Mercruiser 5.7 V8 fuel consumption

willciviceg5

I am looking at buying a sports cruiser with a mercrusier V8 either the 5.0 or 5.7 either carb or EFI and I'm trying to find real world fuel consumption figures. It will be around a 25" bayliner or similar used mainly inland on the Barrow so low speeds of around 6mph and the very odd trip down to the estuary and out towards Dunmore East. Does anyone have any real world figures of what I can expect at low inland speeds? At high crusing speeds I've heard around 8-10GPH, is this accurate? I don't want to buy one and then be afraid to use it if I'm burning through €80.00 or €90.00 an hour!

fergal.b

This should give you an idea.

bonky2018

They say when you ask how many MPG for Royals Royce car you cant afford to own it

Much the same for the large Mercruisers they are 1960s throw fuel at it power solutions when fuel was a few cents a gallon

Mere mortals now know the modern 4 stroke outboards these days can often give double the MPG for the same HP as the older 1960s solutions

Water creates lot of friction and HP to overcome that friction comes from burning fuel burn more fuel make more Hp make more speed

25 foot boat will require about 15 to 30 BHP to motor along a river at 4 to 10 MPH

Using a 300 BHP at tick over to make 15 BHP often reguires the engine parsitic drag of extra pistons and so forth to Burn generate 30BHP of power to that work so the 300BHP can often demand twice the fuel as the 15 BHP engine .

the above graphs show that speed is the rate you burn fuel and MPG can go from as high as 5MPG at 4mph on the best case down to 1.6MPG worst case or 24 gallons per hour at 38 MPH

The happiest Mercruiser I know are rich folk where money is not a problem or party animals who only use the boat as party zone in the Marina and never drive the boat and some excuse it wont start for some unknown reason .

Various Bayliners exist where the Mercruiser was replaced but most of these are abortions heavy diesel engines that upset the balance of the boat or some other abortion like unsuitable outboards saga .
I cant recommend buying into other peoples abortions .

if you do buy one the majority of the boats with outdrives the props were put on without any suitable oil to stop the props fusing to the shafts
The albumin props often fuse onto the shafts and not even tractor pulling on the shaft can break the grip so its cut the prop off job .Many is the prop I had to cut off . A little suitable oil on the shaft with new prop and the problem doesn't repeat itself . The alternator is another problem i can write a whole thesis on .

Mercruisers are up there with the 1964 Mustang convertible you buy it to own that that eras buzz and the smeell of gobs of unburnt fuel going out the tail pipe .

My mates Mustang costs him easy 100 euros a trip and he does that 10 times a year and barely goes 50 miles each trip
You can smell the unburnt petrol fuel for miles around each time he drives it

So if you use the big mercruisers 4 times a year to go 40 miles round trip at full tilt and it costs a mere 300 a trip or 1200 a year in fuel and you got your annual buzz then go for it burn the fossil fuels and make the Irish government richer in fuel tax .

If you want to look the fuel gauge then invest in boat which can take a modern 4 stroke outboard or turbo diesel inboard and still do that journey at those speeds for probably half the money or throttle back a tad and make interesting saving on fuel costs .

The basic rule with old 25 motor boats is its hard to exceed 3MPG at any kind of speeds like 20 Knots plus and modern boats its hard to exceed 8MPG

Down sizing the boat towards 20 feet can up the MPG with modern four strokes towards 15MPG at 30 Knots in calm conditions but hard to find lighter turbo inboard diesels in these smaler sizes

There are many factors at work

There does exist a new diesel engine twin 35 BHP from Austria which is claimed to double the MPG again on a similar Petrol engine and seems to give closer to 40BHP in real power due to larger prop

Jury is out on that new engine

Only moving a t slow speeds with small modern marine engines often diesel inboards in smaller boats can return MPG exceeding 20MPG

willciviceg5

bonky2018 wrote: »

They say when you ask how many MPG for Royals Royce car you cant afford to own it

Much the same for the large Mercruisers they are 1960s throw fuel at it power solutions when fuel was a few cents a gallon

Mere mortals now know the modern 4 stroke outboards these days can often give double the MPG for the same HP as the older 1960s solutions

Water creates lot of friction and HP to overcome that friction comes from burning fuel burn more fuel make more Hp make more speed

25 foot boat will require about 15 to 30 BHP to motor along a river at 4 to 10 MPH

Using a 300 BHP at tick over to make 15 BHP often reguires the engine parsitic drag of extra pistons and so forth to Burn generate 30BHP of power to that work so the 300BHP can often demand twice the fuel as the 15 BHP engine .

the above graphs show that speed is the rate you burn fuel and MPG can go from as high as 5MPG at 4mph on the best case down to 1.6MPG worst case or 24 gallons per hour at 38 MPH

The happiest Mercruiser I know are rich folk where money is not a problem or party animals who only use the boat as party zone in the Marina and never drive the boat and some excuse it wont start for some unknown reason .

Various Bayliners exist where the Mercruiser was replaced but most of these are abortions heavy diesel engines that upset the balance of the boat or some other abortion like unsuitable outboards saga .
I cant recommend buying into other peoples abortions .

if you do buy one the majority of the boats with outdrives the props were put on without any suitable oil to stop the props fusing to the shafts
The albumin props often fuse onto the shafts and not even tractor pulling on the shaft can break the grip so its cut the prop off job .Many is the prop I had to cut off . A little suitable oil on the shaft with new prop and the problem doesn't repeat itself . The alternator is another problem i can write a whole thesis on .

Mercruisers are up there with the 1964 Mustang convertible you buy it to own that that eras buzz and the smeell of gobs of unburnt fuel going out the tail pipe .

My mates Mustang costs him easy 100 euros a trip and he does that 10 times a year and barely goes 50 miles each trip
You can smell the unburnt petrol fuel for miles around each time he drives it

So if you use the big mercruisers 4 times a year to go 40 miles round trip at full tilt and it costs a mere 300 a trip or 1200 a year in fuel and you got your annual buzz then go for it burn the fossil fuels and make the Irish government richer in fuel tax .

If you want to look the fuel gauge then invest in boat which can take a modern 4 stroke outboard or turbo diesel inboard and still do that journey at those speeds for probably half the money or throttle back a tad and make interesting saving on fuel costs .

The basic rule with old 25 motor boats is its hard to exceed 3MPG at any kind of speeds like 20 Knots plus and modern boats its hard to exceed 8MPG

Down sizing the boat towards 20 feet can up the MPG with modern four strokes towards 15MPG at 30 Knots in calm conditions but hard to find lighter turbo inboard diesels in these smaler sizes

There are many factors at work

There does exist a new diesel engine twin 35 BHP from Austria which is claimed to double the MPG again on a similar Petrol engine and seems to give closer to 40BHP in real power due to larger prop

Jury is out on that new engine

Only moving a t slow speeds with small modern marine engines often diesel inboards in smaller boats can return MPG exceeding 20MPG

Thanks for your advice guys I really appreciate it.
After a lot of research and thought I have decided to go for it, the cheapest diesel I could get was €6000.00 more than a similar petrol and I’m unlikely to do anymore than 40 hours in the season so that will buy a lot of petrol.

bonky2018

A good tip is try to always use up all the fuel in the tank for each trip
The original petrol engines were made for pure petrol with either lead or toxic heavy methanol chemistry formula which were not prone to soaking up water from the air .


Most modern petrol fuel sold in the local petrol station garage will contain some small amounts of water that car engines can tolerate .
However most cars are used every day refilled frequently so they dont tend to suffer large water content build up issues in the petrol fuel

Todays modern petrol fuels sold in Ireland contain 95% petrol mixxed with combined 5% Ethanol and sometimes simplex methanol alcohol based fuels.
These modern fuels with alcohol tend to soak up water from the air and can cause issues.
The alcohol molecule always wants to grab water from the air.
These issues are less likely with a boat used every day where new fuel is always going into the tanks .

Getting non alcohol type petrol fuel in Ireland is possible but very expensive per liter typically 3 euros plus per liter . Also engine often has to modified and re-tuned to run that old petrol fuel formulation.
The aviation worlds light aircraft use the lead based petrol fuel as water in their fuel can stop their engines and cause crash landing .


The boat that is seldom used the fuel is more likely to soak up water from the air.

If the tank is near empty for several months and then before going back to being used a full tank is put in the tank it will reduce the overall content of water in the tank to such a lesser amount that low water content may not impact on the engine ability to start and to keep running.

If possible ensure the carburetor remains empty before starting up the engine if there has been a long break in use as the water can be in the bottom of the carburetor bowl .
If you have a clear plastic carburetor bowl any water will be easy to see as it will sit in the bottom of the bowl .
Some carburetors bowls have a tap to drain away this water and fuel so new clean fuel enters the carburetor bowl .
if after use the carburetor bowl is drained the risk for water content is greatly reduced

Water is heavier than petrol fuel so will tend to remain in bottom of the fuel tanks.
The motion of the boat can cause the water content in the bottom of the tank to move around and possibly allow the fuel pick up to suck up a large blob of water causing the engine to flame out .Water in the bottom of the tank can sometimes be seen as balls of water moving on the tank floor when looking into the tank often with suitable lights that wont create a spark to ignite the petrol fumes.Often boat tanks have suitable drain on the bottom of the tank to remove water filled fuel from bottom of the tanks.

Boat petrol engines can keep to run even with high water content in the fuel provided the water content is suspended in lots of small drops of water .The result is the engine might blip a lot or run a bit rough as water impacts a single petrol explosion inside the engine out of many thousands of firing strokes the engine makes per minute.

Marine and other fuel trading shops will also sell fuel additives that can suppress the ability of modern petrol fuels to soak up water .
Some DIY solutions are add more toeline to the petrol often less than 1% is enough.
The Toeline tends to wrap itself around the alcohol molecule so keeping it away from contact with air and water . On pure 100% alcohol fuel 1% added toline will form a air tight barrier scum over the fuel. So 100% ethanol fuel will really be 99% ethanol 1% Toeline


Winterizing the boat best is to remove all the fuel from the fuel tank as the water content in tank can rust the tank . Also treat the engine and drive and props to all winter treatments it need .Remove the batterie/s and put them on a trickle charger for the winter time .

Those V8 engines properly maintained can last donkey years and properly looked after run like clockwork in low hour season use
Water in the fuel badly winterized treated batteries and damp in the engine and damp in the electrics are the main problems for engine starting issues or engine quit issues .

Winter starting anti damp sprays cans for anti damp to spark plugs and winter starting ether fuel spray can often start a engine that is acting up .

The large V8 Mercruisers are probably the most robust petrol engines for boats out there with many happy users even in occasional low hour use .
Most issues I ever encountered were from lack of of following the maintenance schedules .

Given the choose between a Petrol Mercruiser and diesl Mercruiser if my funds allowed for me to choose the large V8 petrol Mercruiser is the king of the marine world for me .


Mercruisers now have a engine rebuild schedule for larger V8 5 liter plus engines which if you still have the engine in few years from now might be interesting to look at.
The engine rebuild uses latest fuel saving systems and all new parts except for the engine block .
The new Mercruiser rebuilt petrol engines could be a better option than the diesel solutions for low hour per season using boats .


Petrol fuels do have one big advantage for low hour use marine boats they are much less prone to getting alga growth in the tanks and fuel lines .
Many diesel engines have been wrecked from alga growth in the fuels as modern diesel fuels for cars unmarked stuff has ~7% vegetable oil in it .
Alga loves to live in vegetable fuels.
Even red or green diesel the marked stuff with no vegetable oil content can still grow alga but the risk is greatly reduced but i have seen it on some boats that use the green stuff .