Which is the most economical kind of small vessel?

the narrow beam(s) cutting wave resistance down is the reason why cats are more efficiently going faster, that even apply's to displacement monohulls. and the bigger a small vessel is; the more economical (froude's square root of the length law). have a look at the hullspeed calculator

yipster

 

I think that Boojum 25 which can be seen at http://www.xsw.com/Boojum/index.html & http://www.kastenmarine.com/boojum25.htm with a range in excess of 2,600 NM and a full load displacement of 18,500 lbs she is a capable passagemaker although admittedly compact.

My personal favorites are the Greatheart 36 http://www.kastenmarine.com/greatheart36.htm and the Wave Runner 36 http://www.kastenmarine.com/wave_runner.htm .

I don’t think the fuel tank with sea water in a bladder is as easy as it sounds and a failure could be disastrous if it occurred far out to sea. OTOH it provides tantalizing potential.

In the size that you are considering, I think aluminum would be a better choice. Steel runs into minimum gage limitations which makes it too heavy for smaller boats. Aluminum is lighter, has a higher strength to weight ratio and the ease of working and lower painting requirements offset the material cost difference.

For full displacement hulls, displacement speed is proportional to the square root of waterline length, however the power required to achieve displacement speed is affected by beam. Therefore, a narrow beam is more efficient.

Regards;
Mike Schooley

 

You're right that the narrow boat produces less wave drag than the wide boat. Even for a narrow hull there is an increase in drag in the vicinity of hull speed. However, the magnitude of the drag increase is far less.

This figure from Leo Lazauskas' paper on solar-powered boats (http://www.cyberiad.net/library/multihulls/solar1/solar.htm) shows the total resistance vs speed for several configurations.



The "Tri 0.6" configuration has three short hulls with the weight well distributed among the hulls. The other three configurations have long, narrow hulls. At 5 knots, the short hulls are experiencing a definite hull-speed problem as the wave drag increases dramatically. In order to get past 5 kt, the "Tri 0.6" configuration would have to have the power for 8 kt or above.

The other configurations show an increase in drag betwen 6 and 7 kt, but it's not very pronounced. The speed at which the drag rise occurs depends on the length for all the boats. But the magnitude of the drag rise is not the same.

Below 4 kt, all the boats can cruise efficiently. The slender hulls can also cruise at 5 kt with a reasonable increase in power. Let's say the total thrust available is 150 N or less. The slender hulls will respond well to adding power, and starting from 4 kt can almost double that speed. But the short fat hulls will hit the wall at 4 kt and all the power available will serve to squeeze out less than one more kt.

So whether or not a boat appears to have a hull speed limit depends on how severe the drag increase is and how much power is available. A typical powerboat also has a very definite hull speed, but has enough thrust to get through drag hump and plane on out to higher speeds. But a classical sailboat doesn't have that power and stays stuck at hull speed. A sailing multihull with the same rig would be able to sail past hull speed because the modest drag increase for its slender hulls doesn't provide the same barrier. The same principles apply to all three but the consequences are different for each type.

 

That's a very clear explanation.

 

Tom;

You state that a narrow hull produces less wave drag than a wide boat? I am not sure about this, is this true if displacement and length remain the same? It seems to me that so-called "low wash/wake" forms are wide, flat, and shallow. Is a lower displacement/length ratio not part of the equation?

I would certainly suspect drag differences due to entry angle, but they would be slight compared to the D/L ratio difference between half a cat and a monohull.

I just did a comparison between a Krogan 58, at 100,000 pounds displacement and 52' DWL and Dashew's FPB at 81' DWL and 100,000 pounds. Theoretically fuel consumption for the longer hull will be half that of the Krogan when both hulls are traveling at a S/L of 1.25. This is around 9 knots for the Krogan and 11.5 for the Dashew.

All the best, Tad

 

You asked:

One basic question: If displacement hull speed is always proprtional to the square root of the length, why does a boat with a narrow beam go faster than one with a wide beam? Surely the wave-making drag is more on the wide boat? Or is it just that the wider boat needs more horsepower to reach its hull speed?

Your question is much like asking:

If the strength of a beam is proportional to its width why does a 2x6 seem stronger than a 2x4.

In both cases there are other factors affecting the process.

 

Beam vs Wave drag

Wave drag is crudely proportional to beam squared, because you have to push sideways more water, faster, to get it out of the way of the hull.

Twice as wide, twice as much water, twice as fast, four times the resistance (again, very roughly).

 

It is also proportional to submerged area. The shape of the hull also affects it. For example a full bow creates a larger wave than a fine one. However, the speed at which the speed increases is roughly the same; it is the amount that changes.

 

Hmmm... my bet would be on #7, other. A blimp? Just thought I'd throw wrench into the tool box. :idea: Ahem, although I would probably think that pacific proa would do nicely.

Jay

 

Once asked for the most "efficent" sail boat design the AYRS answer was a submersablew towed by a box kite.

With the remarkable advances in chute technology perhaps a steerable chute would do even better.

Yes they will work to windward ,
and running lights 75 ft off the water might even be noticed by the one man "crew" of the "Esso Maru".

FAST FRED

 

director

Is there a term used in hydrodynamics for displacement drag