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View Full Version : Fuel Consumption at a given RPM, weighted vs unweighted



trayson
07-02-2017, 03:00 AM
So Imagine 2 scenarios:

1) The boat is cruising with no ballast, the wakeplate is down and the engine is at 3,000 RPM's and the boat in turn is doing a speed around 25mph.

2) The boat is loaded with 3,000 pounds of ballast and is likewise turning 3,000 RPM's at 10mph.


So, with the two scenarios, once the boat is up on plane and cruising; is it using the same amount of fuel for both scenarios because the transmission is a 1:1 and the RPM's are the same despite the different respective speeds?


It seems to me that because our boats have a 1:1 transmission, that if you hold the RPM's constant at 3000 RPM's, that the fuel consumption would likewise be constant and the variable would be the speed that you'd then go as you added weight to the boat.

mmandley
07-02-2017, 02:02 PM
Go run a mile and time it.

Put 40lbs in your backpack run that mile the same speed.

How do you feel?

While the RPM look the same due to 1 to 1 transmission the throttle position is quite different.

trayson
07-02-2017, 02:27 PM
Go run a mile and time it.

Put 40lbs in your backpack run that mile the same speed.

How do you feel?

While the RPM look the same due to 1 to 1 transmission the throttle position is quite different.

No I'm not talking about running the SAME speed weighted vs un weighted. Your analogy would be closer to comparing hiking with 40 pounds on our back at 2mph vs running the same distance and comparing the amount of calories burned. It might be damn close.

zabooda
07-02-2017, 02:39 PM
Your throttle position would be different as you have more energy loss due to the drag which in turn causing more wash at the prop. It is important to determine the best configuration of the boat and cruise speed for getting from point A to B. Efficiencies are thrown out once the water sports start. With ski boats the gas consumption rises with engine efficiencies at higher speeds and in my case when the four barrel kicks open.

gregski
07-03-2017, 04:31 AM
Ditto on the throttle position. If both scenarios happen with the same throttle position then I would expect that they would consume the fuel at the same rate (gallons/hour) but obviously the 10mph scenario would go less than half as far as the 25mph scenario per gallon. Throttle position and rpm = fuel burn rate. So the question becomes what is the throttle position for each of those situations? I'm pretty sure that the loaded/surfing situation takes a lot more throttle/gas for me.

I haven't figured it out exactly but I know I'm burning at least 5 gallons/hour surfing, and I think I'm in 2-3 gallons/hour cruising but I don't cruise enough to really know (not sure about rpms there either).

trayson
07-03-2017, 03:14 PM
Okay, thanks guys. that's the part that I was missing.