Why do sailboats seem to be more fuel efficient that powerboats

[Chuck Losness]

Although I have been reading this forum for awhile this is my first post. I think that there are a lot of sharp people on this forum compared with some of the other forums that I visit so I thought that this would be the best place for my question.
Now for some parameters. I am a liveaboard cruiser and currently in the Sea of Cortez. I have a Gulfstar 37 sailboat, estimated 20000 lbs, 34' LWL (measured during last haulout), 11'10" beam (don't know the WL beam) and 5' draft. I went through my logs since I left San Diego in January 2006 and found that I have been averaging 7.5 mpg (actual miles traveled from gps divided by fuel purchased). The engine is a Perkins 4108. I normally run at 2100 to 2200 rpm which equates to around 6.3 to 6.5 knots on my gps. Prop is a 2 blade maxprop, 16 x 13.4. Information from other crusiers seems to match my fuel ecconomy.
When I read about the fuel consumption on similar size powerboats, i.e. trawlers running at similar speeds, the powerboats are only getting about 2 to 3 mpg. And when I look at the fuel consumption charts such as those listed in Gerr's "Nature of Boats," they seem to indicate that I should not be averaging 7.5 mpg.
So why do sailboats get 2 to 3 times the fuel economy of trawlers? And better fuel ecconomy than what you would expect from the published information on fuel usage?
I am asking this question because I am considering going back to power, specifically a displacement powerboat and plan on traveling 3000 to 4000 miles per year. So fuel economy is a big issue. I have owned go fast express cruisers in the past but never a trawler. And my question does not include planning or semi-displacment powerboats.
Are trawlers just poorly designed from a fuel economy viewpoint? You would think that they should have better fuel economy than a sailboat because they don't drag a deep keel through the water nor a mast and rigging through the air. Am I just missing something? Thanks for your consideration of my question

[masalai]

From a know nothing, many cruisers:-
1 - - seem to be trying to "climb out of the water" - which is a loosing tactic
2 - - have a larger water line beam or otherwise bigger frontal area to push through the water
3 - - may loose some efficiency by angling the shaft down (to clear the bottom of the hull whereas many of the older "comfortable sailboats" will have a horizontal shaft
4 - - Most sailboats are powered and propped to achieve a speed appropriate for that hull shape without trying to push up a big bow wave or get on to the plane.

This is only for starters from a simple fools perspective - the experts will shortly come along to (hopefully for your point of view) howl me down, correct my errors, give you a proper answer along with some technical figures to demonstrate the point...
It would help to post an image of your boat - out of the water, and some of those with which you are making a comparison (still in the water there should be OK)...

[kengrome]

So why do sailboats get 2 to 3 times the fuel economy of trawlers?Trawlers are work boats designed to tow huge, heavy drag nets, so fuel efficiency is not at the top of their list of design requirements. It is more important that these boats have the power and stability to drag those nets safely since that's what they are designed to do.

People who use trawlers for cruising are naturally going to suffer from lower fuel efficiency than those who use sailing hulls. I think trawler hulls can be pushed faster than displacement speeds too -- at an ever more severe fuel consumption cost of course -- which further adds to their low mpg performance.

Are trawlers just poorly designed from a fuel economy viewpoint?I think this is true.

You would think that they should have better fuel economy than a sailboat because they don't drag a deep keel through the water nor a mast and rigging through the air. Am I just missing something?I think its their transom drag that contributes more to fuel inefficiency than a keel and mast. Whenever a boat's transom is in the water it is not going to be anywhere near as efficient as when its transom is completely above the waterline.

If you want an efficient displacement power cruiser, it makes sense to look at sailboat hulls as a starting point -- but of course you already know this ... :)

[Chuck Losness]

Thanks for your response. My question is more general than just my sailboat. I used it because I had good data on fuel economy. I'll try to attach some photo's and my prop shaft has a significant angle to it.

[masalai]

Chuck, that looks like she has sweet lines & would be easy to push - that is why your economy is sooooo good. Open the engine right up for a short test burst and watch the fuel consumption go through the roof... he he he

[lazeyjack]

look at your water plane? small, look a t you engine small? each 50bhp, = 2 gal hour, how many bhp to push yours at cruise? 18? look at at powerboat, big waterplanne, big engine
Talking bout waterplane, your boat must be very tender initially?

[Landlubber]

Your underwater hull shape is designed for little or low resistance, it is basically a canoe stern, double ender, only it does not get above the waterline. The water flow over such a shape is as near to perfect as we can get (so far anyhow) for a displacement vessel, as long as it remains under hull speed (the speed where the waterline wave length is the same as the actual waterline DWL). As stated above, wack in double the required horsepower and open her up, and you too can get to waste fuel like it was going out of fashion.

[terhohalme]

It is all about speed and used horsepowers. What is the speed you are motoring? On what speed the motorboats are going? Sailboat hulls are designed for low speeds to maximize close hauled speed. Motorboats are designed for higher speeds (motoring is so boring that they have to get there quickly). When you are motoring at low speeds, your sailboat hull is more economical. If you could go on high speed, you would need more fuel than motorboats. Remember to compare boats with equal length and displacement.

Terho

[the1much]

most sail boats "sit" ON the water,,,trawlers sit IN the water.,,,is all i can come up with,,,im pretty "simple" hehe ;) NICE ta meet ya, and welcome to the forum hehe ;)

[Guest625101138]

Fuel economy is not a recent invention but it has been a forgotten cause until the cost of fuel started to skyrocket. When a 15HP steam engine weighed 2 tonne the hull designs were different than when you can get 200HP that weighs 200kg and fuel costs next to nothing.

If you have a cruising power boat designed for economic cruising it would get double the economy of your sailboat for the same accommodation because it does not have to drag around the huge keel or push mast and rigging through the air.

The hull shape you are looking for will be long and slender with canoe stern. These sort of hulls are coming back into vogue but not many examples around yet. Alternatively there are powered catamarans which have the narrow easily driven hulls but also plenty of stability.

Ken is right about trawlers being designed to do a specific job. They are more a tug than efficient cruiser. Tugs need big props to grip the water.

THe majority of boat owners have to work to afford them so their time is limited and they need speed to get anywhere in a reasonable time. As the price of fuel goes up I am certain there will be more efficient hulls coming into use. There have been a few on this site trying to work out how they can transform their planing hull to an efficient displacement boat.

Your shaft angle is quite moderate by comparison with some you see in other hulls where the shaft is inclined at 20 degrees. You would never see an airplane with the prop mounted at 20 degrees because they need to work efficiently just to get in the air. Boats don't matter that much - just burn a bit more fuel.

Rick W.

[tom28571]

Chuck,

Why do you want to change to a powerboat like a "trawler"?

More room and comfort?

More speed?

Get rid of keel and mast limitations?

If it is either or both of the first two reasons, some loss in fuel efficiency is the price you will have to pay for the difference. Although cruising trawlers may not have a lot in common with working trawlers, they are far from being the easiest boat to move through the water. All that beam and full ends which give the room and comfort with submerged transoms don't compare with a decent sailboat in power required to move it forward at or below its hull speed. Want more speed in a trawler? Just add power and many will allow you to plow forward at speeds into the teens. Want more room? Just add it and it won't affect the displacement performance very much. Try that with a sailboat and it is likely to become a total slug. Design elements like block coefficient and prismatic coefficient define major differences between power and sail.

In an earlier fuel crunch, a builder (Martin I think) came out with a boat called the Energy 48 (memory may be off in the name) that was designed to give maximum fuel efficiency as an inshore cruiser. It had the efficiency of a sailboat under power (maybe better) but it also had many of a sailboat's disadvantages in comfort like narrow beam, restricted height of superstructure, expensive to dock because of length and pointed ends. He never sold any.

Tad Roberts designed and built the Memory 38 which is a less extreme example of a fuel efficient power cruiser. Beautiful boat and delivers the promised fuel economy. Ask Tad if he ever sold any more of them.

It is not that designers don't know how to deliver what you ask, they just have not been able to interest buyers or sell them.

[Chuck Losness]

Rick I would love to find a powerboat with double the economy as my sailboat but they don't seem to exist. At least I haven't seen any. Can you give me some examples? What about prismic coefiecents and other design parameters to look for. As you said a displacement powerboat should get better fuel economy because it doesn't have to push a mast and keel. This just makes common sense to me but I am not a naval architect so what do I now.
I have thought about a power catamaran. They just seem to be pretty ugly in the smaller sizes. I have a friend with an older 40' sailing cat that has twin diesels, I think 25 hp each, and he claims to get 10mpg at 10 knots. But this could just be the rum talking.
I will be leaving Puerto Escondido today and heading to Santa Rosalia with several stops along the way and will not have internet again until Friday or Saturday. Just thought that I would give you guys a heads up. I appreciate everybodies comments.

[TollyWally]

LOL,
As pointed out before the increasing price of fuel does indeed increase the interest in easily driven hulls. I thought I might point out that the term trawler as used in describing yachts has almost no relevence to an actual trawler other than they are both slow. A trawler yacht might be compared to a slow bus or RV. A fishing trawler is more like a combination of a tractor and a dump truck.

[masalai]

Chuck, have a look at this site to get an idea of what a fuel efficient hull can do - - Does 1.2litres per nautical mile at 12 to 16 knots appeal?

http://www.icecat.com.au/objectives.htm

"Foreign Affair" was in New Caledonia... See page 12 of issue 30 of "The Coastal Passage" - http://www.thecoastalpassage.com/recent_issues.html for a free download... Best wishes and may you enjoy good boating weather...

[Ike]

here's a thread that discusses many of your questions http://www.boatdesign.net/forums/showthread.php?t=5073

[Ike]

here's a thread that discusses many of your questions http://www.boatdesign.net/forums/showthread.php?t=5073

[Knut Sand]

I had an idea... very original (or so I thought... (tm)), a couple of years ago; take a sailboat, modern hull, strip off the keel, strip the mast, put on a moderate, long deplacement keel, small diesel engine, keep the cruising speed at approx displacement speed, put on a top like the lobsterboats or whatever.Then a collegue said; your talking about the "unsailboat". Then he gave me the link to these people:

http://setsail.com/dashew/do_paradigm.html

Like it or not, I think its pretty impressive, even though this is an hull twice the length of what I was blabbering about.

[Landlubber]

Knut Sand,

Thanks for that link, it is very interesting, many special ideas incorporated there, Ta.

[Brent Swain]

Prismatic coeficient. A sailboat has a prismatic coeficient of around .54 which is ideal for a speed wl length ratio of around 1.34 times the sq root of the waterline length, whereas a powerboat has a prismatic coefficent of up to .85 which is efficent for a speed wl length of arount 1.85 or higher. The finer the ends , the more efficient it is at lower speed and the less so at higher speeds. The fuller the ends the more efficient it is at higher speeds( and horsepowers) but the less eficient it is at lower speeds (and horsepowers.)
Brent

[masalai]

Brent, I am displaying my ignorance, but, that may not apply to the modern form of displacement skinny hulled catamarans?... "Foreign Affair" does 20 knots fully loaded and cruises around 12 to 15 knots - way faster than most monohulls can maintain in an ocean passage (Brisbane to New Caledonia for example)...

[Guest625101138]

Brent, I am displaying my ignorance, but, that may not apply to the modern form of displacement skinny hulled catamarans?... "Foreign Affair" does 20 knots fully loaded and cruises around 12 to 15 knots - way faster than most monohulls can maintain in an ocean passage (Brisbane to New Caledonia for example)...

It does still apply. The further you push the hull above hull speed the fuller the ends for minimising drag. Just that with slender hulls you are not going to plane readily so it helps to have the canoe stern to avoid the transom drag.

Rick W.

[FAST FRED]

Chuck, have a look at this site to get an idea of what a fuel efficient hull can do - - Does 1.2litres per nautical mile at 12 to 16 knots appeal?

Looked at that site but was unable to find the fuel milage claims?

To go cheap, all ant reasonable boat needs to do is cruise at the SL (Sq rt of the LWL ,) in Knots.

This will usually give 1/2 the fuel use of SL of SL x 1.34 .

Go slow and even an inefficent shape like a deep V power boat will do well.

If you want to go fast , cheap, it just takes a longer boat.

FF

[Guest625101138]

...
If you want to go fast , cheap, it just takes a longer boat.

FF

Fred
I would add narrow as well - a 100ft blunt end barge doing any speed will not be economical. So long and slender with canoe stern. Then if you want stability without ballast you need to get a cat. Maybe a tri if you can afford the length.

I liked the setsail boat.

Here is a tri:
http://www.youtube.com/watch?v=y91luG9-kn8&feature=related
Not the most economic but fast.

This ballasted mono is designed to cross the Atlantic using 62lb of fuel:
http://www.adventuresofgreg.com/HPB/uploaded_images/overview-701764.jpg

Rick W.

[Steve W]

Displacement powercats are not constrained by the usual hull speed formula because of the very slim individual hulls and probably offer the best fuel economy at speeds not attainable economically even by the most efficient monohulls,they also dont need the overall beam of a sailing cat so it would be possible to have a 38 ft powercat that would fit in a standard marina slip.Just throttleing back on something like the 50ft searay is not going to be practical as it is not good for the engines to be running below their power band which is why large offshore powerboats such as the nordhavns,small ships really ,have tiny engines.There are two good books on this subject that i know of.
"The troller yacht" by George Beuller and "Voyaging under power" by Robert Bebbe. One defining feature of the boats in these book is that even those with large transoms have double ended waterplanes like sailboats.One sailboat hull ive often thought would make a good powerboat would be the morgan "out island" series.
Steve.

[masalai]

Chuck Losness,
Sorry, "pirating your thread to reply to Fast Fred's comment - could not find proof of 1.2 litres/Nmile...

HERE:- http://www.icecat.com.au/summary.htm

[Knut Sand]

Chuck Losness,
Sorry, "pirating your thread to reply to Fast Fred's comment - could not find proof of 1.2 litres/Nmile...

HERE:- http://www.icecat.com.au/summary.htm

Here...:
http://setsail.com/dashew/Power_Range_64.html

Hmmm...
I get 1,78 l/Nm, at 10 knot.
but 1,024 l/Nm ar 8,5 knot. that aint too bad...

Also believe propellers have been changed a couple of times
(maybe the tables aren't updated after the last prop change).

[Brent Swain]

Power boats need huge engines because they don't have sails to fall back on when the wind gets too strong. Using a sailing hull for economical cruising may work well if you have a small engine for normal cruising and a bigger standby one for stronger winds.
Brent

[Landlubber]

"Power boats need huge engines because they don't have sails to fall back on when the wind gets too strong. Using a sailing hull for economical cruising may work well if you have a small engine for normal cruising and a bigger standby one for stronger winds.
Brent"

DUH!

[Knut Sand]

Power boats need huge engines because they don't have sails to fall back on when the wind gets too strong. Using a sailing hull for economical cruising may work well if you have a small engine for normal cruising and a bigger standby one for stronger winds.
Brent

I had a 26' sailboat, with 7,5 hp which in pretty strong headwind and waves, managed 5 knots. Now; strip the mast, the keel, the keel weight, the extra weight for stiffness you will need to buils into a sailboathull, and add a cabin, not too much bigger than the Sprayhood already in place (Seen from the wind/ front side). And a much better propeller than the folding propeller I had.

I do not think you will need an engine bigger than let's say 30 hp to safely push a 32 feet design like that. However, it will be a wet ride, but some fun we must have.

[Chuck Losness]

I'd like to thank everyone for posting their replies. They confirm my initial impression that displacement powerboats are not typically designed with fuel economy in mind but are designed for other parameters such as room and interior layout and are fitted out with larger engines than necessary. Will either have to build or have built a fuel efficient boat or maybe convert a sailboat hull to a power cruiser. That's probably a topic for another thread. Again thanks for your replies.

[FAST FRED]

Will either have to build or have built a fuel efficient boat or maybe convert a sailboat hull to a power cruiser.

Many sailboat hulls would be quite simple to convert .We have a neighbor who did it although he kept a mizzen mast, as steadying sail.

The sunk engine was replaced with an 80 hp j. Deere conversion and he has been quite happy for years.

I would have struck the mizzen as the joy of East Coast US travel , with under 10ft air hight is real time , Full speed , damn the bridges!!

FF

[newinertia]

this is what ive been working on- C-15 Catalina day sailer converted to inboard cruiser. got rid of the swing keel, filled in the slot with 1/8" 5052 Al,
sealed with silicone, and lots of screws. cut the floor out for reinforcing the inside of hull, fitted new plywood for mounting base, opened the front of the hull up for honda civic drivers seat and more plywood flooring and expandible foam to fill all the gaps, THIS IS BEING DONE ON THE CHEAP! everytime I do something that could compromise the hull integrity, I watertest and look for excess flexing and take appropriate measures to reinforce panels.
More pics to come in the next few days!

[newinertia]

Ok as you know, I got rid of the junk tecumseh engine because it was impossible to keep synchronized at idle speeds (2cyl v-twin dual carb), wide open it was a powerhouse, but low rpm, effortless efficiency was the goal.
I traded it out for the KOHLER Courage single cyl. 17hp. engine. (lighter)
It produces about 4 amps of 12 volt dc current at 800 rpm (max 16 amp), and with the reduction belt drive, I am seeing 5.4 mph at the same 800rpm. With the 10x12 prop, I wanted to do a measured fuel test, so I marked a container with accurate marks for measuring. I started with one gallon of fuel and a gps to find speed and range- its close enough for this work. Every hour I marked the level on the container- the ONE gallon container, it took me six hours and 25 minutes to burn that same ONE gallon. (6 hours 25 minutes.) GPS average was 5.2 mph, there was a 6-8 mph breeze and it was a 2-way average. the rear pontoons were removed, so it was just me and my dog and about 100 lbs of gear. top speed recorded so far was 12.2 mph- and that burns alot more! (different day) so- Im sure Im going to be called a liar by a bunch of you, but it shows that a 30 mpg boat is possible for cruising- do the math.
and please start designing boats like this- there IS a market!!!!

see full thread here- http://www.boatdesign.net/forums/motorsailers/20-motorsailer-25454-2.html#post282270

[Sheepy]

I'm not sure if this will help any but anyway...

This boats rough dimensions are 54 feet x 20 feet x 7 feet.

6 cylinder turbo cummins diesel. 4 to 1 gearbox with 42 inch 4 blade prop through a nozzle. She burns from memory 28 to 35 litres an hour at 1500 to 1700 RPM. 8 to 9 knots steaming.

http://i16.photobucket.com/albums/b1/InflatableSheep/PICT0513.jpg

This boat is roughly 34 feet x 12 feet x 6 feet.

4 cylinder detroit diesel. 3 to 1 gearbox to a 3 blade 30 inch prop. She burns 15 to 18 litres an hour at 1500 to 1600 RPM. 7 to 7 1/2 knots steaming.

http://i16.photobucket.com/albums/b1/InflatableSheep/alexd.jpg

As has been said before trawlers are not built for speed or cruising in mind, towing power is what they require.

[ancient kayaker]

Displacement power boats mostly seem to have hard chine hulls rather than the rounded bilges of typical deep keel sailboats. There is an article on fuel-efficient power boats in The Wooden Boat #206 Jan/Feb 2009. comparing the fuel consumption at SL = 1.34 of several power boats. I took a quick look at the numbers and, of the boats for which consumption figures were available, "Imagine" seemed better than the others once it's greater displacement was taken into account. Another had the best overall consumption. Sure enough, a look at the lines revealed these two boats had rounded bilges, whereas the other two had hard chines.

"Imagine" at SL = 1.34 gave 1.58 mpg, with a displacement of 60,9000 lb. From post #22 that might double to 3.1 mpg at S/L = 1, half that of the boat described in the first post, creditable for x3 displacement.

As Spock might have said, be long and prosper.

[masalai]

Ya can't make a silk purse out of a sows rear (or ear), to get passage-making/long-distance cruising efficiency one needs to design from that point of view and design to that end with all other aspects compromises... Have a look at Rick Willoughby's threads on designs and human powered vessels and low powered boats he is TESTING and designing then you may get the idea... very specialised design criteria using "off the shelf" products is possible now, pushing a 20 odd year old design style - you may have more success pushing **** uphill with a small pointy stick...

[philSweet]

Regarding the original question, there are some basic economics involved as well. One HP of sailpower is expensive; and the second one costs more than the first. One HP of diesel is cheap; and the second one is even cheaper. Sailors run into diminishing returns and practical sail carrying and handling limits pretty darn quickly. They pay more attention to (and money on) the hull because of this.

Sail handling is tiring. A more easily driven sailboat is less tiring.

[ancient kayaker]

Power boats are designed and built for speed not for economy, they have been for a long time and it will continue for some time despite economic hiccups and intermittently high fuel costs. Back in the 20's and 30's when powerful engines started to get smaller and cheaper it was no longer worth a designer's while producing a design that would get great economy and cost an arm and a leg to build that nobody wanted to buy when a simpler, cheaper boat would sell if all it could do was go like stink.

People value their time more than the time spent in a power boat, it's a "time's money" thing. Time spent in a sailboat is worth more, a case of "where else would I rather be?" There is challenge and satisfaction in getting someplace using just the wind. In a power boat the challenge doesn't start until the engine doesn't, especially in a busy seaway with the tide heading back out to sea, and from that point on it never gets to be satisfying.

[Brent Swain]

Sailboats typically have a prismatic coeficient of around .54 which is very efficent at a low speed length ratio , but very inefficient at higher speed length ratios. Power boats have a much higher prismatic coeficient , and thus much fuller ends ,which is far more efficent and higher speed length ratios, but very inefficient at lower speed length ratios. That is why with higher horsepower, power boat hulls are far more efficient and reach higher speeds that the low prismatic coefficent, finer ended sailing hulls would , but suck a lot more power at low speeds than a sailing hull would.
Brent

[ancient kayaker]

Brent, I assume you're writing of planing hulls when you describer power boat hulls; what about displacement hulls? How does a typical power boat displacement hull compare with a sailboat hull as far as prismatic coeeficient is concerned? Of course, sailboats have to contend with heeling forces in excess of those that power boats experience.

[Brent Swain]

Displacement power boat hulls also have a much higher prismatic coeficient , and are thus desiged to be more efficent at much higher speed length ratios that sailing hulls , as having more power to do that is assumed. Thus they are also far less efficent at lower speed length ratios.
Brent

[masalai]

Huh? Brent, have a little more thought and that power boat with several world records for speed and distance - very long and skinny - like a sailboat but with "outriggers" for stability all following "sail" type efficiency concepts...
I am over simplifying but you seem to doing likewise, by thinking only of POWER pushing a bathtub, as opposed to elegant efficient design needing only modest (relatively) power to travel at similar speeds over several thousand miles...

[ancient kayaker]

When it comes to efficiency and making the most from a small power source it is hard to beat some canoes, kayaks and rowing shells. These routinely beat "hull speed" over quite long distances.

[Brent Swain]

I was referring to normal displacement power boats , not outriggers. Not enough room in an outrigger to cruise and live aboard comfortably.
Brent

[Pierre R]

I think what Brent is thinking of is what the North American market calls a trawler. 95% of all North American trawler yachts are really trawler styled semi planing hulls or planing hulls with prismatic coefficients in the range of 0.66 to 0.72 and bill themselves as displacement hulls even though they are not. Most owners of these trawler yachts will tell you that they have a displacement hull even though they do not. To this end Brent has a legitimate misbelief concerning trawler yachts born out of experience with talking to these folks.

I own a 26' true displacement trawler driven by a Sabb H2 with controllable pitch propeller. In a crusing configuation I get between 10 NMPG at an S/L of 1.35 or maintaining an S/L of 1.25 in heavy seas to 16 NMPG in canal cruising at an S/L of 1.2

My overall economy is limited by the fact that I have a 130 amp alternator and use power. The fuel to drive the alternator prevents numbers much above the 16 NMPG range.

My boat can easily be continuously cruised on a social security budget.

[carl mccall]

because they use wind for propulsion

[ancient kayaker]

Pierre; "Imagine", the boat I mentioned in post # 35,is 56.5' LOA. Her economy figures are comparable with your boat's, allowing for greater displacement.