Why do sailboats seem to be more fuel efficient that powerboats

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

 

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!

 

Gas consumption numbers- 30 plus mpg! ex-sailboat

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

 

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.



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.



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

 

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.

 

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...

 

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.

 

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.

 

Sail hulls VS power

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

 

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.

 

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

 

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...

 

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.

 

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

 

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.