Outboard gas consumption ?

[Smith Point]

Thanks guys,
I think you are right. I need a pro to check it out. I'm convinced that its a carborator problem, the boat came with a 14-12 prop, higher RPM but tough getting out of the hole, switched to a 14-14 prop the boat popped out of the water, now i have a 14-13 prop good compromise. The hull is clean, and it runs great so i don't think its the plugs. Just terrible fule economy. Is there a website that i can double check RPM vs. MPH?

[Dave R]

Originally Posted by Smith Point

Thanks guys,
I think you are right. I need a pro to check it out. I'm convinced that its a carborator problem, the boat came with a 14-12 prop, higher RPM but tough getting out of the hole, switched to a 14-14 prop the boat popped out of the water, now i have a 14-13 prop good compromise. The hull is clean, and it runs great so i don't think its the plugs. Just terrible fule economy. Is there a website that i can double check RPM vs. MPH?

Not likely any website that has data for your boat. What is the gear ratio of your transmission? With that info we can easily calculate the prop slip for given speeds and RPMs.

My request for spark plug info was specifically to check the carburetor, not the plugs themselves.

Here's chart of speed vs MPG for a Malibu Response ski boat. Note how precipitously the MPG drops as the speed goes above 27 MPH. It drops from 4.3 at cruise to 1.75 MPG at WOT. That's like your boat and typical for a ski boat.



Here's a chart for a similarly sized stern drive boat. Note how the MPG does not drop nearly as badly once it's on plane. The MPG goes from 4.2 at cruise to 3.4 at WOT. That percentage of fuel use difference is pretty typical for a stern drive boat.



Note that both boats cruise at 25-30 MPH with very similar fuel burn, and they both top out at 50 MPH, but the ski boat requires about twice the HP to hit 50 because the prop thrust cannot be vectored with trim.

My advice would be to keep it below 30 MPH and above 20 MPH when possible.

[garysanfran]

I know there is intense research for using this abundant fuel for a lot of vehicles...but boats? I know the infrastructure for delivery is not there, but create a need and the infrastructure will follow.

I also wonder how much the anti-petroleum "it's a politically incorrect fuel" affect has an effect on cost? We've got a lot of petroleum in our ground.

I'm sure there is a fair amount of people who would like to see power boating go away.

[Dave R]

Originally Posted by garysanfran

I know there is intense research for using this abundant fuel for a lot of vehicles...but boats? I know the infrastructure for delivery is not there, but create a need and the infrastructure will follow.

I also wonder how much the anti-petroleum "it's a politically incorrect fuel" affect has an effect on cost? We've got a lot of petroleum in our ground.

I'm sure there is a fair amount of people who would like to see power boating go away.

LNG would be good in boats, but has some drawbacks. It has less energy density than gasoline, so it takes up more space for an equal amount of range, and needs to be in an insulated pressure vessel to stay liquid, so fuel tanks can no longer be made to fit tightly down low in the center of a boat. They need to be cylindrical or spherical for strength and that means they can never be as space efficient as a typical rectangular fuel tank with a deep V profile. Due to these two drawbacks, you could expect, storage space, and stability to all suffer compared to gasoline.

I don't know of any production LNG boats.