Cruising Efficiently and Saving Dollars

With talk of expensive oil in our future, I’ve been looking at ways to get the most out of a gallon of gas. I’m not talking about camping at the marina vs. on the hook with the genset running, or treating guests to Subway vs. going down to Tumbleweed, or adding a mast, solar panels and a fuel cell motor. My objective is to demonstrate how to tweak things to operate more efficiently over a given distance with a given boat, both with and against the current.

I started thinking about this over the winter when I considered taking the boat to Pittsburgh for my vacation in May. Do we really get .65 mpg at cruise as we were told, or is it .40 or .70 or 1.00? Maybe we should have bought the diesel?

Everybody knows that by staying off plane, you can extend your range in calm waters. You’ve seen the graphs in magazines before. The numbers are usually best at idle, if you can stand going that slow, then decrease as you approach “displacement speed,” or “theoretical maximum hull speed.” Once you start to plow, your efficiency decreases dramatically, until planing, where you are riding on top of the water with less friction and at a higher speed.

So how can we determine the theoretical maximum hull speed?

There are several factors involved, such as hull type and beam. To generalize, a deep-V hull with a large deadrise will be affected more by drag than a boat with a flat bottom or a displacement-type hull. Without getting too much into the science, as you push the water in front of the vessel, a waveform is generated. The key is to determine the maximum speed that you can operate without getting into the drag caused by overriding the wave. The formula commonly used is as follows:

V = 1.34 * Square Root(LWL)

Where,

V is the solution, in knots
LWL is the length at the waterline, in feet

I’ll save you the math: 28’ LWL yields 7.09 knots, while 36’ LWL yeilds 8.04 knots.

Let's examine a local Brand-X 37 for example, with a LWL of 34'. The solution yeilds 7.69 knots, or 8.84 statute mph. Practically speaking, you could cruise efficiently up to 8.8 mph in that boat, but if you want to go faster, you’ll be using more fuel per mile. I used to think 1200 RPM = 1400 RPM = 1800 RPM = 3400 RPM (planing) in terms of efficiency, but the data below doesn't support that. Current will also make an impact, which we will examine.

What about planing? Many boats will see their mpg increase again at planing speeds. Tabs up helps, but you need instrumentation to know for sure. For example, used to operate a mid-80’s Sea Ray 340 Sundancer on Lake Erie that was equipped with fuel-flow meters and a fancy sumlog. It would use the same fuel per mile at 3000 RPM with tabs (1/4 deployed to improve the running attitude) as it would at 3400 RPM clean. At 3000 RPM without tabs, the Sundancer would fall off plane. Once the secondaries opened up, of course, the efficiency decreased dramatically. It all depended how fast you wanted to go or how much noise you wanted to make.

The Brand-X 37 described above gets slightly better efficiency on plane with a 3 second burst of trim tabs out at 3400 RPM (as indicated by GPS speed changes). Generally speaking, tabs are used in rough seas to control the running attitude with head or following seas, but in this case we're describing the use of a small amount to improve running attitude and efficiency.

Applying a little geek-factor here, I considered installing a fuel-flow meter on my current boat so that I could monitor fuel flow, improve my operating efficiency and mitigate the increasing cost of fuel. Captain George East, who has some experience with drag racing and dynos, suggested the following as an alternative:

While fuel flow meters are a very good idea for optimum efficiency, the cost may turn some people off. There is another way that is not as precise but will yield good results: Go to any boat magazine that has boat tests that give gal./hr. figures for various RPMs. Be sure to use an engine that has the same displacement as the ones in your boat i.e. 5.7L 350 CI, 7.4 L 454 CI, 8.1L 496 CI, etc. This data that is given in gph is fuel burn under a load and will not significantly vary from boat to boat provided that the engines in the tests turn a similar full throttle RPM that the boat engines do.

Using a GPS, do a two way average to determine speed at these test RPM settings. Simple math will yield the answer on mpg. Also, the physics of optimum fuel burn for internal combustion engines are .44-.45 lbs of fuel per horsepower per hour for gasoline and .35-.36 lbs of fuel per horsepower per hour for diesel. Modern engines have fuel injection systems that meter fuel more precisely through out the entire power range better than carburetors on older gasoline engines.

Using George's suggestion, I ran the numbers on the 37 Brand-X Convertible equipped with a pair of 454 fuel-injected Crusaders. It's a very nice, beamy boat, with the interior space of a small houseboat. The Crusader factory rep sent me information regarding fuel burn at given RPMs on a very similar boat, which happened to have the same wide-open-throttle RPMs. On a no-wind morning, I plotted GPS speed data at 200 RPM increments from 1000 RPMs to WOT. Although I took numbers on the river going both directions, it soon became apparent that the current that day was a healthy and constant 2.4 knots. I compensated the speeds to reflect no-current conditions. For each RPM, I took the speed divided by fuel flow to determine the specific range, or miles per gallon (mpg). These numbers were converted to statute measurements, as the inland waterways are measured as such.

For the Brand-X, the following numbers were obtained with 3/4 fuel, 1/2 water, two passengers, and a dirty bottom that needed new paint after 3 years. Obviously, new bottom paint would improve the numbers.

Looking at the above graph from the Brand-X, the numbers near idle give the best fuel over distance. The mpgs decrease closer to planing speeds, but on this particular boat, the mpg numbers did not improve more than a little bit while on plane. In this case, it becomes a function of hull design. I know from flying that a tailwind or headwind can have a large effect on range, so given that we're focusing on the river, I ran additional numbers accounting for a 2.5 mph current either downbound (DB) or upbound (UB). The difference is greatest at slow speeds. On the above chart, for example, you can see how much the efficiency improves going with the current at slow speeds (i.e., 1200 RPM), but going up-river, it doesn't make sense not to go 1600 RPM or more, as the efficiency is almost the same.

What about the "theoretical maximum hull speed" that we started with? Sure enough, on the Brand-X, 8.8 mph (no current) occured at 1800 RPM. You can see that the efficiency drops very close to that point, supporting the math and physics.

Regarding the cost of cruising, some people say “you don’t want to know,” or “if you have to ask, you can’t afford it.” With good numbers for your engine and a GPS for speed, it's pretty easy to run a spreadsheet and come up with the specific range for your boat. You could go one step further and determine the cost per mile, but that's not as much fun at today's prices. You can easily figure out, however, where your boat performs best for the way you are operating on a given day. The difference can be measured in dollars per mile, which will add up over a distance.

Captain Eric