Volvo 5.0 vs 5.7 difference

[AndieMac]

Interestingly Volvo say they "don't give out fuel consumption figures for gasoline engines".....

Too scary are they?

Too many variables I reckon, the whole ball game changes with heavily loaded boats and rough water.

Neale's figures (35 - 45 lt per hour) seem realistic to cover average conditions, if you want to move two tonnes plus of boat at 25 knots, you are going to burn some fossils.

 

[GrahamHR]

A different manufacturer, but the enclosed compares the performance and fuel economy of boats with different engines and drives. For any given type (e.g. Sundowners) the hull design is the same for the different length of boats in that line-up. OK, manufacturers figures, but they are all manufacturers figures so the different variants can be compared.

http://www.fourwinns.com/past_product/specs

 

[ontheplane]

Yes thank that's really helpful.

I cannot for the life of me see why Volvo couldn't do that - if you look at the same engine in all the boats, for the same rpm the fuel flow is pretty much the same - so I am not sure what all this "too many variables" thing is. If it's propped properly, to max out at peak rpm, then I think the fuel use will pretty much be the same whatever the boat (perhaps displacement boats different I don't know).

I'm amazed, but at 2000 rpm the Injected engines are about 3-4 lit per hr less to run - but thereafter there really isn't a lot in it - and then at the very top end they use a tiny bit more (but size for size they are then producing a fair bit more power).

So no, I am not sure lack of Injection is the deal-breaker I thought it was. Also, I am pleasantly surprised that at a cruise (30mph would be more than enough for me) 38lit per hr doesn't actually sound all that bad - given you probably only cruise for an hour or two a day (more at tickover of course up and down the river to get to the sea). I do think the Injected engines start easier and run smoother - however not so much I wouldn't buy a non-injected boat now.

It does show an impressive improvement in the performance of the Duoprop though - good 2-3mph faster in the normal cruising speed range for the same fuel burn

That is a brilliant site though - I have favourited it for future ref.

 

[Spi D]

It comes down to Brake Specific Fuel Consumption (BSFC).

An engine converts the energy contained in the fuel to power. Some of the energy is used for overcoming internal friction etc. and some is turned into heat in the process, but at the end of the day you can calculate how much fuel is needed to develop any amount of hp. Petrol engines reach an efficiency of 30-36% - newest inventions slightly better.

Diesels reach 40-47%

By maths you'll be able to calculate the consumption when your engine produces eg. 200 hp, based on a set of facts:

Conventional petrol engines burn 0.230-0.276 liter/(hp·h), an average of 0.253 liter liter/(hp·h)

By multiplication you see, that eg. 200 hp burns (200*0.253) = 50.6 liter an hour.

Ofcourse the big joker in this calculation is to know how much hp you demand from your engine. Puttering along at just above idle takes almost nothing where as accelerating x tons of boat plus y tons of persons, gear and equipment up to and maintaining planing speed takes a lot. As a matter of fact it takes all the hp you have on tap... until you decide to reduce to eco-cruise at 'the lowest speed where you can keep a stable plane'.

Best mpg is at idle, second best is at eco-cruise.

All as a rule of thumb

 

[GrahamHR]

... until you decide to reduce to eco-cruise at 'the lowest speed where you can keep a stable plane'.

Best mpg is at idle, second best is at eco-cruise.

All as a rule of thumb

The fuel flow meter built into the GPS plotter on my planing boat tells a different story. The boat will stay on the plane at around 15 mph. The petrol gph at 15 mph is less than at 30 mph. However the mpg at 30 mph is significantly higher than it is at 15 mph, presumably as there is less boat in the water at the higher speed. ! Above 30 mph the mpg drops.

 

[Spi D]

Consumption over time (lph or gph) should not be mistaken for consumption over distance (mpg, km/liter l/nm etc.).

You will find hull design/driveline/weight combos that do not follow the rule of thumb, but in general planing boats have a sweet spot when 'slow planing'.

 

[neale]

The fuel flow meter built into the GPS plotter on my planing boat tells a different story. The boat will stay on the plane at around 15 mph. The petrol gph at 15 mph is less than at 30 mph. However the mpg at 30 mph is significantly higher than it is at 15 mph, presumably as there is less boat in the water at the higher speed. ! Above 30 mph the mpg drops.

That suggests to me that your boat isn't fully on the plane at 13 knots (15mph) and therefore not at it's most efficient.

 

[ontheplane]

Mmm... I didn't see this before posting my previous comment.
IIRC, the FL came with the old style "original" VP outdrive, doesn't it?
And otoh, the Gi on the 21' has the newer "OMC style" outdrive, right?
Which means that probably the first has alu props and the latter s/steel.
If so, that's definitely a point in favour of the 5.0 - more than the injection per se, imho.

Yup - this boat comes with the Older type Duoprop drive - I'm trying to find out whether it has S/S props or Ally.

Does it make that much difference (Never owned a boat with Stainless Prop(s) )

Thanks

 

[gjgm]

Boattest.com used to have test figures on loads of boats-maybe have a look there if they still publish. Even if your boat isnt listed, you can look at others for hints.
Bear in mind that you probably will day boat, and mainly to nearby bays. Not many diesel boats are doing much cruising either with these fuel costs
That being the case, you dont actually run for that long in a day, unless you are into watersports.Much time is lazing about at anchor etc, when fuel consumption is limited...
I think many of us agree it shouldnt be like this, but we tend to think where to go for the day, and think of the costs. Have a think what you might actually do in a normal day out, and make a ball park fuel cost. All I am suggesting re the two engines is that if one makes a day out seem painful each time, you find you have the boat you want, but cant afford to use it without crying! You might also think that £100 out locally is fine, but £250 just to bbq on the next beach means you had better not burn that food.

 

[Scarron]

Consumption over time (lph or gph) should not be mistaken for consumption over distance (mpg, km/liter l/nm etc.).

You will find hull design/driveline/weight combos that do not follow the rule of thumb, but in general planing boats have a sweet spot when 'slow planing'.

I think it depends very much on the efficiency of your drive system in terms of hydrodynamic drag. I agree that a shaft driven boat will be most efficient just 'over the hump' generally at about 16 - 18 knots, however the hull is not truly on the plane at that speed and a more efficient sterndrive will produce it's best MPG considerably faster generally somewhere between 23 and 35 knots depending on the hull form.

 

[MapisM]

Yup - this boat comes with the Older type Duoprop drive - I'm trying to find out whether it has S/S props or Ally.

Does it make that much difference (Never owned a boat with Stainless Prop(s) )

Well, yes and no.
I mean, steel is better in all respects, but it's only with high performance boats that it really makes a big difference.

My previous concern on alu props of the old style VP outdrive was more specific, and based on the fact that those props are total rubbish.
Not that the whole outdrive was brilliant, but if nothing else it was popular and well known by most mechanics...
I managed to find this old thread where I explained my experience with VP alu props in more detail, if you're interested to have a look.

 

[AndieMac]

if you look at the same engine in all the boats, for the same rpm the fuel flow is pretty much the same - so I am not sure what all this "too many variables" thing is.

What I was alluding to was perhaps Volvo didn't want to state fuel consumption figures calculated during flat calm (perfect running and trimmed) test conditions on a light ship, when they will change considerably in rough head seas, or when loaded up to, and beyond maximum capacity and incorrectly trimmed in the hands of an amateur helmsman.

 

[Spi D]

I cannot for the life of me see why Volvo couldn't do that - if you look at the same engine in all the boats, for the same rpm the fuel flow is pretty much the same - so I am not sure what all this "too many variables" thing is.

The variables that decide what sort of fuel economy you'll get (vars that are unpredictable for engine manufacturers as for everybody else), include but are not limited to

Boat weight
Hull design, trim tabs or not
Wet area
Wind speed and direction
Boat size above waterline (wind)
State of sea
Amount of growth
Antifouled or not?
Engine condition
Single or twin installation
Driveline loss
Prop material, type and design (slip)
Prop dimension related to the combined load based on the above
Prop running condition in water (running height, disturbed stream)

When you read boat reviews you'll see results listed for that specific review, on that day and under that conditions.

If manufacturers were to state a consumption (like for cars), there should first of all be a standard measuring method and then they'd have to make reservations for all possible variations to explain why nobody gets the same figures in real life.

The only comparable measure is the mentioned BSFC, and it is commonly used by manufacturers.

"Does this mean that all makes burn about the same same fuel to perform the same job?" No. Conventional petrol engines burn 0.230-0.276 liter/(hp·h) which in reality is a 20% difference. Late designs and innovations (not least EFI) are better.