Roughly... most economical planing speed should be somewhere in the low to mid 3,000 RPM range. Compare that to the speed you get at that RPM. 35mph sounds high for an economy cruise on that boat. I'm guessing somewhere around 25mph.

 

Sounds about right to me.

 

I would go with what you typically cruise at as this will provide the most realistic eta and fuel burn on the Navionics app.

 

Slim, I think Lazy nailed it. There are actually a few different speeds for any boat and the fuel economy will vary with each speed, sometimes dramatically.

The lowest of those speeds is just about idle, but boats tend to wander side to side quite a bit and require constant steering effort to maintain a heading.

Next up is "hull speed" and this will vary with the length of the hull at the waterline. There's a formula to figure it: 1.34 x square root of the length of the hull at the water line (LWL). That will be a very efficient speed.

As you go above that speed the stern of the boat tends to sink down into the water because you're outrunning your bow wave. The bow rides high and the boat is plowing through the water. This is VERY a inefficient speed.

As you increase speed your boat will transition from plowing through the water to riding on plane on top of the water. This speed, "planing speed" is also efficient and may be your boat's most efficient way to run. The bow drops and the stern of the boat is supported by the water running beneath the hull. This efficient speed may well be from about ~3200 rpm's up to ~4,000 rpm's.

From there, the faster you run the less efficient your boat will be. WOT speed is REALLY sucking down the gas.

 

Thanks guys. I'll try to pay closer attention to rpm at different speeds while on plane and see what the lowest rpm I can run at and stay up on plane.

 

To help ballpark consumption I used this site.

http://www.boat-fuel-economy.com/mercury-mercruiser-3.0-181-fuel-consumption-us-gallons

I don't have fuel flow meters on our engines so instead I went for test runs at all different RPM's and recorded the GPS measured speed at each. I did idle, 1200,1500,2900(planing),3000,3200,3400,3600,and WOT

Armed with the speed numbers I went to the page for our twin engines and was then able to calculate rough GPH/MPG at different speeds. Using a few different fill ups and tracking the mileage traveled I was able to verify that site is pretty accurate. Flow meters would be best, but they're expensive, so I thought this method worked pretty well.

 

 

Here is a fuel flow chart for a 350 mag.
As you can see, from about 3300 to 4200 RPM, you are using roughly the same amount of fuel per mile traveled, with the option of speeding up or down for conditions, bladder urgency, or dinner time. I was surprised that the efficiency was so high over 4000.

 

So now you guys have me scoring the internet for something I have for my car. I have a bluetooth adapter that plugs into my data port on my car that connects to my head unit running Android and I can see real time engine data on the screen. I see Fusion has something similar but does anyone know of an Android enabled app and adapter that for Mercruisers that works in a similar fashion?

 

https://fox-marine.com/

 

Yeah they don't have an android app yet. I run Navionics on my tablet and it would nice to flip back and forth between the 2 apps or even run them is split view. I would love to use something like this https://www.amazon.com/Panlong-PL-B...5&sr=1-5&keywords=bluetooth+obd2&dpID=51c3Ty9HDPL&preST=_SY300_QL70_&dpSrc=srch

 

Yeah they don't have an android app yet. I run Navionics on my tablet and it would nice to flip back and forth between the 2 apps or even run them is split view.

Ah sorry I didn't read it close enough, I saw Android mentioned but didn't realize it was coming soon.

 

We don't pay much attention to a partucular cruising speed when plotting a course since that plotting usually takes place in a slip the day before going on the cruise. Once the course is in the computer, we will sometimes add different speed data to get various ETAs for further planning purposes. However, cruising speed on open large bodies of water is always heavily dependent on wave heights. You end up making the best speed that is comfortable, and the ETA is displayed based upon real conditions what ever those happen to be on the day of the trip.