Fuel consumption at low speed?

chicagoq said:

I've read on line that sailboats can get 8MPG under power(no sail) ~ at low speed like 5-6knots.

Obviously power boats have very different engine, hull design etc. I wonder in general what kind of MPG can a power boat get at low speed? Or could you tell me for your boat, what max MPG can you extract? At what speed and what kind of engine? Thanks!

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I am one of those that does not think in terms of MPG for the boat. My 40' sailboat burns about .8 gal/hour when doing 7 kt through flat water with no wind. So that is about 8.75 nm/gal or about 10 MPG. Sailboats tend to be extremely efficient, just like "trawlers" with FD hulls and small engines.

My next boat will likely only get about 4 MPG in comparison.

We keep it simple and cruise normally- when ever possible - with 1400 - 1500 rpm of our twin FL120. That 1400 - 1500 rpm gives us 7.2 - 7.4 knots (Beebs S/L slightly above 1.2) through the water (calm condition) with a fuel consumption of 1.5 l/nm or 2.5 nmpg.
Lower rpm would obviously reduce fuel consumption but would result in an unreasonable low load of the engines IMO. Therefore we regard this throttle as our optimum throttle on long distances (which are typically not more than 80-100 nm).
As long as there are no constraints like we have to pass a certain point - lock / bridge / bar - latest at xyz time we keep this throttle also when cruising in head currents i.e. we take the disadvantage of reduced SOG if we cannot avoid the head current by an appropriate planning.

Concept: constant throttle, constant speed through the water, resulting in SOG and consequently ETA depending on the conditions around. Throttle adjusted only in high seas or in case of other constraints ...


best regards / med venlig hilsen
wadden

psneeld said:

What is important is thinking....not just letting the rest of the day hit you in the face.

We see it all the time.

A good example of tide planning is the Delaware River run upstream.

In a 6 knot boat, running the 60 miles or so to the next reasonable anchorage or marina against a 1.5 knot tide on average will take 13.3 hours and will burn 27 gallons of fuel in my Lehman.

If I catch the tide, I am now making 7.5 knots and make the same trip in 8 hrs and burn 16 gallons.

So at dinner time in Chesapeake City, I have refreshed, had a cocktail, made a reasonably early dinner and have an additional $27 to spend on dinner. Only if I was wise enough to plan the tides.

Sure flexibility is good, sure relaxing instead of extensive planning is good, but thinking ahead probably never hurt anyone when it came to making smart decisions.

P or J......at some point they overlap....just like type A or B...

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And I guess that is part of it. Not so much P or J or A or B...just the fact that you planned and ended up scoring a little bit extra because of it!!! One can do a happy dance!!!! Or you can NOT plan and show up late and down some cash....

Baker said:

And I guess that is part of it. Not so much P or J or A or B...just the fact that you planned and ended up scoring a little bit extra because of it!!! One can do a happy dance!!!! Or you can NOT plan and show up late and down some cash....

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After 20 years of meticulous, military planning of missions...that became continuously interrupted by search and rescue mission that required instant adaptability from liftoff.....

....transitioning to "where we gonna spend tonight" wasn't all that big of a deal.

Yet...if travelling along the ACIW during the snowbird rush....if you want to stay in a particular place and not anchor...you better be thinking days if not weeks away. If you live in remote areas and anchor all the time...sure life is less complicated. Just be ale to "think ahead" when the time comes is still necessary for any cruiser.

psneeld said:

So at dinner time in Chesapeake City, I have refreshed, had a cocktail, made a reasonably early dinner and have an additional $27 to spend on dinner. Only if I was wise enough to plan the tides.

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Yes, if cruising in tidal waters an appropriate trip planning saves fuel and gives the opportunity of a reasonable early dinner. When traveling on the river Elbe we always try to run with and not against the tide.
We can easily make those 55 nm from Cuxhaven to Hamburg up the river within 6.5 hrs with our 7 kn through the water, the tide is 7.5 hrs running with us on that direction.
Down the river the tide runs only 5.5 hrs with us. Starting in Hamburg at high tide we usually make a stop at Glückstadt which is only half the way and have this reasonable early dinner. Otherwise we would have to run the last 10 nm in a developing head current and the last 2 - 3 nm to Cuxhaven can then be a PITA.


best regards / med venlig hilsen
wadden

waddenkruiser said:

Yes, if cruising in tidal waters an appropriate trip planning saves fuel and gives the opportunity of a reasonable early dinner. When traveling on the river Elbe we always try to run with and not against the tide.
We can easily make those 55 nm from Cuxhaven to Hamburg up the river within 6.5 hrs with our 7 kn through the water, the tide is 7.5 hrs running with us on that direction.
Down the river the tide runs only 5.5 hrs with us. Starting in Hamburg at high tide we usually make a stop at Glückstadt which is only half the way and have this reasonable early dinner. Otherwise we would have to run the last 10 nm in a developing head current and the last 2 - 3 nm to Cuxhaven can then be a PITA.


best regards / med venlig hilsen
wadden

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Very good point...same with the Delaware river...

You can ride the current for the full 6 or slightly more hours (I don't remember exactly without looking it up)....

But going down river you punch through the tide change in less than a 6 hour run so you have to fight the current at some point. Because of the river shape and the current varies quite a bit from channel to the banks...you can still play it to your advantage.

I know that 20 minutes of planning to save hours of open water droning in possibly rough conditions seems too much...but I do it every time...

Looks like the whole extent of this discussion is that mostly we could just wing it and do just fine or we could do a huge amount of calculations and make a precise float plan. Personally I'm a wing it kinda guy, but, I'm not when it makes a difference. Just coastal cruising, with 1000 gallons of fuel, I'm not gonna get to excited about economy or the next fuel stop. Put 3000 miles of ocean in front of me and my perspective changes dramatically. Now I MUST plan, and try to know and do everything possible to eek out every mile from every gallon of fuel. Wind and current can be a major factor. If I could carry 10,000 gallons I wouldnt worry.

kulas44 said:

Looks like the whole extent of this discussion is that mostly we could just wing it and do just fine or we could do a huge amount of calculations and make a precise float plan. Personally I'm a wing it kinda guy, but, I'm not when it makes a difference. Just coastal cruising, with 1000 gallons of fuel, I'm not gonna get to excited about economy or the next fuel stop. Put 3000 miles of ocean in front of me and my perspective changes dramatically. Now I MUST plan, and try to know and do everything possible to eek out every mile from every gallon of fuel. Wind and current can be a major factor. If I could carry 10,000 gallons I wouldnt worry.

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Actually if you carried 10,000 you would worry because then you'd have a boat that required 10,000. I'll give you an example of one. A Westport 130 has 9,965 gallons of fuel. At it's cruise speed of 20 knots it has range of 1300-1400 nm. At 15-16 knots, 1800-1900 nm. At 12 knots, 3000 nm. So, crossing oceans you would definitely be watching your fuel consumption and wanting accurate information. Even their 164' with has range of 5000 nm at 10 knots and has 20,000 gallons of fuel, but you'd never make it across at it's cruising speed of 20 knots.

Many large boats that cruise at 12-14 knots, require being run at 10 knots for their ocean range. Some even slower.

Basically if you're going to try to make long distances that challenge your range at certain speeds, then you'll be aware. For coastal cruising, fuel is generally not a concern, especially when doing so at speeds very little above hull speed. The boat we're using for the Loop can not be run all day (12 hours) at it's cruise speed on a tank of fuel. It has range at cruise of about 10.5 hours.

We have a lot of Bayliners and Mainships on this site. Most of those would be fuel challenged if they ran all day at their cruise speeds.

Managing currents is much like driving a car up and down hills. More fuel to go up, less fuel to go down, averages out to dang near flat travel. The only difference is with boats and current you can sometimes plan for advantage. But often you just have to deal with what you've been handed.

I try to make the best of the current: timing, course or otherwise. But if there is no clear advantage to be taken, just plug on and deal with it. All summed up, works out near a wash.

With the right tools, it isn't extensive planning...it is one or two minutes in many cases here on the ACIW.


Tomorrow morning is a good example for me. I can leave to be mostly up 10 miles of river that have several knots of following current, pick the 1.5 hrs where there is nearly slack water, or fight an average 2-3 knot current for 3 hours.


My choices for picking my departure time are:


1. cover 10 miles in a bit over an hour.
2. make it in an hour and a half
3. or make it a trip of over 3 hrs.


Not a big deal for fuel...but make those decisions over long trips, years or otherwise...and the fuel and certainly engine hours add up.


Total planning time... ONE MINUTE. When entering a tidal river mouth, looking at a couple tidal current graphics lets you see the time and duration of fair or foul current. So between 2 sips of coffee this morning...I decided a departure time for tomorrow. It ALLOWS me to have a rough, not in concrete idea of what I can do that day.


Now the rest of that 30 mile trip (20 miles) in just plain old behind barrier island ICW where the tidal currents change every inlet every few miles and there is really no data to plan with either...so that I don't worry about and let Ski's up and down the hills exaplme do it's thing.


Yep Ski, the might Cape Fear River...see you soon.

Yep, the Cape Fear, the Wacamaw and behind Myrtle Beach- Those areas it is well worth it to time your trip to take advantage of the current.

And it is not quite like hills. As see in the math, a boat's response to current is not linear. Taken in it's extreme, with a favorable current you can turn the engine off. Calculate the NMPG there!! And with a bad head current and a slow boat you could make zero headway. Again calc NMPG there!! Same with trip time. So not quite like hills in a car. But except at the extremes, things do tend to wash out towards an average. But still good to time the longer rivers. You are stuck in them for much longer.

In the PNW there are hundreds if not thousands of sailboats cruising around that seldom use their sails for getting places. Fluky winds and ever changing currents turn these small tankage vessels into very careful fuel management conveyances. Smart skippers here.

Whereas I may worry about where to fuel up next week these non sailing S/V guys keep an eye out on tomorrow. SOG and a watch are their best friends. Some even use a chart plotter!

A calibrated sight tube or dip stick can tell us M/V guys very accurately how much fuel remains. With a watch and plotter fuel burn per hour or per mile can be simply calculated. Codger's neat system tells us how much fuel we are using. As a last resort, a call to Psneeld for assistance because we know nothing seems a popular choice too.

FWIW, even in cars you don't gain as much speed or fuel economy going down a hill as you lose going up the hill. It is not a zero sum game.

This has kind of become angels on pins. Many could argue that it is silly to worry about a bit of extra fuel cost or engine wear since those are tiny compared to the 100s of $1000s spent on the boat to begin with. OTOH, some of us get a certain amount of satisfaction or joy by running our boats as efficiently as possible. Since most of us own a boat in search of "joy" in our lives, I think that is OK. Others get their enjoyment by not worrying about little stuff and just go out and have fun.

I do my own maintenance on my engine on my current boat because I enjoy it (in a sick kind of way). I empty my own holding tanks. However, there are a lot of times when the pollen is falling thick and heavy that I will pay someone to wash my boat because I don't enjoy spending my limited time on that chore (not to mention it destroys my back). Same with spending the 1 minute considering the effects of current on our travel time or costs. We do it because of how we experience "joy" in our boating endeavors.

I normally dont do any calculations, and have enough fuel that it wouldnt make a difference anyway on usuall trips. But, it takes much more than a few minutes to lay in a plan to say, the Galopagos from southern California. Or even the east coast to Bermuda. Now your gonna need to dig out that old musty Bowditch, if you can find it. Get your big chart for the area, your dividers, etc. Done correctly it will take an hour or more. Or we can just wing it, set a course on the chart plotter and go. I would do both. That way I know where I am on the paper chart at regular intervals.

Actually, open ocean is a bit less planning in my experience. Total distance and reserve with caution plus any time spent in any specific ocean current.

Weather is the single most issue, and that past 7 days is iffy at best.

Dhays...
Do a trip on the ACIW at 6 knots for 1500 miles and see if it is Angels on a pin to you when all those hours could be spent visiting places or relaxing instead of pretty tedious driving .

Planning to me stopped being a joy decades ago, I do it because it gives me hours of MY choice what to do for the afternoon. One minute planning is even hard to consider a joy or a hardship...to me it would be crazy not to on many levels. If one hates it and takes them hours, then I guess I can see skipping it.

kulas44 said:

Now your gonna need to dig out that old musty Bowditch, if you can find it.

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Search "The American Practical Navigator" (or probably just "Bowditch") on the NIMA site for softcopy.

-Chris

Turbo Charged engines should get better fuel economy as they use exhaust energy to increase the intake pressure. Without a turbo charger the exhaust energy that the turbo extracts is just wasted. Of course you have to be running at a high enough RPM to get the turbo to "spin up". Depending on the engine you could increase your fuel economy by around 10%.

Planing boat, light compared to a trawler, twin diesel 370hp stern drives.

Sweet spot: 1300 rpm, 8 knots, 3.5 gph.

Go fast: 2800 rpm, 23 knots, 26 gph.

Just got back this afternoon from a three day trip and did 80% of the trip at trawler speed.

And where did our warm water go? A very different summer than last year. I miss the El Niño.

Turbos are for getting more hp out of a smaller engine. Boat builders use them to make a smaller engine space and more living space in yachts. Also to go fast when economy isn't an issue. But turbos are fuel hogs at anything near full power. I have Detroit 671 mains that at absolute full power burn about 6 gallons an hour. The high hp turbo version of the same engine burns up to 30 gallons an hour at full power and 2.5x hp. The 12v71 natural (non-turbo) burns about 12 gallons an hour at about the same hp as the turbo 671. And you have a turbo or twin turbo, maybe an intercooler all needing maintenance. Turbo engines used at high power require overhaul more often. High exhaust gas temps eat up cylinder components.

Lepke - I agree with you on the maintenance issues but not sure about your statements on fuel consumption. ("The high hp turbo version of the same engine burns up to 30 gallons an hour at full power and 2.5x hp. The 12v71 natural (non-turbo) burns about 12 gallons an hour at about the same hp as the turbo 671.") At http://boatdiesel.com/Engines/Engines.cfm?TZ=-4&SC=1366.768
I found that the 671M burns 6 gal/hr at 80hp and the 671TI burns 5 gal/hr at 80hp both at 1400 rpm and I took the data from the prop hp curves. So the turbo does burn about 20% more fuel than the non turbo but not twice the fuel.

I believe that in the case of 4 stroke diesels (at least some) the turbo charged version gets a little better fuel consumption. I'll check the data sheets and post when I get more time.

Happy cruising - -

Fletcher500 said:

Planing boat, light compared to a trawler, twin diesel 370hp stern drives.

Sweet spot: 1300 rpm, 8 knots, 3.5 gph.

Go fast: 2800 rpm, 23 knots, 26 gph.

Just got back this afternoon from a three day trip and did 80% of the trip at trawler speed.

And where did our warm water go? A very different summer than last year. I miss the El Niño.

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How effective is an autopilot at trawler speeds on you boat?

Lost Horizons said:

How effective is an autopilot at trawler speeds on you boat?

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For an autopilot to be effective at very different speeds and even in different conditions, you need to fully learn all the capabilities of the one you have and the adjustments. You may need the sensitivity set very different at 8 knots vs. 25 knots. When it impacts you more at trawler speeds is in non-calm conditions. On plane you just power through in a straight line. At trawler speeds you can tend to waddle through. Also if the waves are from your stern, then at planing speed you're running fast enough that they don't hit you, while at trawler speed they may. You may have to adjust speed down or up and the autopilot sensitivity.

I recommend practicing with all your autopilot can do. Many just set it one way and never adjust it. Some have more capabilities and flexibility than others.

Lost Horizons said:

How effective is an autopilot at trawler speeds on you boat?

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Just went from Charlevoix MI to Port Huron MI ~320 miles. At 1200 RPM/6.2 Knots my auto Pilot worked very well. The PO had done the setup. We had 2' to 3' quartering seas off the bow one day and 6' following seas the next. Auto Pilot held a steady course both days.

We're naturally aspirated, and glad for it!

DonF said:

Just went from Charlevoix MI to Port Huron MI ~320 miles. At 1200 RPM/6.2 Knots my auto Pilot worked very well. The PO had done the setup. We had 2' to 3' quartering seas off the bow one day and 6' following seas the next. Auto Pilot held a steady course both days.

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I have no doubts about Grand Banks steering ability at displacement speeds. It is 25-30 knot hulls with tiny deflectors instead of rudders that I am wondering how they manage at low speeds without having to constantly pull the throttle.

Lost Horizons said:

I have no doubts about Grand Banks steering ability at displacement speeds. It is 25-30 knot hulls with tiny deflectors instead of rudders that I am wondering how they manage at low speeds without having to constantly pull the throttle.

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I have one of those. And the answer is....not very well!!! Granted...the P29 in my signature also had a "deflector" and not much of a rudder. But it did okay!!!

Baker said:

I have one of those. And the answer is....not very well!!! Granted...the P29 in my signature also had a "deflector" and not much of a rudder. But it did okay!!!

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Interesting. Is there a systematic reason why some deflectors are better than others?

Lost Horizons said:

Interesting. Is there a systematic reason why some deflectors are better than others?

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Nope...pure physics!!! That P29 I referenced was a single engine with a ton of windage. It made me a better boat handler. But why they are that way????...I guess it is just the way they build them!!!

So, is it safe to say that the class of boats that includes Carvers and Sea Rays cannot be trusted with an autopilot maintaining a course in moderate conditions at 6-7 knots?