Any way short of fuel flow meters to figure an economical crusing speed? Twin Detroit 671-N at 2050 RPM 16 knots 22 - 25 gph. Sometimes not in that much of a hurry and would like to save some fuel. Does lowering to 1200 or 1500 rpm improve mpg?
Best Crusing Speed?
- Thread starter woodchuck
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Considering that you have not mentioned most of the variables that are needed to answer the question ( 
) I'll present the generic answer: Your best nmpg will be at idle.
Your best practical speed will be at just below "hull speed" or approximately 1.34 x the square root of your waterline length.
I haven't a clue about where RPM figures into it...
) I'll present the generic answer: Your best nmpg will be at idle.Your best practical speed will be at just below "hull speed" or approximately 1.34 x the square root of your waterline length.
I haven't a clue about where RPM figures into it...
Or, if you really want to save fuel, cruise at the square root of your waterline length, which is below your "hull speed" but is frequently the place where will go the furthest on a gallon of fuel--of course this is pretty slow, a 49' LWL boat would be at 7 knots. Really this is not such an easy question, since there are a lot of factors in what constitutes a comfortable cruise speed in any given boat. It might help if you mentioned what kind of boat you have and how you typically use it. There might be somebody on the forum who has a similar boat and can give you some real numbers.
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"Any way short of fuel flow meters to figure an economical crusing speed?"
Of course, you could always do it the old fashioned way: Make repeated runs over the same course ( both directions then average ) while measuring the actual fuel consumed. This, of course, takes time and effort, and will produce good, but still somewhat inaccurate numbers. But your goal, to find the actual consumption at various speeds, is an important one. ( One every boater should know about their boat ). After many identical runs ( except for speed ) you can produce a chart with your nmpg per speed or RPM, for your vessel, under known conditions. You can adjust from the chart curve for different conditions.
Knowing max fuel consumption, as well max range, can be of huge importance for safety. Still, without knowing the details of your vessels hull ( getting down to the real nitty-gritty details ) conditions, fresh, brackish or salt water, and so on, for a bucket full of variables, all we can give you is a "rule of thumb". A fuel consumption meter will give you real time answers and allow you to achieve the best results for real-time conditions. All else will be a guess and/or approximation.
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Note that the impression of these meters being expensive is a deceiving one. Consider that fuel will likely, in the near future be $5/gallon. If you can use the meter(s) to improve your mileage by 10% you will soon recover the cost, perhaps in one or two seasons.
Of course, you could always do it the old fashioned way: Make repeated runs over the same course ( both directions then average ) while measuring the actual fuel consumed. This, of course, takes time and effort, and will produce good, but still somewhat inaccurate numbers. But your goal, to find the actual consumption at various speeds, is an important one. ( One every boater should know about their boat ). After many identical runs ( except for speed ) you can produce a chart with your nmpg per speed or RPM, for your vessel, under known conditions. You can adjust from the chart curve for different conditions.
Knowing max fuel consumption, as well max range, can be of huge importance for safety. Still, without knowing the details of your vessels hull ( getting down to the real nitty-gritty details ) conditions, fresh, brackish or salt water, and so on, for a bucket full of variables, all we can give you is a "rule of thumb". A fuel consumption meter will give you real time answers and allow you to achieve the best results for real-time conditions. All else will be a guess and/or approximation.
---
Note that the impression of these meters being expensive is a deceiving one. Consider that fuel will likely, in the near future be $5/gallon. If you can use the meter(s) to improve your mileage by 10% you will soon recover the cost, perhaps in one or two seasons.
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your profile shows a 43 footer... your hull speed is probably around 8kts... you should burn around 6 to 7 GPH at that speed, RPM depends on props and gearing... probl around 1200 or so. check your speed with GPS on a day with calm wind and no current.
at 16kts you're barely on plane and pushing a big wake. at that speed, make sure your trim tabs are all the way down to help get her up... sometimes, accelerating a little beyond cruise speed to fuylly get on plane and then backing off back to your cruise can help.
at 16kts you're barely on plane and pushing a big wake. at that speed, make sure your trim tabs are all the way down to help get her up... sometimes, accelerating a little beyond cruise speed to fuylly get on plane and then backing off back to your cruise can help.
Since ypu are obviously concerned about economy at cruising speed, I'll suggest you develop a table of RPM vs speed on plane. It should be obvious when speed begins to increase at slower rate vs RPM at which point econmical cruise has been surpassed. That may be considerably more clear with some engine/hull combos than others. Clearly best range will be at slow < hull speed, but that may not be the optimum get-there speed. .
at 16kts you're barely on plane and pushing a big wake. at that speed, make sure your trim tabs are all the way down to help get her up... sometimes, accelerating a little beyond cruise speed to fuylly get on plane and then backing off back to your cruise can help.
Yes, Pascal thats what I have to do to cruise 16 knots. 14 into wind, current, 18-19 with.
Boat is a 43 Viking DCMY and I'll figure waterline length and do the math. If we're only going 5-10 miles to hang for the day 8 knots won't be to bad. If I ever decide to winter in Florida fuel flow meters would be the first thing I would add. The install seems like it would be a project, I would do it myself. I guess the one that ties into the gps would be the best for nmpg.
Yes, Pascal thats what I have to do to cruise 16 knots. 14 into wind, current, 18-19 with.
Boat is a 43 Viking DCMY and I'll figure waterline length and do the math. If we're only going 5-10 miles to hang for the day 8 knots won't be to bad. If I ever decide to winter in Florida fuel flow meters would be the first thing I would add. The install seems like it would be a project, I would do it myself. I guess the one that ties into the gps would be the best for nmpg.
Forget all the math. That predicts maximum displacement speed with big fore and aft waves and has nothing to do with economy despite incorrect conventional wisdom. It predicts the maximum speed of a displacement hull that cant possibly plane. Adding any power above that required for that speed will only be wasted.
Your maximum speed for economy will always be at idle but for practical purposes your maximum speed with decent economy will be the speed just below the point where the bow starts to lift. keep the bow down, no lifting at all. At 7-7.5 kts I expect you will get 2+ NMPG but that will drop quickly as soon as the bow starts to lift. Another clue is found by looking at the transom. Calm wake with little frothing and no separation is what you want.
If you are technically inclined a graph of speed vs. RPM from idle up will be linear to the point where extra fuel is being used to try to push water around instead of increasing speed.
Your maximum speed for economy will always be at idle but for practical purposes your maximum speed with decent economy will be the speed just below the point where the bow starts to lift. keep the bow down, no lifting at all. At 7-7.5 kts I expect you will get 2+ NMPG but that will drop quickly as soon as the bow starts to lift. Another clue is found by looking at the transom. Calm wake with little frothing and no separation is what you want.
If you are technically inclined a graph of speed vs. RPM from idle up will be linear to the point where extra fuel is being used to try to push water around instead of increasing speed.
It's pretty much impossible to measure fuel useage on a boat without a fuel flow meter. On my previous trailer boat, I fuelled at a roadside gas station so I could use the auto shutoff at the gas pump, but you still end up using vastly different amounts of fuel for the same distance depending on wind and currents. Also, the more fuel you have on board, the higher your fuel burn will be just to move the weight of the fuel.
If you're fuelling at a marina, the ones I've been to don't have an auto shutoff so it's just about impossible to fill the tanks to the top without a lot of spillage and they don't seem to like that.
Fuel flow meters for a diesel seem to be about $1 each so it's going to take a lot of fuel savings to pay for them.
Hull speed should be the most economical, it's all downhill from there.
If you're fuelling at a marina, the ones I've been to don't have an auto shutoff so it's just about impossible to fill the tanks to the top without a lot of spillage and they don't seem to like that.
Fuel flow meters for a diesel seem to be about $1 each so it's going to take a lot of fuel savings to pay for them.
Hull speed should be the most economical, it's all downhill from there.
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let's say it is the speed which offers the best compromise between speed, economy, and engine longevity. beyond hull speed, your fuel burn increases drastically faster than speed and below hull speed you may have issues keeping larger diesels warm and fuel properly vaporized.
indeed if you want best economy, slowing down 20% below hull speed will be even better.
indeed if you want best economy, slowing down 20% below hull speed will be even better.