Does Fuel Efficiency = Engine Efficiency?-gctid385834

[Guest]

Alright, this might possibly be the dumbest question to date, but I'll throw it out there anyway.

Depending on conditions, my boat can hold plane at 3200rpm and as I under stand it the engine runs the easiest and with the least stress at about 3800rpm.

Now, if my best fuel economy were at 3200 as compared to 3800, would that suggest that 3200 is the ideal speed for the engine in terms of work load put on the engine?

[Guest]

Gilligan wrote:
Alright, this might possibly be the dumbest question to date, but I'll throw it out there anyway.

Depending on conditions, my boat can hold plane at 3200rpm and as I under stand it the engine runs the easiest and with the least stress at about 3800rpm.

Now, if my best fuel economy were at 3200 as compared to 3800, would that suggest that 3200 is the ideal speed for the engine in terms of work load put on the engine?

It is your boat that determines fuel economy.

Usually it has 2 sweet spots, one at low speed and one at high speed, usually when the tabs are full up, the spray is at about 2/3 back on the boat and the outdrive is properly trimmed.

[yachtman]

There are many things that determin economy. The friction or drag of a boat through the water has a big play in that thus a planning hull vs. a semi displacment hull.

Once the parameters are set IE.... what type of hull and engine you have then yes engine preformance can =economy. For example if your engine is out of tune it can be using more fuel to get the same power. Now other things that effect economy will be condition of the entire drive train. Transmission drive and propellor. The condition of the bottom of the boat is also a factor.

If all is perfect with the other systems on the boat then your equation is more or less true but there are many factors that can and do effect economy.

[captharv]

[QUOTE]Gilligan wrote:
Alright, this might possibly be the dumbest question to date, but I'll throw it out there anyway.

Depending on conditions, my boat can hold plane at 3200rpm and as I under stand it the engine runs the easiest and with the least stress at about 3800rpm.

Now, if my best fuel economy were at 3200 as compared to 3800, would that suggest that 3200 is the ideal speed for the engine in terms of work load put on the engine?[/QUOTE]

in one word: yes

Now, longer answer: The engine has a sweet spot, USUALLY 1000 RPMs less that WOT. The boat has a planing sweetspot, whatever speed is determined by design. If you can get the two to be in the same sweet spot, the MPG does peak there. ( On my 2452, its 3600 RPM at 26 MPH=2.35 MPG) The fuel usage is directly proportional to HP used. Law of physics. Anything which increases MPG is taking a load off the engine.

All that said, how do you intend to accurately measure MPG?

[Guest]

[QUOTE]captharv wrote:

Gilligan wrote:
Alright, this might possibly be the dumbest question to date, but I'll throw it out there anyway.

Depending on conditions, my boat can hold plane at 3200rpm and as I under stand it the engine runs the easiest and with the least stress at about 3800rpm.

Now, if my best fuel economy were at 3200 as compared to 3800, would that suggest that 3200 is the ideal speed for the engine in terms of work load put on the engine?[/QUOTE]

in one word: yes

Now, longer answer: The engine has a sweet spot, USUALLY 1000 RPMs less that WOT. The boat has a planing sweetspot, whatever speed is determined by design. If you can get the two to be in the same sweet spot, the MPG does peak there. ( On my 2452, its 3600 RPM at 26 MPH=2.35 MPG) The fuel usage is directly proportional to HP used. Law of physics. Anything which increases MPG is taking a load off the engine.

All that said, how do you intend to accurately measure MPG?

I'm measuring with the speed the GPS gives me and the GPH the Flowscan gives me.

[Guest]

Gilligan,

Dumbest question to date? Far from it. It probably ranks right up there with one of the most intelligent ones. Also one of the hardest ones to answer.

Here's why.

I think that I can be safe in assuming that you have a gas engine (engines?). This being the case, engine efficiency is usually best at or near peak torque RPM, Wide Open Throttle. What this means is that the Brake Specific Fuel Consumption is at minimum. Least amount of fuel burnt per horsepower/hour produced. At WOT, BSFC increases with increasing or decreasing engine speed. At part throttle loads, BSFC increases a whole bunch. The less load that is on the engine, the more inefficient it becomes. However, you really don't want to want to run your engines at WOT on a continuous basis. Typically, gas engines produce more than enough power to hurt themselves if run at WOT for long periods of time. This is not good for engine life. It should be limited to about 80% load or less, which lowers the efficiency. That's the engine part.

Now we have the boat part. A planing boat will typically have two points at which it will be most efficient to run regardless of what engine and gear train combination is driving it. The first will be about 2/3ds of the calculated hull speed, which is usually quite slow. The second will be just past the point where the boat is truly planing. On a true planing hull, this is the point where she is " on the step" enough that you can back off the throttle a bit and still maintain the same speed.

This is the efficiency part. The economy part is the compromise between engine efficiency, hull efficiency, and the speed that you would like to run. Engines have a rather narrow efficiency range. So do boat hulls. Matching the two together isn't all that difficult if the owner has a particular speed that he would like to run. If the speed range is anything but narrow, the compromises limit everything.

How all of this relates to your boat/engine combination will depend on the torque curve of the engine and the actual load on it at the different RPM points.

That probably didn't answer your question, but I hope I have been successful in trying to explain why.

[ksanders]

Gilligan wrote:
Alright, this might possibly be the dumbest question to date, but I'll throw it out there anyway.

Depending on conditions, my boat can hold plane at 3200rpm and as I under stand it the engine runs the easiest and with the least stress at about 3800rpm.

Now, if my best fuel economy were at 3200 as compared to 3800, would that suggest that 3200 is the ideal speed for the engine in terms of work load put on the engine?

Thats the way I've always looked at it.

[SomeSailor]

That's exactly the way to look at it. Efficiency is nothing more than energy delivered as a factor of energy supplied. More fuel in for the same work over a period of time means less efficiency.

Follow the fuel burn over distance (or time) and there ya have it.

[Guest]

Only with regards to holding plane is 3200 rpm the sweet spot. Hull speed most likely increases at 3800 rpm so isn't the trade off fuel economy verses how fast you get there or did I miss the point?

[Guest]

Fish-a-Palooza wrote:
Only with regards to holding plane is 3200 rpm the sweet spot. Hull speed most likely increases at 3800 rpm so isn't the trade off fuel economy verses how fast you get there or did I miss the point?

If we are talking strictly FUEL economy, then you are hitting the point. I'm asking about two efficiency ratings and how they relate, or where they might overlap.

RJH and others seemed to go right down the throat of my question and I think I understand. I'll re read some of these posts in an effort to fully digest it. Thanks... very helpful! :worth

[SomeSailor]

Just remember that hull speed is just a mathematical approximation of where your hull should be most efficient. It will vary a bit, and you can force the boat beyond that speed without planing, but you pay the price in fuel economy.

[Guest]

[QUOTE]captharv wrote:

Gilligan wrote:
Alright, this might possibly be the dumbest question to date, but I'll throw it out there anyway.

Depending on conditions, my boat can hold plane at 3200rpm and as I under stand it the engine runs the easiest and with the least stress at about 3800rpm.

Now, if my best fuel economy were at 3200 as compared to 3800, would that suggest that 3200 is the ideal speed for the engine in terms of work load put on the engine?[/QUOTE]

in one word: yes

Now, longer answer: The engine has a sweet spot, USUALLY 1000 RPMs less that WOT. The boat has a planing sweetspot, whatever speed is determined by design. If you can get the two to be in the same sweet spot, the MPG does peak there. ( On my 2452, its 3600 RPM at 26 MPH=2.35 MPG) The fuel usage is directly proportional to HP used. Law of physics. Anything which increases MPG is taking a load off the engine.

All that said, how do you intend to accurately measure MPG?

I WISH I got that kind of mileage at that speed.