fuel consumption

[reelrecles]

reelrecles here...just wonderin, i have a 1989 340 sundancer twin 454 v drive configuration, trying to figure out where im gonna get my best fuel consuption, is that going to be at 20 knts or 8-10 knts, and can anyone tell me what the hull speed for this boat would be. thanks have a nice day

 

[Alex F]

I don't know what's the sweet spot for your boat is, but 8-10kts sounds like almost worst consumption numbers b/c you'd be using high RPMs (I'm guessing over 1700RPM) and not being on the plane.

Slow speed does give you best fuel economy, but when you go like 5-8MPH ranging from 1100-1300RPM. While all boats have diferent sweet spots, I think majority would still be applicable to these numbers.

 

[duman95]

I'll go out on a limb.. But I would imagine any RPM range, cruising, that keeps the secondary's on the carbs out of the equation is a good start. I would think that a 20K-22K cruise around or just under 3000 RPM would give you the best economy..

Well by 'economy' I mean around 30-32 gph... @cruise...

 

[VinceR]

Your most economic speed is “hull speed” the formula is as follows:

V=1.34x sq. root of LWL


use 30 feet as LWL which is “legnth of water line”

V=hull speed

7.339=1.34x5.477

 

[fc3]

reelrecles here...just wonderin, i have a 1989 340 sundancer twin 454 v drive configuration, trying to figure out where im gonna get my best fuel consuption, is that going to be at 20 knts or 8-10 knts, and can anyone tell me what the hull speed for this boat would be. thanks have a nice day

Generally with a planing hull the most efficient planing speed is the slowest speed where the hull runs at a modest bow-up angle. Too much bow angle and the engines are just burning fuel to dig a hole in the water. Past that point and friction losses increase substantially. Bring the boat up onto plane. The speed at which the bow comes down to a good running angle is about the right speed.

Your most economic speed is “hull speed” the formula is as follows:V=1.34x sq. root of LWL

use 30 feet as LWL which is “legnth [sic] of water line”
V=hull speed
7.339=1.34x5.477

No. That is a simplistic estimate for displacement hulls. Does not apply to planing hulls. Back to naval architecture school.

There is no hull speed for a planing boat. Sub-planing speeds are all speeds below that which the bow is not climbing onto the bow wave.

Best regards,
Frank

 

[VinceR]

Frank,

 

[osd9]

uh oh!!!! here goes that bow wake discussion again!!!!!

 

[VinceR]

Frank,

I disagree. That formula, although best used for a displacement hull, should be good for a planing hull. I find my best MPG is at 1000 to 1200 RPM at about 6.5 MPH. Close to the formula. I do not know the exact LWL of my 340 but I would guess at about 29 to 30 feet. Frank, I have never been to Naval Architecture School. I am just an amateur that loves boats.

 

[VinceR]

I do not know about bow wave but I have researched my performance with GPS and fuel flow. By the way, Frank, you are my hero. When I grow up I will have a 400SB with diesels.

 

[NorCal Boater]

I have that same boat and I can give you what I have discovered.

I try and get the boat on plane as fast as possible and then throttle back to somewhere between 2800 to 3000 rpms. If I am running light on fuel I can run 2800. If full fuel and a full load I need to run a bit higher rpms. This allows me to cruise at 21-24 mph according to the GPS. I also found that I get my best results with a bit of tab down. You will need to play with it a bit.

I have no scientific proof but I estimate I'm burning about 25-30 GPH at that speed. It's currently a bit worse than that as the bottom is due for cleaning which will happen next month.

As I said I have no scientific data to rely on. I have experimented a lot and the numbers I stated are where the boat just seems to like to run. It will go faster. The GPS has indicated 33 mph a couple of times with a light fuel load. And on the other side she seems to start to fall off plane at about 19 mph.

Now that fuel prices have come down a bit we are planning some longer trips and I am planning on doing a better job of recording the data.

Good Luck.

 

[Tail Chaser 661]

Engines are most efficient at Max TQ
The 454ci max tq is at 3200rpm.
So you don't want to spin faster then that.
Spinning high rpm is your worst enemy.

My sisters two 30ft Fountains with twin 502s and 454ci get best mpg by....
Getting boat up on plane.
Then backing off the throttles. But still maintaining plane.
That will get best mpg.
That's what they found out using fuel flow gauges.

Her boats get better mpg then mine :smt017

 

[berth control]

Comparing a 30ft. go-fast that is made to be light and fast to your boat is not really apples to apples. Economy is more about power, speed, weight, beam, and hull design than length. Your boat has a steep deadrise at the transom, causing a smooth ride but sacrificing economy.

My opinion on the original question is that the best economy will be at no wake speed, but the next best will be at a slow plane, like others said. I am not sure for that specific boat, but that seems to be the trend on most planing hulls that I have researched.

 

[Tail Chaser 661]

Its hard to believe twin big blocks, In any configuration.
Can and do get better MPG then my single 5.7 engine.
Problem is, I am underpowered.

Just like twin 5.7 engines would get the same or better mpg
Over your twin 4 cyl engines. With a big performance improvement. Its the Power/Tq to wgt ratio.

Them 30' Fountains weigh about the same as my 26' Dancer.
I'm aware of there more efficient hull. Fresh water also.

Anyway!......Above 3200rpm the big block really starts sucking fuel.
Backing off the throttle to as slow as you can tolerate should get the best MPG.

I read an engine test on the....
Merc 8.1 375 hp
Volvo 5.7 320hp
Merc 6.2 325hp
At 2500 rpm. All engines burned the same gph fuel rate.
WOT with the big block. Burned massive amounts of fuel.

 

[berth control]

You think twin 5.7's would get better fuel economy than my 4 cylinders?

 

[Hampton]

Its hard to believe twin big blocks, In any configuration.
Can and do get better MPG then my single 5.7 engine.
Problem is, I am underpowered.

That's what this thread is all about. If you can run your engine in its efficiency window, you can maximize your fuel economy. If you have to run your engine at the high end of it's operating envelope to stay on plane, then a larger engine will likely give you better economy. Now, 1 small block compared to 2 big blocks, that's less likely.

 

[berth control]

Does anyone else think the twin 350s in my boat would be better economy than my 4cyls? the 4's keep the boat on plane at about half throttle. I can plane at 13mph, i think largely because they light aluminum motors. I cruise at about 25mph on gps and max at 35, so they are not really working that hard to keep the boat on plane.

I am just curious, I would have never thought the bigger motors and more weight would provide better economy, but maybe I am wrong.

 

[NorCal Boater]

Does anyone else think the twin 350s in my boat would be better economy than my 4cyls? the 4's keep the boat on plane at about half throttle. I can plane at 13mph, i think largely because they light aluminum motors. I cruise at about 25mph on gps and max at 35, so they are not really working that hard to keep the boat on plane.

I am just curious, I would have never thought the bigger motors and more weight would provide better economy, but maybe I am wrong.

The answer to your question was explained to me this way:
Assume you want to get your boat on plane and maintain 28 mph. If you give the boat a lot of throttle to get the boat on plane and then back off to the desired speed the bigger engine is somewhat loafing while the smaller, lower HP engines are working their little butts of to get that weight on plane and to maintain speed.

The smaller engine works harder to do what the larger engine does easily. The same comparison works when comparing big block to small blocks.

 

[rondds]

I have no scientific proof but I estimate I'm burning about 25-30 GPH at that speed.

My exhaustive study of this question on my fat tub supports what Shawn (Norcal) above said. We have very different boats but the same engines and at 2800RPM you'd be burning probably 13gph on each engine (total of 26gph). At that RPM I'm dragging my ass but you DAs and ECs'll be hopped up and moving at a nice clip.

Stay out of the secondaries, stay around 3250 and run all day. According to Floscan and GPS, at that clip, I'm burning about 28GPH and breaking the sound barrier at a hair over 18mph.

For best efficiency, you need to run at around 1400RPM. This is where the boat will do better than 1 MPG (ie: approximately 8gph burn making 8mph)

 

[Tail Chaser 661]

Does anyone else think the twin 350s in my boat would be better economy than my 4cyls? the 4's keep the boat on plane at about half throttle. I can plane at 13mph, i think largely because they light aluminum motors. I cruise at about 25mph on gps and max at 35, so they are not really working that hard to keep the boat on plane.

I am just curious, I would have never thought the bigger motors and more weight would provide better economy, but maybe I am wrong.

Hi berth control,
If I remember correctly. You said you were getting like 1.4 mpg. That's about what my underpowered single 5.7 gets.

Unfortunately i don't know the Tq of a 4 banger.
I know they really don't have much Tq.
They just spin High rpm.

In V8 gas power, Every 50ci seems to get 50 more ft/lbs of tq.
So you can push more prop at less rpm. Getting better mpg.

In our boats say 7500/8000 lbs or more.
Every 50 ft/lb Tq increase is good for 2" prop pitch increase. Lowering your cruise rpm by approx 400 rpm.
Getting better mpg.

Most Merc 5.7s get 350/355ft/lbs of tq
Having Twins pushing 700 ftl/bs of tq.
Your 27' Sea Ray would have a nice power to wgt ratio.
Spinning Big props with low rpm cruise speeds.

IMHO
The worst scenario may be... You get the same mpg with a huge performance increase.

If you ever repower it may be worth it.
Your 10 ft beam May fit twin V8s nice.
4 cyl to V8 motor mounts lining up may be a nightmare.

If I were you berth control.
I would do online searches of similar wgt and length boats as yours with twin engines.
Just out of curiosity.

It seems the 4 bangers should get great mpg.
But If you are getting 1.4 mpg. That's not so great.
If your getting 1.8 mpg or better.
Never mind, I just wore out my keyboard for nothing :smt021

You said you cruise at 1/2 throttle with the 4 bangers.
With the added power of twin 5.7s
You may be cruising at 1/4 throttle or less with the same MPH cruise speed.

Makes you kind of wonder, :smt017 At them throttle settings.
Which package would get the best MPG:huh:

I should have stuck with dirt bikes :smt101

 

[Tail Chaser 661]

That's what this thread is all about. If you can run your engine in its efficiency window, you can maximize your fuel economy. If you have to run your engine at the high end of it's operating envelope to stay on plane, then a larger engine will likely give you better economy. Now, 1 small block compared to 2 big blocks, that's less likely.

Both my sisters twin big block 30" Fountains get 1.6 mpg
She told me.
She is a fresh water river rat.

My boat has never been on fresh water or flat seas.
Thats probably why I get 1.4 mpg. Usually spinning 3200rpm at maybe 18 kts. Not completly on plane.

So far, I have never heard of any 26' boat getting less then 1.4 mpg.