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Old 14-02-2009, 05:52   #1
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Lagoon - New Hull Shape Question

We are on day three of Miami strictly sail and are actually quite impressed with a variety of what is on offer.

Lagoon in particular has dramatically reshaped the underside of the bridgedeck with a V shaped protruberance which they claim breaks up any wave buildup and eliminates slamming.

Does anyone know if this actually works. I can see how it might slow the boat down somewhat as it looks like it lowers the bridge deck clearance but I don't know.

Any thoughts??

sk
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Old 14-02-2009, 07:27   #2
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I have one of these "V shaped protruberances" on my Cat, located at the rear of the bridgedeck. It works as advertised. I find it interesting that my hulls were designed in the 1980's and it was determined back then that this was necessary. There is a good photo of it on the website in my sig. Scroll to the bottom of the page.

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Old 14-02-2009, 08:22   #3
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This concept was taken to extremes with the Prout snowgoose, with the central nacelle in the water when the boat was loaded for cruising, providing much more load carrying, but significantly slowing the boat.

This design was reduced to a sensible protruberence just to calm the slamming in the 39' Escale and later 38' Manta

The V in the catalac 8 and 9m hulls was only included when the boats were originally delivered with an outboard as the engine. Those delivered with two inboards did not have the V. - It was added as the place to put a long shaft outboard and provided the transom much further forward than some designs.
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Old 14-02-2009, 16:49   #4
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protruberances

talbot,
Interesting that these were developed by Manta. There are two mantas at the show and neither have anything that I would call a protruberance. There are a variety of "things" hanging off the bridge deck on a variety of boats at the show but other than the Lagoon nothing that resembles a mini hull shape.

sk

PS if you ever consider going to the Miami show pay the extra for thursday or go friday the weekend is to put it mildly a zoo
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Old 14-02-2009, 18:12   #5
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Surely if they were designed with sufficient BD clearance so as to prevent slamming
( eg 800mm+ for 30 to 40ft and 900+ for 40ft and above) , then there would be no need for this "V shaped protuberances".

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Old 14-02-2009, 18:58   #6
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It seems to me a compromise where if it was designed better in the first place, something that pierces waves would not be necessary at all.
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Old 15-02-2009, 02:32   #7
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Quote:
Originally Posted by shawnkillam View Post
talbot,
Interesting that these were developed by Manta.
Prout Manta not the Manta boats

The Prout Manta was the forrunner of the BB38, then the 385.
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Old 15-02-2009, 06:10   #8
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Cat man do,
my understanding is that anything short of the sydney harbour bridge will slam under the right conditions going straight down wind. Higher clearances will certainly reduce that and if I was to either make or have made a boat I would be leaning that way. However you run into the problem the problem of windage. A couple on a PDQ/antares told me last week that with the amount of freeboard on there boat they are blown about pretty easily.
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Old 15-02-2009, 06:17   #9
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That is the fundamental design issue.

Bridgedeck clearance
v
Headroom
v
Windage
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Old 15-02-2009, 06:31   #10
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Quote:
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Cat man do,
my understanding is that anything short of the sydney harbour bridge will slam under the right conditions going straight down wind.
Never experienced slamming on my last cat and have never experienced it on any other performance cat with 800mm plus. Have had slaps, but never slamming, have bottomed out and slid over waves when pushing hard but never slamming.


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Higher clearances will certainly reduce that and if I was to either make or have made a boat I would be leaning that way. However you run into the problem the problem of windage. A couple on a PDQ/antares told me last week that with the amount of freeboard on there boat they are blown about pretty easily.
Reducing cabin height and cabin sole height reduces windage as well, for example on my new one I have shaved 200mm off of its height both at deck and cabin and still have 6ft headroom throughout.

Most newer modern cats are sleek and have sufficient clearance, they can be comfortable and perform without looking like a box, they are a boat, not a hotel.

Others are more resort hotel and less boat.

I'll be honest, I would rather have a slight windage problem than a slamming problem.

I can stay out and anchor behind something if worrying about maneuverability due to windage in close confines of a marina, even though the wide spaced twin engine installation on a cat makes this less of an issue but low bridgedeck clearance can be dangerous.



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Old 15-02-2009, 10:17   #11
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What comes to mind is the criticism from someone on here on the Privilege 39 in that it had disappointing headroom. I think some folks ought to spend time on Cats before they decide on what's important in their design.

All boats are a compromise...
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Old 20-02-2009, 08:53   #12
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Quote:
Originally Posted by shawnkillam View Post
A couple on a PDQ/antares told me last week that with the amount of freeboard on there boat they are blown about pretty easily.
Was it the Antares 44 ?..Do they subscribe to this site ?..I'd love to chat with them.

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Old 20-02-2009, 09:05   #13
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A couple on a PDQ/antares told me last week that with the amount of freeboard on there boat they are blown about pretty easily.
We all get blown around. It's called windage. Catamaran design is guided by the design balance between bridge deck height, interior Headroom and windage.
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Old 25-02-2009, 09:28   #14
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I used to blame windage too, when I was racing a Hobie 14. It may have had more to do with my lack of, er, perspective....

I guess submarines may completely escape windage, when submerged, but then why do they call that high part the "Sail"?
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Old 28-02-2009, 11:23   #15
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My Prout Snowgoose was a severe slammer and I don't think the nacelle "dividing the seas" helped - probably made it worse. More cleareance is better and the center nacelle simply reduces the clearance.
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