Oh no ... another Diesel vs. Petrol thread – Fuel Burn?

[moresparks]

Apologies ... encouraged asking this question from another thread.
Assuming the same boat i.e. 28 foot stern-drive sports cruiser; One with twin 170 hp Diesels (say KAD32’s) the other with twin 170 HP Petrol’s.
Both engines working as they should with no issues; leaving aside availability, safety, maintenance and reliability for one moment.
Both engines at cruising speed of say 22 knots... What is the fuel burn difference between the 2?
If you are retired you can use the boat a lot more but you will more than likely be on a fixed income (Pension). If you were to say undertake a couple of short cruises in the week of say 50 miles each then the cost of the petrol boat is likely to be around £75 a trip. Ouch.. that’s a big hole in your budget and takes the edge of the enjoyment which may need a serious thought about the type of boat used. It would be interesting to know the same costs for the Diesel boat? I know for a fact that at 22 knots the petrol boat gets round 2.6 miles per gallon.

 

[lynallbel]

Petrols are lovely and quiet, but they are never going to be as a efficient as a diesel, going on what I know about cars (which aint a lot!) I would say there is at least 25 percent difference on same sized/bhp engines.
Mate had a mondeo 1.8 derv 45/50 mpg easy, then got a new mondeo 1.5 petrol, rep told him it would do same mpg as derv model, it does not! Iirc he is struggling to get 35mpg.

Previous boat (Fairline 23) was petrol with a fuel flow meter, vp 4.3 single engined, 1.6/2gph at 6 knotts, 7/10gph at 20 knotts, and 13 gph flat out at 30knts, no idea what that equals to mpg wise? but she was a thirsty old girl.

 

[ST840]

This may sound harsh, but it’s not supposed to be. If your already worrying about fuel consumption and costs then you probably can’t afford to enjoy the boat you’d like to have. Why not re think your boat choice for a single shaft drive that is displacement and will be more frugal on fuel but at lower speeds?

 

[Croftie]

If you were to say undertake a couple of short cruises in the week of say 50 miles each then the cost of the petrol boat is likely to be around £75 a trip. Ouch.. that’s a big hole in your budget and takes the edge of the enjoyment which may need a serious thought about the type of boat used. It would be interesting to know the same costs for the Diesel boat? I know for a fact that at 22 knots the petrol boat gets round 2.6 miles per gallon.

Bit of man maths there I think.
50 miles @ 2.6 mpg = 19.23 gallons
Petrol say £1.20 / litre = £5.40 / gallon (app)
£5.40 x 19.23 = £103.84
Sorry to spoil your £75 per trip unless you are paying less than 90p/litre for petrol.

 

[BruceK]

I have heard that the modern petrol outboards are very frugal and return great economy by comparison to petrol inboards but until very recently the debate between petrol vs diesel is not so much about cost but size. As you approach and pass 40 foot petrol just doesn't seem to have the torque to get you going. (Until recently where we are seeing multi engined boats sporting 3 -5+ outboards from factory and that may prove a game changer) Sub 30 foot it's always been moot with imo petrol being a better engine choice bar safety, pump price and availability and all the other caveats thrown in. If I were to re-engine mine I'd definitely go petrol this time round.

 

[Momac]

I know for a fact that at 22 knots the petrol boat gets round 2.6 miles per gallon.

That seems very good seems as good if not better than a diesel? Unless you mean per engine ?

 

[Bouba]

£75 will take you a long way......slowly.
Retirement means more time, take advantage of it?

 

[Rappey]

Would be interesting to throw in a 350hp outboard or two 175,s to the twin 170 petrols and diesel inboards !

 

[Whippersnapper]

A recent 8 metre boat made here in NZ has been tested and is available with 270hp Mercury diesel and stern drive or twin Mercury V8 outboards. per the review. Yes more hp in the ob version but a single outboard wouldn’t have enough torque. The manufacturer told me at trolling speed the outboards are about double the consumption of the diesel. Here in NZ diesel is a no brainer as petrol is much more expensive per litre than diesel so the the consumption difference combined with the economy difference means the cost is usually more like double per mike covered.

Cruising at 24 knots, the diesel burns roughly 33 litres per hour, but it sips just 7lph trolling at eight knots. Integrated Lectrotab trim tabs are fitted to both boats, but we found little use for them on the sterndrive. Its hydraulic steering is smooth and the Simrad autopilot will get plenty of use as Graeme and Karen cruise from place to place.
The twin 250hp outboard version of the 821 is a different beast. Good for 51 knots, its hole shots are eye-wateringly quick, so it pays to hang on whenever the skipper applies a handful of throttle. Those extra horses need more feeding, but this boat has a 400-litre fuel tank to keep them satisfied. Cruising along at 24 knots, fuel consumption is a pretty decent 40lph, both engines combined.

 

[AndieMac]

I have heard that the modern petrol outboards are very frugal and return great economy by comparison to petrol inboards but until very recently the debate between petrol vs diesel is not so much about cost but size. As you approach and pass 40 foot petrol just doesn't seem to have the torque to get you going. (Until recently where we are seeing multi engined boats sporting 3 -5+ outboards from factory and that may prove a game changer) Sub 30 foot it's always been moot with imo petrol being a better engine choice bar safety, pump price and availability and all the other caveats thrown in. If I were to re-engine mine I'd definitely go petrol this time round.

If you compare a new MPI inboard to a new four stroke outboard in the same boat, I believe the fuel consumption will be very similar.

 

[Scarron]

The like for like fuel burn of the diesel is something in the order of 17.5-20% fewer litres per mile/hour burned. However factor in where you buy either type of fuel, garage in Jerry cans or on the water. Then the big variable of cost vs depreciation of the two boats. Add cost of capital, servicing and repair cost differences then the whole thing becomes more complex.

My boat had twin 170hp petrols when I bought it, I ran it that way for a few years and later when I re-engined it I had the option of single petrol or diesel.

I fitted a single big petrol V8 with modern fuel injection and am very happy with the resulting performance vs fuel cost compromise.

 

[pumpsmynads]

Go to 12:30 on the video. Ok so it’s not Stephen Hawkin‘s level of scientific, but the basics are there of what the difference between petrol and diesel may cost.

 

[B58]

Apologies ... encouraged asking this question from another thread.
Assuming the same boat i.e. 28 foot stern-drive sports cruiser; One with twin 170 hp Diesels (say KAD32’s) the other with twin 170 HP Petrol’s.
Both engines working as they should with no issues; leaving aside availability, safety, maintenance and reliability for one moment.
Both engines at cruising speed of say 22 knots... What is the fuel burn difference between the 2?
If you are retired you can use the boat a lot more but you will more than likely be on a fixed income (Pension). If you were to say undertake a couple of short cruises in the week of say 50 miles each then the cost of the petrol boat is likely to be around £75 a trip. Ouch.. that’s a big hole in your budget and takes the edge of the enjoyment which may need a serious thought about the type of boat used. It would be interesting to know the same costs for the Diesel boat? I know for a fact that at 22 knots the petrol boat gets round 2.6 miles per gallon.

Please take into the formula that prop type and gear ratio will differ between petrol and diesel through different ratio's, power band and RPM. Further to this, one engine will run counter clockwise with slight gear ratio change and prop performance.

 

[CLB]

Go to 12:30 on the video. Ok so it’s not Stephen Hawkin‘s level of scientific, but the basics are there of what the difference between petrol and diesel may cost.

got to take into account that those figures are for single diesel .

 

[CLB]

Bit of man maths there I think.
50 miles @ 2.6 mpg = 19.23 gallons
Petrol say £1.20 / litre = £5.40 / gallon (app)
£5.40 x 19.23 = £103.84
Sorry to spoil your £75 per trip unless you are paying less than 90p/litre for petrol.

marine petrol is more like 1.50 a litre. There is also the little problem that the 2.6mpg that the op states as fact, is far from fact. A twin V6 petrol boat going 22knots will be getting between 1.0 and 1.5 nmpg. The same boat with twin KAD 32s will be getting something between 2.0 and 2.5nmpg

a single V6 in a smaller boat will get around 2.6mpg

 

[[2068]]

My previous S28 with twin KAD32's was burning 40-45 litres an hour at a fast cruise.
S28's were also fitted with 225hp V6 petrols, which would burn in the region of 70 litres an hour.

 

[alandalus11]

If you are retired you can use the boat a lot more but you will more than likely be on a fixed income (Pension). If you were to say undertake a couple of short cruises in the week of say 50 miles each then the cost of the petrol boat is likely to be around £75 a trip. Ouch.. that’s a big hole in your budget and takes the edge of the enjoyment which may need a serious thought about the type of boat used. It would be interesting to know the same costs for the Diesel boat? I know for a fact that at 22 knots the petrol boat gets round 2.6 miles per gallon.

I am not rich by any means but if you are retired and you are wincing about £75 a trip then in reality I would be thinking either it's time to give this up or pootle around up and down a river occasionally. If you are going to be concioussly worrying about how much fuel you've spent your boating is not going to be fun so why do it. I said this in a past thread this is meant to be fun and if you are always thinking about how much fuel you are going to use and the cost, you wouldn't leave the marina.

 

[CLB]

My previous S28 with twin KAD32's was burning 40-45 litres an hour at a fast cruise.
S28's were also fitted with 225hp V6 petrols, which would burn in the region of 70 litres an hour.

If by 'fast cruise' you mean around the 22 knots in the OP, that confirms my figures in the post above yours. Petrol circa 1.4mpg, diesel circa 2.3mpg

 

[moresparks]

To be honest fuel cost are just a “mindset”. In reality it pales into insignificance when you compare with maintenance and mooring costs etc.
I was genuinely interested in fuel difference costs between petrol and diesel especially in the light of forthcoming red diesel “review”.
Maybe my engines are particularly economical they are ageing straight 4’s DOHC Volvos.
It’s a twin Sterndrive boat weighing around 3.5 tons
I have a fuel flow meter connected so a fairly accurate idea and admittedly the runs were taken in ideal conditions with some speeds in “no wake” zones. The consumption was around 2.6 miles per gallon combined. The last trip I measured I did 40 miles and used a total of around 70 litres.
Also if I push the throttles further forward then fair enough the fuel consumption rises exponentially – 14 to 18 gallons per hour ( 7 to 9 gallons per engine) above 25knots which I assume may not be the case of diesels?

 

[CLB]

To be honest fuel cost are just a “mindset”. In reality it pales into insignificance when you compare with maintenance and mooring costs etc.
I was genuinely interested in fuel difference costs between petrol and diesel especially in the light of forthcoming red diesel “review”.
Maybe my engines are particularly economical they are ageing straight 4’s DOHC Volvos.
It’s a twin Sterndrive boat weighing around 3.5 tons
I have a fuel flow meter connected so a fairly accurate idea and admittedly the runs were taken in ideal conditions with some speeds in “no wake” zones. The consumption was around 2.6 miles per gallon combined. The last trip I measured I did 40 miles and used a total of around 70 litres.
Also if I push the throttles further forward then fair enough the fuel consumption rises exponentially – 14 to 18 gallons per hour ( 7 to 9 gallons per engine) above 25knots which I assume may not be the case of diesels?

There is definitely something amiss with your figures. I assume you have AQ171's, and they do not have any specific qualities that make them economical. In fact, being carb fed, they are decidedly un-economical. I guess a section of the journey at no wake speeds will skew the figures. Your boat may well jump right up 10-15nmpg at proper displacement speeds. Are you measuring miles, or nautical miles?