Most economical cruising speed

[CruiserSteve]

Hello,

Fuel consumption data, twin 8.1 big blocks. I joined this forum months ago because I am shopping for a Carver 390, 400 or 404. I currently own a Regal 400 Commodore. This boat is LOA 42', beam of 13'1", approx. 17,000 lbs. As mentioned previously in this topic the engines have more to do with fuel consumption than which boat they are in, therefore i am sharing what 8.1 Horizons get in my Regal. This boat was re-powered around 2009 with Mercruiser 375HP MPI 8.1 (496CI) and new ZF Huerth V-drives.

Here is my info. I wish I would have recorded the gph at 3200rpm, which is a good cruise speed but when reviewing photo's and video's of my gauges over the past season I don't see where I recorded that RPM/speed. I was surprised at the change in fuel usage at the lower speeds, 2mph can make a rather big difference. At higher speeds I will need a bigger wallet.

MPH recorded by GPS (Navionics and Simrad). Consumption data by Mercury SmartCraft gauges with fuel flow. The engines have 230 hours on them. Props are 19 X 22.

Fuel on board was 160 gallons, water tank about 100 gallons, waste tank empty. Very smooth water (no wind), 74 degree's air temp. GPH is for both engines.

6.0 mph is 3.5 gph (900 rpm) All GPH listed is total fuel consumed, both engines are added together.

8.6 mph is 5.6 gph (1420 rpm)

9.5 mph is 7.2 gph (1610 rpm)

16 mph is 20.6 gph (2540 rpm)

37 mph is 54.0 gph (4200rpm)

The 400 Commodore is an express cruiser style boat. No Aft cabin, no Flybridge.

[km1125]

QR_BBPOST...
6.0 mph is 3.5 gph (900 rpm) All GPH listed is total fuel consumed, both engines are added together.
...

You could do the ENTIRE Great Loop, filling up less than 26 times!!!

[CruiserSteve]

Hello KM, the Regal has two fuel tanks, 140 gallons per tank. Six miles an hour would be painfully slow but close to 9mph, basically trawler speed, the range exceeds 400 miles.

[bud37]

QR_BBPOST As mentioned previously in this topic the engines have more to do with fuel consumption than which boat they are in,

I think you will find that the boats design, LWL, super structure and especially the hull design ( displacement trawler versus flat aft vs steep deadrise ) has a great deal to do with fuel usage. Remember fuel usage is a result of the amt of hp being applied. Some hulls require less HP to run at the same speed as others due to waterline length and hull shape, just the way it is IMO. The eternal boating struggle....

[Viper]

Ya hull design matters. It's always a compromise between performance, riding comfort, stability, etc. In simple terms, performance is how the hull rides in the water and on top of it, and the power needed to get it on top of the water and keep it there, all of which in turn affects engine performance variables. A lot of other variables must be taken into account as well such as a clean or dirty bottom, type of antifouling paint, running gear, overall weight, water conditions, etc, etc.