Fuel economy?

[DSR]

Hi everyone,
I have this post on my build thread but I think it kinda got buried and I'm actually pretty curious about this, so I wanted to post it here and see what people have been coming up with on their setups.....
I know that there's not going to be any "apples-to apples" comparison on this but say, a Flying Saucer or Zip with a Mark 55 (or a newer 4-stroke), or a Mighty Mite with a 4-cylinder inboard? Just very curious I guess.....

"OK, I've gone off on a tangent and I have a question.
One of the reasons I chose a Honda R12X for powering the TNT is that it is one of the more fuel-efficient skis built, despite having a not so efficient hull design. According to my research, this 842 lb ski with lots of deadrise will burn approximately 3.1 gals/hr at a 35-40 mph cruise. With the TNT being both lighter and a more efficient hull, and using a slightly more aggressive impeller to take advantage of that, I'd guesstimate <3 gal/hr at a similar speed. With jet drives having a reputation as gas-guzzlers that doesn't sound horrible to me, but all my previous experience is with full-size 18-20' boats with big-blocks of anywhere from 500-1000 hp so economy was, let's just say, never a priority.....

I guess my question is how does that compare to other outboards and inboards here on the forum?

I like the idea that with the two 5 gal side tanks I have slated for the build that cruise range would theoretically be in the neighborhood of 125+ miles (as long as I can keep my foot out of it.... )"

Thanks everyone!
Dave

[gdcarpenter]

I have never calculated or measured my fuel consumption, but I can make 5 round trips between the cottage and the marina, about 50 miles, on a 6 USA gallon (5 Imperial or 'Canadian' gallon) cruiser tank, so 8+ MPG (US) comfortably. That's spending most of the time running between 3/4 and full throttle.

That's on a ZIP, that I have weighed out at a truck stop scale, that's a a little under 800 lb. fully loaded for a days outing, not counting people aboard. I am running a 1959 Mark35A (35 HP @ the crank so comparable to a current 30+ HP @ the prop) with a 3 blade 10" diameter, 10" pitch prop and at 5,600 RPM @ WOT I run 30 MPH. With a Mark58A I have hit 33 MPH.

My understanding is that jets are inherently less efficient at converting power to thrust so for a given boat more jet HP would be required for the same speed. Perhaps some Jet Squirt builders can pipe in.

[DSR]

Hi GD, thanks for your reply.
I guess what I'm looking for is some perspective regarding this question I've had and your response definitely helps. Thank you!

[galamb]

There is always exceptions to the (rule), but in general when talking "marine engine" you will burn about 10% of the horsepower, in gallons per hour if your run at/near full throttle.

So if you run a 30 horse at full tilt you burn 3 gallons per hour - at "whatever" speed that gives you.

If I run my 140 all out it burns close to 15 gallons per hour - back it off to about 3/4 throttle (often called "cruise speed" - a point where you have reached max timing advance on the motor but still have throttle left) and fiddle with the trim (both on the motor and the tabs) then the fuel consumption drops to about half (mid 7 gals/hour according to my fuel flow meter) - of course, the speed backs off as well.

All by itself a jet will not burn "more fuel" than a comparable outboard. If you have a 90 horse (at the crank) Jet and run it full bore it will burn 9 gallons per hour. If you have a 90 horse prop job it will likewise burn 9 gallons per hour at full throttle (and that doesn't matter if it's 2 stroke or 4 stroke - 4 strokes are only "efficient" compared to their older cousins if you keep the rpms below about 3500'ish - when you wind them up they burn just as much gas).

Why the Jet is seen as "burning more" is because of the inefficiency in converting that 90 horses to forward thrust. Unlike with a prop, a jet loses about 1/3 of it's power simply converting that power to forward motion.

So with a conventional prop if you want 90 horses "at the prop" you buy a 90. With a Jet if you buy a 90 horse you only get 60 horses (at the prop end) - so if you really wanted 90 "at the bottom of the transom" you would have to run a 120 horse powerhead to get the same speed as you did with the 90 horse propped motor.

So to get the same "performance" your Jet would burn 12 gallons per hour compared to the 9 gallons per hour otherwise.

Again, the numbers will vary "a little" and these only apply to hard chine planing hulls but even with modern engine innovations most marine engines still burn 10% of their horsepower in gas if you wind them up....

[Roberta]

I agree. I was getting about 4 gph running my old Merc 500 (50 hp) at about 4000 rpm on the river runs in TN. That got me about 27-28 mph.

Roberta

[DSR]

Hi Graham and Roberta, thanks for the replies, it definitely helps!
Graham, it makes a lot of sense about fuel consumption. It boils down to taking a certain amount of fuel to make a specific amount of horsepower, regardless of the engine, the variable is how efficiently that thrust is generated, if I've got my head wrapped around it correctly.
So 3 gal/hr at a speed of 35-40 mph is not bad at all. I haven't found anything showing the rpm this engine is running at that speed but I'm guessing in the ski it's probably turning less than 4000 rpm. At WOT the ski runs a redline of 6100 and burns 12 gal/hr at 62-64 mph, which still isn't horrible for a 165+ hp engine but a big difference from cruise mode.
Now I'm really curious to see what happens in a lighter, more efficient hull........ I guess there's only one way to find out.......

Thanks so much for your help!!
Dave