Originally Posted by europaflyer
I'm looking in to choosing an engine
and I've come up against a rather interesting question. Here goes...
I'm looking at a repower
from an 18hp diesel
. The boat needs around 10hp by my estimation to be pushed at a nice cruising speed. I've been looking at Beta engines between 16-25hp.
To produce a given HP output, the larger engines need to run at lower RPM
, ie. with a greater pitched or larger propellor. However, the larger engines seem to have lower fuel consumption
to produce a given power. I know fuel consumption
is not linear with RPM
, but this seems surprising.
To put out the 10hp (which I estimate will produce a nice 5kt) these are the figures. The graphs supplied aren't the best, so these need to be treated with caution.
Beta 16 = 1900 RPM, 0.7 litres per hour
Beta 20 = 1700 RPM, 0.5 LPH
Beta 25 = 1400 RPM, 0.5 LPH
A little surprising the larger engines don't use more fuel, but hey ho. Putting out 16hp is where it gets interesting though.
Beta 16 = 3600 RPM, 3.5 LPH (full power)
Beta 20 = 2600 RPM, 1.8 LPH
Beta 25 = 2100 RPM, 1.2 LPH
The fuel consumption is lower with a bigger engine. Not just a bit - a lot. To prove a point, here's 20HP.
Beta 20 = 3600 RPM, 4.5 LPH
Beta 25 = 2600 RPM, 2.2 LPH
Beta 30 = 1900 RPM, 1.0 LPH
That last one particularly got me. Going from a 25HP to a 30HP engine HALVES the fuel consumption needed to produce 20HP.
I appreciate that an engine running at full RPM is not at its most economical power. But this seems to be true over the whole range.
So am I right? Because fuel consumption and RPM aren't linear over any part of the RPM range, it will within reason be more economical to fit a larger engine, and by a big margin? Surely not! Am I just reading too much in to sketchy graphs?
Here are the data sheets
There is not quite enough information here to make a logical conclusion but some things we can assume. The note that says "theoretical based on full speed matched propeller" is key.
The performance chart is not made using an infinite mumber of propellers. They use a dynamometer brake, so when the engine is producing X torque at Y RPM horsepower can be calculated.
Basically it says if you buy a 30 hp motor
and match it with the correct prop that extracts 10 hp at this RPM, then yes, in theory, you get Z fuel consumption. A prop sized for 10 hp on a 30 hp engine will likely be under pitched and will peak out in rpm etc...
It is hard to make a car anology because we are pushing a boat through water
and a prop is the "perfect" slipping transmission
but I will try. You have a 30 hp engine and decide that you only need 10hp. You have only one gear
in the transmission
. You have to choose 1st gear
to get the power out of the engine. When you are on flat ground the car accelerates and soon you are going along at 1500rpm and extracting 10hp. Now you head
uphill. You floor the gas pedal but the short gear is now your enemy. The car runs out of RPM and you can't get the 30hp. So you put a longer gear on it. Now you are not getting the 10hp at low RPM with the theoretical fuel consumption. The prop is not efficient at that speed so you basically have to push the throttle to get the 10hp out now fuel consumption goes up because the prop is "slipping". Not a perfect analogy and what really happens to consumption is not known because when the engine is tested it is tested with a "perfect" load.
I would be looking at the torque curve for a better answer and the ability to compare more apples to apples. Take the peak of the torque curve for each engine.
B16 13hp 1.5ltr/hr 2500rpm. Steeper torque curve
B20 12hp 1.5ltr/hr 2500rpm. Steeper torque curve
B25 19hp 2.0ltr/hr 2550rpm. Wider torque curve
B30 24hp 2.2ltr/hr 2600rpm. Wide shallow torque curve
Note these are with a matched prop. You could in theory put a little shorter prop on the B25 and B30 but at the risk of running out of RPM at higher speeds. However you can see the the B25 and B30 are characteristically different engines than the other two. Their torque curves are different.
Also - you need to work the problem from the other direction. In theory, flat water
, no wind
etc. your boat may need 10hp to push it along with a matched prop. Add wind
, chop a dirty hull
and you might need 15hp. To accellerate better you might want bursts of 20hp. What is also interesting is the idea of X liters burn for Y hp output hold together better when you get up on the torque curve regardless of displacement
All graphs should be taken with angrain of salt
at their "theoretical" extremes.
I am no expert in selecting an engine for your boat and there are other considerations. Weight, access, size, installation
angle (can I get the prop shaft far enough away from the hull
to swing a big enough prop?
In the end there is no replacement for displacement
. I would be looking at the B20 but the B25 could be compelling if the installation
issues are not a factor. Especially because it has a flatter torque curve and a wider selection of prop pitches should in theory be possible without sacrificing too much efficiency.
Finally as others have mentioned, you need to work a diesel. If you end up cruising around at hull speed
all the time at 1400 RPM you may not be getting to temperature and burning all the carbon out. One more consideration is noise
. If I have to run the B16 at 3000 RPM all the time I wont be happy either.