Gallons per Hour . . .

[LtBrett]

It looks like you guys are arguing the difference between an impeller and a propeller. A propeller with 20" of pitch will move 20" forward through the water for each rotation, less slip. An impeller with the same pitch will move 20" of water through its blades every revolution. What's the difference? A propeller moving forward acts on water that is already moving. An impeller assumes static thrust. Just like adding head pressure to a pump, having the water moving before it hits the prop blade makes the prop's job easier. The faster you move the water towards the back side of the prop blade, the easier job the prop has. In an extreme example, think about freewheeling while sailing. Say you get 1000 rpm. Now throw the engine in gear and rev to 1000 rpm. Is fuel consumption the same in that scenario as it would be if you pushed against your dock at 1000 rpm? Load matters. It takes more power to turn a prop 1000 rpm on a static vessel than on one moving forward.

Brett

[Dockhead]

Originally Posted by LtBrett

Load matters. It takes more power to turn a prop 1000 rpm on a static vessel than on one moving forward.

Brett

Indeed, as the formulas say.

On an airplane travelling at 300mph the difference is very great. On a planing-hull power boat it's less, but still measurable. On a displacement hull sailboat, the difference between power required at a given RPM tied up to the dock versus travelling at 6 knots is a few percent. Which is why the tables don't even consider speed; just RPM.

That's why, as the manufacturers explicitly say, the fuel consumption curves are highly correlated to reality for displacement hulls, but all bets are off on planing hulls.

[Chief Engineer]

Fuel tamk sending units...with the arms are notoriously unreliable.

WEMA Tank units are better.

But give me a sight glass or stick and I am a Happy Engineer.

Originally Posted by Charlie

I like the simple stick method too. Problem is my tanks are not rectangular. They are five sided with a small flat spot at the bottom of the hull then they angle up the hull and finally turn up parallel to the inboard side of the tank. I have a stick that is tiked off for every five gallons. The first five gallons takes 4", second and third take 2 3/8" of stick, fourth and fifth 2 1/8", then 1 7/8" and finally 1 3/8". It would have taken me a long time to do it for each gallon but maybe it is worth it.

[hpeer]

Chief,

I've got a question for you. I have a steel tank and not stick (now inaccessible under muffler.) Can I drill a hole in the top of my diesel tank and put in a stick? What are the fire/explosion risks? Minimum if the tank if full I think?

[Chief Engineer]

I am sure others will come up with an idea.

You could put in one of those Garboard Drains in the top of the tank....with a bronze plug....3/8"?

I have also seen pipe couplings welded to the top of the tank...with a square head plug.....the center of the plug was drilled out and a rod inserted as a dipstick

[gon2sea]

its more complicated than a simple equation... for 10 years on my 78ft schooner with a 120 ford lehman with a 3:1 gearbox and a 29" prop i would get 1.4 gph at 6 knots pushing 85,000lbs... one thing is certain, big, slow turning props are way more efficient... and these are REAL figures, i had a range of 1100 miles under power ( 3 times thru panama ) so these figures are not from week end trips or in and out of a harbour... waterline lenght helps greatly as well.

[Delfin]

Perhaps someone else has made this observation, but it appears there is some confusion between maximum horsepower with prop demand in this discussion. Maximum hp is the total output at a given rpm. Prop demand is the horsepower used at a given rpm with the limited load required to spin a prop in water. The fuel consumption is very different at the same horsepower because one is at maximum load and one is at a relatively minimal load. At max rpm the curves come together, but at lower rpms they are way different. For example, my CAT 3306 requires 69 hp burning 3.8 gph at 1400 rpm to spin a prop. Put a load on the engine that will allow the motor to turn at 1400 rpm, where any increase in the load would cause the engine to lug, and you need 204 hp and will burn 11.7 rpm. At full load, a diesel will burn around .5 gallons per hour per 10 hp. At prop demand in the mid range of operating rpms, the number is still about .5 gph, but the amount of hp to turn the prop at those mid range speeds is significantly less.

www.delfin.talkspot.com

[handmer]

Originally Posted by Dockhead

Indeed, as the formulas say.

On an airplane travelling at 300mph the difference is very great. On a planing-hull power boat it's less, but still measurable. On a displacement hull sailboat, the difference between power required at a given RPM tied up to the dock versus travelling at 6 knots is a few percent. Which is why the tables don't even consider speed; just RPM.

That's why, as the manufacturers explicitly say, the fuel consumption curves are highly correlated to reality for displacement hulls, but all bets are off on planing hulls.

Nudging the keel into a mud bank on the weekend while under power reminded me of this thread

Over about 5 seconds I went from 5 knots to 0 knots. I could hear the engine note change as the motor was loaded as I slowed. And yes, the prop was entirely clear of the bottom. I did not hang around long enough to check fuel consumption

Dockhead, you are mostly correct except for thinking that the speed of the boat makes only a few percent difference in engine load.

The thing here you need to consider is the speed of the boat compared to the speed of the water being fed out the back by the prop.

With a plane the speed of the air fed out the back is very high, as is the speed at which the plane travels.
A boat travels much slower but also creates a much slower stream of water.

In a perfect world a prop at a fixed RPM will accelerate a fixed quantity of water to a fixed speed.
So it takes a fixed amount of energy to accelerate the water to a fixed speed.

Lets imagine that at a fixed RPM but against a dock your propeller creates a stream of water travelling at 10 knots. The load of the engine is constant. It used a fixed amount of energy to accelerate the water.

Lets say under way at the above fixed RPM your boat does 5 knots. At the same fixed RPM the prop still accelerates the water to 10 knots. However relative to the prop the water is already doing 5 knots. It requires half the energy to accelerate that water compared to when the boat is not moving.

So lets imagine you have a headwind that drops the boat speed to 2.5 Knots.. Your engine still has to accelerate the water to 10 knots, expect now the incoming water is only moving 2.5 knots relative to the prop. So it takes 50% more energy to accelerate this water compared to at 5 knots.

Even if your headwind only drops the boat speed by 0.5 knots you still need 10% more energy to accelerate the water. So, 10% more fuel use.

Of course this is in an ideal world and does not take into account changing drag, prop efficiency etc etc. I also don't have any hard numbers on actual speeds of water created by the prop. From my research it seems for a displacement boat the difference is typically less than double the cruise speed.

[markpierce]

This is what the designer of the 35-foot Coot trawler says about fuel consumption for it.

"A typical diesel will develop almost 20 HP for an hour on one gallon of fuel. I’d tend to cruise at about 1.2 or so speed/length ratio. 6 1/2 to 7 knots is a good clip, and the engine will easily do it. Notice the difference between 7.6 knots and 9; that translates to 1.2 gallons an hour to over 70 an hour.....



S/L Ratio..... Knots..... HP
1 ...... ..........5.63........ 3.9
1.1............... 6.19....... 6.0
1.2 ...............6.75....... 9.5
1.25............. 7.03...... 12.8
1.3............... 7.32...... 17.3
1.35.............. 7.6........ 23.5
1.6................ 9.0 ...... 149.3
Thank goodness the standard engine on the Coot is only 85 horsepower.

[sbmar]

BSFC ~~ FUEL Consumption & Horsepower Produced


It seems that we have so many opinions of just how much a diesel engine burns, I thought I drop in another version of how it really works.. I originally put this together over 10 years ago and posted in on another forum after some editing, as just like now, nothing much has changed about all the perceptions out there as to how much fuel does a diesel use..

See if this makes any sense, as really, this is how it all actually works.. And of course, if you can’t buy into this tutorial, you can always hit up Wikipedia, “BSFC” , and get their take on it.. I wrote this before Wiki was born, and yes, I did leave pleanty room for improvement…


There is a direct relationship between the horsepower produced or extracted from all engines in relation to the fuel they burn. This is not a bold statement, it's just a fact, and I certainly didn't write the rules. There are many different efficiency factors involved and many different types of fuel, conversion factors, etc... But it all comes down to the conversion of heat energy to mechanical energy. For this discussion, I'll make a few very basic claims (?) that are generally accepted principles in the operation of the types of most common diesels used in marine service ( or any service for that matter) .

1. BSFC (brake specific fuel consumption) is an accepted and universally used measurement for gauging power output in relation to fuel consumed. Typical units used by Cummins, Caterpillar, Yanmar, Isuzu, etc. would be lbs/hp/hr or grams/kW/hr. A standard wt for #2 diesel might be 7.001 lbs per gallon at 60 degrees F. Most manufacturers publish graphs showing BSFC at various rpm/load levels, and that, along with some interpolation, should enable one to determine that generally all diesels are their most fuel efficient (lowest BSFC) at peak torque. Of course, in marine applications, you should NOT be able to load the engine at peak torque. This is because a vessel with the correct prop and reduction ratio (propped to reach or exceed rated RPM under maximum loaded conditions) will prevent the aforementioned condition from ever occurring.

2. All the diesels in marine service ( and other service too) that I have come in contact with over the past 25+ years fall in to a BSFC ranges of .450 to .325 (lbs/bhp/hr.) With a little math, one can derive the "magic number" of 20 hp/gallon/hr (.355) as this is a BSFC that matches (a high average) the amount of hp that is produced by a modern 4-stroke, direct injected, turbo charged/after cooled high speed diesel of modern design. At the far end of this scale (lousy BSFC) you will find normally aspirated 2-stroke diesels ( whose design characteristics date back to pre-WWII w/ many being mechanically supercharged although called NA's) and a few NA, indirect injection, 4-stroke diesels (which are not nearly as old).

You will also find at the best end of the spectrum modern engines typically designed and used for the very heaviest duty applications. By coincidence, the most efficient engines today used to produce rotational energy for marine applications (and, I believe, the most efficient heat engines available for any off the shelf application, are large (10,000+++ hp) diesel engines of the 2-stroke cross head design that operate at low speed levels (40 to 300 rpm), but do burn heavy fuel oil ( higher BTU content per of fuel?) . The are used mostly in large containership and oil tankers to move extremely large shipments across the oceans.
3. For comparison purposes, consider the following and give this some thought:
a) A carbureted gasoline (60's through 80+'s design) engine used in an automotive application, or adapted for another application (your typical 454 Chevy/Mercruiser), will deliver LESS than 12 hp/gal/hr of gasoline consumed under the best of conditions.

b) A two-stroke outboard engine (carbureted) may give you 6-8 hp/gal/hr.

c) A two-stroke high output motocross bike (Honda CR 250, Suzuki RM 250, etc.) might give you 4-5 hp/gal/hr if you are lucky.

d) Your Cox .049 model airplane engine running on methanol/castor bean oil mix is off my scale. I think more fuel comes out the exhaust than is used to make hp.

The above relationships are for comparison and may help some readers to understand why a diesel powered vessel goes farther on a gallon of fuel than a gas powered vessel of similar size/design.

A much misunderstood concept related to this is that the propeller moves the boat and not the engine. The engine just rotates it. It takes a certain amount of hp to turn any given propeller under any given circumstance, so if you have an engine that produces the particular hp needed for that circumstance, then a diesel engine will do it with substantially less fuel burn. Less propeller (diameter or pitch) mean less HP will be used to turn the prop. More propeller , more PROPELLER RPM, or more vessel mass or resistance to movement means more HP is needed, hence more fuel per hour is used.. In other words, it IS NOT how much HP you have available in your engine, it how much HP you are asking for the engine to develop that equates to GPH.. A 100 HP engine in neutral at 3000 RPM burn less fuel per hour than a Yanmar 3GM at 2500 RPM moving your sailboat at 5 knots…


4. Typically BSFC is better (7-10% is common) with engines that are "turbo-ed" due to the capture of some wasted exhaust energy that is then mechanically used to compress air (oxygen) which is then fed to the engine. This allows an increase of air during combustion and increases the extraction of energy from the fuel allowing more fuel to be burned properly in a given engine which means more hp output can be extracted from that engine, if needed.. Cooling the compressed air between the turbo and the intake of the engine is referred to as "aftercooling" or "inter-cooling" (I don't want to argue the semantics of the two words/terms).. This further allows more air to be put into the engine because as the air is cooled, more oxygen is available do to the density increase from cooling.. Again, more fuel can be burned efficiently to make more hp, if needed. There are other benefits from aftercooling besides; it reduces thermo loading/stressing in the combustion areas of the related components and promotes less nitrogen oxide (NOx) to be produced during combustion. A popular misconception regarding turbocharging is that it increases stresses in the engine. To some extent it does raise the compression ratio, but for the same hp output from the same basic engine, turbocharging in itself is not the culprit. It's the fact that more hp is available to use that MAY add to a shorter lifetime of the engine. From my practical experience with modern engines like the B & C Series Cummins and others specifically designed for turbocharging, I don't feel this point contributes to how long the engine lasts.. Rather, it's the "nut behind the wheel" concept, that is much more the determining factor in the overall life of a modern diesel engine today.

Most boaters are concerned about fuel consumption and should be. Although there are some who take exception to using diesel engines in smaller boats as they feel the benefits of reduced fuel consumption don't even come close to justifying the cost of upgrading to diesels, I'll leave you with this thought. There are many (one, very visible in this industry) skeptics that preach that gas is the only practical choice for smaller boats (under 35 ft or so) based on many of their own perceptions and observations over years of "experience" All of these reasons when read and understood, are really just based on a skewed perception & dollar bills. I wonder if in all of the negativities that they preach about things or "basic principles", if they have even considered the fact that to some of us, just the inherent safety of diesel fuel (besides a dozen or so other valid reasons) could mean more to some than that wonderful old dollar In closing, there is a very definite relationship between the amount of fuel burned and the amount of horsepower produced. This is an important concept to understand, especially for you 'forum readers' as discussions center around various competitive diesel engines, fuel burn rates, performance and range.

The internal combustion diesel engine still remains to this day the most fuel efficient engine designed, built and used in a variety applications. Quite a nice idea that Rudolf had back in 1880's.

--------------------------------------------------------------------------------

Tony Athens / October 1st, 2000

[wolfenzee]

I have been told by a mechanic that an engine should be able to reach it's maximum rpm, if not the prop is pitched wrong.
I have a Vetus m4.14, a 4 cylinder 1415cc Misubishi K4E
rated at 33hp max rpm 3000rpm, max torque 78Nm (57.5ft/lb) at 2100rpm
fuel consumption is listed at 260 g/Kw-h (gph?) at 2000rpm.
My boat is 30' w/25' lwl technically hull speed = 6.75kt
1000rpm=4kt, 2000rpm=6.5kt, 2200rpm=7kt
I have a 16"x9"" three bladed prop, because I have room for such a big prop could I replace it with a 2 bladed prop with more pitch and achieve the same thing...I could line up the prop with the keel using a shaft brake while under sail and cut down drag. I have seen two bladed props with squared off ends to achieve more area.

[model 10]

1/2 lb. per hour per hp holds true for just about any engine. Turbines a little more, modern cars a little less

[wolfenzee]

My engine is rated at 33hp at 3000rpm (max rpm) at 2100rpm, the height of the torque curve. 57.5ft lbs I am doing hull speed....how do you figure hp from torque and rpm?

[model 10]

I do not know how to figure that out. Use a percentage of max power to come to some sort of conclusion.

[Panope]

Originally Posted by wolfenzee

My engine is rated at 33hp at 3000rpm (max rpm) at 2100rpm, the height of the torque curve. 57.5ft lbs I am doing hull speed....how do you figure hp from torque and rpm?

If your published torque curve is based on "propeller load" then you have good torque information. If your published torque curve is based on the maximum torque available for each rpm, then we really do not know the torque without more info.

If you believe your torque data to be accurate then you can use this formula.

Horsepower = torque x r.p.m./5252

Steve