Diesel VS 4 stroke outboard fuel consumption

CatBuilder, not being a sailor I didn't think about drag, just saw the shiny prop. You're right of course, all hardware under water causes unwanted drag.

And let's hope someone invents a new power source, because looking at the consumption figures of present day outboards makes me very sad.

 

Good point CDk the diesel is direct cooled.

Found another outboard site testing 115 hp outboards:Honda and Yamaha are 4 strokes,will ignore the 2 strokes as they are just too horrible.
From 9 years ago,2001
http://www.popularmechanics.com/outdoors/recreation/boating/1276841

At WOT they use:
Honda BF115 AYLA: 9 gph which = 34 l/hr
Yamaha F115 TXRY: 9 gph


Going to yanmar 125 hp,think they came out about 2001
http://www.yanmar.com.au/express/Dealer/Prodspec/ENGINE PDF/4JH3_DTE_TechData.pdf

Going to the Yan fuel chart (on the right),34 litres an hour.... ooops once again the yanmar at 125 hp doesn't even get to 34 l/hr- it peaks out at 26 l/hr at 125 hp.

At the outboard's comparable 115 hp (go to the left chart)the yan is at about 3200 rpm...so going back to the right chart again,we plug in 3200 rpm and the Yan is using....ooops...16 l/hr.

So from the literature, yet once again :

115 hp gas engine needs 34 litres an hour to make ~115 hp.
The Yanmar diesel needs 16 litres an hour to make 115hp,and being optimized with a turbo it uses 47% the fuel.

No.s 1 and 3 here:
http://www.urbandictionary.com/define.php?term=put that in your pipe and smoke it

 

WesVanHan
If you put those 2 115 engines in a boat the gpm would not reflect the wot gph as the lighter outboard would be going much faster or much closer in gph at a similar speed. Dont buy engines by comparing wot specs in a brochure.
You need to know how they actualy perform on the same boat.

 

West Van,
Who cares what they burn and make at WOT.
Nobody runs there anyway so we've got apples and oranges again.
Can you produce fuel/power numbers at 75% load?

Easy Rider

 

Why don't YOU produce some gas outboard fuel/power charts???
I already have posted one,if you had read.
And I've been posting the diesel's continuous rating which is not WOT.

As for the 2 115 outboards vs a single diesel,thats silly as they will be 2 different boats.

But if you must.....sigh....the outboards would be 2x34 so 68 litres an hour,and I know for a fact (I have one) they are about 400 pounds each so 800 pounds.

Seeing as Yan has no 230 hp engine and there are few around we'll go with the 720 pound VW marine 225 hp ( I have one) plus a ZF at 100 pounds and a shaft so call it 850 pounds.

http://www.vw-m.de/fileadmin/PDFs/engine_performance_TDI_225-5_e.pdf

Where's the huge weight disadvantage?

The VW produces 225 hp uses 48 l/hour- ~850 pounds.
Two 115s produce 230 hp uses 68 l/hour- 800 pounds.

So 70% the fuel consumption for maybe 50 pounds more.


The Thread is called "Diesel VS 4 stroke outboard fuel consumption"

 

WesVan
Sorry I meant those 2 engines as in, the same hull with a single 115 outboard versus the same hull with a 115 hp diesel inboard.
There are plenty of boats where you have a choice of outboard versus sterndrive.Both will have a different fuel curve so that complicates the comparison even more.
We all know diesel is more efficient than a SI engine BUT they weigh more, they cost more and you cant readily get a diesel outboard.
Without comparing a hull with each engine, looking at the specs is almost useless.
All the specs are GPH, a comparison must be GPNm.

 

Sorry Van I don't have the time to look all that stuff up. Using Yamaha's and Honda's seem to be good examples but the Yanmar is a bit of a freak as far as hp to weight so we would need to change the title of the post to include only Yanmar diesels. If you substituted a typical diesel the weight would go way up and you'd be close to apples and apples.
Powerabout, No, specs are not useless at all. They tell us that a specific amount of fuel will produce a specific amount of power. Set up the two examples at a reasonable power loading relative to the power/burn curves and all youv'e got for confusion is the weight factor. You couldn't ballast the gas OB boat as that would eliminate the advantage of lightness to the gas OB. But you've stated OB/ v/s inboard so the difference in efficiency of each needs to be taken into account. It's hard to compare since there's no diesel OB and there's no OB powerhead driving an inboard propeller shaft. The OB is sea water cooled and the inboard's propeller is less efficient. As has been said we all know the diesel engine is more efficient but w the more efficient propeller and lighter weight the difference is probably less than most of us think. A better starting point may be to compare inboard gas and diesels then compare OB drive gear and standard inboard drive gear.
powerabout, I think gph is good as that value is used w the hp hour value.
Wer'e talking how much work can be done in an hour consuming how much fuel in an hour.

Easy Rider

 

Easy Rider: scroll back. My 1st operating cost post compares a diesel outboard to a gas outboard.

 

The original question was about an outboard 4 stroke vs inboard diesel in a SAILING boat.

In this context most of the difference comes probably from the propulsion efficiency. A typical inboard has a decent sized propeller with low enough propeller shaft rpm. It will have much higher efficiency and not suffer from cavitation. Furthermore it will not suck air in the waves. An outboard, even a "high thrust", is far from optimum regarding shaft rpm and propeller size for displacement speeds.

Also diesel is more efficient as an engine, especially on partial loads. This is clearly shown by looking at the consumptions of cars. Diesels consume 30-50% less at typical partial load. At optimum load the difference may be much smaller. If you are interested in fuel consumption, you should use the engine on rather low load on a sailboat and have 70% of power just as a reserve for bad weather or harbor maneuvering.

 

Where do you get a diesel outboard from today?
Anyone got one in stock?

I know in the offshore oil n gas game several fields have banned petrol so there has been a scramble to find diesel outboards and the offshore guys are paying 10's of thousands to get ones in any condition.
Some compnay in the US is trying to get a tier2 turbocharged engine going on a Merc trunk and lower unit, it could be a winner if it goes into production.
Cheers

Re specs if the 2 outboards CI and SI have the same prop diameter then you might be able to use the spec sheet but with differerent rpm ranges and gear ratios you will not be able to make direct comparisons

Spec sheets are only relevant when looking at a boat with a known prop curve ( displacement only)and you are comparing 2 engines that have the same operating rev range and they will have the same gearbox and prop, then you can say apples v apples

Poor old outboards have been installed on too many boats where some designer just said horsepower is horsepower.

 

Joakim,
OK good. 1st we've got to know if the propeller on the inboard is aft of the rudder as on an Albin 27. They are practically useless maneuvering in close quarters so here the OB would be anyones choice. The inboard propeller may be larger and slower turning but the water flow through the blades is at quite an angle. The pitch will need to be low and the blades may have no pitch at all on one side of it's swing and much too much pitch on the other side. So much for efficiency. The OB with it's smaller and faster propeller will probably be more efficient. To get a comparable boat one will need to stretch it as in make it bigger to make up for the space lost with the diesel engine inside the boat. The bigger boat will need more power ..of course. If one needs to stack up all the fly scat we'd be here till Christmas but why is this question on the table? Not only is there going to be a small difference but the amount is going to be so small in the first place one hardly needs to know.
You are right about light loads reducing consumption but one shouldn't under load a diesel and if one were to chose a smaller diesel loaded correctly the consumption would be lower yet.
Yanmar did make a 70hp diesel OB in the 90s but I don't know if it's still available (never was in the US).

 

Powerabout,
Almost any two different propulsion systems will require different gear ratio's. You chose an engine, chose a propeller and that dictates a gear ratio. So quite the contrary ..you will need different gear ratios and rpm ranges to make
direct comparisons whereas both units are optimized. Your'e not likely to find an gas OB and a diesel inboard w the same engine speed and gear ratio's and if you did one would be far from optimized so an objective comparison would be pointless.
And yes, horsepower is horsepower. An amount of work done, the work being the force required to raise 33,000lbs at the rate of one foot per minute. Four stroke two stroke gas diesel hp is hp. But since you can't go to the store and get a gearbox for your OB to turn the ideal propeller at the ideal speed and with the ideal diameter you must compare the OB as is. So a question needs to be answered .. with the average "high thrust" OB how much slippage is present at the speed of our sailboat. Is the high thrust unit optimized for a 6 knot sailboat or a 27' OB sport fishing boat w twin 225hp OBs as a trolling engine or a "get me home" engine? What I'm saying is that the OB engine necessary to make this comparison meaningful probably does not exist.

Easy Rider

 

I can attest to this other handling characteristic of my outboard hanging off the stern of my sailboat. It gave me some concern for the seaworthiness of this approach... When trying to make your way into a seaway to get out of trouble, excessive bucking can result in very poor performance as the prop "sucks air". I'd expect this to happen at the worst possible time, like when crossing a bar into a stiff headwind with cross currents and other such hazards. Murphy lives for these moments!

A good inboard with the prop forward of the rudder is the more seaworthy approach, IMHO.

 

I have never sailed a boat with inboard propeller behind the rudder, but some older boats may have them.

Inboard with a shaft does have some angle, but usually only ~10 degrees to horizontal and ~20 degrees to bottom, which is really not a big deal regarding efficiency. Majority of new installations have a sail drive with horizontal shaft.

E.g. my boat has a Yanmar 1GM10 with 10 hp, saildrive and 14x8 propeller. At cruising speed of 5.7 kn it has a drag about 500 N and efficiency is 55% according to a propeller software. The propellers of a 10 hp 4 stroke OB's are clearly smaller. The biggest one I found was 10x7 for Mercury "big foot". Yamaha has 8.5 x 7.5. The same program predicts 50% efficiency for Mercury and 42% for Yamaha. Yamaha also shows cavitation already at cruising speed, thus the real efficiency is even lower.

5.7 kn and 500 N = 1.5 kW = 2 hp, thus 55 % means 3.6 hp, 50 % 4 hp and 42 % 4.8 required engine power. Thus Yamaha OB would reguire over 30% more engine power than my Yanmar.

Yes the inboard takes same space inside, but it also charges batteries and can even be used to make warm water.

Is underloading a diesel really a problem? I usually cruise at 5.7 kn (2500 rpm) while the full speed is about 6.4 kn (3450) rpm. That is about 40% of full power. If I had a 18 hp engine, I would still use the same cruising speed, thus only 20% of full power. But I don't use the engine that much, less than 50 hours/season.