Why do sailboats seem to be more fuel efficient that powerboats

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

 

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.

 

Knut Sand,

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

 

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

 

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.

 

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

 

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.

 

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.

 

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).

 

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

 

"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!

 

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.

 

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.