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  1. #1
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    Theory on fuel consumption

    As I am meandering down the lake in my thirsty old carbed motor hearing the gas pump clicking away as I go (I am old enough to remember that sound). I wonder which uses less gas. Scenarios: I'm running 30mph @ 3200 rpm after coming on plane. Will I use more gas trimmed higher @ same throttle position when RPM climbs to 3800 or do I need to trim then throttle back to prevent more fuel from being sucked in by higher speed engine even though butterflies are in same position? I'm guessing I NEED to throttle back to save fuel but I am curious, will it use MORE fuel trimmed up and turning faster? Thanks.

  2. Member berudd's Avatar
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    #2
    Quote Originally Posted by dentpusher View Post
    As I am meandering down the lake in my thirsty old carbed motor hearing the gas pump clicking away as I go (I am old enough to remember that sound). I wonder which uses less gas. Scenarios: I'm running 30mph @ 3200 rpm after coming on plane. Will I use more gas trimmed higher @ same throttle position when RPM climbs to 3800 or do I need to trim then throttle back to prevent more fuel from being sucked in by higher speed engine even though butterflies are in same position? I'm guessing I NEED to throttle back to save fuel but I am curious, will it use MORE fuel trimmed up and turning faster? Thanks.
    Probably use more fuel. And to complicate it more for you, as your going faster you will arrive at your destination sooner. So while yo are burning more gas per minute, did you get there fast enough that the total trip burned less fuel?
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  3. Member DrewFlu33's Avatar
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    #3
    Pretty sure that you're going to use the same amount of fuel no matter the RPMs given a specific throttle position. You can think about it in a couple different ways.

    If you can come up with an example where RPMs going up does not increase fuel consumption, that's a good place to start for me. The easiest example is gearing down in a vehicle when coasting down a hill. Your RPMs go up, but you're literally using zero fuel until your foot touches that gas pedal again. Maybe not a perfect comparison, but "proof" to me at least that higher RPMs does not equal more fuel consumption.

    Another thing that came to mind for me was thinking about a workout regime you might undertake. Obviously working out and a motor pushing a boat down the lake are different things, but when you get right down to it, they're both burning fuel (gasoline or calories) to do work (move an object, whether that's lifting weights or turning a prop/moving a boat). If the force required to move an object goes down, you can move it further for a given amount of fuel burn. If the force required to move an object goes up, you move it not as far for a given amount of fuel burn.

    As a specific example, think about riding a stationary bicycle. If the resistance is really low, you can turn those pedals a bunch of times without burning many calories. If you crank the resistance up, you'll burn a hell of a lot more calories to turn the pedals the same amount of times, or will burn the same amount of calories as before by turning the pedals fewer times. The motor is the same way - trimming up reduces the force required to turn the prop (analogous to less resistance on the stationary bike), so it can turn the prop more times for a given amount of fuel burned.

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    #4
    Quote Originally Posted by DrewFlu33 View Post
    Pretty sure that you're going to use the same amount of fuel no matter the RPMs given a specific throttle position. You can think about it in a couple different ways.

    If you can come up with an example where RPMs going up does not increase fuel consumption, that's a good place to start for me. The easiest example is gearing down in a vehicle when coasting down a hill. Your RPMs go up, but you're literally using zero fuel until your foot touches that gas pedal again. Maybe not a perfect comparison, but "proof" to me at least that higher RPMs does not equal more fuel consumption.

    Another thing that came to mind for me was thinking about a workout regime you might undertake. Obviously working out and a motor pushing a boat down the lake are different things, but when you get right down to it, they're both burning fuel (gasoline or calories) to do work (move an object, whether that's lifting weights or turning a prop/moving a boat). If the force required to move an object goes down, you can move it further for a given amount of fuel burn. If the force required to move an object goes up, you move it not as far for a given amount of fuel burn.

    As a specific example, think about riding a stationary bicycle. If the resistance is really low, you can turn those pedals a bunch of times without burning many calories. If you crank the resistance up, you'll burn a hell of a lot more calories to turn the pedals the same amount of times, or will burn the same amount of calories as before by turning the pedals fewer times. The motor is the same way - trimming up reduces the force required to turn the prop (analogous to less resistance on the stationary bike), so it can turn the prop more times for a given amount of fuel burned.

    Fun with analogies.
    Dang. Started out cruising down the lake and ended up riding a stationary bike. Took all the fun right out of it!

  5. Member berudd's Avatar
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    #5
    Quote Originally Posted by DrewFlu33 View Post
    Pretty sure that you're going to use the same amount of fuel no matter the RPMs given a specific throttle position. You can think about it in a couple different ways.

    If you can come up with an example where RPMs going up does not increase fuel consumption, that's a good place to start for me. The easiest example is gearing down in a vehicle when coasting down a hill. Your RPMs go up, but you're literally using zero fuel until your foot touches that gas pedal again. Maybe not a perfect comparison, but "proof" to me at least that higher RPMs does not equal more fuel consumption.

    Another thing that came to mind for me was thinking about a workout regime you might undertake. Obviously working out and a motor pushing a boat down the lake are different things, but when you get right down to it, they're both burning fuel (gasoline or calories) to do work (move an object, whether that's lifting weights or turning a prop/moving a boat). If the force required to move an object goes down, you can move it further for a given amount of fuel burn. If the force required to move an object goes up, you move it not as far for a given amount of fuel burn.

    As a specific example, think about riding a stationary bicycle. If the resistance is really low, you can turn those pedals a bunch of times without burning many calories. If you crank the resistance up, you'll burn a hell of a lot more calories to turn the pedals the same amount of times, or will burn the same amount of calories as before by turning the pedals fewer times. The motor is the same way - trimming up reduces the force required to turn the prop (analogous to less resistance on the stationary bike), so it can turn the prop more times for a given amount of fuel burned.

    Fun with analogies.
    The throttle on a carburetor only controls air. Gasoline is drawn through the jets in a carburetor by the vacuum created as the pistons pull in air. When RPMs increase, the velocity of the air through the venturi increases. The volume of air pulled by each piston is roughly constant so more intake strokes in the same amount if time, ie higher RPMs, means more air is pulled through the carburetor. Thiswill pull more fuel through the jets. If this didn't happen to some extent the, the engine would go lean which, if it got bad enough, could cause engine issues or perhaps not even run at all except in a very narrow RPM range. With carbs, the fuel ratio is not perfect throughout the RPM range but increase the velocity of the air will pull more gas.
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    #6
    Motors an air pump, your carb meters fuel based on airflow not throttle position/butterfly position!!!!!!!!! if you were WOT but running at 750 rpm your not burning WOT fuel... though i imagine you would be running a little rich and not efficient.

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    #7
    Quote Originally Posted by berudd View Post
    The throttle on a carburetor only controls air. Gasoline is drawn through the jets in a carburetor by the vacuum created as the pistons pull in air. When RPMs increase, the velocity of the air through the venturi increases. The volume of air pulled by each piston is roughly constant so more intake strokes in the same amount if time, ie higher RPMs, means more air is pulled through the carburetor. Thiswill pull more fuel through the jets. If this didn't happen to some extent the, the engine would go lean which, if it got bad enough, could cause engine issues or perhaps not even run at all except in a very narrow RPM range. With carbs, the fuel ratio is not perfect throughout the RPM range but increase the velocity of the air will pull more gas.
    This is what I thought but I didn't think about arriving at the destination sooner like you mentioned before.

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    #8
    With cars , it's mile per gallon, with boats it's gallons per hour. In the old days we were always told to run 3/4 throttle for best gas consumption. a lot of it depends on your load.

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    #9
    If I have to worry about fuel mileage, I'll buy a canoe.

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    #10
    Gas consumption cannot be predicted in a boat with similar means as a car or truck due to drag caused by friction of water with the hull. Let off the gas in neutral in a truck it slows down slowly. Let off the gas in a boat and it stops rather quickly. If your boat is on the pad, there is less friction than if the hull was fully in the water. The economy gets quite a break compared to RPM's when it gets up to speed, but not WOT.
    Each hull and motor will have a "sweet spot" where it gets the best economy with reasonable speed.

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    #11
    I thought going fishing was for NOT worrying about stuff like that.

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    #12
    my theory:
    Put gas in, go fishing!

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    #13
    Quote Originally Posted by dentpusher View Post
    As I am meandering down the lake in my thirsty old carbed motor hearing the gas pump clicking away as I go (I am old enough to remember that sound). I wonder which uses less gas. Scenarios: I'm running 30mph @ 3200 rpm after coming on plane. Will I use more gas trimmed higher @ same throttle position when RPM climbs to 3800 or do I need to trim then throttle back to prevent more fuel from being sucked in by higher speed engine even though butterflies are in same position? I'm guessing I NEED to throttle back to save fuel but I am curious, will it use MORE fuel trimmed up and turning faster? Thanks.
    The direct answer to your question is "NO" . You do not burn more gas if the throttle is the same but RPM's go up due to raising trim and decreasing drag. You actually keep your gph consumption the exact same but increase mpg.

    I run digital GPH as I run down the lake, for my best compromise of fuel consumption and a reasonable speed I run throttle to 9.5 gph then raise trim to get the higher rpm there for more speed and more mpg. Ball park 45mph is 9.5 GPH and 68mph is 22 GPH.

  14. Member LTZ25's Avatar
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    #14
    Don't be fooled all of them use fuel . My SHO does better going down hill .
    Last edited by LTZ25; 08-07-2019 at 07:29 PM.

  15. Member Bassnailer's Avatar
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    #15
    The less hull you drag through the water, the more efficient you will run, thus trimming as high as you can without porpoising will get you your best fuel economy. I have Mercury vessel view mobile which is great for determining how much fuel you are burning, your mpg, and gph. Other than idling, I find that I get the best economy at two different spots; planed out and trimmed up as high as I can go at either around 3800 RPM or 5800 RPM (WOT).
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    #16
    I have an old carbed motor too, when all the kids (3) moved out, I stopped hearing that clicking noise.

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    #17
    There are a lot of factors that are going to determine the fuel economy. Water and wind drag, weight, prop slip.

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    #18
    Quote Originally Posted by Riccochet View Post
    There are a lot of factors that are going to determine the fuel economy. Water and wind drag, weight, prop slip.
    Yes, but all of those are the same in the op's question of rpm vs. fuel economy.

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    #19
    Quote Originally Posted by billnorman1 View Post
    Gas consumption cannot be predicted in a boat with similar means as a car or truck due to drag caused by friction of water with the hull. Let off the gas in neutral in a truck it slows down slowly. Let off the gas in a boat and it stops rather quickly. If your boat is on the pad, there is less friction than if the hull was fully in the water. The economy gets quite a break compared to RPM's when it gets up to speed, but not WOT.
    Each hull and motor will have a "sweet spot" where it gets the best economy with reasonable speed.
    ^This, most engines will be +/- 3500 rpm for an outboard. Mainly because you can get a lite lift without using a lot of H P.
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    #20
    With a boat it is best to not think about fuel economy.
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