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Go Back   SailNet Community > General Interest > General Discussion (sailing related)
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  #11  
Old 07-24-2007
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Quote:
Originally Posted by SanderO View Post
A displacement hull does not plane...
Yeah they do, just not very well and for very short periods of time.

All multihulls displace water just like any other hull and plane when pushed harder than "displacement speed" just like any other hull. The thing is that the dynamics of the event start to alter as the beam to length ratio decreases (and the wetted surface area, weight etc etc alters). I have had it explained to me and it gets complicated, so I am not the one to explain, but my understanding is that the relationship or "multiplier" constantly alters... i.e. its a curve not a fixed ratio and it is far more efficient at the narrow beam end of the scale.

Try asking at www.boatdesign.net there are plenty of designers there that will explain if you ask nicely.

Last edited by MeanzBeanz; 07-24-2007 at 06:14 PM.
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For purpose of motoring however - just go at the square root of waterline length - ie: 36 foot WL - motor at 6kts - to get max range per gallon anything above this you will go faster but consume much more fuel.
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  #13  
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I think that one thing to consider with performance cats and modern dinghies etc is that they displace relatively little so they are starting the process of planing at much lower speeds and with much less resistance. I remember the first "Sports Boat" that I sailed slipped onto the plane so quickly and easily I was not sure it had happened... its a fantastic sensation. If you are a heavy displacement sailor and never done it I'd recommend getting your tail on one of these things for a ride at least once and talk to the guys about "how they sail"... the rules change a little with these things (more like a stable dinghy!).

Last edited by MeanzBeanz; 07-24-2007 at 06:33 PM.
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Quote:
Originally Posted by krguarino View Post
For purpose of motoring however - just go at the square root of waterline length - ie: 36 foot WL - motor at 6kts - to get max range per gallon anything above this you will go faster but consume much more fuel.
I would consider a displacement boat a little "over propped" if you can push to much beyond that kind of speed.
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Please accept my appologies, but it looks like that when I attached the links to the sites I somehow messed up..

Well I fixed that, and here are the links again..sorry

Please check these threads bellow


HULL SPEED


GZ

and this site. In this site if you place the cursor over the rectangles, it explains what is what.

SAILCALCULATOR

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Destroyers are not exempt from the principles of naval architecture. While they and other warships are capable of a good turn of speed, and btw the older one's were faster-speed is no longer the priority it once was, they do so at considerable expense. They will not effectively plane, and the stern squats substantially, often on the order of twelve feet. Once the stern begins to submerge all extra horsepower is of little use, in fact, it is counter productive. "Real" figures on naval vessel top speeds is mostly conjectural, but the naval architect has as good a guess as any since hydrodynamics will always have it's day.
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No catamarans that I know of plane.

The general formula of 1.34 x SQRT (LWL) doesn't generally apply to most multihulls, including the "non-performance" cruising multihulls.

Most cruising catamarans have a waterline-to-beam ratio of 6:1 or so. Most trimarans have amas with a hull ratio of around 12:1, and main hulls of around 8:1 or so. While they are technically displacement hulls, the high waterline-to-beam ratio often means that they are not subject to the 1.34 * SQRT (LWL) limit, as keelboats generally are. In many cases, they have a hull speed formula of 2.0 * SQRT (LWL) or higher, depending on their exact hull design.

On my 28' trimaran, I've seen speeds of over 15 knots under sail, and generally we can run down most of the monohull sailboats that sail in the area around us.

Monohull keel boats can often exceed the calculated hull speed due to several reason. First, when they are sailing down the face of a wave, or surfing, they can exceed hull speed. Second, when they are being pushed by a following current, they can exceed hull speed as their Speed Over Ground, but may not be exceeding their calculated hull speed relative to the water. Finally, some of the smaller keelboats can plane...but not too many of the larger keelboats can or will.
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Quote:
Originally Posted by sailingdog View Post
No catamarans that I know of plane.
Mine does...

Think about it! Anything doing that speed must build up pressure under the hull and generate lift. Cats are light, high powered (well some are ) and their hulls are narrow, the combination means that you are planing more easy (no noticable "speed hump")... so much so that its hard to notice the transition (some sports boats are that good U hardly know it). When I look off my transom's at 15 knts+ it looks just like looking off the back of any other high speed planing vessel. You might not be generating similar lift to a mono hull at that speed but then you have not got the drag either.

My old Paper Tiger (hard chine cat) planed more noticeable than the round bottom boats I sailed against but at some point they generated lift. We had a big cat (60'? Plywood? ) locally called "Big Bandicoot" that actually had V type planing hulls like the Paper Tiger (more power boat style)...

I suppose the question is what do you call planing? How much lift is planing? Sitting right on top of the water? or merely exceeding the hulls displacement speed capacity.

Just like all hulls displace water there is a speed at which all hulls will generate enough lift to rise up out of the water. Physics demands it! The only questions are things like...

At what speed?
How much force does it take?
Can the hull take it?
How stable is the hull form at that speed?

Quote:
A multihull's speed comes from several factors. Multihulls plane on the water rather than plowing through it like most monohulled sailboats. Multihulls also use the wind in their sails more efficiently because the extra hulls keep the boat from heeling over excessively. Multihulls are lighter than comparable monohulls, so their lighter weight also increases their overall speed.
Chris White, excellent multihull designer

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Quote:
Originally Posted by sailingdog View Post
Monohull keel boats can often exceed the calculated hull speed due to several reason. First, when they are sailing down the face of a wave, or surfing, they can exceed hull speed.
Yes, that is planing... albeit in short bursts due to the hull's inefficient shape. Thats talking heavy displacement yachts, I have had many mono hulls planing for hours... the Adams 10's love it when pushed hard under kite, they just go like a great big Malibu surf board.

Quote:
Originally Posted by sailingdog View Post
Second, when they are being pushed by a following current, they can exceed hull speed as their Speed Over Ground, but may not be exceeding their calculated hull speed relative to the water. Finally, some of the smaller keelboats can plane...but not too many of the larger keelboats can or will.
Agreed about the GPS thing, not a good measure of hull speed.

Most of the Sydney Hobart Fleet over 60' can plane these days, look at the Volvo 60's etc etc...
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MeanzBeanz-

Planing is generally when the hull has generated enough lift and is mostly out of the water... which is generally not the case with most multihulls. Which Catamaran do you have??

BTW, citing boats that compete in the Sydney-Hobart race that are over 60' LOA and the Volvo 60's is relatively irrelevant IMHO, since these are mostly pure racing machines, and in most cases far out of the budgets of most sailors. I seriously doubt that anyone cruises on one of these designs, since they are not really designed for comfort to any degree.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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