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Old 19-11-2007, 14:16   #1
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replacing keels with daggerboards

right , i am grinding my teeth in anxiety at the prospect of removing my cats keels and installing daggerboards ! the boat is a lerouge design , 11.5 m with 1 metre draught 500mm of which is the keel , 1900mm long . the daggerboards would be mounted inboard side of each hull to a nominal draught of 1100 mm there are two excellent locker spaces waiting and enough clear deckspace above. now the questions are : vertical boxes or canted in/ out from centreline ? swept aft or perpendicular ? the whole idea is better pointing and greater hull speed , running down wind , less wetted surface friction , and shallow water access . has anyone had any experience of this ? all opinions / ideas appreciated
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Old 19-11-2007, 14:41   #2
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I imagine your biggest challenge will be the structure of the box - will you reinforce it to the extent that a collision with anything will not open you up like a zipper? Determining this may likely dictate the rest of the geometry.

Do you really think this is worth the elective surgery?

Dave
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Old 19-11-2007, 15:44   #3
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Very ambitious, but IMHO your two design goals may be at odds with one another. Dagger boards will improve pointing. Where is it written that they have anything to do with speed other than slow you down? They in fact offer more resistance as that's their job. So now you need to increase sail area, and I then have to ask which marine design group ran the numbers on the correct sail plan, center of effort and so on...etc..? Perhaps the boatdesign forum may assist you.

I 2nd 2hulls comment. Sell the boat and find a Catana. You own a very good cruiser, not a racer.
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Old 19-11-2007, 17:56   #4
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Originally Posted by rickm505 View Post
Very ambitious, but IMHO your two design goals may be at odds with one another. Dagger boards will improve pointing. Where is it written that they have anything to do with speed other than slow you down? They in fact offer more resistance as that's their job. So now you need to increase sail area, and I then have to ask which marine design group ran the numbers on the correct sail plan, center of effort and so on...etc..? Perhaps the boatdesign forum may assist you.

I 2nd 2hulls comment. Sell the boat and find a Catana. You own a very good cruiser, not a racer.

Daggerboard are faster than minikeels downwind because they have less resistance than keels because you RETRACT them when running downwind. So there is no need for more sail etc... Still going from keels to daggers will be a lot of work.

One way to halve the amount of work would be to only have one daggerboard. There is only a very slight loss in pointing ability compared to two boards.
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Old 19-11-2007, 18:05   #5
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As far as I know there is no practical advantage to canted daggerboards. Swept boards might in theory shed crab pots and kelp better but you would probably have to have quite an angle to have much effect. While you may gain a bit of speed and pointing ability with daggerboards my feeling is that the time and money would be put to greater effect by investing in new sails. Save the enamel on your teeth too.

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Old 19-11-2007, 21:48   #6
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Are you converting a racing cat or sailing a cruising catamaran? I missed the brand of Cat you are sailing??

There are several items that I would rate more important than redesigning hulls and sail plans just to squeeze out an extra knot of speed.

Considering

Safety
sea keeping ability
load carrying capability
ease of sail handling
manuverability
Live-aboard comfort

lastly

speed

And a quick comment: Of course the boards are up on a run, I haven't read anything which claims greater efficiency than multichined hulls on a run. But, we don't even know what kind of boat this is or what type of hull design. Or for that matter how much the sail area would have to be reduced so the boat doesn't trip in a blow.

On a reach or a run, I have to believe that there would be little advantage to installing boards and destroying his keels, certainly not worth the trouble to reengineer his hulls.

Especially a home brew job.
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Old 19-11-2007, 22:02   #7
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Originally Posted by rickm505 View Post
Are you converting a racing cat or sailing a cruising catamaran? I missed the brand of Cat you are sailing??

There are several items that I would rate more important than redesigning hulls and sail plans just to squeeze out an extra knot of speed.

Considering

Safety
sea keeping ability
load carrying capability
ease of sail handling
manuverability
Live-aboard comfort

lastly

speed

And a quick comment: Of course the boards are up on a run, which makes his hulls no more efficient than they are now.

If a man wants boards, he desires better windward ability. On a reach or a run, there would be little advantage to installing boards and certainly not worth the trouble to reengineer his hulls.
Of course his hulls will be more efficient on a run. Boards retracted means a clean underwater shape, less wetted area, and no interference drag between the hull and mini-keel. Even reaching, with one board half down, you will likely have less wetted area, and yet still have more lateral resistance.

There are those who would argue that being able to greatly reduce lateral resistance when required does make for a safer and more seaworthy boat.

Whether the gain is worth the work and expense is another question altogether, and something only the person doing the work and spending the money can decide.

Personally I doubt if I would do it, but if someone else wants to I wouldn't be critical of them.
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Old 19-11-2007, 22:07   #8
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Depending on where you sail, being able to reduce your draught by 500mm might be an important benefit too.
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Old 19-11-2007, 22:13   #9
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The sail area wouldn't need to be reduced "so the boat doesn't trip in a blow", because the boat would be LESS LIKELY to trip in a blow. In case you still haven't got the idea, when it is blowing hard you RAISE THE DAGGERBOARDS. Even if you are sailing to windward, if you have reefed sail you should also have reduced board. You can also completely raise the leeward board, and leave some windward board down - so you still have the ability to point higher, but have NO RISK of tripping.
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Old 19-11-2007, 22:19   #10
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Originally Posted by 44'cruisingcat View Post
Personally I doubt if I would do it, but if someone else wants to I wouldn't be critical of them.

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Originally Posted by gramos View Post
has anyone had any experience of this ? all opinions / ideas appreciated
The man asked for opinions.

I don't know any professional who would recommend that a guy take a chainsaw to his keels. Recommending or advising a hull design change without engineering is illadvised. That's my opinion.
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Old 19-11-2007, 22:29   #11
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Who is recommending aything?

If I were to consider doing something like Gramos is I would most certainly get advice from either an MA or a well known catamaran designer. He will need that advice anyway, to determine exactly where to position the boards, so I assumed he was planning on getting it.

All I was doing here was correcting some of the things you said that were simply incorrect.
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Old 19-11-2007, 22:31   #12
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I simply don't understand what you are trying to say in this post.

Most Catamarans that have gone over, have boards, and have had them down at the wrong time with too much sail area utilized. I'd say 100% of the wind cause accidents had boards. Of the remaining is an even split with no one knowing for sure what happened. Cat's without boards unload excessive winds by slipping sideways. The very reason that boards down make for better performance, is their Achilles heel. In a sudden blow, short handed crew can not do everything fast enough to prevent a disaster. In every article I'm referencing, this was the case. Everything happened too fast to react.

How many printed news reports from the USA would you like me to post here on this very topic, before you will please concede this point. I think I can post 3 or 4 articles before I go to bed tonight and look for more tomorrow evening. I think I have two Geminis saved on this computer.

As they say, I have no dog in this hunt. I'm just stating the facts.

You are very much entitled to prove me wrong. Please show me all these cats without boards that have gone over. Please, printed news reports only.

Quote:
Originally Posted by 44'cruisingcat View Post
The sail area wouldn't need to be reduced "so the boat doesn't trip in a blow", because the boat would be LESS LIKELY to trip in a blow. In case you still haven't got the idea, when it is blowing hard you RAISE THE DAGGERBOARDS. Even if you are sailing to windward, if you have reefed sail you should also have reduced board. You can also completely raise the leeward board, and leave some windward board down - so you still have the ability to point higher, but have NO RISK of tripping.
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Old 19-11-2007, 22:44   #13
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http://www.cruisersforum.com/forums/...gedy-7141.html

There are others, I don't have the time just now. The fact is, boats with minikeels go over, as do boats with boards. Most catamaran capsizes occur in races, and most race boats have boards. But for a cruising boat, boards can have advantages, and if used properly they can actually REDUCE the likelyhood of capsize.
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Old 20-11-2007, 02:29   #14
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44' Cat found up-side-down
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Old 20-11-2007, 07:28   #15
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Quote:
Originally Posted by gramos View Post
right , i am grinding my teeth in anxiety at the prospect of removing my cats keels and installing daggerboards ! the boat is a lerouge design , 11.5 m with 1 metre draught 500mm of which is the keel , 1900mm long . the daggerboards would be mounted inboard side of each hull to a nominal draught of 1100 mm there are two excellent locker spaces waiting and enough clear deckspace above. now the questions are : vertical boxes or canted in/ out from centreline ? swept aft or perpendicular ? the whole idea is better pointing and greater hull speed , running down wind , less wetted surface friction , and shallow water access . has anyone had any experience of this ? all opinions / ideas appreciated
Gramos,
I would first contact Eric Lerouge:

Erik Lerouge Yacht Designs

Here's and interesting article as well:

Articles - Daggerboards

You may want to reconsider positioning the boards inboard and canting them out. Better to cant them in, therefore the outboard position is likely. The thinking is that when the hull lifts, the canted in board is already becoming less effective and the hull slips. In a real blow, best to use the weather board, which has a good grip if canted in. Also, mounting perpendicular is fine, either inboard or outboard.
Building the box is not difficult; mounting it is the trick. What is the hull material? Have you decided on a foil section? Adding daggerboards and boxes can also be heavy, so you'll want to pay attention to weight as you install them.
You will have better performance and Lerouge's cruisers have a relatively high level of performance to begin with. What design is it? Any pics?
Good Luck!
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