Bridgedeck centreboard why don't they work???

http://evelyn.australialisted.com/ad/gallery/18264645

a dart keel is part of the hull , either with s- shaped frames in the front- and midsections , or more often elliptical frames. in it's aft sections a dart keels frames are s-shaped .The dart keel is part of the hull , so it begins more or less at the stem. Please see post 397 and 400.
The keel under the International 23 is a stub keel , a LAR keel with a radical angled leading edge. A stub keel is a wing, more or less glued under the conventional hull with semicircular frames .
A stub keel can be a NACA foil, not so a dart keel.
The dart keel is a lifting body, the LAR keel is a profile; a wing.

https://en.m.wikipedia.org/wiki/Lifting_body

The astonishing Knife edge flight--fuselage is lifting body.
http://www.google.de/search?q=knife...g6vSAhUDEpoKHesKD98Q_AUICSgB&biw=1024&bih=594


pogo

 

Thats a pretty interesting question. Its sought of half way. Certainly not a foil yet tacked on and not really part of the hull shape.

And yes the International 23 sails rather well.


For those still offering opinions on what works and what doesn't, I always like to check out the VYC yardsticks, from data based on actual results over multiple years and races in varying conditions.

Sort of "free tank testing" shall we say.

From boats in the 5 meter range , with similar technology/design era , we have ....

Hobie 16(asymmetric hulls) on 81.5
NACRA 5 (dart keels) on 81
Cobra (centre boards) on 81

All very close in performance yet wildly different leeway control.

The big performance gains came later with reduced weight, higher aspect rigs ,minimum wetted surface and the addition of spinnakers, such as the F16 on 68 ,approximately 15 % faster than the historic Hobie 16, yet features that are difficult to harness fully on a short handed cruising boat.

What are the chances of a F16 lasting repeated cartwheels in the surf, or being sailed up the beach flat chat year after year?

 

Clarity Needed

Please organize your posting(s) just a little bit more clearly, before jumping up and 'tweeting' right away.

Are you saying that the Dazcat 1495 has both a Dart appendage and daggerboards?...seems like it??
I looked pretty thoroughly thru there website and saw none of their models that utilized both of these features concurrently.

And what are you saying "deep forefoot= not so great draft" ?

 

taken from the Dazcat website for the 1495
"A combination of optional dagger boards and dart shaped keels are optimised to create "
"as the keel shapes are designed to reduce the yaw effect that can be experienced in larger seas sailing downwind."
assume you will check with the Dazcat site to confirm http://www.dazcat.com/d1495.html
D1295 - no mention of dart keels only daggerboards
ditto D1195 and D1095.
of interest. No draft numbers are given on their web site.
see http://www.boatdesign.net/forums/mu...d-why-dont-they-work-57051-29.html#post799043 for what is available.
is 2 ft 7" shallow draft? see page 63 https://ia601907.us.archive.org/0/items/Yachting_World_March_2016/Yachting_World_March_2016.pdf
my error. they must mean with boards down and boards up.

 

Perhaps their 'claims' are mis-spoken, or they have mistakenly made the claims for their whole line of designs in that one statement. In other word you can have the option of daggers or dart shaped keels. LOOK thru all the photos on their website and find me one of their designs that has BOTH daggers and dart keels on it?? I don't think you will find one. You might say they have an error in their own description.

While we are on the subject of their website claims,

I believe what they are saying here is that the keel shapes (vs daggerboards) act to help reduce the yaw effect. I could agree with that statement, but contrary to what you are insinuating that the daggers and dart keels act to accomplish this. As I have said before one would NOT want any daggerboards nor centerboards down when running downwind in a big sea. You don't want a grip on the sea in these situations

BTW, UpOnStands have you ever been out in a big following sea ??
I've been out in this sort of sea for 3 days....(but is was on a 47' monohull ketch)



or maybe something like this:
https://www.youtube.com/watch?v=hQ-svmgOxqw&index=5&list=PLhH_subO2puxkSFg8IHY6zjo9CZrOe5A1


The point I want to make here is I have serious doubts that I want those 'new age' slender reverse bows on my cruising cat. I can well image them getting buried rather easily as I surf down the slope of a big wave,...the bow steering the whole boat,...extremely dangerous.





So when Dazcat makes these claims I have some reservations,...

 

BTW here is a pretty nice view of the Dart hull model

 

Very interesting post, and I agree with you. Three cats for the same use made with the same technology of the period, three different sail plans, three different solutions for leeway.
And after hundreds of races, the same rating...each has its strong points, each its weakness. All three can survive years of abuse.

To go faster you need a technological leap and that has drawbacks; price, complexity and durability.
You'll see it at the prices in the used market, a high tech F16 sees its price falling down totally after 5 years. The reason is very simple; the catamaran hulls are worn out.
That doesn't mean that it was badly built, that means that it was built shaving the weights so there is no more margin for durability. A very different use.

The old ones were "family" boats lasting many years, the last ones are kleenex for racers who accept to pay the high price.

Is that comparison useful about a genuine cruising catamaran? Yes. First there is no need to copy the racers technical solutions unless you're a shipyard trying to sell your stuff to newcomers.
Kevlar, carbon and exotics, high aspect rigs, enormous gennakers. deep daggerboards are useless on a cruising boat. There are even nuisances.
The mind being freed of the fashion plague, the future owner can think rationally over his SOR, and choose the technical solutions that fit his real needs.

An example. A friend of mine wanted a retirement boat to voyage all around the Pacific and Indian Oceans. it called the project; "My geriatric catamaran".
Background of the guy; aeronautical engineer at Dassault, aerodynamics Ph D, designing fighters. Also aero-hydrodynamics consultant in the French Flying Circus of big multihulls. 45 years sailing from the 420 dinghy to the maxi catamaran. Bathing in hightech every morning.
So we can presume that the guy has some knowledge about boats. he designed himself the cata.

His SOR; cheap catamaran for a sexagenarian couple and their cat. Comfortable amenities AT SEA. Big enough to be safe in high sea in any weather. Easy to sail. No worries about hardware. Low draft, able to go upriver and be left at a tidal mooring. Easy to maintain and repair anywhere in Asia. And good performances as he hated slow boats.

The boat; A long 14m light and thin hulled catamaran. A 12 meters with 2 meters added in the middle. 6.80 meters wide. Dart keels, he made several calculation about before picking this solution. Beach cata style rudders.
Material. Composite wood. Strip plank bottoms, compounded plywood for the top sides. Big bolted arms in plywood with plenty of place inside for the material.

No bridgedeck but a very comfortable central pod cockpit with a dog house. All sheets and ropes coming back to cockpit. 4 winches, two of them 3 speeds. Amenities in the center part of the hulls. A lot of details too long to describe. A big white tent at the mooring with hammocks. The tent was kept in the rear arm.

Weight empty; 3800 kg. Weighted when finished at my shipyard, not an estimation. Full displacement 5500 kg. Never arrived to this displacement. Yes well engineered wood composite can be light.

Engines and electricity; two outboards Yamaha 30 HP 4 Stroke on side pods. A 30 Hp weights less than 200 pounds...You'll find gas and spare parts anywhere in Asia. Honda 2 kw genset. Solar panels. Windmill.

Rigging. Schooner. 2 mainsails 35 m2 fully battened on wishbones. Self tacking jib 12 m2. Gennaker 35 m2. For very light wind a very big gennaker. No roller furling, for the gennaker same system as a beach cat. Alu rotating masts 13 m. Very wide spreaders.

Perfs; Extremely good in very light wind, the slims hulls have something in that. In the very lazy way at 5100 kg displacement; 8 to 10 knots upwind, 12 to 15 knots downwind. Pushed very hard by three brutes in a good 25 knots breeze ; 10-12 knots upwind, 100 degrees tacking angle, 16-18 knots constant downwind, bursts 20 knots. Not bad for a geriatric boat with a schooner rigging. Slim long hulls pay, that shows also that a well designed boat has good mean speeds without effort.
The owner reported days of 250-300 miles in the trade winds, sleeping 8 hours day and more occupied by cuddling his wife than tuning the sails.
Very stable, very easy going boat. The autopilot could be replaced by 2 sandows on the wheel in good conditions.
Taking reefs was a breeze and the cata behaved very well in trials with 40-45 knots wind and very strong seas at the Mer d'Iroise. Very safe. No big problems for a couple in these conditions (that doesn't mean it's a cocktail party, strong winds and sea are never comfortable and always dangerous).

What he did? Vagabonding during 6 years in all Pacific Asia and Polynesia. Upriver the Mekong until Cambodia and many other rivers. Many miles as they were always sailing.

For me it's the perfect example. A very educated guy used to high tech and complexity thinks over technical solutions for a voyaging boat in remote places.
After defining carefully his SOR, he designs a geriatric catamaran with small sails very far from the mainstream, and chooses simple and low tech in a same way that a sensate guy takes a 4 cylinders sedan instead of a V8 sport car for everyday use.

If the SOR was different, he would had picked very different solutions. You do not make a geriatric cata when you want a racer cruiser.

 

Hissy Fit has "dart shaped" keels and daggerboards. Note: Not "Dart" shaped meaning like the Dart beach cats (which are integral part of the hull, almost deep V) but dart as in a triangle shape.

other Dazcats have round bilge hulls, kick up rudders and daggerboards, no keels. it depends on the design. And as I always say, if you want inboard engines you need keels, with/without daggerboards as well. As on for example, my Transit 38

I won't post photos to prove it as you can easily google the Dazcat site and Facebook page. As i say, I know Dazcats well and have known Darren, Biffa and Derek for over 30 years.

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Brian "or maybe something like this:
https://www.youtube.com/watch?v=hQ-svmgOxqw&index=5&list=PLhH_subO2puxkSFg8IHY6zjo9CZrOe5A1


The point I want to make here is I have serious doubts that I want those 'new age' slender reverse bows on my cruising cat. I can well image them getting buried rather easily as I surf down the slope of a big wave,...the bow steering the whole boat,...extremely dangerous."

About the video; a scary situation...

About the cata. I'm far more worried by the large opening slim window with sharp angles at the topsides, the huge and slippery front window ready to break a leg and the very uncomfortable place of the mast's base. Inclined, nothing to grab..I imagine lowering the main sail in a good breeze. Ah the guys are supermen now with special vacuum suckers boots.
For the bows you have to try. That can work pretty well.

All these curves and the cockpit that looks like a shoe box...

 

The daggerboard, or centerboard,
i remember that it is Newick's idea ( " Three Cheers ") . A centerboard in the bow section, geometric a delta, mainly for compensating leeward helm under top genoa on the cutter rigg , and under top drifter/screacher or chute ( nowadays named code 0 and genacker)

http://yp.stel.ru/gulfstreamer/newick_yacht.html

pogo

 

how about the launch of an actual boat D1395? Dart plus daggerboards
https://www.youtube.com/watch?v=8R6G-3ZVrSc

how about some photos and interview with the designer? Yachting World March 2 2016.

 

"Hissy Fit has "dart shaped" keels and daggerboards. Note: Not "Dart" shaped meaning like the Dart beach cats (which are integral part of the hull, almost deep V) but dart as in a triangle shape."
An apex...I use Dart because of Rodney March. As I'm old and often repeat myself, very interesting hydrodynamics.

 

sounds very very interesting. #1 did he use dagger boards?
Did he / does he have a web site? anything public? Even the SIG 45 just launched with carbon everything is 5.5 tonnes so his weight, with dog house, is great. I assume he did not use CNC machines or industrial autoclaves etc etc. just great design and careful fabrication.

 

Isn't the keel on this D1395 an integral part of the hull?
edit: I suppose my real question is "what hydrodynamic feature constitutes a Dart keel?"

 

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