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Old 06-01-2008, 00:00   #1
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Thumbs up Hull clearance from the water

I am looking at the options re; mono or multi and over 40 feet , This is for a world cruise , I have been reading as much as I can regarding multi's they all say the more clearance the better the ride, overall takeing in hull clearance , speed and finish what would everone recomend ??
I will admit I do have a preferance for dagger boards

Graham
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Old 06-01-2008, 00:04   #2
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You mean freeboard? On a monohull, you don't want too much. Some of them are towering; makes getting on and off a pain, and increases your windage. Obviously you want enough to keep your cockpit and the such out of the water, but excessive amounts causes its own problems.

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Old 06-01-2008, 00:07   #3
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The distance from under the bridge deck to the water
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Old 06-01-2008, 03:20   #4
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You mean freeboard?
No I think he means what is known in the cat world as bridge-deck clearance. That is a distance from the water to the underneath of the deck that connects the two hulls. It is the most important issue in a cruising cat design, and the reason that very few south african boats will ever make my list of possible boats. To answer your question - the more the better, its also a function of length and beam, but rough guide, about .075 times the length of the boat should get you started and be a fair starting point. Beware any boat that has you stepping down from a cockpit into a saloon.

Chris White's (The cruising multihull) book does a reasonable job of dealing with the subject.
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Old 06-01-2008, 04:08   #5
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Bridge-Deck Clearance shouldn’t be examined in isolation from other design considerations; such as:
Hull Beam (buoyancy), Shape, & Separations (Overall Beam)
Bridge-Deck Length (How far forward the deck extends. A set-back deck allows “venting” between hulls)
Nacelle shape

Excerpted from "Catapaulting to the Future" ~ by Steve Callahan
Cruising World, Callahan, Catapulting to the Future

“... Cat Facts On Bridge (Wing) Deck Configurations, Underwing Clearances And Freeboard:

Short Bridge Deck With Low Cabin Or No Bridge Deck Benefits: weight and windage for better speed and pointing ability; minimizes weight in ends to reduce pitching; sail plan can be carried lower to increase stability for safety and sail-carrying power; large passing waves can vent through open areas of connective, reducing pounding and overturning moment from storm seas; a low-profile bridge with underwing high off the water for less (or no) pounding from normal offshore waves can still contain berths with sitting headroom.

Longer-To-Full Bridge Deck With Full Cabin Benefits: Greater interior space; well-lit saloon with good view unites hulls' interior layout; more rigid support for rig (especially if extended to jibstay attachment) which reduces shock loads and fatigue of fittings; longer bridge deck spreads stress between hulls and connectives, keeps deck drier and eliminates area of inter-hull trampoline, which must be replaced periodically.

Benefits Of Heights Off The Water: Freeboard provides a drier ride and more interior volume for headroom and accommodations in hulls; bridge-deck cabin height provides headroom; raised bridge-deck underwing reduces slamming by waves for comfort, speed and less structural stress.

Benefits Of Low Heights Off The Water: Lowers the center of gravity, increasing the range of stability (important offshore in storm conditions); lowers rig height, giving more sail-carrying power and dampened pitching; minimizes windage for better pointing ability; more pleasing aesthetics. Low freeboard gives less wind drift and wave impact in heavy seas; easier boarding and disembarking from the side to promote docking.

Bridge-deck underwing clearance forward is vital, but it is important throughout the length of the bridge deck. Clearance is less of an issue in flat water, but offshore it is imperative for comfort and safety. Waves slamming underwings have damaged many boats. When the going gets tough, slamming of the wing can really rattle things around in the boat -- it has even bounced people out of their berths -- adding structural and psychological stress and slowing the boat, perhaps even preventing progress to weather. Wider-spaced and longer hulls span more ocean, allowing swell and chop to affect the underwing more and demanding more wing clearance. Many boats use complex curves under their wings, which helps to dissipate any slamming and excess loads.

Multihulls appear as ugly boxes to some sailors while others think they are as elegant as seabirds. One catamaran aficionado wisely directed his designer to draw up a boat with a typical 50-footer's accommodations, then make the boat 65 feet long. The result was elegant and not much more costly. Freeboard for a given length is one key to catamaran aesthetics. A heavy monohull sits deep in the water, so freeboard to provide headroom need not be excessive. A light catamaran's hulls cannot be very deep, so freeboard tends to be high. Even a trimaran's freeboard is lower because the boat's weight is supported by a single deeper hull, and its freeboard is masked somewhat by sleeker outer hulls. Many multihull designers note that it becomes exceedingly difficult to create an elegant cat with a luxurious accommodation and standing-headroom across the bridge deck in a vessel less than 40 feet long...”
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Old 06-01-2008, 06:00   #6
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Bridge deck clearance

I think the comment "all South African built cats have a low bridge deck clearance" is absolute rubbish.
Here's the facts on a few: -

Leopard 46 770mm
Dean 441 660mm

I think all the tosh talked about SA built cats is due to the Voyage 440 which has only 350mm and yes, it does slam.

All the above have been measured by myself and are when fully laden with fuel, water and supplies.

Adaero
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Old 06-01-2008, 07:08   #7
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Originally Posted by KIWI View Post
I am looking at the options re; mono or multi and over 40 feet , This is for a world cruise , I have been reading as much as I can regarding multi's they all say the more clearance the better the ride, overall takeing in hull clearance , speed and finish what would everone recomend ??
I will admit I do have a preferance for dagger boards

Graham
Hi Graham - what's your budget?

As for bridge deck clearance, be biased for higher, consistent with all the other compromises you'll be making if you choose a cat. After chartering an R&C (Simonis design Moorings 45) I swore never to seriously consider a low slung cat. That said, my son absolutely LOVED all the water gushing up through the tramp on a hot day. [sarcasm] It was very wet - but at least it was slow. [/sarcasm]

IMHO, boards separate the real sailing cats from the also-rans.

Dave
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Old 06-01-2008, 07:41   #8
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As Graham has said above it is all budget dependant.

Boarded cats that come to mind: -

Catana (Graham will be able to tell you all about them)
Dolphin 460
Outremer
Gunboat (If money is no object)
Freydis/Soubise

There are loads of Australian built with boards but someone from down under should be able to add to the list.

Adaero
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Old 06-01-2008, 11:21   #9
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I can add the following Cat to this list
Fastcat 435 / 460 bridgedeck in front 850 mm in the rear 1250 mm where most of the waves actually slam
Daggerboards are a option
and the FastCat 560 950 mm in front under the saloon and 1350 in the rear under the cockpit and both have a very low weight so they follow the waves better

gideon
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Old 06-01-2008, 11:24   #10
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Please keep in mind that a lighter cat for the same size and volume will have considerable less slamming over a heavy cat that punches holes in the waves and has much more slamming
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Old 06-01-2008, 14:24   #11
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High bridgedeck clearance is important to avoid slamming in rough seas, but the beam of the boat is also important.

For the same bridgedeck clearance, a beamier catamaran may slam more on a beam reach than a narrower beam catamaran. A cat with an extremely wide beam will straddle seas coming on the beam, and as the hulls straddle waves, there will be bridge deck slamming.

I've talked to fellow cruisers about when they had bridgedeck slamming, and some folks told me it was in folowing seas and quatering seas on the stern where they had problems with slamming. Our cat is relatively narrow with a maximum beam of 21 feet, and we never had slamming in following seas, quartering seas, or beam seas. All of our slamming was when we were saling to windward.

Just because you have a high bridgedeck doesn't mean that you aren't going to have slamming. It will help, but they only way to tell whether a particular design is going to slam is to take it out on a rough day, and test in on all points of sail.
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Old 06-01-2008, 14:35   #12
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you are quite right the more the beam the higher the bridgedeck needs to be. also for a heavy cat the same works , more slamming. A cat with a lot of volume in the front part of the hulls will slam less because of the lifting these hull will do when heading into waves. We use a percentage of width between the hulls for bridgedeck height in our case 15 % of the beam between hulls under full load 5.50 meters beam x 15 % =
82.5 cm with margin 850 mm
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Old 06-01-2008, 17:33   #13
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The bridge deck clearance is always stated but what is missing is the weight of the vessel at which it is measured. Is it max displacement, empty or somewhere in between. Schionning uses max displacement but having seen the smoke and mirrors that some of the production builders use to claim lighter weights I would think it may be the same for this measurement as well

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Old 06-01-2008, 19:25   #14
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WOW, I have just come home from work to some very interesting reading,
Thank you all for your imput, To my Aussie freinds over the ditch, The schionning cats has me interested because I like the way they bullt and they are light and fast??
As for my budget about 500.000 US and if it works out around the 45-48 feet mark.

But thanks again , You all have answered my questions in detail

My intention is to join the rally in Darwin in 2009 for the Darwin-Indonesia rally so it gives me this year to sort out a boat already my house is on the market.

Graham
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Old 06-01-2008, 22:13   #15
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Kiwi
Why don't you have a chat with Lorraine Schionning. They have set up some sort of joint venture with some expat boat builders living in Indonesia calles SPC. Building times are supposedly quite quick and overall cheaper than a pro build in Aus.
If you buy one be sure to tell them who sent you, I might get a commission

With the way the US dollar is going if you hold off a couple of weeks you will probably have 1 million US

Mike
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