perkins 4.270 fuel usage?

If an engine is matched to it's prop and the hull displacement/shape shouldn't it be using most of the power it makes at a given rpm?

In a strong headwind or seas and without increasing the rpm then my boat speed decreases (but not the revs). Which implies to me there is no more power available at those revs?
 
Supertramp said:
If an engine is matched to it's prop and the hull displacement/shape shouldn't it be using most of the power it makes at a given rpm?

In a strong headwind or seas and without increasing the rpm then my boat speed decreases (but not the revs). Which implies to me there is no more power available at those revs?
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This is because the propeller drive is in effect a fluid drive and the propeller slip would increase

If it was a rigid drive like on my diesel car the engine revs would reduce for a set throttle setting and more fuel would be injected in an attempt to keep the revs the same
 
geem said:
My fuel consumption is identical to the Perkins published graph for my engine
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This is power and consumption curev I found in a Perkins document on page 5

https://s7d2.scene7.com/is/content/...6.1061864965.1688055412-1920585326.1688055412

It shows 2 curves RED maximum power of various speed and in BLUE the fule consumption

The BLUE curve is looks like a propeller curve and in fact it refers to propeller law consumption which is the power consumed by the propeller at the particular engine speed and you can then look at the engine power that is being generated
 
rogerthebodger said:
This is power and consumption curev I found in a Perkins document on page 5

https://s7d2.scene7.com/is/content/...6.1061864965.1688055412-1920585326.1688055412

It shows 2 curves RED maximum power of various speed and in BLUE the fule consumption

The BLUE curve is looks like a propeller curve and in fact it refers to propeller law consumption which is the power consumed by the propeller at the particular engine speed and you can then look at the engine power that is being generated
Click to expand...
The fuel consumption is exactly what I told you in my earlier posts.
 
geem said:
The fuel consumption is exactly what I told you in my earlier posts.
Click to expand...

That's fine so the fuel consumption is related to the power absorbed by the propeller at the engine speed selected by the throttle and not the Maximum power that the engine can develop as the selected speed

This is the important point so the approximate actual fuel consumption for any setup and not the maximum power the engine can develop and max speed or any selected speed in between

This is very interested to me as I haave a Perkins 4 236 which is the 3,9 lit version of your 4.4 lit 4 270
 
rogerthebodger said:
That's fine so the fuel consumption is related to the power absorbed by the propeller at the engine speed selected by the throttle and not the Maximum power that the engine can develop as the selected speed

This is the important point so the approximate actual fuel consumption for any setup and not the maximum power the engine can develop and max speed or any selected speed in between

This is very interested to me as I haave a Perkins 4 236 which is the 3,9 lit version of your 4.4 lit 4 270
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You told me a defied the laws of physics in post#37. So now what are you telling us? My fuel consumption is still the same as the Perkins graph as I told you it was.
 
The graph seems to show 6 lit / hr at 20 bhp 8 lit / hr at 40 bhp at 15 lit / hr at 75 bhp

The question is how close does the propeller curve match the propeller used by geem
 
rogerthebodger said:
The graph seems to show 6 lit / hr at 20 bhp 8 lit / hr at 40 bhp at 15 lit / hr at 75 bhp

The question is how close does the propeller curve match the propeller used by geem
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are you reading a different graph as it looks like 75hp is around just under 8ltrs/ h at about 1750rpm...?

too slow.......
 
The engine curve is the maximum power that the engine can generate when fully fueled. This is the red curve

The lower blue curve is the fuel consumption curve at the power the engine is actually generated against the resistance of the propeller in the water driving the boat along.

It the engine generated nore power than can be absorbed the speed of the engne will increase until the power absorbed equals the power that the engine can generate.

When I used to tune engines mainly petrol we would put a running engine on a dyno and open the throttle and load up the dyno to absorb the power / revs we wished to measure.

If we wished to measure maximum power we would open the throttle fully and set the dyno load to the specified max revs

To measure the power curve over the full engine speed range the throttle would be closed partially and the dyno would be set to to required speed and the read the power output on th dyno.

With a diesel the speed control sets the engine speed and the govener sets the fueling to maintain the set speed dependent on the power resistance imposed by the load.

Consider a diesel powered non inverter generator . The speed of the engine is elated to the frequency required 3000 rpm on a 50 Hz 2 pole alternate 3600 rpm for 6 Hz 2 pole alternator. Half those speeds on a 4 pole alternator.

As the more current drawn the more the govener sends more fuel to the engine to keep the speed the set for the frequency required. More fuel more power is developed to balance the power drawn from the alternator

"The governor's purpose is to control the fuel to the engine cylinders so as to control the speed of the unit, holding the speed constant for all conditions of load imposed on the generator conditions of load imposed on the generator being driven by the engine."

This may explain it better

Speed Controlling Device- Governor for Marine Diesel Engine

or this


https://www.nrc.gov/docs/ML1122/ML11229A131.pdf
 
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geem said:
This is the only bit that is relevant to the OP or the post
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Yes that is true but it also applies to all boat engine insulations so any fuel consumption will depend on the exact arrangement as the engine ,gearbox. propeller and hull design and weight will b different
 
Tranona said:
No, its a rule of thumb to provide a guide and usually overestimates actual. My last 3 30hp nominal engines cruised at about 20hp and consistently burned just under 2l/h. The point that I was making was that the previous estimates of 2-3 gallons an hour were way out. Very few people actually run their engines in auxiliary yachts at constant speeds for any length of time so the constant consumption shown on the graph is not really particularly useful. An average for mixed use is more useful.
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Mine, md22, over the years I have owned her uses 1.7 ltrs per hour consistantly
 
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