Bulbous bow.

http://en.wikipedia.org/wiki/Bulbous_bow is a fairly basic explanation of what it does and how it does it.

From memory and I do not have access to my textbooks on this subject with me at the moment but the most effective speed of a vessel with a bulbous bow is 0.85 and 0.9 of the hull speed ( Sq root of L.W.L. in feet X 1.38 ) being the theoretical hull speed. So a displacement hull vessel with a waterline length of 100 ft would have a theoretical hull speed of 13.8 knots and if fitted with a bulbous bow ( if the hull is suitable ) at between 11.7 and 12.4 knots she would require 12% to 15% less power than the same hull without the bulbous bow. Not much when you are looking at a smallish vessel but when you start looking at a 400foot ship or larger it gets to be a big saving.

Also Icebreakers do not have bulbous bows, in fact I do not believe that an ice class vessel can have a bulbous bow.

Hi,

Ice Breakers have a definite rtumble home to give a guilotine effect.

I have been told and experienced the vibration at low speed big dispalcement variations of CPP's that lead me to beileve that the heavy duty Ice Breakers rely on displacement tonnage and ****ty machinery balance and alignment to get where trhey are going.

Good editing tool Carl

The Bernoulli Principle can be applied to all fluids. This explains how the low pressure is created on either side of the bow, and as water is incompressable a trough would form. However as the vessel moves trought the water this trough is filled with what would have been much of the bow wave, so in effect a smaller bow wave is produced, this carries on and the trough and stern waves are also reduced. Well thats it in a nutshell boffins write really thick text books about this stuff.

Take a look at this bulbous bow from a Benetti. It has what looks like strakes running along it. Wouldn't this prevent the water from riding up and over the bulb? Or is it the opposite, where the water gets "trapped" on top and can't run directly off?
Capt Tom

Hi,

I think you will find these are to reduce the amount of damage done to the coating on the bulb by the Anchor Chains.

The anodes on he Amevi are monumental, but theyre not that big

Hi,

Here is a Bulbous Bow

http://upload.wikimedia.org/wikipedia/en/0/0d/Zadm_drydock.jpg

Patrick Bray is a real expert in bulbs up here. He has retro fitted them to existing yachts to improve performance, done numerous tank tests and even incorporated them on hulls of 47' long, so not just for large boats.
Check his website, I think it's...www.brayyachtdesign.com

Wow, all questions are answered already...

One that remains: empty or not?
Very often the bulb is part of the forepeak ballast tank. As it's so far forward, you need less ballast to correct the trim.
But I can tell you, it's a special experience crawling around in the bulbous bow with the usual 10 cm drab all over the place. Even the disposable head-to-toe boiler suits don't keep all the mud out.

On ice breakers: newer models have an ice-breaking stern. They go stern-first when breaking ice. This helps as their azi-pods suck the water out from under the ice. When the ships weight then slides on top, it's easier to break the ice. I guess for light ice, they just go bow first.

I've seen them in action. There is hardly a more impressive sight. In northern Finland you can take "ice-breaker cruises" for those interested in experiencing it. Included in the trip is a swim in a survival suit amidst the ice blocks.

Bruno

Gosh, maybe he meant to say 'shifty'

Francois, in the realm of expedition/trawler vessels under 100 feet or so, the bulb has a greater job in reducing pitching in a head sea rather than any gains in hull efficiency.
Then, there are those troglodytes who insist that the absence of a bulb, especially on a larger boat, denotes a severe lack of manliness.

troglodytes