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Old 24-08-2008, 01:53   #1
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Bridgedeck clearance Leopard

Hi guys, I have been told that the Leopards lack the bridgedeck clearance for comfortable passagemaking and are designed for coastal use. This seems t make sense considering where they are most seen but I was wondering if I could have your opinions / experiences please?
I am considering a 470 for a world cruise.
What is the bridedeck clearance? I am told 7% of the LWL is the minimum and that it should be no less than 70cm.
Also, what is the difference between the clearance at the bow and the stern as the Leopard slopes...?
Thank you!
AM
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Old 24-08-2008, 03:59   #2
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It is interesting to note that the more recent Leopard 40 designed by morelli and melvin has a much higher bridge deck clearance (around 850-900mm) than other Leopards. A guide to whether a cat has sufficient bridge deck clearance is whether the manufacturer lists it, if they don't then it hasn't.
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Old 24-08-2008, 23:45   #3
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A guide to whether a cat has sufficient bridge deck clearance is whether the manufacturer lists it, if they don't then it hasn't.[/quote]

Love it !!!
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Old 24-08-2008, 23:52   #4
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Hear him! Hear him!

Quote:
Originally Posted by DtM View Post
A guide to whether a cat has sufficient bridge deck clearance is whether the manufacturer lists it, if they don't then it hasn't.
Love it !!![/quote]

So true, and so true of every statistic, I might add. The so called Fastcat 435, when on its marks according to published figures on Multihull Maven, has the rather high displacement length ratio of 153.5 - a figure you won't find on the Fastcat website! The Outremer 65, to give some perspective, has a D/L ratio fully loaded of 86.5. Smaller D/L = lighter for its size. The Lagoon 67 has a lightship D/L of 82, so if you add, say 12,000 pounds of paylaod, it has a D/L, loaded, of 107. So, the performance of the Fastcat reported by Gludy is no surprise.
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Old 25-08-2008, 04:29   #5
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So, the performance of the Fastcat reported by Gludy is no surprise.
Was to me!!
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Old 25-08-2008, 09:42   #6
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Was to me!!
Hi, Gludy - With a fully loaded D/L of 118, the St. Francis is actually quite a bit lighter than the Fastcat 425 is at 153.5, lighter relative to its size, that is. This is why I do these calculations, because without them, you don't really know what is light or heavy in proportion. Since you and the Admiral wanted a lot of comforts aboard, the only way to get speed is to go up in size, which of course, you did. The lower D/L ratio quantifies this, and predicted faster sailing loaded. You also got a 5 foot longer waterline, and of course, when sailing at speed, the key determinant is the square root of the waterline.
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Old 25-08-2008, 10:16   #7
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Quote ----the key determinant is the square root of the waterline.----

I don't know that the hull speed formula means much with slender hulls - my PDQ32 regularily sails in double digits - sailed 11.4 yesterday in about 17 knots of wind.
Unfortunately with the trend to squeeze in more accomodations the wide hulls do tend to calc out more like a mono.
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Old 25-08-2008, 10:17   #8
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A couple of points: All Leopards are delivered from SA on their own bottoms - this is hardly "coastal"!

Personally, having sailed and delivered yachts for many years now, I find that centi-metric differences in bridge-deck clearance dont make a great deal of odds: if its rough and you are going to windward, you are going to slam anyway. Having said that, there are some slightly more sea-kindly shapes in the under body which dissipate (for want of a better word) the slamming.

Lastly - Big Cat - I always understood that the Square root of water-line length ration to give theoretical hull speed (note; theoretical, not actual!) is not true of catamarans and that they dont have a theoretical - they just go faster with more wind. Tony
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Old 25-08-2008, 10:24   #9
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Hull speed is relevent for displacement vessels. Both Catamarans and Monohulls are displacement vessels - to an extent. Once the vessel reaches hull speed, the only way it can go faster is to rise up out of the water - surf. Catamarans do this with relative ease compared to monohulls due to weight.
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Old 25-08-2008, 10:26   #10
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Waterline length matters in multihulls

Waterline matters with catamarans, but you don't multiply the square root of the waterline by 1.35, you multiply it by an adjusted factor that takes hull beam into consideration. I don't know the hull beam of the St. Francis 50, so I can't compare the theoretical maximum speeds. You can read about the adjusted multiplier in this article:

http://www.southwindssailing.com/art...iseMulti.shtml

Also, Terhohalme posted a chart about this on boatdesign.net:

http://www.boatdesign.net/forums/sho...t=22529&page=5

No, those are different, but interesting, charts. I meant:

http://www.boatdesign.net/forums/sho...t=22529&page=3
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Old 25-08-2008, 10:59   #11
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Sailing catamarans don't surf

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Hull speed is relevent for displacement vessels. Both Catamarans and Monohulls are displacement vessels - to an extent. Once the vessel reaches hull speed, the only way it can go faster is to rise up out of the water - surf. Catamarans do this with relative ease compared to monohulls due to weight.
Sailing catamarans do not surf, except for the rare few equipped with hydrofoils. They are displacement vessels. Surfing is like skipping a stone across the water, you are using dynamic lift caused by speed to raise the vessel partly above the surface of the water. Sailing catamarans are fast because they are narrow, and as you might expect, it is easier to push something narrow through the water than it is something wider.
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Old 25-08-2008, 11:09   #12
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Thats all good but does anyone out there have first hand experience of the Leopard 470 or know what the clearance is? I cant find the info anywhere!!
Thanks again!
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Old 25-08-2008, 11:49   #13
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BigCat,

If cats don't surf, what do you call the increase in boatspeed speed that follows the cresting of a following sea under your transom?
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Old 25-08-2008, 12:03   #14
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Thanks for explaining why the St Francis is so much faster than you know what - the mystery for me is cleared up.I can now see why a light slow cat with a short sharp sea and well over 20 knots of wind and all its sails up needs engine power to even tack! There was not enough speed built up to overcome the braking effect of the sea.

I have only one mystery left to solve .... why each thread turns into the same subject even when i do not raise it!!
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Old 25-08-2008, 12:12   #15
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"Surfing" is not surfing

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BigCat,

If cats don't surf, what do you call the increase in boatspeed speed that follows the cresting of a following sea under your transom?
Abaco, are you serious, or are you quibbling for sport? It's called surfing, but the phenomenon is not to be confused with planing, which a fast planing power boat does. I have surfed down 25' waves on a 12,000 pound 35' monohull, but I wouldn't say that it was a planing boat. It takes a huge amount of power to make a boat plane. Vessels under sail just can't summon that much power unless they are very, very light, much lighter than any cruising catamaran. See:

http://www.2hulls.com/archive/Gen%20...erplaning.html
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