sailorman
Well-Known Member
Sailorman - Absolutly. But not Power guesstimate!
one = the other
Consumption guesstimate perkins 4108?
- Thread starter pcatterall
- Start date
one = the other
ithet
Well-Known Member
pcatterall
Well-Known Member
Blimmy!! sounds thirsty to me!!
ianj99
Well-Known Member
I looked at the 4108 power curve and used propcalc. I'm confident its a reasonably accurate guesstimate in as much as you can never know exactly what bhp any engine is developing in use.
Its academic anyway as the OP only wanted a rough idea.
sailorman
Well-Known Member
reeac
Well-Known Member
I'll add my bit to this prolonged dabate: published engine performance curves for smallish marine diesels generally show a minimum specific fuel consumption of 190-200 g./kW which, at 0.746 Kw per bhp and fuel density of 835 g./litre and 4.54 litres/gallon comes to 20 bhp.hour./gallon. That's all just arithmetic. The knotty problem is how many bhp. you are typically taking from your engine and that's where the guesswork has come in. To take the quoted case of the diesel car:. at 80 mph it won't be developing the power shown on its engine curve at that engine speed because it will be at part-throttle. Cars need surprisingly little power when travelling at constant speed - if you assume that power required is proportional to cube of speed then a 120 mph. car needs only one-eighth power at 60 mph.
To sum up: don't get confused by the bhp numbers on the engine curve - they are all full throttle figures.
You will gather that I support the 20 bhp.hours/gallon rule of thumb.
To sum up: don't get confused by the bhp numbers on the engine curve - they are all full throttle figures.
You will gather that I support the 20 bhp.hours/gallon rule of thumb.
jdc
Well-Known Member
power used versus rpm
I completely agree with this and think it's the source of the diferences of opinion expressed so far. To put it another way, the power your engine outputs is rpm x what the torque happens to be, which is not necessarily the max HP it would produce were the load to be the maximum it could drive. However engine manufacturers usually publish only this latter information, which is unhelpful imho.
To illustrate this point here is the HP curve from the Beta 50HP technical data sheet, in red, and in blue the actual HP this very engine delievers into the prop shaft on my boat. They agree at max rpm - where my prop is well matched to the engine - but differ widely at normal revs.
Using this curve, and 0.295 lt / kWhr consumption (this figure being that used also by reeac and ithet, albeit they've converted to imperial units), I get calculated consumption which is spot on what I mesasure, and I in which have great confidence.
I completely agree with this and think it's the source of the diferences of opinion expressed so far. To put it another way, the power your engine outputs is rpm x what the torque happens to be, which is not necessarily the max HP it would produce were the load to be the maximum it could drive. However engine manufacturers usually publish only this latter information, which is unhelpful imho.
To illustrate this point here is the HP curve from the Beta 50HP technical data sheet, in red, and in blue the actual HP this very engine delievers into the prop shaft on my boat. They agree at max rpm - where my prop is well matched to the engine - but differ widely at normal revs.
Using this curve, and 0.295 lt / kWhr consumption (this figure being that used also by reeac and ithet, albeit they've converted to imperial units), I get calculated consumption which is spot on what I mesasure, and I in which have great confidence.
contessaman
Well-Known Member
My beneteau has a 4108 and uses about half a gallon per hour at cruise. 6 knots 1200 rpm.
of course being a 4108 it leaks about half a gallon of oil per hour into the bilge too!
but hey its reliable and smooth as silk compared to a modern lightweight diesel. Im going to try a tip off an american site of putting baby nappies under the engine to soak the oil up.
of course being a 4108 it leaks about half a gallon of oil per hour into the bilge too!
but hey its reliable and smooth as silk compared to a modern lightweight diesel. Im going to try a tip off an american site of putting baby nappies under the engine to soak the oil up.
