Fuel Consumption

[Offset]

I have a 22' Classic with a 454 Magnum. Much of our boating in the past has just been cruising a little over idle and puttering along. However I wonder with the design of the Classic hull is there an optimum cruising speed for fuel efficiency?

I realize that there are many factors in what is perhaps a simplistic question. Prop, drive and gear ratio and the motor itself. Weight in the boat I would think critical as well.

Anyone ever done any testing on this?

Have a great boating season everyone.

[Ghost]

Others may have real experience or measurements.

I'm guessing that despite the large range of factors you mention, peak miles-per-gallon would come between 27 and 33 mph. (Rationale: my experience is that most efficient speed is the slowest speed once the hull planes out well. In the lower 20s, I suspect a 22 Classic is still squatting too much, wasting fuel. So somewhere from upper 20s to low 30s would be peak.)

Further guesses:

1. Your peak MPG will fall between 2300 and 3000 RPM.
2. Your peak MPG will be somewhere in the ballpark of 3.6 MPG.
3. Anywhere from about 27 to 33 MPH, your MPG won't vary a whole lot. Whatever RPM first reaches a happy, non-squatting plane, to about 500 RPM more, will be ideal. Above that the dropoff will start much more significantly.
4. Trimming the drive up/out as much as possible without porpoising or other adverse effects will be most efficient.

If you take some solid RPM measurements and GPS speed readings, you can probably calculate the answer pretty closely. BoatTest has plenty of data for your motor, with Floscan gallons-per-hour measurements at 500 RPM increments. So if you dial in at 2000 RPM and record your GPS speed, then dial up to 2500 and do it again, on up through 4500, you can use the BoatTest fuel consumption data to figure pretty good estimates of miles-per-gallon.

I'm interested in anyone's real-world experience or measurements that susbstantiate or refute any of these guesses.

[Walt. H.]

Do exactly what Ghost suggested, that's the best advice explained in the simplest manor for just about every boat.

[osur866]

Mines not a 22 rather a 18 and I can tell you anything under 3,000 RPM's will be much happier on the gas gauge, fuel consumption is gonna go way up 3,000 and above!

[zelatore]

Ghost pretty much nailed it.

For any planing hull you'll see the best economy at very low speeds, then a big dip as the boat comes up on plane, then it will generally improve somewhat as the bow drops, then it will steadily get worse as the speed continues to increase.

When I'm training new owners on larger boats, I usually simplify it down to 'look at your wake - the bigger it is, the worse your economy'.

That an over simplification, but it helps them visualize the 'bad spot' where the boat isn't fully planed off yet. It's easy for them to understand that pushing all that water can't be good.

[Pismo]

2500 rpm/34-35 mph with a MPlus 25" prop in a 22 was the highest mpg in the Powerboat Mag test. 3.2 mpg with an HP500. I would think a little better with a 454.

[craigdskilling]

Mine's a 18, simple don't open the 4 barrel carb.I can run all day at 2700 to 3000rpm at $30 maybe.

[MOP]

@3300 doing 43 mph gps I get a tiny tad over 4 mpg, that is just before the back barrels open it is also 22 but with a 383 SB.

Phil

[Offset]

Thanks to everyone for your knowledge on fuel consumption.

[Carl C]

You bought the wrong kind of boat.....

[Pismo]

The quadrajet was great for fuel consumption. Those tiny front barrels. If you did not get into the secondaries, it would use little gas. If you did get into the secondaries then it would use a ton.

[Rumblefish]

As I set out for a lunch run I tell the wife.. " its a 40 mile run so im going to take it easy and save fuel"

I plane out at 3000 .. trim er up watch the rpms climb to 3300 and set back the rpm to 3000..

Aaaah this is great im thinking.." we could cruise all day like this I tell her"..

Minutes later my hand is without my knowledge pushing forward on the throttle, as the sound increases so does my joy for cruising 50-55mph and now I have chosen to get there faster instead!!!

After all thats why I love my Donzi....

[Offset]

You bought the wrong kind of boat.....

Carl, I sold a high performance outboard to buy the Donzi. Based on years of experience with that boat (and several others) I wanted something with some freeboard so the smallest wave would not soak everyone on board. I wanted something that would handle rougher water that was comfortable and if need be get us home quickly. I wanted a boat that I could afford that was quality built and retained some of its resale value that was trailerable. Above all I wanted a boat that looked good.
While I like to go fast like everyone else I do enjoy just idling along enjoying the sights. Speed at least for me is not what it is all about. So based on that what boat should I have bought?
If I have the knowledge of how to save a little fuel that just means more time on the water. That was my reason for asking.

[glashole]

is there any real world numbers on using more fuel to run faster

but using less fuel fbecause the boat is running for less time

[Ghost]

is there any real world numbers on using more fuel to run faster

but using less fuel fbecause the boat is running for less time

A good point. If I understand the question correctly, I think examining everything in miles per gallon takes care of this. You can go at idle for ten times as long, with very low burn. You can go at WOT in no time flat. Reducing to miles per gallon takes care of the time difference, giving you the fuel burned to cover the distance.

On to the question, for real world numbers, I think the optimum usually remains the slowest speed where the boat has planed out happily with the bow down, but with the most trim it will take. Commonly around 3 to 4 miles per gallon in small single engine boats like Donzis. With I/O performance boats, I think WOT gas mileage usually drops between one third and one half from peak. 40% is probably a decent guess for any performance boat. So, maybe 1.8 to 2.7 miles per gallon for things the size of classic Donzis. I think this compares favorably with inboards and other less-sleek designs, which suffer more from hydrodynamic inefficiency at higher speeds. (The "pushing a brick" syndrome.

To add some color around your point (faster with greater burn rate, but for less time), consider an example of a 23 degree deadrise 23-foot performance boat I just pulled off BoatTest:

• Peak mileage at 2500 RPM, 27.8 miles per hour, 8 gallons per hour, 3.48 miles per gallon
• WOT at 4800 RPM, 62.4 miles per hour, 32 gallons per hour, 1.95 miles per gallon

So, at WOT it was burning gas at 4 times the rate of the best cruise. But the boat was going well over twice as fast. So WOT gas mileage dropped about 44 percent from the peak.

I think this is very typical. WOT sees 4 times the burn, more than twice the speed.