bullnose type hull for sailboats?

[sww]

Hello,

just new here and was wondering if anyone heard of using "bull nose" type bow hulls for sailboats. My understanding is that most containerships use those designs nowadays apparently to increase waterline (?)

Would this not also be interesting for sailboats. Does anybody know what effect (roughly) those extensions have compared to a hull without the bullnose "underwater" bows.

thanks for any information anyone may have. Please don't spend too much time on the theoretical background if you happen to know this.

-sww

[sharpii2]

Hi
They have been tried on "Americas' Cup" 12 meters. But not for long. Such a bow protrusion is only good for Length/Speed ratios of 0.75 to 0.85 to my best recollection. Since the Length/Speed ratio is the square root of the waterline (measured in feet) expressed in knots. This means that if you have a 9ft waterline length dinghy, a Length/Speed ratio of 1.00 will be 3 knots. This dinghy should be able to reach speeds of 3.75 to 4.00 knots. That would be Length/Speed ratios of 1.25 t0 1.33... At theses speeds the bulb does nothing for performance and ADDS drag.
For a 900ft waterline container ship, the situation is different. This ship has to get by with as little as 0.75 horsepower per ton. At this low a power rating, a Speed/Length ratio of 0.75 to 0.85 is quite atractive. It means 22.5 to 25.5 knots and, with the bulb itself, brings a net savings of fuel consumption of around 4.00 percent. And those ships use a lot of fuel.
I hope this answers your question.
sharpii2

[sww]

Thanks Sharpii2 for the explanation. I may have you lost somewhere here. Doesn't the beam anywhere come into place for this? I am looking at a catamaran of 33 feet WL and a 4' BMAX per hull. Now this cat can cruise between, lets say, 3-11 kts in its current config. I would imagine that extending the stern by about 3' and possibly adding a bullet nose should add some benefit. The cat is 19 1/2 ' wide and has the usual interaction with waves for this config. Added drag at that speed should not be as critical versus the added buoancy at the bow.
-sww

[SailDesign]

One of Europe's largest builders (at the time) marketed a beast called the Coronet/Elvstroom 38 (coulda been 34) with a bulbous bow/ram bow thingie. Well tank tested, etc. Nobody was interested, and it really didn't sail that well compared to boats of that length without one. Downsides were numerous, including that it was the first thing to hit any submerged object and was not terrifically strong.
Steve

[Andy P]

Bulbous bows were used in International moths in UK in the early 1990's
eg Magnum 9, Axeman 3.
Advantages were sharp entry, fine waterlines, volume carried fwd.
But disadvantages in handling ( ie nosediving) and poor performance in waves, where the bulb comes in and out of the water.
Current designs have vertical topsides.
Some A class cats, and Hobie cats have topsides that are past vertical ie narrower at deck level. Here the advantage is less wind and wave drag, less drag in a nosedive for the same fwd volume.