How to save fuel? cruising speed the only way?

Glowplug007 · Apr 5, 2011

Its not a HUGE concern for me because boating costs what it cost, but im about to launch my first boat and I have read some conflicting information in various places. I have read that cruising speed is theoretically where your boat is most efficient, but I equate that as cruising speed being the fastest you can go with best fuel economy. Is that accurate?.......Then I read that going slower then cruising speed actually burns more fuel! Is that true? If so why?

elkhunter338 · Apr 5, 2011

Re: How to save fuel? cruising speed the only way?

Yes, I got to see a fuel consumption chart for several boats and at about 1,000 rpms fuel mpg was pretty good, just above about 5mph mpg dropped off then rose again then droped off at wot. Each boat was alittle different but if I recall most got their max mpg fuel economy at around 65% of wot speed. so for example if your boat would go 100mph wot, max mpg was at 65mph.
I know for a fact my boat gets about twice the fuel economy at about 3/4 wot. at about 10-15 mph I get around 2mpg, at 20 mph I get around 4 mpg, at wot who knows because I do not run it there, but wot is 28mph. This is with a 21foot cuddy cabin boat, 181 ci chevy, merc outdrive. I always go 5-20 miles from dock each way several times a summer and have run the same boat for the past 6 years.

rallyart · Apr 5, 2011

Re: How to save fuel? cruising speed the only way?

For any boat the best mph figure at slow idle because it takes little work to move slowly through the water, unless there is wind or current to deal with. For planing hull boats the next best mpg figure will be at the slowest speed at which the boat is fully on plane. That's usually just faster than it feels like it is up on plane. That is where the hull is most efficient. It can be at the torque peak RPM of the engine if it's on plane because that is where the engine is at its most efficient. You can see fuel use charts at boattest.com for quite a number of different boats and may find one like yours.

Silvertip · Apr 5, 2011

Re: How to save fuel? cruising speed the only way?

The only way you will ever determine what cruising speed actually is and where the best fuel economy is will be if you install a fuel flow monitor. Otherwise everthing you do and every way you measure will be only and "average" and the number will be valid for only that trip.

Home Cookin' · Apr 5, 2011

Re: How to save fuel? cruising speed the only way?

first, stop letting it be a HUGE CONCERN. Boating is not where you save money; if it worries you that much, sell the boat and take up bird watching. Worrying about gas mileage in a boat is like fretting over the mileage your Suburban gets. It's terrible and that the end of it.

If you idle across the lake for 4 hours because it saves gas, you may burn more gas in the 4 hours than if you ran at 30 MPH for 20 minutes. And a whole lot less fun.

That's not to say you shouldn't be smart about it; study your boat's performance charts and listen for the sweet spot. Then relax. You're out there to have fun, not save money.

Glowplug007 · Apr 5, 2011

Re: How to save fuel? cruising speed the only way?

please read my post........"again"

nikon · Apr 5, 2011

Re: How to save fuel? cruising speed the only way?

^^^ He said it's not a huge concern.....

Edit: op beat me to it.

Fireman431 · Apr 5, 2011

Re: How to save fuel? cruising speed the only way?

The short answer is: The engine in full tune, the correct prop (and pitch), don't overload the weight in the boat, and run at moderate speeds.

DuckHunterJon · Apr 6, 2011

Re: How to save fuel? cruising speed the only way?

Bird Watching!!! Too funny.

As everyone said, there's two local optimums. One at just above idle, and one just above planing speed. If really concerned, travel at one or the other. Now to maximize smiles per gallon, I'd suggest there are also two optimums. The first is engine off, floating at the sandbar, sipping your favorite beverage - the second of course is at WOT on calm day.

Andy'sDelight · Apr 6, 2011

Re: How to save fuel? cruising speed the only way?

Think of it like driving a snow plow with a moderate amount of snow on the ground. If the plow just idles in gear, it pushes and moves the snow aside because it's not accumulating fast enough in front of the plow, so the engine is not "working" to move the load. Step on the gas a little more, more snow accumulates on the plow, the engine has to work harder to keep moving the weight of the truck and the accumulating snow forward and uses more fuel. Keep stepping on the gas and eventually the momentum of the truck starts assisting with moving the snow and the engine doesn't need to use as much energy to keep moving forward.

It's the same with a boat. At low idle speed, the water moves out of the way easily enough that it doesn't take alot of energy to move forward. Start idling up and now you're "plowing" that water forward and the engine needs to work hard to do so, thus consuming alot of fuel. Once you plane, you now ahead of your own wake and the momentum of the boat is helping to ease the work on the engine and eases the fuel consumption.

robert graham · Apr 6, 2011

Re: How to save fuel? cruising speed the only way?

Maybe check; Yamaha Outboard Performance Bulletins, plug in your boat/motor combination for charts showing GPH, MPG, MPH for any speed/RPM. Like for my 90HP jon boat, most efficient cruise is right about 3500RPM's. Of course this chart only applies to certain outboards/boats combinations, but bottom line, seems like "just up on plane" is the most efficient. It's an interesting subject and with price of gas, becoming more "interesting" all the time$$$

Home Cookin' · Apr 6, 2011

Re: How to save fuel? cruising speed the only way?

I like that snowplow analogy.

I read an article about "hypermilers" who try to get the best MPG in their car as possible, by going to extremes. But one thing that came out of it was determining the most efficient speed for the time on the road--obviously speeding uses more gas, but going too slow does too, because you are running the engine longer. I think they found that 48-53 mph was the best speed for their typically small engines. The same principle applies to boats; too slow may reduce MPG but increases GPH to a tipping point.

One of the things that distinquishes (most) boats from (most) cars is the shift to better efficiency when the boat gets on a plane, so sometimes the way we think doesn't apply. I guess the closest thing may be driving on soft sand. I've seldom seen a snowplow on a plane, but they try!

OP sorry I misread your introduction to your question. My bad; sorry. But there are a lot of people who think that way and discussions on these forums are generally to be helpful to a wider audience than the first question.

hungupthespikes · Apr 6, 2011

Re: How to save fuel? cruising speed the only way?

Cruising: to fly, drive, or sail at a constant speed that permits maximum operating efficiency for sustained travel.

Ned L · Apr 6, 2011

Re: How to save fuel? cruising speed the only way?

The 'most efficient speed' is probably not arbitrarilly just above idle, but rather at hull speed (~1.34x square root of the waterline length), which for many boats is just above idle.

rallyart · Apr 6, 2011

Re: How to save fuel? cruising speed the only way?

Andy'sDelight has a really good answer.
Glowplug, the reason the boat gets horrible gas mileage below the cruising speed is that if it is just running below a planing speed you are just plowing the water which you avoid, in part, when the boat is up on plane. The guy wake surfing is using a lot of fuel because he (it might be me) is pushing the most water he can to make a big wave.
When you watch the bowrider cruising slowly along the shore looking at everything with his bow up you can be sure he is using a bucket of fuel every couple minutes. If he slowed down or sped up his boat be working less hard.

QC · Apr 6, 2011

Re: How to save fuel? cruising speed the only way?

Just for clarity . . . this is almost never true:

Home Cookin' said:
If you idle across the lake for 4 hours because it saves gas, you may burn more gas in the 4 hours than if you ran at 30 MPH for 20 minutes. And a whole lot less fun.

This is dead on:

Ned L said:
The 'most efficient speed' is probably not arbitrarilly just above idle, but rather at hull speed (~1.34x square root of the waterline length), which for many boats is just above idle.

The only thing I would say about planing hull efficiency is that it is more about hull efficiency than engine efficiency. Although peak torque is a factor, it just happens to be around where typical I/Os and some OBs, and the typical planing hulls they are installed on, converge. With a single gear, it is almost an accident. Good one, but nobody sits down and designs a hull, gear ratio, propeller and drive together and says "make best cruise speed and peak torque line up". They say "make it go the fastest it can". If you did the former, there would only be one engine combo available that achieved it.

Another indicator of this separation of engine and hull efficiency is within the idle and hull speed thing. The fact is, Idle is the least efficient point for an Otto cycle (throttle and spark plugs) engine. So why would it be the best MPG in almost all cases?

QC · Apr 7, 2011

Re: How to save fuel? cruising speed the only way?

I was thinking about this thread a little more, and if you really look at the title and the OP's question, the answer is more this:

Fireman431 said:
The short answer is: The engine in full tune, the correct prop (and pitch), don't overload the weight in the boat, and run at moderate speeds.

And of the others are right I think weight is probably the factor that can have the most influence, and the one you have some control of. If you absolutely wanted the best fuel economy with a given combination start with the least possible amount of fuel, and nothing in the boat but a throttle jockey. No passengers, tools, liquids, spares, stuff, anchors, fenders, lines, water toys, seats, life jackets, nuttin'. Of course that's neither safe nor practical, but maybe a way to think about trimming unnecessary weight is to reassess what' you need and use on board. And it might be a good way to tell the Admiral that your mother-in-law isn't welcome

And I don't think trim has been mentioned, which is also somewhat about weight distribution. You need to be able to get the bow up a little at cruise too. Obviously important for top speed, but more speed with the same throttle input is an efficiency gain, and can be huge. More than 5 mph is an easy difference between down and up at cruise. That's 15% plus at typical 30 MPH cruise speeds.

tboltmike · Apr 7, 2011

Re: How to save fuel? cruising speed the only way?

This is the perfromance data from my trihull using the Lowrance fuel mgemt system. It was under favorable condition and light load. Will do another with more load. Scroll down page for chart.

View attachment Performance Data.pdf

dingdongs · Apr 8, 2011

Re: How to save fuel? cruising speed the only way?

yes it is true because if you have a planing hull and its not on the plane you will be using a lot of fuel trying to push a wall of water in front of the boat at all but tickover where there is less resistance

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