Gas Consumption.

[85SeaRay]

Hi all, I am having a discussion with a fellow SeaRay owner about gas consumption. MPG. We're planning a 330 mile trip in our boats. I say my best gas consumption is at half throttle (mid range cruising). He believes if we're not up on full plane that it wastes more gas. I know the few times I've followed him I went through way more gas but got their quicker. What say all of you?

 

[Lucky's]

Hi all, I am having a discussion with a fellow SeaRay owner about gas consumption. MPG. We're planning a 330 mile trip in our boats. I say my best gas consumption is at half throttle (mid range cruising). He believes if we're not up on full plane that it wastes more gas. I know the few times I've followed him I went through way more gas but got their quicker. What say all of you?

I'd say you just answered your own question. If time is money, you getting their quicker is valuable. But in boating terms and to answer your own question, if you're both making it to the same destination and he's going slower (taking longer) and you're going through more fuel, clearly going faster is using more fuel.


Shaun
Sent from my iPhone

 

[Lucky's]

Btw, welcome to CSR!!


Shaun
Sent from my iPhone

 

[Big Island Lifer]

The question is what is more efficient? Every boat and motor has a sweet spot. It's not the fastest but the most efficient at cruise. Being barely on plane is not efficient. If your were going to travel at displacement speed (having all the time in the world for you 300mile trek) you would be the most efficient. Your efficient cruise is probably 1.0-1.5 mpg or 20-30 gph. Just depends on how you want to look at it.

 

[KC_Sundancer]

I've found while running on plane in what feels like the "sweet spot", that if I bump rpm up a few hundred speed increased such that the shorter running time to a destination was actually saving me fuel.
As you know every boat reacts different though based load placement, engines etc...
Now if idling to the destination at 1000 rpm it takes all day but fuel use is less. Although at that speed you might as well be sailing. :smt101

 

[Getaway]

Bring her up on plane, get the tabs and trim set for a comfy ride, then back her down until you close the secondaries. That's about as good as you are going to get.

 

[KC_Sundancer]

Bring her up on plane, get the tabs and trim set for a comfy ride, then back her down until you close the secondaries. That's about as good as you are going to get.

Exactly :thumbsup: - I forgot to mention that I listen for the secondaries closing too, a narrow RPM window with the 185 v6's in our 270.

 

[CV-23]

When do the secondaries kick in on the 7.4L 454? I'm thinking above 3600rpms? My best cruise is 3600-3800rpms.....WOT is 4600rpms. My 350 in the CV-23 seems about 3300rpms. Best cruising is 2900rpms for 4.5mpg. That's an awesome number vs. the 454 that gets about 1.25mpg, Mike.

 

[rcon]

Any ideas on rpm /secondary range for 4.3l V6s?

 

[Getaway]

Not sure about what rpms they kick in. I listen for them.

You can hear them if you listen closely.

 

[northern]

With my 454s the secondary cut in at just above 3200 as that is when fuel consumption goes way up on the fuel flow meters

 

[270win]

Bring her up on plane, get the tabs and trim set for a comfy ride, then back her down until you close the secondaries. That's about as good as you are going to get.

This is the best way to find the sweet spot...in our AJ with a single 5.7, I'm at 3000 RPM (which seems to be the point where the secondaries are just about to open on every 5.7 I've ever owned) with a touch of tabs. It's getting about 17mph and the fuel computer shows 13.5gph fuel burn. Or 1.25mpg...which, compared to our 32' Express Cruiser with 2 454s, is outstanding!!!

 

[rcon]

What exactly am I listening for?

A change in pitch for the motor, or a mechanical opening of valves in the carbs themselves...

Thanks

 

[Tritone]

I've got to pay attention an hear where the carbs open up. Some of you seem to be getting more milage than I knew possible. I cruise around 21 knots at around 3050 rpm. My best estimate is I get 0.7 miles per gallon. Maybe I have a little room to bump it up keeping the secondaries closed and not lose milage.

 

[play dough]

I've got to pay attention an hear where the carbs open up. Some of you seem to be getting more milage than I knew possible. I cruise around 21 knots at around 3050 rpm. My best estimate is I get 0.7 miles per gallon. Maybe I have a little room to bump it up keeping the secondaries closed and not lose milage.

That's the difference in outdrives vs V-drives. I have the same hull and basically the same boat as yours except with twin 5.7L/Alpha1 I/O's ('89 300 DA). Depending on my load, at your 3050 rpm, I'd be real close to 30mph (26 kts). You'll out maneuver me at the dock, but I'll burn a lot less fuel getting there. I typically cruise around 2900rpm moving about 27mph and burning 16-18gph.

 

[Getaway]

Like northern said, on the big blocks the back doors start to open around 3200.

 

[hottoddie]

On my Crusader's the secondaries start in @ 3100-3150. You can see it happening on the Garmin flowmeters. Think they open slightly early due to the size of the boat and having to use a little more throttle to obtain any given rpm.

 

[JVM225]

I cruise somewhere between 28 and 30 mph (depending on conditions) at about 3100 -3200 RPM with the secondary's closed. I can hear when they open, and see it on my Garmin 4210 with the GFS 10's.

 

[IanBat]

What exactly am I listening for?

A change in pitch for the motor, or a mechanical opening of valves in the carbs themselves...

Thanks

Hi Rcon

Have a read of this page it explains the secondaries pretty well. http://en.wikipedia.org/wiki/Carburetor
You need to scroll down to the multi-barrell carburettor section.

May pay to drop the flame arrestor off your carburettor and have a look at the carby's with the article, ar search on google for the opearting instructions of what carby you have fitted. With out the engines running,have someone shift your throttle levers and note the movement of the butterlys etc, it will give you an understanding of what is happening. I find if you can grasp an understanding of how it is operating then you have more chance of hearing feeling what is happening.

Basically with a four barrell carburettor, with only primarys operating, two barrells are open, so the engine is sucking air (air fuel mix) in through 2 barrels (openings) through the carbburettor.
When the secondaries (the other 2 barrells) open it is sucking through 4 barrells. Far more air and fuel entering, thus higher revs.

To understanding this, Positition your mouth as is if you are going to whistle except instead of blowing, suck through your lips, you will feel a restriction to the airflow. Now open your mouth wide and breath in, no restriction, and air enters much quicker.

So when the secondaries open, the resultant more air entering the engine will generally change the engine noise, you may detect more of a roar, more air bing sucked in.

As others have said here, flow guages can make it easier. I run the basic Navman fuel flow gauges, one per engine. I can jump the boat up onto the plane and then check my fuel flow meters, by slowly bumping the throttles back I can drop the fuel flow back without dropping much speed, so generally I am looking at both the speed and the fuel flow at the same time. There is not much speed difference in mine between 76Lph per enging and 38Lph. Thats the difference of Primarys and Secondarys.

It is well worth investigating fuel flow meters, I fitted one on my Haines Signature with a 140Hp, and ths one came withem fitted. I would be lost without them. And I am a mechanic and believe I have a fairly good ear for engine running, but the cost for the flow meters for what they give you is well worth while

 

[rcon]

Thanks, Ian. That's some good reading. Think I might go do some playing this weekend.

Fuel flow meters high on wish list, need to find some compatible with our raymarine gear…