comparitive fuel efficiency of inboard vs outboard?

[fredrussell]

Did a google search but nearly all results related to mobo's with planing hulls so...

In a yacht, how do petrol outboards compare to inboard diesels with regard to mpg? I assume diesels get more mpg but does anyone have an idea of how much difference there is for, say, a modern 10hp inboard (with folding prop) vs a modern 10hp outboard (that lifts clear of water) on the same boat.

I'm well aware of the advantages/disadvantages of both types of engine so if we could avoid that whole area - I'm only enquiring about the relative fuel efficiency of one to the other.

 

[Topcat47]

That's easily done, I can cross the Channel with my 1GM10 on around 15 litres of diesel ( around 1 -1.2 l/hour). My brother in law has a 10hp Honda 4-stroke outboard on a displacement hull on the Broads and uses around 4 litres/hour.

 

[Tranona]

Inboard uses way less fuel. Yanmar 1GM less than 1L an hour. on this size of engine fuel consumption is hardly worth bothering about apart from having a big enough fuel tank for the range you expect.

 

[fredrussell]

So, obvious next question: why is there such a difference? I mean diesel cars are generally more fuel efficient than petrol ones but not to that extent.

 

[AngusMcDoon]

So, obvious next question: why is there such a difference? I mean diesel cars are generally more fuel efficient than petrol ones but not to that extent.

In calm weather at 5.5 knots my 30' trimaran at 2 tons uses 1.6 litres per hour which is about 16 nm per gallon. I don't recognise the 4 litres per hour figure at all. Maybe they are revving the nuts off the engine or got the wrong prop or something.

 

[fredrussell]

In calm weather at 5.5 knots my 30' trimaran at 2 tons uses 1.6 litres per hour which is about 16 nm per gallon. I don't recognise the 4 litres per hour figure at all. Maybe they are revving the nuts off the engine or got the wrong prop or something.

Petrol outboard or diesel inboard Angus?

 

[Heckler]

So, obvious next question: why is there such a difference? I mean diesel cars are generally more fuel efficient than petrol ones but not to that extent.

Very simplistically, the "heavier" the fuel the more bang per buck. So gas powered cars do less mpg than petrol and diesel does more!
So even given the sophisticated systems used today to get fuel in to engines a diesel engine will always be more fuel efficient. An interesting fact that came out when they started taking sulphur out of diesel, because sulphur actually combusted and gave some power, the resulting "clean" fuel was slightly less "powerful"
Stu

 

[Tranona]

So, obvious next question: why is there such a difference? I mean diesel cars are generally more fuel efficient than petrol ones but not to that extent.

Fuel consumption is a function of the power demanded by the prop and a diesel extracts more power per unit of fuel than petrol. Additionally in boats the diesel with its larger slower revving prop converts the power more efficiently into forward motion. At cruising revs the Yanmar is producing less than 6hp.

 

[Topcat47]

Well there's also the issue that I have a straight connection between the engine and the 15" prop, which doesn't cavitate, unlike the outboard. Also I have a sailing boat, while my brother-in-law has an open daybook with a different (shorter) waterline length and that will work in favour of the diesel.

 

[johnalison]

Well there's also the issue that I have a straight connection between the engine and the 15" prop, which doesn't cavitate, unlike the outboard. Also I have a sailing boat, while my brother-in-law has an open daybook with a different (shorter) waterline length and that will work in favour of the diesel.

Don't you just love autocorrect?

 

[thinwater]

All true.

Weight and dragging props are another factor to consider. With my boat (PDQ 32 catamaran) it is well-known that the diesel versions are 1 knot slower under sail due to prop drag and weight. Thus, they motor more often. With a monohull this difference would be less noticeable, but an outboard will generally be faster under sail.

Location also matters, even in terms of fuel efficiency. Transom outboards pitch, while center-mounted outboards (PDQ for example) do well in all conditions and generally use less fuel.

(I've seen boats with PHRF ratings from 85-210, depending on engine, sails, and tweaks. Multihulls can be funny this way.)

 

[Topcat47]

I missed that one. I seem to have to re-edit half my posts due to that particular "assistant".

 

[LittleSister]

I don't know the full explanation beyond what's been said above, and I don't have exact figures, but on the outboard powered Hurley 22 I had (8hp 2 stroke twin outboard) carrying enough fuel, making it last, and paying for it was always an issue when cruising (not to mention the earache and fumes!). My current boat, a Samphire 23, is not too dissimilar to the Hurley (bit bigger, heavier and longer keel, though) and the 14hp diesel on that is a joy in comparison. I don't even think about the cost of fuel (and I'm not well off), and the tank holds enough to run for nearly a couple of days non-stop.

 

[davidpbo]

I am pleased with additional economy going from an 8HP 2 Stroke to a 9.9 4 Stroke. Not to mention quieter and less fumes.

 

[Iliade]

A rule of thumb I use is 10hp 2T o/b uses a gallon per hour, a 10 hp 4T o/b half that and a diesel inboard half that. It is surprisingly close to reality.

Then when you consider that it took me some eight hours to motor from Shoreham to Bembridge, that worked out at about 6.5 gallons with my 8hp 2T, costing today about £33. The 9.8hp Tohatsu 4T would do that same journey for about £18. But the 10hp Nanni does it for about £6 on red diesel with no bleeding ears, far fewer pot lines [1. Now I've gone and done it...] more reliability [2. Hmm.] No explosion risk and very little fire risk, better electricity generation - just, better weight distribution, easier and legally stored fuel and more prop drag.

Furthermore, being quite light, it's only a tiny bit more hassle to remove it from the boat compared to the o/b in a well with controls connected to the boat. [See 2.]

[1. The o/b in a well on the fin keeled Listang was forever collecting fishing gear, plastic bags and seaweed. The transom mounted o/b on the bilge keeled Centaur collected a few bags and bundles of seaweed but not so bad and never a pot line. The inboard has now picked up the tender's non-floating painter once (stupidity on my part, fitting a long sinking painter and trying to anchor in too small a cove) clogged the water intake with fine riverine weed and overheated twice (Time for a more modern water strainer), but only collected seaweed around the prop a couple of times and a quick blast of reverse has sorted that. No fishing gear, old ropes, plastic bags or the like. Yet...]

 

[dom]

A rule of thumb I use is 10hp 2T o/b uses a gallon per hour, a 10 hp 4T o/b half that and a diesel inboard half that. It is surprisingly close to reality.

These numbers are counterintuitive, but probably correct. I have an 8hp 2T o/b for the dinghy and if one runs it hard when there is too much weight to plane I would say that 5l per hour is not too far off the mark.

Great engine, but thirsty little beast!

 

[AngusMcDoon]

Petrol outboard or diesel inboard Angus?

4 stroke petrol outboard. Here are some reasons why outboards of the same power use more fuel than inboards...

2 strokes lose a significant proportion of their fuel unburned, but that's well known, so I'll look at losses from 4 stroke outboards.

Petrol less energy dense than diesel

Petrol engine compression ratio lower

Diesels fuel injected, small outboards carbureted, so fuel metering not so good

Cylinder capacity of equal power outboard is smaller so less thermally efficient because of surface area to volume ratio of combustion chamber greater

Outboards higher revving so increased friction losses

Outboard power measured at prop, inboard at output shaft before the gearbox, so comparison of equal power not quite true to detriment of outboard

Outboard engines have to be compact so smaller sized air input & exhaust pathways than optimum.

Gearbox has smaller gears to be compact, running faster so greater losses

But after all those, this is the big one...outboard engines are designed to push a light boat fast, inboard a heavy boat slowly, so the prop shaft speed on the outboard is faster. Even putting a fine pitch prop on, the rotation speed is too high to be optimum, so there are higher losses converting rotary motion to thrust.

 

[Snowgoose-1]

After many years with inboard diesels my preference is now with outboards. 4 stroke has made all the difference compared with 2 strokes.

Much better fuel economy, less noise, less smoke and I can even carry a small 4stoke back up (well) .

The thrust is excellent now that you can get sail props i.e. mickey mouse ears with less pitch.

I reckon I use the same amount as diesel, primarily because I clean the prop and leg every time I go sailing.

 

[geem]

After many years with inboard diesels my preference is now with outboards. 4 stroke has made all the difference compared with 2 strokes.

Much better fuel economy, less noise, less smoke and I can even carry a small 4stoke back up (well) .

The thrust is excellent now that you can get sail props i.e. mickey mouse ears with less pitch.

I reckon I use the same amount as diesel, primarily because I clean the prop and leg every time I go sailing.

My experience is a little opposite. Having started off with 4stroke outboards on our catamaran i found them noisy, heavy on fuel and not reliable. By comparison the diesel we now have is super fuel efficient and quiet. Different boats but I cant see an outboard solution for a 19 ton yacht being viable......

 

[paul-essex]

GPH don't mean a lot when amount of miles covered is not in the equation