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

[Alicatt]

My 6.5m ex Royal Marine dory with a Johnson 85hp V4 2stroke petrol on the back returned about 9gallons per hour at around 20 to 24 knots wide open throttle, top speed was around 36 knots but we seldom achieved this mainly as we were carrying 6 divers, and she was scary at anything over 25 knots driving it alone .

 

[rafiki_]

A petrol Engined 28 ft boat is going to use at least 50% more fuel than a diesel. If your boating is on a river then this will not be a problem, as neither engines will use much fuel. At planing speeds the difference will be dramatic. There are other issues like availability of petrol waterside, plus the inherent fire Fisk of petrol over diesel. Petrol is more carcigenous than diesel too. A diesel boat will cost a lot more to buy in comparable condition, but will also retain far more of its value.
Actually fuel costs were a long way down the lists of costs for my boat ownership. Even if you forget depreciation, mooring was by far my biggest single cost, followed by maintenance then upgrades.

 

[moresparks]

I don’t doubt I may be wrong and in less than ideal conditions and longer runs at 22 to 25 knots may well push the figures way up. The distance is in nautical miles via the plotter tracks.
Interesting responses though – many thanks.
I must admit that I also thought that a reasonable fuel consumption return would be around 1.5 nmpg, so it surprised me that I was getting around 2.6 nmpg on the trips I “measured”.
This would make a big difference to me ... not because of costs but because of fuel range. So far I have only used it as day cruiser but keen to explore further afield. Waterside petrol is in short supply around our area so would limit my cruising areas.
Looks like more “extended” testing required ... otherwise it’s another boat.....

 

[Hot Property]

Whilst a diesel engine is undoubtedly more efficient than a petrol one, maintenance of them and the normally associated stern drive is a big cost factor.

It is interesting to read the posts on the Motor Boat forum - 99% of them relate to inboard engine maintenance. Risers, gimbals fuel filters seem to feature regularly.

By way of comparison, outboards are simple to maintain and most of the time they are out of the water with just the engine bracket immersed.

The long term trend is for boats up to say 40ft being outboard powered. Yes, petrol is expensive and sometimes difficult to obtain, but the advantages of just dropping the engines and going boating outweigh these factors in my mind.

Maybe there are only a few members with outboard powered boats which could account for limited posts about "problems". Or maybe they are just out boating!

I get 3 nmpg from my 23 ft sports cruiser which weighs in at around 2.5 tons. That's cruising at around 20 knots. As previous posts have mentioned fuel cost is behind my mooring costs...

 

[LBRodders]

Whilst the question is particular, I think you need to look at annual engine costs.

We went to a single engine, purely because the service and maintenance costs outweigh the fuel difference.
We installed a brand new 350 petrol and enjoy the reliability.
Fully loaded slack water we got 2.5mpgish at about 21knts. Have been known to g over 3mpg.

Some figures in the video..

 

[CLB]

I get 3 nmpg from my 23 ft sports cruiser which weighs in at around 2.5 tons. That's cruising at around 20 knots. As previous posts have mentioned fuel cost is behind my mooring costs...

Probably important to point out that your 3mpg is, I’m guessing, with a single engine.

 

[Hot Property]

Probably important to point out that your 3mpg is, I’m guessing, with a single engine.

Correct.

 

[Scarron]

25ft deep vee classic express cruiser weighing around 3000kg, single V8 petrol with outdrive.

Gentle cruise, 23knots, 2.8nm per UK gallon
Fast cruise, 40knots, 1.9nm per UK gallon
Flat out 1.5mpg

I have plotted the graph and it’s a fairly straight line once you get properly established on the plane at about 27-28 knots

 

[Hot Property]

I have attached a graph of fuel flow vs speed that I created when I was testing some props.

This one is for an Enertia 15" and you can see fuel does rise faster than speed but not exponentially..

 

[oldgit]

Petrols may make sense for local day trips or slow non tidal cruising but simply do not make much economic sense for anything else.

 

[Farmer Piles]

Regardless of any build and reliability issues, a diesel is more fuel efficient than a petrol engine - especially with a turbo and intercooler. Hence every truck, tractor, fishing boat having one. How much so in the original scenario I wouldn't know.

 

[LBRodders]

Petrols may make sense for local day trips or slow non tidal cruising but simply do not make much economic sense for anything else.

Nonsense ?

 

[LBRodders]

Regardless of any build and reliability issues, a diesel is more fuel efficient than a petrol engine - especially with a turbo and intercooler. Hence every truck, tractor, fishing boat having one. How much so in the original scenario I wouldn't know.

^^ all commercial applications.
Cost of installation, maintaining and servicing easily levels the playing field.

 

[CLB]

Petrols may make sense for local day trips or slow non tidal cruising but simply do not make much economic sense for anything else.

I think its not as clear cut as that, and will be even less so when Red finally goes. I have a twin diesel, my friend a single V8. Boats are roughly the same size, ie around 28ft ad we both cruise in the low 20's. Our fuel burn translated into nmpg, is around the same. He benefits by having a single engine. Assuming no problems in availability, his boats will cost a similar amount, once the derogation goes, to run at a decent cruising speed, so there really is little argument against petrol in this instance. When it comes to servicing, his costs will be half mine. Put twin V8's into the equation and things would be very different indeed.

 

[BruceK]

^^ all commercial applications.
Cost of installation, maintaining and servicing easily levels the playing field.

Yes, but, no but, maybe. As previous much depends on the size and weight of the boat. The smaller and lighter it is the more the pro petrol argument holds ground. There is a reason the yanks put 4 plus outboards on boats that traditionally would have had 2 diesel inboards and it's not all about speed.

 

[LBRodders]

Yes, but, no but, maybe. As previous much depends on the size and weight of the boat. The smaller and lighter it is the more the pro petrol argument holds ground. There is a reason the yanks put 4 plus outboards on boats that traditionally would have had 2 diesel inboards and it's not all about speed.

I've been trying to stay OT with OP spec of 28ft. ?

 

[Farmer Piles]

^^ all commercial applications.
Cost of installation, maintaining and servicing easily levels the playing field.

Agreed, but I was specifically referring to fuel efficiency

 

[BruceK]

I've been trying to stay OT with OP spec of 28ft. ?

Yes but you are still only viewing it from your angle. You have a monster single V8 to his twin installation and can lay down the torque without butchering the throttle. And you compare your fuel burn relative to your peers, and as we are more than twice your weight (8.5, 10 and 12 ton to your 4) on twin diesels your consumption seems frugal by comparison. But it's not less than half by any means.

 

[ari]

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 .

At 18 minutes there is a cost comparison between two identical twin engine boats, one twin diesel, one twin petrol.

It's about 3x the cost in fuel at planing speeds for the petrol.

 

[Hot Property]

Regardless of any build and reliability issues, a diesel is more fuel efficient than a petrol engine - especially with a turbo and intercooler. Hence every truck, tractor, fishing boat having one. How much so in the original scenario I wouldn't know.

I agree, Diesel engines are of course more economical than equivalent HP petrol ones.

However, when you take into account capital cost, maintenance & repair costs then the equation swings back somewhat.

Modern outboards are highly reliable and cheap to diy service.

My last service in March comprised oil and filter, & leg oil change.

I checked the impeller, thermostat which were fine.

I changed the serpentine belt which I had as a spare but only as a precaution. I kept the original as an emergency spare.

I have a racor fuel filter upstream of the main engine that reduces filter changes...

Spark plugs are iridium and last for years.

Overall cost about £100 and a mornings work.

Compare that to the numerous service items on an inboard engine of either fuel type and the pendulum swings towards outboards again.

Just down the road is a brand new sports cruiser of probably 35 ft with twin 400 Verados on the back. The trend is for bigger boats to be outboard powered.......