Outboards Fuel Consumption
Similar to inboards, you can calculate the most efficient operating speed for your vessel. The same applies that you must go out on the boat and measure the boat speed at a range of operating RPMs. Then compare these to the manufacturers fuel consumption rate vs RPM.
The following is a graph of Mercury outboard engines comparing fuel consumption vs RPM of one 175 HP engine vs two 175 HP engines vs one 350 HP engine. It shows that the two 175 HP engines consume about the same fuel amount as one 350 HP engine. The calculation for the two 175’s was done by multiplying the consumption of one 175 by two. But in actual fact both engines will be each working a little less and thus the fuel consumption is likely to be less than double.
To calculate the most efficient speed, again use a table similar to the below fictitious table of a boat operating with a single 350 HP outboard engine. The table as an example shows that this particular boat would best operate at 3000 rpm. Also, notice the significant jump in speed from 19 knots at 2500 RPM to 30 knots at 3000 RPM. This is likely because the boat got up on a plane which significantly reduces the water drag on the boat. Also, notice the significant jump in fuel consumption rate from 2500 RPM to 3000 RPM – but this rate is offset by the big increase in speed.
If you own your own boat, you are well advised to create a similar speed fuel consumption table yourself – this is assuming you can get the manufacturer’s data on RPM vs fuel consumption rate.
