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Old 26-03-2007, 01:09   #1
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hull shape - bulb versus narrow

Hi,

I came across a post on this site that had an article about cat design. I can't find it now for reference, but my question is actually about the validity and to discuss owners observations of large production cats. The article talked about bulb shaped hulls and how they affected pitching, stability and heavy weather. The designer talked about how the design philoshophy changed from narrow bows to more bulb like in shape.

Can anyone help explain how/why this is valid, and perhaps compare it to current FP or Lagoon cats. In looking at the lagoon 380 which is on my short list the bows are very narrow, so it seems to go against the article I read. Im curious to hear some of your opinions.

thanks!
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Old 26-03-2007, 18:37   #2
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No answers yet. I was hoping to learn something here too. From what I can tell there was a lot of experimentation with bow bulbs, particularly if you look at older Crowthers. Most accounts I've seen suggest they make the boat a little more sea-kindly and reduce pitching, but that seems to be more gut-feel than scientifically-tested. As it is more expensive to produce, but doesn't provide outstanding benefits, it has fallen into disfavour. That said, I think there are still a number of plans still out there that either incorporate bulbs, or have the option to.


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Old 26-03-2007, 22:19   #3
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I think bow bulbs proved more effective on large ships that ran at a constant speed with the bulb always immersed at the same depth thus the shape could be fine tuned for the most efficiency and helped streamline the hull.
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Old 27-03-2007, 02:51   #4
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Yacht Design Explained:
A Boat Owner's Guide to the Principles and Practice of Design ~ by Steve Killing

offers a short explanation of the bulbous bow on page 37.
View it at:
Yacht Design Explained: A Boat Owner's Guide to... - Google Book Search

A bulbous bow (or “bow wave depressor" - a protruding bulb below the waterline at the bow of the ship) reduces drag, and affords an increase in speed, range, and fuel efficiency generally in the range of 10 to 15% (over a formerly “conventional” bow).
The purpose of the bulb is to reduce the bow wave system, by creating a counter-wave that interferes and dampens otherwise larger bow wave.

At higher speeds (generally above 6 knots), wave making resistance accounts for the greater portion of the drag.
At lower speeds (generally below 6 kts), proportionally more of the resistance is taken up by wetted surface area frictional drag.
Accordingly, at slower speed, the bulbous bow actual increase drag.

A bulbous bow also increases directional stability - which may be advantageous (or disadvantagous).

Extensive tank testing has determined that the benefits of a bulbous bow are most noticeable on boats 50 feet and longer. Because there is a relationship between the ideal diameter of the bulb and the area of the mid-ship section, a number of issues become inherently impractical with smaller boats. If your craft is over 50 feet in length and is a displacement vessel,, and you plan on doing some very long passages at a constant speed ~ then you might consider a bulbous bow.
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Old 27-03-2007, 05:03   #5
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My Coronet-Elvstrom 38 has a bulb bow. The tank tests on the hull showed a 17% reduction in drag - compared to the same hull without the bulb. It also found an unexpected reduction in pitching motion.
However, adding more waterline length would reduce the size of that advantage - unless the bulb protruded further forward, which would be a mooring and anchoring issue. Also the hull shape provides less internal volume. The bulbed hull will plane less easily, I suspect. Waves make a big difference as to how well the bulb works, so on a large ship, where on average the sea's waves are smaller than the bow wave, the bulb has advantages. For small craft, the bulb is really only working as a drag reducer in calm water. However, the pitch reduction effect is noticeable in waves.
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Old 27-03-2007, 10:17   #6
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thanks guys! Not only did I learn something about hull shape design, but I learned about google book search - SWEET!
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