Bridgedeck centreboard why don't they work???

I do agree heartly. Obese cats with tons of useless gadgets, sails so badly tuned that have the aerodynamics of worn potatoes bags, not maintained hardware plus people moving all time the poor tiller over 90 degrees.
The one design racing is the best school. You'll learn tuning, the good use of a tiller, to tack fast, to choose the good tack at the good time, to keep always the best VMG and a lot more skills. Far more important that the last hyperlaminar daggerboard in carbon fiber cooked twice.
After you're able to extract the last tenth of knot from a cruising boat correctly designed in any situation.

 

Dazcats are not slow. Certainly a lot quicker than Outremers, Schoinnings and Catanas. Check the Fastnet and Round Britain races results

Hissy Fit, for example, has daggerboards and skeg keels. That is because it has inboard engines and is kept on a drying mooring in Cormwall

I think only the first Dazcat 28 had asymmetric boards

(Daz and I go way back, he built one of my 14ft Pixies when at school, sailed the prototype Windsong to Norway in 1986(?). He also built the daggerboards for my Sagitta and the hulls for our 22ft Wizard while still working in his fathers garden)

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Have Dazcats ever sailed these races against Schionnings, Catanas and Outremers ?
I guess perhaps an Outremer ? Same class ?
But , i agree, Dazcats are not slow.


pogo

 

On a good multi there is no need of flaps, well sailed they have enough speed to get a good flow on the hull and appendices. Occam razor again. A good repartition is around 80-85 % of the total lift on the hull/keel/board appendice and 15-20% on the rudder blades. That gives a very lively boat which needs very small corrections on the tiller and has immediate answer to the rudders angle. A good forgiving profile of the rudder blades is the WORTMANN FX 76-100. It's also a very strong one with lots of mechanical inertia.
If you add an ackerman steering geometry you'll get an autostable catamaran but keeping a good and fast answer at the price of a small parasitic drag. That can be corrected with asymmetric foils but it's another story. The most important is the general equilibrium.
And you get a F40 catamaran that you control with just the weight of one hand on the tiller and the other hand tuning without effort the angle of the 81 m2 mainsail, keeping the ciphers of the knotmeter jumbo between 19.95 and 20.05 during 2 hours. A good training and the absolute nirvana while convoying an outstanding design...

 

Yes they have. I raced a Schionning 38 against several Dazcats a couple of years ago. They were all quicker than we were. I have also raced against (and on) Catanas and Outremers, neither are fast boats, except compared to FP's and Lagoons etc

RW

 

Could someone please explain the hydrodynamics of a dart shaped keel to me please

I assume it has a naca type profile ? Is it the Delta shape that makes them work ?

I assume they aren't " all the rage" because unlike the stub type they don't support the boat on the hard ?
Or is there another reason ?

 

Such a "hull intergrated keel " gives a deeper forefoot and volume for a motor .
similiar to ancient longkeelers the keel is the hull and vice versa. The frames in the fore-and midsections ,are more ore less , of S-type--more surface.
http://sailboatdata.com/viewrecord.asp?class_id=4542
Note props at trailing edges:
http://www.boats.com/sailing-boats/1989-beneteau-blue-2-5953791/
http://en.advisto.com/auctions-classifieds-sale/boats-107217.htm


pogo

 

dazcats now use daggerboards. only the 1495 model has a dart appendage supposedly for relief from broaching in following seas. Main foils are daggerboards. Deep forefoot = not so great draft.

 

More volume in the forefoot is going to help keep the bows from digging deeper in big following seas,...that will help with broaching. Daggerboards are going to be at a disadvantage if utilized as they will provide a pivot point about which the vessel will try to swing.

Remember the rotation of the water is in the opposite direction at the top of the wave verses the bottom of the wave. This is what contributes to broaching. I don't really think the new style bow designs are that great at preventing broaching. I'd have to be shown in person.

 

Hope you don't mine, but I chose to excerpt a few of those skeg photos from your links,...because they are 'for sale items', those links will disappear.

 

Deep forefoot=smoother ride upwind in a chop.
One reason why longkeels are still popular amongst ocean-cruisers

Cutaway longkeel:
http://www.western-horizon.co.uk/boat_view.php?boat_id=322


pogo

 

Yepp, thanks !

pogo

 

you're right !
A rudder hanging at the trailing edge of a keel is a flap.
The whole foil ( keel and flap) becomes asymmetric when the rudder is laid.

One can see this pretty often on longkeelers motoring backwards--with the rudderblade to port the boat goes to starbord!
The rudder acts in this case as a slat , making the unity of keel and rudder - the foil - asymmetric with lift to starboard.

In the 60s-80s flaps at the trailing edge of finkeels were pretty popular. These little additional rudderes ( on upwind courses always positioned to leeward of course --like the real rudder) were named trim tabs.

A thread about an Atlantic w. trim tabs :
http://www.multihulls4us.com/forums/showthread.php?3772-Chris-White-Atlantic-47-alwoplast

pogo

 

I'd have to see an accurate lines dwg to be sure but it appears that the max depth is reached mid point of the hull so there is little emphasis on fwd volumes. The Dart keel simply stops the aft of boat from skewing sideways and thus takes some of the loads otherwise resisted by the rudder.

 

Delta keel


pogo