Efficient Displacement Hulls

[BOATMIK]

Originally Posted by yacht371

The most efficient combination of speed and fuel economy is found in power catamarans such as the MC27 built by Bond Yachts in Poland. This boat is as efficient as a sailboat at slow speeds but can do 24 knots with 100 HP. If is built lightly and has narrow hulls. www.motorcat.com

Certainly true of the speeds you mention. But at lower speeds it may be worth going for a single hull of the same displacement to take advantage of the lower wetted surface.

Also as multis get smaller they have a lot of hull and superstructure surface which can make it hard to keep the weight down.

I suppose also that a reserve of extra power is needed for mutis to deal with the extra windage when conditions get bad. Bigger engines, perhaps more fuel depending on the operational radius = more weight.

For really big boats - carrying big loads long distances - whether megayachts or bulk ore carriers, lower speeds and a monohull form are more "efficient"

But I would certainly agree with you in the speed regime and type of use that you are mentioning here as multis being "most efficient"

Best Regards
Michael Storer

[fcfc]

I do not like powercats.

That said, look the efficiency of theses monohulls:

http://www.parker-marine.com/commuterpage.htm
20 kts with 90 hp.
http://www.classic-boats.com/gamme/i...AND10&chglg=en
15 kts with 60 hp.
http://www.stadtdesign.com/products/vds608_6531.htm design 653.
Not yet built.

The key to efficiency is to be long and light, not to have 1, 2 , 3 or even more hulls.
The MC27 is 2.2t. ANY monohull with the same weigth and same length WILL have the same efficiency or even better. (less wetted area and single prop).

[fcfc]

BTW, have you even seen catamaran rowing boats, which need the best efficiency possible, since power is rather painful to generate.
skiff, kayak, canoe all are monohull. Some time not very stable, but monohull.

Now, if you say a skiff is very effcient, but I want bigger so put two side by side :
NO wrong physics.
Get one (cubic root of 2) times bigger (about 1.26) . You will have twice the volume and weigth, but not twice the drag at same speed. You will only have 1.58 time the wetted area, not 2 as the catamaran. And you will have a Froude number divided by 1.26, since the hull is longer by that.

[Willallison]

Definitely one of the more quirky production cats made! Personally, I'm not fond of the styling - but I love some of the features - like the drop-down bow 'ramp'.

[yacht371]

That is the MC30 you are looking at. The boat I'm talking about is the MC 29, found at www.aviadesign.com. There are photos there, the first boat left the factory to be shipped to Vancouver BC today. Arriving mid July.

The one we are getting has a single volvo diesel with duoprop.

fcfc is correct, a long narrow monohull is more efficient than a catamaran of the same displacement. But the MC29 has a hull length to beam ratio of 13, equivalent to a 65' monohull with a beam of 5 feet. What would keep it upright? Even allowing for the cats having two hulls, a 30' monohull would have to be about 5' wide to reach similar efficiency levels.

Hardly any boats are designed for maximum efficiency, you always are forced to put in decent accommodation. Also the people who REALLY care about operating costs usually have little money, so they build their own or buy used, which does nothing for the builder of new boats trying to earn a living.

I pay more than $3600 a year to moor my 37 foot sailboat, and spend about 200 a year on fuel (figure triple that if I never used the sails). A 75 foot boat of the same weight would go faster and burn less fuel, but I would pay 7200+ for moorage, far out weighing the fuel savings.

Another fallacy that I learned the hard way. Half a lifetime ago I had a lovely 26' traditional folkboat. No engine (although I later put on a tiny outboard as the harbor patrol objected to me sailing under the Lion's Gate Bridge). No electrical gear or battery, everything kerosene. It weighed about 6000 lb. in cruising trim, and sailed extremely well. I had a great sail inventory, including a huge drifter and assymmetrical, so she moved in a wind you couldn't feel.

Anyway, I had the idea that if I took the same weight in materials, and made a 34' boat with an 8' beam, fin keel and spade rudder, and a bigger rig it woulkd sail a lot faster, have more room and comfort and not cost a lot more. All of this was true except the last point. In fact she cost 4 times as much to build because of the care needed to build a light boat, and although faster she was also tippier and more work to sail, and the speed increase was less than I had imagined. Oh yes, the folkboat sold for more than she cost to build, the custom boat dropped in value like a stone.