Fuel Consumption

[JORIDAPILOT]

It would seem that there are too many factors to predict fuel consumption and of course there is the tide and currents to factor in.
Perhaps it is best to not think about it and just enjoy yourself

Sometime difficult when you get back to the fuel pump and have to get your wallet out.

[dpb101]

Thats a detailed plot. Did you do this yourself with data picked up from the engine etc?

The gph figures were arrived at by plotting the figures from a range of tests, mostly from boatest.com, which had the same engine and averaging them out and interpolating. The figures for each rpm were fairly consistant across the tests.
The mph figures were actual tests on the water using a gps for speed and tacho for revs.

[Ed]

Ah, I see thanks

[Centaur]

I've done similar graphs for some of my boats.

My old Maxum 1900 SR3 was fitted with a Mercruiser 4.3L MPI V6 220hp motor. I used a GPS to measure speed and utilised the motor's SmartCraft connectivity to show fuel flow and revs direct from the ECU. I plotted these at various revs and with several different propellers as follows:



My current boat is a Nautique SV-211 fitted with a 6.0L ZR6 V8 375hp motor. This also shows engine management data but not fuel flow. However, it does show range in hours based on current fuel flow and fuel remaining in the tank. By keeping the tank full during testing I could use the range feature to relatively accurately compute the fuel flow. It is somewhat more thirsty but, being a wakeboarding boat, it has a much larger wetted area when planing and therefore requires much more horsepower. Again, I have plotted these with two different propellers:



As can be seen from the graphs, the Maxum was capable of around 4.0-5.5mpg across a range of cruising speeds, whilst the Nautique gets about 2.5-3.5mpg (4.0mpg at the sweet spot with the higher pitched prop). These figures could be borne out in reality when just cruising about but, for wakeboarding at around 20-22mph, they both only achieved around 3.0mpg - or less! (using 13-15lph / 3gph)

[dpb101]

Interesting prop comparisons.
Interesting also to see how the sweet spot for the little 4 cyl engine is 750 / 1000 rpm higher than the bigger V6 and V8s.
I wonder if that is true in general. Mine is a big boat for the engine size so the prop is pitched accordingly, maybe when fitted to smaller boats with a larger pitch there is less of a difference?

[Centaur]

The difference in revs is mostly down to gearing. The Alpha 1 sterndrive with the V6 had a 1.63:1 reduction ratio, whereas I would be surprised if yours was not 2.00:1.

At 20mph, about the minimum comfortable planing speed on most sports boats, the V6 Maxum was turning about 2,400rpm but my previous Monterey 3.0L would be doing about 3,000rpm at the same speed (with the same propeller). This is simply a gearing issue as 2,400rpm / 1.63 * 2.00 = ~3,000rpm. Incidentally, at 3,000rpm the 3.0L can produce a maximum of around 100hp, whereas at 2,400rpm the V6 could make up to around 120hp. This is not just a coincidence, as the gearing is selected primarily to ensure that the engine is capable of producing sufficient horsepower to propel the boat. At 2,400rpm the 3.0L can only produce around 80hp, which might simply be inadequate to propel the boat at 20mph, or get it out of the hole and onto the plane in the first place.

Flat out, my Monterey would do just over 40mph at 4,800rpm WOT, where it would be producing its maximum 135hp. The V6 could push the larger Maxum at 40mph at only around 3,800rpm, where it was capable of producing up to around 190hp. The V6 was therefore being worked relatively less hard both in terms of revs and torque output - this is why larger engines (in boats at least) do not necessarily use much more fuel than smaller ones. This is especially true where the motor is frequently being worked hard, such as when towing wakeboarders!

This 'spare capacity' of larger engines tends to let them run relatively more efficiently, dropping the sweet spot a little lower down the rev range. This, combined with the significant gearing difference, is why the sweet spot of your boat is that much higher up the rev range. Increasing the engine size in any given boat generally tends to reduce the revs at the sweet spot but the dynamics of the hull usually come into play here too - the hulls of many sports boat seem to be optimised for a best cruise speed at around 25-30mph.

[Centaur]

Here is another interesting illustration. The following chart shows the fuel flows over the range of engine revs for various propellers:



As can be seen, the lower geared propellers use less fuel for any given engine revs than the higher geared propellers. At first sight, this seems counter intuitive as you would expect better economy from the higher geared props. However, it is easier for the engine to turn the lower geared props at any given speed as there is less load - less load requires less torque and therefore less fuel. The analogy is rather like riding a bike - it's easier to pedal quickly in a lower gear than a higher gear. The difference, of course, is that at any given revs the boat will be travelling at different speeds with the various propellers. Although the higher geared propellers require higher fuel flows, they are propelling the boat at higher speeds. Taking this factor into account, mapping economy (mpg) against speed (mph) for the various propellers demonstrates the greater efficiency of the higher geared props:



The moral of this is that, for maximum efficiency, there is a target boat speed not target engine revs. Finding the optimum cruise speed and matching this with a suitable propeller is the way to go.

[Ed]

Good thread this. Its interesting to see all this info. I didn't expect people to have such detailed logs.

[markanddawn]

Very intersesting, all your different experiences.

I have run the engine for 5 hours and fuel guage has only just started to flicker off full mark.

I will refuel on Monday and divide litres by hours and post on here for anyone interested.

Performance figures achieved: On GPS - with 70 gallons of fuel

Tickover - 4.5 knots

Fast Idle - 6 knots

Slow Cruise - 20 knots

Fast Cruise - 27 knots

Flat Out - 35.3 knots

I reckon only 1.5 hours (out of the 5 hours run) at 20 knots or above in reality, so the 22 gallons per hour figure is wholly irrelevant in the real world.

[Overfinch]

Very interesting thread this particularly interested in the maxum data as I have the same boat with the 5.0mpi.
Centaur are you certain that your maxum had a 1.63 ratio I thought the V6 had 1.81 reduction. Mine is a 1.62 and I spin a 21" SS 3 blade prop to a higher top speed usually 56-59 mph at @ 4900 - 5000rpms depending on load.

[Centaur]

No, mine had the same 1.62 (not 1.63!) reduction ratio as the 5.0L MPI V8 - I think that the 1.81 was only fitted to the 1800 SR3's. The only difference between the V6 and the V8 was that the V6 came with a 4-blade 19" stainless Mercury Vensura propeller, whereas the 5.0L MPI came with a 3-blade 21" prop. I did have a factory order in for a V8 myself originally but there were problems with the supplying dealer and I eventually had to source a boat elsewhere from UK dealer stock and had to settle with the 4.3L MPI. As I used it a lot for watersports, I did rather regret this and would have liked the extra grunt of the V8. I should think that the 1900 SR3 with 260hp and all that extra torque must be a very rapid machine!

Incidentally, I still have the hugely expensive SmartCraft wiring harness that connects SmartCraft gauges into the motor's ECU and the (again hugely expensive) Navman Gateway harness that connects any SmartCraft-enabled Navman / Northstar gauge into the SmartCraft harness. I kept hold of these when I sold the boat as the new owner did not need them and I may well utilise them again in the future...

I have a Northstar GPS which could take fuel flow from the SmartCraft and GPS speed to calculate instantaneous fuel economy data, plus it also calculated fuel used and fuel remaining. Additionally, there is a wealth of other engine management data available via SmartCraft as well as the fuel data. If you are anywhere near Rutland, you would be welcome to borrow the harnesses.

[Overfinch]

It took me quite a while to find a 19fter with an mpi v8 and it does go well especially with the smart tabs to get its heavy ass out of the water.
Thanks for the offer of the loan of the smart craft gear would be interested to see what is actually going on, subjectively my MPG appears better than Gazjen's (same hull) V6 on carbs probably slightly worse than your 4.3mpi but I doubt there is much in it.

[Centaur]

I noticed after my earlier post that you are based not far away in Nottinghamshire. If you haven't got a SmartCraft compatible gauge / instrument, I could be persuaded to bring my Northstar and the cables over and rig it up for you - I would be interested to see just how well the 5.0L MPI goes! Do I take it that you ski on the Trent or do you have to trailer the boat over to the coast?

[Overfinch]

I do live in Notts but the boat is in south Devon usually launch in Torquay in fact I am going down on the 24th and will leave it on the water for several day's you are welcome to a trip out.

[Centaur]

The distance is a bit of a problem in term-time, I'm afraid, although we could perhaps arrange something in the holidays. My in-laws have a holiday home down near Dartmouth and so we might also be down there for a week or so later in August.

Since we sold the Maxum and got a dedicated wakeboard boat we haven't bothered taking the boat down there - it would have to be very calm to want to venture far out of the river. Recently it has got me thinking of something more suitable for open water use - perhaps a cuddy? It's a real pain being so far from the sea...