Outboard gas consumption ?

[upthesaukee]

With my 135 HP Optimax I had on my old boat, I also had the "smart gauges" that were available, one setting included GPH and total gallons used. (I almost wrote total gals used!!! that would have started some off topic quips ). Now, I have a carbureted Merc I/O and per Mercury, the smart gauges are not available.

I know I can get a flow meter installed, approximately $200 plus installation, but will wait until my boat disposable income can best be used on that luxury. It was handy to see at a glance where my best economy was, MPH VS GPH.

[Smith Point]

Hi Jack 2,
I have a 22' supra ski boat (inboard) W/ a 454, holly 4 barrel, WOT 4200rpm. how do i find my MPG? I use about 1/2 tank from Smith Point to Wolfboro and back ($50?) whiskey tango foxtrot. Any info would be appreciated.

[Charlie T]

Originally Posted by Smith Point

Hi Jack 2,
I have a 22' supra ski boat (inboard) W/ a 454, holly 4 barrel, WOT 4200rpm. .

Hi Smith Point. I believe you may have an issue with either your prop or your tune up. 4200 at WOT is way too low for a 454. Most of the marine V8's are calling for a WOT of around 4800-5000 rpm. Has the max RPM always been that low or is this something that has happened over time. You may want to find a good marine mechanic to sort out your issue.
FYI I've got a 25' Chap stern drive with a 454 and I'm at 5000-5200 wide open at a little over 50mph. I don't drive at that speed but that's where I end up with the "hammer down"

Good Luck

CT

[Baja Guy]

Charlie is right, you may be getting some economy back once the motor will turn it's right rpm.
Also check that the hull isn't all loaded with marine growth, that will drag a lot in some boats.
I think you could pay back some boat care in savings in a reasonable time if you are whipping off $50 trips all the time!
My Chevy 350 gets it torque right in the middle and that's a nice low plane around 23 or 25 mph with my boat. I don't know how much effort the motor needs to give to keep your boat moving efficiently but my guess would be 200 to 500 rpm above plowing, just on an easy plane.
I'm not going for a flow meter unless I run out of higher priorities first.
I still want a bimini and some trim tabs. Plus I don't really have the right pitch prop. I have one that I have borrowed but it's not fair to keep someone else's stuff. I have one that's too small and 2 that are too big.
My borrow prop is a 21, gotta find someone to trade my 19 or my extra 23 for a 21 of my own.

[Baja Guy]

delete, I'm way off topic!

[Dave R]

If it's an oval port 454, 4200 RPM is OK but not great. They top out at 4600. The square port big block Chevys breathe more freely and are have cams and reciprocating parts that let them exceed 5000 RPM. The 8.1/496s are a whole different breed with heads that split the difference between oval and square ports in flow and have 5000ish RPM red lines.

Smith Point, check your spark plugs for correct tan/gray color. If they are black you are running too rich and that will hurt power and economy.

A ski boat has a disadvantage compared to I/Os and O/Bs in that it can't trim the prop up to get the bow out of the water. Fuel economy will suffer dramatically as you go faster than minimum planing speed because unlike boats you can trim, going faster will only push the bow down harder.

[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.