Boat fuel milage sucks

[rockdog57]

Just got back from a five day trip to Lake Powell. We went with my nephew and his kids. He owns a 1984 FourWinns 20 foot openbow. It has a 5.7 liter with quadrajet carb, 25 gallon tank. We have a 1989 Crestliner crusader 20 foot openbow with a 5.0 liter with the Mercruiser two barrel carb, 35 gallon tank.
His boat obviously has more power. But here's the kicker. His boat gets way.......... better fuel economy. I'm talking, we burned thru 35 gallons of fuel and he has used like 15 to 17. Both boats carrying similar loads. Both running props that have the wot at 4200 rpm.
Could the two barrel carb make that much difference? I mean they are both merc outdrives and motors. Both basically the same motors (Chevy small block)
My boat is in great shape with not a lot of hours on it, so engine worn out isn't the issue.
What am I missing here that would make that kind of fuel consumption difference?

 

[QC]

Re: Boat fuel milage sucks

Fuel use is a somewhat squishy thing to be comparing. Things like load, speed, trim, hull condition, prop condition, hull type are all variables that could negatively affect your boat vs. another. Was all else the same? Distance? Time? Speed? This could be bad data too even though you are the collector. Of course engine condition can make a difference, but that kind of difference is a little beyond a poor running engine. That bad and your exhaust would be horrendous.

 

[LippCJ7]

Re: Boat fuel milage sucks

I would be interested in the weight of the two boats, due to the age I am guessing that neither boat is owned by the original owners so I wonder if rockdogs boat is retaining water? add a thousand pounds of water to any runabout and it would seriously affect fuel mileage. maybe go get them both weighed?

I would check the Carb though, may be time for a rebuild, I wish you had captains call, I would ask if there is a lot of soot in your exhaust or something that would indicate your running rich....

On another note, you just got back from Powell? I'm so jealous, you suck

 

[cr2k]

Re: Boat fuel milage sucks

Unless he was running on hard with the all 4 open his primary (front 2) venturis are way smaller then your 2 bbl.
You could have more waterlogged foam then he does and more weight. Different hull design. Too many variables.

 

[americaneagler77]

Re: Boat fuel milage sucks

if your both running together your boat might be working harder to maintain pace than his. this is the same reason in a dodge truck the 5.7 hemi gets better mileage than the 4.7 magnum.

 

[Silvertip]

Re: Boat fuel milage sucks

Put a vacuum gauge on both boats and you will find the 2bbl has significantly less vacuum and hence is sucking more fuel (vacuum goes down as throttle opening increases). Why?? As pointed out, the 4bbl has very small primaries compared to the 2bbl. If the 2bbl is open to the point where the power valve is also open, then that makes the situation even worse. However, just speculating on condition, weight, etc., is not sufficient to form reasonable conclusions on why the difference. Only thorough inspection, engine tuning, proper prop selection etc., and more "scientific" testing can pinpoint the reasons for the difference.

 

[DuckHunterJon]

Re: Boat fuel milage sucks

Unless he was running on hard with the all 4 open his primary (front 2) venturis are way smaller then your 2 bbl.

Bingo - this would be my guess. For a while, a friend of mine had a 68 Cougar with a 302 and dual 450 carbs. Since at a cruise, only the primaries on one carb were open, the car got amazing gas mileage. Floor it and all 8 barrels would dump gas about as fast as you could pour it in. Your buddy's boat was loafing along and getting great mileage.

All the other factors mentioned will contribute as well.

 

[rockdog57]

Re: Boat fuel milage sucks

No water in the hull. Hardly a couple of cup fulls after five days. Both boats were carrying about the same weight. Same distance covered as we were traveling together. But the motor running rich and two barrel vs four barrel with only the primaries open sounds pretty logical. And the smaller engine vs bigger makes sense. Hmmm, might be time for a intake and carb swap.

 

[LippCJ7]

Re: Boat fuel milage sucks

No Rock your missing the idea, there are openings in your hull that you do not have access to, the manufacturer filled these with foam and over time the foam could have become water logged, its pretty common with boats that are stored outside or various other factors, but one of the quickest ways to find out if you do have this problem is to weigh your boat, completely empty your boat of equipment and go to a local landscape yard or scrap yard that has a scale and get the weight, we can find out how much the boat weighs at manufacture and subtract the remaining gas and trailer weight and compare the two numbers.

Problem is the more I think about it with the fuel mileage numbers you put up I'm leaning towards a carb issue

 

[sschefer]

Re: Boat fuel milage sucks

In your original post you said both of you are turning 4200 RPM are you both getting the same top end speed? If you are, then your fuel consumption on the 5.0L will be higher simply because you require more fuel to produce enough hp to reach 4200 RPM than the larger displacement 5.7L does. The 5.7L motor is a very efficient when it comes to it's ability to produce horsepower per cu inch of displacement. It would seem that the 5.0 should also have this ability but it doesn't because the geometry is not as well matched. This is a layman's answer to your question, I hope it's sufficient. Just for grins I'll throw this in, the geometry of the inline 6 is the closest thing to a perfect engine that there is and believe it or not it's all about balance.

 

[TilliamWe]

Re: Boat fuel milage sucks

4200 rpm is on the very low end of the WOT RPM range also, isn't it? Those motors should turn 4400 at least, maybe 4600. Your boat, with 45 less cubes and the same load might be working extra hard to maintain its cruising speed. But that's a lot of variables.

 

[Silver Eagle]

Re: Boat fuel milage sucks

A lot has to do with the grade of gasoline you are using. Out boards use 87 octane, I/O's shouldn't use anything less then 89 octane. If your using 87 octane that could be one of the problems. put a couple of gallons of Hi test in the tank and see if that makes a difference wind drag could be a problem. Did you have your top open.

 

[QC]

Re: Boat fuel milage sucks

There is an awful lot of speculation here. Why does the smaller engine require more fuel for the same horsepower? It doesn't. Air to fuel ratio same (if right), load same (maybe) and displacement is smaller so throttling losses are less, so should be better efficiency. Yes, the carb could be all f'd up, but this displacement stuff is arse backwards. If your theories are right then a 502 would get better fuel efficiency than a 3.0 in an 18 footer . . .

 

[haulnazz15]

Re: Boat fuel milage sucks

A lot has to do with the grade of gasoline you are using. Out boards use 87 octane, I/O's shouldn't use anything less then 89 octane. If your using 87 octane that could be one of the problems. put a couple of gallons of Hi test in the tank and see if that makes a difference wind drag could be a problem. Did you have your top open.

I've never seen a requirement on any I/O to run 89 octane. Some require 91/93 in high-performance applications, but 87 runs fine in most I/O's. They are usually low-compression engines (like 8-8.5:1 static).

 

[TilliamWe]

Re: Boat fuel milage sucks

I've never seen a requirement on any I/O to run 89 octane. Some require 91/93 in high-performance applications, but 87 runs fine in most I/O's. They are usually low-compression engines (like 8-8.5:1 static).

Exactly haul. Silver Eagle can waste his money all he wants, but it doesn't make it right. My 1997 Mercruiser 5.7LX EFI Gen+ recommended 87 octane. This notion of improved fuel mileage with 89 is even more flawed when you realize that to get to 89 octane, most (all?) gasoline refiners add ethanol to it. It is a proven fact that ethanol LOWERS fuel economy.

 

[AZBoatDreamer]

Re: Boat fuel milage sucks

I would be interested in the weight of the two boats, due to the age I am guessing that neither boat is owned by the original owners so I wonder if rockdogs boat is retaining water? add a thousand pounds of water to any runabout and it would seriously affect fuel mileage. maybe go get them both weighed?

I would check the Carb though, may be time for a rebuild, I wish you had captains call, I would ask if there is a lot of soot in your exhaust or something that would indicate your running rich....

On another note, you just got back from Powell? I'm so jealous, you suck

Agree about the Powell Jealous comment. Havent been there yet.