Bulbous bow.

Francois · 10-24-2006, 11:42 AM

Hi .Can someone please tell me whats the real purpose of these bulbs at the bow of megatachts? Also are the empty for bouyency or is their some weight in it.Some folks told me it was to act as an icebreaker and I did not beleive it.
Thanks for the great site.

Francois

Kevin · 10-24-2006, 11:45 AM

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

Francois · 10-24-2006, 11:58 AM

Kevin thanks for the link,I can now see the way it works .I will seave through it .Some very interesting reading there.

Francois

Garry Hartshorn · 10-24-2006, 05:30 PM

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.

K1W1 · 10-24-2006, 06:37 PM

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.

K1W1 · 10-24-2006, 06:38 PM

Good editing tool Carl

YachtForum · 10-24-2006, 07:34 PM

LOL! We'll use our imagination for those astericks.

YachtNews · 10-25-2006, 11:43 PM

As was noted, icebreakers do not have bulbous bows because they use a method of riding up on top of the ice sheet and use the tonnage weight to crack and break the ice. Sometimes when the going gets tough then will back up and speed up and ram the ice.

On bulbs----Displacement yachts carry bulbs, from my experience, they create a low depression on both sides of the bow that are in contact with the water...allowing the bow to slice through better with 'less' friction. As was alluded to before it can save you some good money on fuel. I think it give good hydrodynamics too...Im not an expert but i think that is some general idea. As for whether they are full solid or hollow.....that is a good question!........However bulbs only work efficiently at certain speeds..they are good wave breakers though.

Garry Hartshorn · 10-26-2006, 05:33 PM

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.

CaptTom · 10-26-2006, 09:25 PM

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

YachtNews · 10-26-2006, 11:22 PM

I'm not sure but those might very well be formed there on the bulb to help with the channeling of the water around the bulb...again hydrodynamics.....On the new Amevi she has thos same lines on her bulb...Sacrifical Anodes?...but so far up though?

K1W1 · 10-27-2006, 12:33 AM

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.

tantetruus · 10-27-2006, 05:40 AM

Quote:

Originally Posted by yachtluver

I'm not sure but those might very well be formed there on the bulb to help with the channeling of the water around the bulb...again hydrodynamics.....On the new Amevi she has thos same lines on her bulb...Sacrifical Anodes?...but so far up though?

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

K1W1 · 10-27-2006, 05:51 AM

Hi,

Here is a Bulbous Bow

http://upload.wikimedia.org/wikipedi...dm_drydock.jpg

Francois · 10-27-2006, 08:27 AM

You guys are the best.I have learnt so much from you and thanks for the links.

Nice clear picture there Capt Tom.It shows the bulb very well.Thanks.

Francois