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-   -   Dagger boards, mini-keels or swing keels (https://www.cruisersforum.com/forums/f48/dagger-boards-mini-keels-or-swing-keels-237224.html)

dstraton 19-07-2020 14:24

Dagger boards, mini-keels or swing keels
 
Most cruising catamarans have either mini-keels or dagger boards. The mini-keels are fixed and do not obtrude into the hulls. The dagger boards provide better performance, especially upwind, and permit lifting to create a smaller draft. But they require a boxy daggerboard case which obstructs some of the living area in the hulls. A grounding at any speed risks breaking off the dagger board, or slicing the hull open like a can-opener.

Can anyone explain to me why there are not more cruising cats with swing centre-boards? They could live inside a smaller mini-keel and the hull below the floor boards. They would provide the performance advantages of dagger boards, with the accomodation advantages of mini-keels. In addition they could swing up in the event of a grounding, rather than break or slice open the hull.

Clearly there would need to be some mechanism for lowering and raising them, and servicing the hinge, as well as protecting the slot from foreign objects.

Fore and Aft 19-07-2020 16:35

Re: Dagger boards, mini-keels or swing keels
 
dstraton good question. We have a swing keel yacht and love it. Originaly we had a lift up rudder but after breaking it we went to a swing up rudder and have no more issues.

Cheers

catsketcher 20-07-2020 04:03

Re: Dagger boards, mini-keels or swing keels
 
Swinging boards are a pain. The case slot needs to be long and this produces a weak spot just where you need strength. Then you get barnacles and growth in the case. Also you can't get the board tight in the case because it needs to rotate so it will bang when sailing. Also the big slot causes lots of drag. Then you have the problem of sealing the top of the board case, which is usually just above the waterline. When sailing fast you will hear lots of turbulence in the case and this can even blow the top off a poorly secured lid.

Even if you do touch the bottom you better be going forwards only. If you are sailing to windward then you still can't pull the board up as it has lots of side force on it. The only time it works really well is when you are sailing downwind with the board down and you hit the bottom. Then it can come up.

My first multi had a swinging board and the next four have all had daggers. I have never, for one second, regretted the choice. Daggers are silent, low drag, easy to operate, strong, produce strong hull bottoms, great foil shapes, highly efficient sailing, low maintenance and usually in the up position. I rarely run aground when sailing because I sail with the depth sounder on. I run aground all the time when putting about in shallow spots because I can, but the boards are always up then.

As for grounding at any speed causing the boat to split - well that is wrong. I have sadly run aground in my small cats (without depth sounders) often and have a damage free board and case. I have broken one board in 40 years. And that is when a wave pushed me sideways in my trimaran. Even a swing board would have done the same. There was damage to the case but I sailed 500 miles home and waited a month for slipping. My more modern cat has much stronger cases and the bottom 30cm of the daggers cut off and then reglued back on again. This provides a break point. It has never broken after 20 years and many thousands of miles.

In a modern cat the board cases will take up no usable room. Mine are outboard of the walkway and intrude only into the cupboard space - there is no issue with them inside. A swing board case would take up much more usable room inside.

Some great boats have pivoting centreboards. Searunners are one example but they would be faster with a dagger. (Although it would be a pain the in the cockpit)

chrisr 20-07-2020 04:18

Re: Dagger boards, mini-keels or swing keels
 
frankly iv'e often thought the same thing ie recess a swing c/b inside the mini keel

however i'm very much in the "no centerboard at all thanks - they are dangerous" group. iow, i would not have a centerboard, of any configuration

despite this, i do think it's an interesting idea...and if you want centerboards, it's worth further investigation

cheers,

valhalla360 20-07-2020 05:16

Re: Dagger boards, mini-keels or swing keels
 
All keel configurations are a compromise.

In some ways a mini-keel with swing keel inside is the worst of all worlds.

Mini-keels: The advantage is they are tough and maintenance free. The downside is they are the least efficient and minimum draft is deeper.

Dagger Boards: They are the most efficient for sailing and allow shallower draft when retracted. The downside is they intrude substantially into the cabin, sometimes going thru the upper deck and they require design features to fail safely if you hit bottom.

Swing Keels: If done properly, they are more efficient than mini-keels but not as good as dagger boards. They intrude into the cabin but less so than dagger boards. They allow for shallow draft. Yes, they do work on grounding (having a depth sounder if far from a guarantee you will never hit bottom).

Trying to fit a combination mini-keel/swing board that doesn't intrude into the hull: You get the deeper draft of mini-keels, the swing board has to be rectangular and the bottom has to come completely clear of the mini-keel, so there is an open slot behind the deployed board reducing efficiency. Because it's limited to the keel, it can't be very long or the torque could rip the mini-keel off (regular swing boards have more hull structure to resist the side torque). You still have the deeper draft of mini-keels. Access to check on it and do any work would require hauling the boat as everything is well below the waterline.

We had swing keels on our Gemini and they were integrated into the cabinetry such that they had minimal impact on the living space. They didn't rattle in the case (once sailing, the forces hold them firmly so they don't rattle) but we have had shallows cause them to lift. They are held down by a friction fitting. Some of the early ones had the pivot bolt at the waterline but that was moved up, so there was no issue with leaks and the top never blew off...there was a pressure relief hole with a hose so no splashing either. You could check on the board by removing the top of the case. This also allowed work on the up/down haul ropes without hauling the boat. Some replaced the top board with Plexiglas, so you could visually confirm the board position.

montyp 20-07-2020 06:52

Re: Dagger boards, mini-keels or swing keels
 
Swing keel also requires maintenance that a dagger board does not. The bolt holding the swing keel can wear or loosen causing leaks. The lifting mechanism can wear. Inspection is more difficult than a dagger board.

Swing keels are also prone to knocking around in the swing keel box at anchor.

That said, I've run aground with both and the swing keel was a much more pleasant experience :biggrin:

MIRELOS 20-07-2020 11:59

Re: Dagger boards, mini-keels or swing keels
 
I believe McCognaghy Yachts does that to one of their catamarans, they use a rather unconventional designs...quite interesting....of course all carbon boats.....very pricey but nice.
But I agree DStraton, it does make sense

Paul Howard 20-07-2020 13:30

Re: Dagger boards, mini-keels or swing keels
 
Gemini cats have swing centerboards, as do other older designs like the British Sailcraft Iroquois.

I have watched a friend replace the lifting line for the swing centerboard on a Gemini and can vouch for the fact it is a trial of determination and difficulty.

I have owned Cats with minikeels, sacrificial as they were attached to the bottom of the hulls, not integral to the hulls, as well as a cat with daggerboards.
The minikeel cat was great for long distant cruising to remote places as it would dry out on a sandbank for cleaning or repair.
The daggerboard cat is much faster to windward and more flexible with lateral resistance adjustment, but I cannot dry out on a sand bank with the fixed rudders.
So...

dstraton 20-07-2020 13:32

Re: Dagger boards, mini-keels or swing keels
 
Thanks Mirelos. I found this video that discusses the issue. https://www.youtube.com/watch?v=bY-JM4GS5cw

MIRELOS 20-07-2020 14:04

Re: Dagger boards, mini-keels or swing keels
 
Yes that's how I found out about it, good of Gregor to enlighten us with catamaran knowledge. if you have never met him, look him up, he is usually at the major shows, I always enjoy talking with him.

catsketcher 20-07-2020 14:26

Re: Dagger boards, mini-keels or swing keels
 
I always find topics like this interesting. Gregor says that hitting a container with a daggerboard boat may have meant "he would not be here today". Why? It is just engineering to beef up the back of a daggerboard case. You keep adding unidirectional laminate until the hull is incredibly strong. It's not hard, just a couple of hours of work. My cat has wraps of unis around the back of the case to ensure you don't split the hull. The boat will stop or the board will break.

Minikeels are a great option for many. But I would be worried about Gregor who states that glassed on keels are a worse option than a glued on keel. Again you can just keep adding glass to a keel till it becomes bulletproof. My parents cat ran up on a reef at 6 knots with its keel with no damage. Gluing on minikeels will make them less suitable to sitting on the sand as sometimes the loads are offline. My parents cat once put one keel in a hole and the other on the sand. It stayed there for a hours at a weird angle but was fine. Because it was well glassed on.

After watching Gregors video I feel that his advice is not very sensible. The MC 50 has centreboards but the vast majority of board boats don't. And it is not just complexity in fitout or construction. Centreboards can be harder to use in normal use.

cheers

Phil

Mike Banks 20-07-2020 15:17

Re: Dagger boards, mini-keels or swing keels
 
There is a third alternative. They are called LEE boards. You need an anchor point for holding up the top, a stern line for retaining it if something goes amiss, and a forward line to hold it against the pressure of the sea, and designed to be weak enough to snap if the board strikes something when down. The board rests against two rub rails set longitudinally into the hull and placed and sized to make the board vertical in the water when the vessel is at laden rest.

Lee boards have another advantage in that they can bu moved along the hull to the best point for balanced sailing--something no other type of keel arrangement can accomplish.

Mind you--I would still have a long bronze-shoed beaching keel under each hull, just in case I want to drag her up the beach on rollers, or park her on a gravel shore between tides.

The rest of the time in port the lee boards hang out of the water from the rails or the scuppers--and you need TWO of them because depending on the direction one is sailing, the board in use is always on the LEE side of the LEE hull.

You can make them simply out of sheets of plywood, and as complex or fancy as you like--but for me--simplicity is best. Easily replaceable too!!

https://www.leeboards.com/

longjonsilver 20-07-2020 15:28

Re: Dagger boards, mini-keels or swing keels
 
Quote:

Originally Posted by catsketcher (Post 3190027)
Some great boats have pivoting centreboards. Searunners are one example but they would be faster with a dagger. (Although it would be a pain the in the cockpit)

Have you ever seen a Searunner with a daggerboard?

catsketcher 21-07-2020 00:50

Re: Dagger boards, mini-keels or swing keels
 
Quote:

Originally Posted by longjonsilver (Post 3190512)
Have you ever seen a Searunner with a daggerboard?

No but they sure are fine boats. I was going to build a 31 and almost bought a 37.

There is another type of leeway reducing device that has been trialled. Designer Albert Sedlemeyer used a pivoting centreboard mounted under the bridgedeck. It was surface piercing so it was less effective than other boards but it was simple and had no impact at all on the interior.

The boat's usage also has to factor into what you want. I like fast cats. We have clocked up high 19s on our family cruiser and have twice sailed at a ten knot average for over 15 hours. So our boat can really get it on. So I like boards that can let me do that. If you don't need that, and as ex racer I like being able to do so when everything lines up and it is safe to do so, then you can have a slightly slower set up and be really happy with it.

cheers

Phil

johnn33 21-07-2020 06:52

Re: Dagger boards, mini-keels or swing keels
 
A modernised Dutch leeboarder
https://www.yachtingworld.com/yachts...msteraak-81869
John


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