Bridgedeck centreboard why don't they work???

from Dazcat website page on the 1495
WA combination of optional dagger boards and dart shaped keels are optimised to create exceptional upwind performance and highly controllable downwind performance, as the keel shapes are designed to reduce the yaw effect that can be experienced in larger seas sailing downwind."http://www.dazcat.com/d1495.html

Dart 18 catamarans was the starting post http://www.boatdesign.net/forums/mu...d-why-dont-they-work-57051-25.html#post798855

 

Upwind--are dart shaped keels more effective than NACA stubkeels ( same draught) ?
Do they make the boat significantly slower , ' cause of their additional surface ?

( cruising) advantages of dart shaped keels :
---normally less draught than stubkeels
---motor in belly ( under cabinfloor)
---deeper forefoot
---better grounding/beaching



pogo

 

looks as if Dazcat thinks they are slower than the daggerboard approach.
The design guidelines tell us that semicirc hulls have the least wetted area per volume and fin keels must have the greatest WA/vol.
A NACA LAR keel is not as efficient a foil as daggerboards and I am not sure how to calculate the effective foil area and shape of the Dart keel.

 

Yepp,

but thousands of beachcats with deltakeels perform very well.

Are dart keels the better compromise for bigger cruiser/racer cats ?

pogo

 

IMHO, nope. Of course, if I was sailing exclusively around the British Isles and Northern waters I certainly would not take a Carib-styled condorama.
There was/is a 1998 Dazcat open bridgedeck cat for sale that is reputed to have sailed very competitively in British open water races, it has regular hull shape with daggerboards. see here http://www.boatdesign.net/forums/mu...d-why-dont-they-work-57051-26.html#post798877

 

Again,
are delta keels the better compromise for bigger cruiser/ racer cats ?

Please note that i am NOT talking about open racers with daggerboards , cruiser/racers with daggerboards
or condomarans.

pogo

 

are delta keels the better compromise? no

 

why ?

i think you compare the deta keels with daggerboards only .
Compare them with NACA stubkeels as found under Lerouge's boats please.





ok, ok , forget it.


pogo

 

Delta hulls have the wrong volume distribution for cats.

 

Who is talking about "delta hulls " ?

i'm off now.



pogo

 

Yves Loday, a former champion on Tornado and designer of several very competitive sport catamarans doesn't think like you. Rodney March designed several good catas using this shape, and several other NA. And I think they are more qualified than you...
It's known than daggerboards are the most efficient solution, it's the reason it's the quasi universal solution. But many times you have requisites as mooring in high tides harbors, navigation in "rocky" waters, beaching without worries and/or the quest for simplicity and sturdiness so you make a compromise. The Dart compromise has proved since 41 years to be a good one. Like any solution for the problem of leeway there are pros and cons, and it has a restraint domain of application.
This solution of low aspect appendices on catamarans is called DART, not delta, from the name of the first beach cata using this feature and designed in 1976 by Rodney March.

So may you explain why the repartition of volumes is bad? I would like to read a very solid argumentation.

 

Pogo wants to use the term delta -- tried to explain we were talking about dart but -- whatever.
The below is for a cruiser, not a racer that wants to fly a hull.
OK, a delta is a fin that springs from the aft of the hull. its starting point is the turn of bilge - lowest point of the keel. Its greatest span is at its aft end.
to develop lift the foil must have chord and span and the span is determined by the depth of the keel. If the keel is only shallow, end effects will destroy the lift so the keel has to be deeper than is true for regular hulls. If the hull is deeper in the middle the displacement of the ends has to be reduced commensurately. You can make the mid-hull part thinner but this creates problems with internal space.
Yves Loday has several small cats. the most recent two in 2004. one has asymmetrical hulls one has daggerboards. His Extreme 40 has daggerboards.

 

First there are thousands of profiles, and NACA is only one producer of profiles among dozens. On boats the NACA symmetric 00XX series, created in 1933, were used simply because these profiles were in the public domain in the 1950s and were a huge improvement over the vaguely profiled planks used in this time.
Creating profiles was a very hard task and the results were kept secret.
There are now thousands of profiles in the public domain, and the tools for 2D simulation work now in a simple PC.
Add the problem that water is about 800 times denser than air, and can generate cavitation, a very destructive phenomenon well known on propellers. Plus the Reynolds numbers are often very different as planes are generally far faster than boats, and their wings have larger chords than daggerboards or rudder blades.
So the 2D airflow simulations on PC are very only vague indicators that maybe that will work in water even when recalculating with the density of water. There are surprises...
The NACA symmetric 00XX serie work rather well, is pretty tolerant for fabrication of the daggerboard or rudder blade but there are symmetric profiles far superior on fast multis. But these profiles must be respected with very close tolerances and have excellent surfaces.
Lifting foils are another story...And only recently, good foils for sail boats can be calculated and simulated. It's a work for a qualified hydrodynamician.

Ordinary plane profiles are useless on a Dart appendage as it works in a different way of a classic wing. With a such low aspect of the appendage and the interaction of the hull, separation of the flow and vortexes are used to generate lift.
Its a rather similar way to generate lift to the gothic (not delta!!!) wing of the Concorde, apexes (F18, Su35 fighters, Berkut 360 sport plane, and some lifting bodies for example. Make a search in Internet about gothic wing, lifting bodies, apex and vortex aerodynamics.

The goal is to separate brutally the flows of the "extrados" and "intrados" on a "apex" and provoke a lifting vortex on the "extrados" (side to the wind on a cata) as water on the low pressure side is sucking with some difficulty the water of the high pressure side (the "intrados" or side under the wind).
The goal is to keep a good organized vortex along the Dart apex shape, not a disorganized drag. Apexes are very tolerant and work on a rather wide range of angles thus do not stall easily.
Note that the angle of the leading edge of the apex (or Dart shape on the cata) is small, thus very different of the about 60 degrees angle of a delta wing which works with different aerodynamics.

Like always there are pros and cons.
The pros are the tolerant angle of attack, the lift produced, and the easiness to make it work in conjunction with the rudder. It works from upwind to downwind. Apparent simplicity, low draft and "toughness" are key pros. It's a protection of the hull and can survive very hard hits.
The cons are serious. First and main con, it's not so easy to design, and at my knowledge there are not computer tools. Empiricism and understanding of its peculiar hydrodynamics are essential. A dart shape is a fixed not orientable appendage, so a mistake is a costly affair.
A Dart shape drags more than a daggerboard and needs more angle of attack to generate enough lift. So in theory a daggerboard points better upwind. In practice it's the ensemble sail/hull/appendages that gives the ability to point.

On a multi it's better to get an optimized VMG and let it run as it generates its own wind. A good multi has its apparent wind coming from the front as it "creates" wind, and downwind is the worst allure. Experienced skippers on multis tack downwind to optimize the VMG.

So the Dart shape is a good solution when it meets the goals of the SOR. Low draft, strength, simplicity, protection of the hull, ability to beach safely with very decent sailing performances.

That would be my choice in conjunction with rudders on pins on the stern (like a sport beach cat) on a simple tech voyaging explorer catamaran far from "civilization". Little to break, and easy to make, maintain and fix. A Franciscan catamaran (private joke on Ockam or Ocam who was a Franciscan monk).

"Or is there another reason ?" No, a good dart shape is practically as good on a cruising boat, but it looks rustic, not exciting like a carbon fiber daggerboard that can be shown to the people like a rolex watch.
Simply customer buy what they know and it's based on the short knowledge given by racing merchandising of the mags. Very few have first hand experience of different types of boats. So the parallel comparison between cruising and racing boats suppress a lot lot of technical possibilities. Try to explain to non "expert" customer that daggerboards are not the best mandatory thing on a cruising boat, that kevlar is not so useful on a cruising cat, that only a few pounds of carbon fiber are useful etc...
The customer saw the last racing monster and he wants the same, surfing at 30 knots on a wild monster. He wants a F1 but he needs a easy going smooth station wagon. And he wants all the amenities that make a catamaran an obese short breathing and slow poor thing.

 

Aka "ogival wing " , "ogival delta", or "ogee wing"

For those that are able to understand it ( not me :
http://www.google.de/search?q=ogiva...AqWe6ATinbjQBw&start=10&sa=N&biw=1024&bih=644


I like the compromise dart keels ( i propose to use the term "dart keels" , credits to Rodney March ; Dart catamaran)
100.000s of beachcats can't be wrong.
I don't know any well performing beach cat with stub keels.
http://www.google.de/search?q=topca...G0yanSAhUjYJoKHe39COYQ7AkIRQ&biw=1024&bih=644
Do we have missed something for our cruiser/racers, performance cruisers ?



pogo

 

Some of the boats being discussed here were primarily designed for (a) one-design fleets and/or (b) commercial/rental fleets.

Attributing technical/performance capabilities to various components of those boats may lead one down the garden path - as many of the requirements for these boats, especially those intended for commercial/rental fleets, result in boats built more with an eye toward low initial cost, low maintenance costs, low breakage rates, minimum parts count, and low insurance liability risks.

Just as an example, viewing the Dart catamaran's very low aspect bottom fin as an example of some underappreciated bit of hydrodynamic wizardry might be fraught with excess dependence on marketing hyperbole of the day than proven performance characteristics compared to other means of providing windward ability.

Just saying.