallon. In the below formula, this will be referred to as FSW (fuel specific weight). It is also necessary to know that the specific fuel consumption (SFC) of a four-stroke gasoline engine is, on average, 0.50 pounds of fuel per hour for each unit of horsepower. For a diesel engine, this number becomes 0.4 pounds. This, of course, does not take into consideration any outside factors or loss of efficiency.
GPH = (SFC x HP) / FSW
For example, if you have a 350hp gasoline engine, you would use the following formula:
GPH = (0.50 x 350)/6.1 = 175/6.1 = 28.7 GPH
A simpler, yet less accurate formula you could use would divide the total engine horsepower (total HP) by 10 for gasoline engines and .06 for diesel. This would give you an average of your gallons per hour at wide-open throttle.
Gas Engines: Total HP / 10
Diesel Engines: Total HP / .06
The amount of fuel consumption will depend on both external and internal factors. From how your boat is built to sea conditions, all these elements can increase or decrease the time in which it takes for your fuel to deplete.
Rough sea conditions can drastically reduce your fuel efficiency. Pounding or plunging due to a rough ocean can force your boat to slow down and come off plane. Make sure you check out the sea conditions before heading out to properly account for potential fuel usage.
Depending on the boat you own, you may have dual motors, which would be more powerful and beneficial for boat maneuvers. It would also come in handy in case one motor breaks down while you’re out on the water. This will certainly change your fuel consumption rate though, so keep that in mind when performing your calculations.
Horsepower is another element to consider as the more power your boat has, the more fuel it will use. When the throttle is wide-open, you will obviously consume more fuel. Throttle back, save on fuel.
Speed is a factor that can either increase or decrease fuel consumption. Driving at a cruising speed, for example, will decrease your boat’s fuel consumption. Depending on your boat’s specifications, size, and engine, there is a recommended speed you should take into account. These pieces of data will help you ensure fuel efficiency.
Your boat’s fuel type can also have an influence on how much fuel is consumed. As mentioned above, gasoline tanks burn an average of 0.50 pounds/hour for each unit of horsepower compared to .4 pounds/hour for a diesel.
One important thing to keep in mind is that gas tanks cannot be completely filled, nor completely emptied. Filling the tank up to the maximum isn’t a good idea – as there’s an overflowing risk. Because of the boat’s movement, there is also the risk of fuel spilling out of the tank vent.
There is a fine line between a tank that is full and one that is overly full. This is why when filling your tank, you should never go straight to the number you know is specific for your tank. For example, if you have a 20-gallon tank, it would be recommended to fill it to around 18 gallons, to avoid spilling.
Boats are all different and knowing how much you need to fill your boat’s gas tank will always go down to trial and error. However, keep in mind that just because you know your tank has a certain number of gallons, the risk of overflowing is still present.
If you don’t have a fuel monitor installed, these formulas could be your saving grace for planning a safe journey out and back home. Fuel consumption will depend on many factors, both external and internal, but figuring out your boat’s average usage and how quickly your fuel depletes can benefit you in more ways than one.
