Optimal Cruising speeds for best gas milage

Kevin Kidd

Kevin Kidd

Member
Oct 16, 2017
41
California Delta
Boat Info
270 Sundancer 2000
Engines
7.4 Mercruiser w/ Bravo 3 outdrive
We have a 2000 270 Sundancer powered by a single MerCruiser V8 7.4 MPI with Bravo 3 outdrive. Only had her since late Oct. of this year, so we are still getting to know each other.
I find that about 3400 RPM seems to be her sweet spot for fast cruising. My wife wants to cruise slower to enjoy the ride and conserve fuel. What are your opinions on speed? What do others "slow" cruise at? I will add that I have always had runabouts and this is my first "big" boat. I've been told by a mechanic that I drive her like a ski boat. Meaning that I get on the throttle and I do tend to take fast turns, something I am working on correcting. I understand that depending on the weight in the boat will affect mileage, as will the tank levels etc. I'm looking for what speed folks cruise at for a leisurely ride. We are on the California Delta so its a lot of channels and sloughs, so a lot of water that's not wide open.
 
Once you are on plane the gas mileage (measured in miles per gallon) will be the same from on plane to about 3500rpm. At cocktail cruise speed, say 10mph the Mpg will be about the same, but you will be going a lot slower. Planing boats are not fuel efficient. So if you don't have an agenda or destination to get to, cocktail speed will save you money. If you are going somewhere specific, your 3400 probably is about as good as it gets in MPG. Maybe 3100 to 3400.
 
Hull speed is the best you can normally do at low speeds.
Maximum Hull Speed
Maximum hull speed (in knots) = 1.34 x the square root of the waterline length (in feet)
When we had a 28 footer (water line) beam 10.5 feet we found at 24 knots we burnt 1 liter a minute IE 60 an hour or 15 US GPH. Boat had upper and lower helm. Weight was about 8,000 pounds at hall out.
 
Kevin, one way to figure out what your best slow cruising speed will be is to watch your hull.

If you are moving at idle through the water your hull will be riding flat. As you step up the speed the bow tends to ride up on the bow wave and the stern digs in.

The more the stern digs in the less efficient you are running. Part of that comes from pushing a big bow wave, part from dragging the stern through the water instead of it gliding over the water as it would if the hull were flat.

If your water line length is 22' (for example), using the formula northern provided (which is very accurate BTW) you'd have:
Hull Speed = 1.34 x (sq root of 22)
= 1.34 x (4.690)
=6.28 kts
=6.28 x 1.15 (convert kts to mph)
=7.22 mph.
At any speed above that you're going to find your stern settling down in the water and your fuel economy going down.

To give an example of what I'm talking about, here's my boat at about hull speed, about 7-8kts....
A_P7280145.jpg

and here she is at a very INEFFICIENT speed, about 10-11kts....
A_P7280165.jpg

In the first picture the stern is being supported by a wave that is generated by the bow as it moves through the water. In the second pic the boat is outrunning that wave and the stern has settled way into the water. Not a good way to cruise.

If you look at my avatar pic, I'm running well below hull speed, about 5-6kts. Note the very small bow wave and small wake. Very Efficient and you won't spill drinks at this speed. :)
 
Gofirstclass said:
Kevin, one way to figure out what your best slow cruising speed will be is to watch your hull.

If you are moving at idle through the water your hull will be riding flat. As you step up the speed the bow tends to ride up on the bow wave and the stern digs in.

The more the stern digs in the less efficient you are running. Part of that comes from pushing a big bow wave, part from dragging the stern through the water instead of it gliding over the water as it would if the hull were flat.

If your water line length is 22' (for example), using the formula northern provided (which is very accurate BTW) you'd have:
Hull Speed = 1.34 x (sq root of 22)
= 1.34 x (4.690)
=6.28 kts
=6.28 x 1.15 (convert kts to mph)
=7.22 mph.
At any speed above that you're going to find your stern settling down in the water and your fuel economy going down.

To give an example of what I'm talking about, here's my boat at about hull speed, about 7-8kts....
A_P7280145.jpg

and here she is at a very INEFFICIENT speed, about 10-11kts....
A_P7280165.jpg

In the first picture the stern is being supported by a wave that is generated by the bow as it moves through the water. In the second pic the boat is outrunning that wave and the stern has settled way into the water. Not a good way to cruise.

If you look at my avatar pic, I'm running well below hull speed, about 5-6kts. Note the very small bow wave and small wake. Very Efficient and you won't spill drinks at this speed. :)
Click to expand...
Thank you, I love the visual.
I'll be trying this out this weekend.
 
northern said:
Hull speed is the best you can normally do at low speeds.
Maximum Hull Speed
Maximum hull speed (in knots) = 1.34 x the square root of the waterline length (in feet)
When we had a 28 footer (water line) beam 10.5 feet we found at 24 knots we burnt 1 liter a minute IE 60 an hour or 15 US GPH. Boat had upper and lower helm. Weight was about 8,000 pounds at hall out.
Click to expand...
Thanks!
 
I have a 1994 270 with the same engine and drive. I have a fuel flow meter. Here are the flows at different RPM:
600 rpm = .01 gpm @2.9 mph or 4.8 mpg
1200 rpm = .06 gpm @ 7 mph or 1.9 mpg
3200 rpm = .26 gpm @ 23 mph or 1.4 mpg
3400 rpm = .28 gpm @ 26 mph or 1.5 mpg

7 mph is hull speed and is the most efficient in mpg on plane 3400 rpm is most efficient.

Hope that helps.

Happy New Year!
 
We had twine V6, 3.8s. To go 24 knots we ran at 3,800 RPM.
Based on Mike's numbers seem the same.
We now travel at 6 knots and burn 1 US gallon per knot. Mostly that speed to watch for logs. If wind picks up we go 8 knots and burn about 9 GPH.
 
Install a fuel flow meter so you can monitor usage in real time and it will likely pay for itself.
You’ll find that different sea conditions, boats and power combinations have different optimal speeds.
My rule of thumb is to operate at 3 basic speeds.
1. No Wake Zone speed dictated by location and signage.
2. Cocktail or Hull Speed which is generally around 1200 RPM on my current boat. My old boat was similar.
3. Plane speed which is about 2350 RPM on my current boat, and was about 3100-3200 for the 5.7 gassers on my old boat.
The first you can’t do much about, but you can optimize fuel useage at the other two with slight adjustments as dictated by fuel flow monitors.
I had Garmin electronics on my old boat and installed their relatively inexpensive GFS 10’s on each engines fuel line. They were fantastic. I wish there was a relatively inexpensive option for the diesels in my current boat.
 
JVM225 said:
Install a fuel flow meter so you can monitor usage in real time and it will likely pay for itself.
You’ll find that different sea conditions, boats and power combinations have different optimal speeds.
Click to expand...

I agree. I refer to the best cruising speed as a “moving target.” My meters not only allow me to find the best speed in any given circumstance, but by calculating range and fuel onboard with incredible accuracy, I can carry less fuel.
 
carterchapman said:
If I run hull speed (800 RPM - 10 kts), I burn 6 GPH total. At 80% load (2000 RPM) I burn 64 GPH Total for 24 Kts...
Click to expand...
Dang, Carter, those Mans are pretty thirsty. I don't get to those kinds of fuel consumption figures (64 GPH Total for 24 Kts) until I'm running at WOT at 2150 rpm's at 30.3 kts. I love my kitties.
 
I miss my 3126's in One Sweet Day...1.3 MPG - better than my gasser 340...

Reward weighed in at 78,000 lbs during survey, so all in all, .4 MPG is not so bad.
 
carterchapman said:
.4 MPG is not so bad.
Click to expand...
Dammmmm Carter, you should have warned me to be ready for that. You just about made me choke on my first sip of coffee. Mine's not a helluva lot better with 65gph at WOT at 30.3kts, but I just HATE seeing that tiny little number expressed like that.

Ahhhh, the joys of owning a bigger boat. Thank heaven mine likes to cruise like it is in my avatar photo.
 
image.jpeg
image.jpeg
I have the same boat and a fuel computer so I can answer this based on my experience. I have 22 props and no a/c or Genny. The pic above was on a trip to block island with
approx 2 footers.
I find when I slow down I lose some mpg but keep in mind you will reduce some gph. My computer is hooked up with my gps so I get a real sense of best fuel economy. The truth is my gut was right on. You develop a sense of your sweet spot as you learn your boat. My motor has 1000 hrs so it's not in its prime but she runs perfect for me. If I remember correctly this trip was burning around 16-18 gph. As you can see I leave the display on mpg as that is what I monitor mostly.
Let me know if you want more info.
Mark
 
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