A couple of questions for diesel consumption

Fuel consumption is only affected by how much HP you extract at a given RPM ... that being 1000 RPM or 2000 RPM ..... the only constant is a given gramme, (or volume per HP hour, and is dependant on engine's tune and overall efficiency).

As a general rule, you can use 220 grammes per HP/hr for a modern(ish) diesel engine. Calculate diesel weight as 0.86 grammes per liter, then you are getting close.

This means when you extract 10 HP out of an engine, it will consume approx 2.5 Litres per hour... the only ones where you cannot use this equation is large diesels at low rpm, where at low rpm, the mechanical friction is % wise relatively high... after 1200 RPM + these engnes can still use the formulae with a reasonable degree of accuracy.

As a result, we can assume that the propellor load curve is essential to know for your vessel/engine configuration.

Also, a diesel engine's governor will attempt to keep the engine's rpm static irrespective of what you have set the throttle at. A result of this is that the propellor load curve will fluctuate as the vessel moves through water and is affected by waves and tidal streams and growth (hence the importance for MOBOS to calculate with 20% + reserves etc).

So not a straight answer, but do use the formulae as a general rule and do your own observations for your boats/engine configuration and conditions.... you could be surpsised how accurate it is, but please be careful to rely upon theoretical calculations only.

Happy sailing.
 
We arrived at the fuel consumption in litres/h - over three tank refills - simply by dividing the fuel used (ie the diesel pump reading to refill the tank) by the engine hours run, as recorded on the VDO RPM and engine hour gauge.

Not an exact science admittedly, given the accuracy of diesel fuel pumps and the fact that "tank full" is somewhat subjective. However we certainly weren't measuring gallons/h.
 
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Theory says that a 10hp inboard diesel engine will consume around 1lt/hr, 20hp 2lt/hr. 30hp 3lt/hr and so on.
1. At what "speed" this is achieved? Cruising speed which means around 80% of maximum or less?
2. As long as you keep the same rpm (cruising) will consumption be affected by waves, tide or whatever changes your SOG?
Thanks

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Theory is not always right. Hubby works on 1.6lt/hr at 5kts, that's about 1/3rd throttle on our Perkins 4-107. That engine is 43HP. If we push to 5.5kts and slightly above cruise setting we get about 2 lt/hr. Of course if I want to push hubby and he opens it right up then consumption goes crazy to possibly 4+ lt/hr.
Basically consumption is not as some would like to say based on maths alone. The combination of hull form, propellor and environment create varying figures that only your trial can determine.
 
There is an awful lot of confusion here. Fuel consumption is a direct function of power used in a given time period. That is what the specific consumption curves are showing. In the Yanmar case at least they show consumption measured by weight. Diesel is lighter than water (where 1kg = 1 litre) with an SG of between .86 and .96 dependent on grade. So a KG of diesel is between 1.04 and 1.16 litres. For simplicity I used 1 to 1 in my estimates earlier because I don't know the actual SG of Derv or Red.

As Divemaster says modern diesel g/HP-h is around 220 (Yanmars are about 210). So, if you are consuming 10hp you will burn just over 2 litres an hour. In my earlier calculations I used engine hp, but maybe one should use the propeller power curve. This makes a big difference as on a 2GM at 2600 the propeller curve shows about 7HP and the engine curve just over 12!

Maybe a real expert rather than an armchair commentator who can read graphs and use a calculator can advise on this point!

All the other issues are about how efficiently the boat translates this energy (however measured) into forward motion. Most of the variables have been identified and we can see that they result in significant variations in actual consumption.
 
Hubby has direct connection with Fuel Industry and also we do not look at fuel per HP calculatuions at all when deciding if enough fuel in tank to make destination.
There are references to MDO (Marine Diesel Oil) which is a different product to what you are all using in your engines - so as Hubby says - don't confuse issue even more. MDO is a poor cousin to Red or other diesel as we use. Data from his files, shoosh I peeked.
Depending on market and Cetane rating density - NOT SG - is ranged from 0.8200 - 0.8600 kg/l and 0.8300 - 0.8800 kg/l, all at 15 C.
SG is as he told me a unit used by engineers because they don't understand ASTM tables.

The reference you require for Diesel specs is either Fuel Oil Domestique (France) or the EU EN590 revised.
 
Thanks for that. I did look up the tables but there is no specific reference to fuels available at road or marina pumps, which is why I put in the range.

Understand that it is not easy to use specific fuel consumption to calculate fuel usage because you do not know how much power your engine is producing at the revs you are running at, nor how much the propeller is absorbing.

The original poster was asking about consumption in relation to the HP of the engine. What is clear is that you cannot have a simple rule of thumb, and certainly not one based on maximum advertised HP as that is a meaningless figure. Only empirical evidence of a particular boat is of any real value, and this seems to vary enormously!
 
In heavy seas this does not increase noticeably but you cover less distance per hour.

My experience is entirely different. In heavy seas we can go from 5 to 9 ltr/hr using the same revs. I don't think the engine size is relevant .
 
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In heavy seas this does not increase noticeably but you cover less distance per hour.

My experience is entirely different. In heavy seas we can go from 5 to 9 ltr/hr using the same revs. I don't think the engine size is relevant .

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You're right there ... with the throttle you "ask" the engine governor to be fuelled to a certain RPM .... once obtained, the governor wil endavour (up to max load) to keep the requested rpm, irrespective of load ...... which means that you can go from anything between 50 - 100% load at the given RPM.
 
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