Not really a topic that is discussed much on the internet - PWC fuel economy. We've touched on it a little in the Best Jet Ski for Fishing article, but i think it's time to take a closer look at the subject.
For starters, gallons per hour (gph) is the unit of measurement typically used for boats and water craft. This can be a difficult number to work with when trying to plan how many miles you can travel in a trip offshore since miles aren't even part of the measurement. But with some simple math, and some sea trials on your ski to determine fuel burn at different speeds, you will start to get a good idea of capable range.
For this Article we're going to look at the fuel burn on a 2015 Kawasaki Ultra LX. The ski used has over 100 hours of operation, and was tested on the ocean loaded down for fishing. To get a full range of fuel economy we took readings in 5mph increments, from the gph gauge built into the ski's computer. Correlating these numbers on a graph with the gph on the Y axis, and the mph on the X axis. Then divide the mph by the gph number to get mpg at the different speeds, plotting this information on the same graph gives this:
As you can see from the orange plot, the highest mpg is achieved between 25 and 35 mph at roughly 5.8-6 miles per gallon. Now to figure out the breathing fumes, stranded in the ocean, calling for a tow range, take your fuel capacity and multiply that by the mpg at the cruising speed you intend on running. For the Kawasaki, assuming it can run at 30 mph steady, the 20.6 gallon tank provides for a 123.6 mile range before you'll need a tow.
Now let's inject some reality - you won't be able to run at optimal speed with steady throttle for your entire trip, especially if the weather kicks up. I've ran through the Astoria Canyon when the river influence had some decent chop rolling for miles and could only muster 14-17 mph on the outbound trip, running that speed only gives a range of 86.5 miles till stranded - that's nearly a 40 mile difference in range from 10mph decrease from optimal speed. And on the other side of that, on a flat day, with the ability to open it up a bit and stretch the legs out, a 10mph increase out of optimum to 45mph your dropping the range nearly the same amount.
To make matters worse, in the rough conditions you'll be on and off the throttle - full throttle for chop correction gets really thirsty and and can push 20gph while accelerating on the bigger ski's!
So when planning a long range trip, look at that 40 mile decrease in range as your buffer zone. What i'm referring to as the "buffer zone" is for planning purposes: Consider that 40 mile lower range as the stranded, out of gas, hoping your VHF works and someone can hear you, having to explain to authorities, or worse your wife, cutoff point - instead of your actual really out of gas breathing fumes level.
Staying with the Kawasaki in this example, from a 120 mile range, subtract off the 40 miles of buffer for an 80 mile range until "out of gas". And take every effort to keep steady throttle in the optimum 25-35 mph zone. Now you'll still want some reserve left in the tank so lets figure on 3 gallons left over reserve. 3 gallons at optimal speed would get 18 miles, so subtract that off the 80 mile "out of gas" number for a total round trip cut off range of 62 miles.
Now that we've covered the hard way - The tried and true third rule (1/3 outbound 1/3 return 1/3 reserve) would suggest to leave 7 gallons reserve on the Kawasaki. 20.6 gallon main tank minus 7 leaves 13.6 gallons, multiply that by the non-optimal 4.2 mpg, and get 57.12 miles range - really close to what we figured above, just without all the "this is why we do it this way" explanation. The key here, is to not use the optimal burn number, since you cannot guarantee that you'll be able to maintain that speed for the duration of the trip.
If you're looking at running long range trips, figure out your optimal speed as shown above, and try to maintain steady throttle in that range. Then give yourself some reserve fuel and figure on a buffer for less than optimal conditions.
Stay tuned for the next fuel article where we'll discuss adding auxillary fuel to increase range.