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Old 22-03-2008, 17:29   #1
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Daggerboards, symmetric or asymmetric?

Been doing some reading on daggerboards, and am not decided on symmetric or not. I am leaning a bit towards asymmetric, as you mostly only sail with the leeward board down I think? I have no experience with daggerboards, so any advice is very welcome. Also regarding aspect ratios and profiles, widths etc.

These are for a 49 foot performance cruiser weighing around 7,2 tons empty and 10 tons fully loaded.

I have looked at having a single board like Shuttleworth does, but prefer the redundancy of 2 boards.

All input is welcome

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Alan
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Old 22-03-2008, 18:15   #2
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Having assymetrical boards kind of defeats the redundancy argument a bit doesn't it?

It would be of questionable value for a cruisng boat IMHO - the crew are probably not going to be wanting to switch boards for every tack like they would in a racing situation.
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Old 22-03-2008, 22:10   #3
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No need for asymmetrical. As 44'CC says, it defeats the redundancy issue, it's a pain when tacking and frankly the gains over fixed keels etc is so significant that the subsequent perceived gain of asymmetry is limited. I have 2 boards and generally only use 1 or 1 and a half boards and never bother to change things. The water doesn't know which board is the leeward board!
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Old 23-03-2008, 02:41   #4
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I considered & came to the symetric, redundancy was one issue & am making my boards symetrical side to side & same top to bottom as well - this also minimizes tooling cost, when sailing they have an attack angle due to leeway & can, if fitted to a squared out case have guides that can be replaced for angle experimentation in regards to hull centreline or fitting of asymetrical boards, personally I think I'll never bother But good to have the option. Regards from Jeff.
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Old 23-03-2008, 03:03   #5
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Originally Posted by 44'cruisingcat View Post
Having assymetrical boards kind of defeats the redundancy argument a bit doesn't it?

It would be of questionable value for a cruisng boat IMHO - the crew are probably not going to be wanting to switch boards for every tack like they would in a racing situation.

I see 2 levels of redundancy with boards, one where you can actually move them from one side to the other, the second is that you can always sail on one board regardless which side its on...


I can understand that having 2 boards exactly the same, is an easier option, but do you think there are any benefits to asymmetrical boards perofrmance wise or otherwise. e.g. could I go with lighter and smaller asymmetric boards, compared to symmetric? (Easier handling)


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Old 23-03-2008, 17:57   #6
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Yes, I wasn't actually suggesting you would want to swap the board over from side to side for each tack, if you had lost the other, more that the remaining board would actually be working against you on one tack if it was assymetric.

Also as I suggested, and cchesley backed up, for cruising most people wouldn't want to have to raise one board and lower every time they tack. The people I know who have cruising boats with boards, tend to set both boards at a depth appropriate for the wind strength and just leave them there. (When sailing to windward)
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Old 24-03-2008, 02:44   #7
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Symmetric sounds like the way to go, as there probably isn't much performance to be gained from asymmetric, given the hassle involved.

Any suggestions as to size, chord, profile and draught, are there any guidelines?

regards

Alan
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Old 24-03-2008, 06:46   #8
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Not entirely off topic > I recall the old MacGregor 36 had gybing boards. The forward portion of the trunk was intentionally too wide for a snug fit enabling the symmetrical board to "gybe" depending on which tack you were on, thereby angling the board. This supposedly resulted in lift to weather, mimicing an asymmetrical board. I have no idea if this worked as intended or just slowed you down.

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Old 25-03-2008, 08:45   #9
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Been doing some reading on daggerboards, and am not decided on symmetric or not. I am leaning a bit towards asymmetric, as you mostly only sail with the leeward board down I think?
Alan
There are two schools of thought on this. For performance keeping the leeward daggerboard down is usually prefered. For safety upwind in bad conditions the windward dagger down. This way the leeward hull can sideslip and the windward hull digs in, preventing the boat from tripping.

On a cruising cat, I would skip the assy. Daggers. It takes options away as far as possible setups. I doubt it makes a huge difference.
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Old 25-03-2008, 10:01   #10
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For beaching, servicing ashore etc, I have been thinking about having "stubby" keels about 30 cm deep, and letting the boards go down through these.

Any comments on this setup?

I want to go a bit longer on the daggerboard chords, to keep the height when lifted down, so this will probably be about 50-60 cms. Adding 20 cms fore and aft for the stubby. I want to set the whole thing outboard of the centre line to minimise hull intrusion.

Regards

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Old 26-03-2008, 02:48   #11
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Alan, a chord length of 50-60cm is pretty typical of the boats I've worked on & I,d be wary of incorperating the dagger into a "stubby" keel, its gunna be trickyer to make/alighn/engineer over a dagger case that simply exits the hull skin with appropriate reinforcement & closed out. Some cats mount their "mini" keels into a molded socket- glued in with some "bog", this gives a safety valve of sorts for the integrity of the hull as the keel can bust out on accidental/severe grounding Maybe a shallower docking/strip keel may be good for you. All the best from Jeff.
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Old 31-05-2008, 06:51   #12
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Asymmetrical CB's, nacelle mounted

...from another forum where I brought up this idea

One item of your thought processes caught my attention in particular; your desire for a single centerboard, and real shallow draft capabilities. I'll certainly second that motion, that shallow draft idea. One of the greatest attributes of multihulls is their capability to really go exploring ALL the water areas including those tributaries, lagoons, reefs, etc. That's why I had kick-up CB's in each hull of my design.

BUT, what you may not have noticed was my alternative to the CB's in each hull. Look at the attached drawing, (or the very bottom profile drwg that denotes "asymmetrical CB's, nacelle mounted". First, imagine a flat plate, on edge, mounted down the centerline on the underside of the bridge deck. This flat plate will act as a rib to strengthen the fore-to-aft rigidity of the vessel, a somewhat weaker characteristic in a catamaran structure vs. a keeled monohull. If a tow bundle (rope, etc) of carbon fiber (kevlar, PBO, etc) was laid along the bottom edge of this flat plate, the rigidity could be even greater (sort of akin to a bottom truss structure, or a flange of an 'I' beam). Now on either side of this flat plate I propose to mount a centerboard, not a single, symmetrical one, but rather two asymmetrical ones; sort of like a single board split in half. The flat sides of these asymmetric boards would fit up against the flat plate nacelle, and rotate on oversize (possibly 1-foot) diameter bearings. The flat fit & big bearings would together supply a great big surface for the large bending moments to bear against. Only one board at a time would be lowered. In fact the two could be linked together such that the act of lifting one automatically lowers (& powers) the other down. And they both could be rigged to 'kick up' upon hitting any solid object and/or for shallow cruising. The control lines (cables) could be routed right up to the cabin top and back to the cockpit.

There are several advantages to an asymmetrical shaped centerboard. First, it requires less total board area to develop a leeway reducing force....so the board size is reduced. Secondly, since it is asymmetrical, it does not require an angle of attack (does not require the boat itself to be sailed at a skewed angle) to develop the 'board's lift' (leeway reducing force). This actually may result in the vessel making less leeway. Plus the drag forces associated with the CB lift forces are on the centerline of the vessel, rather than offin one hull that produces turning moments about the center of the vessel.

This centerline mounting may also improve the tacking capabilities of the vessel as it allows the 'clean' hulls to slip a little while pivoting about the central board.

The front of this nacelle/plate could be configured to act as a wave splitter to actually attack, up front, the formation of those peaky waves under the tramp areas that eventually slap at our bridge deck underside. We kind of slice those waves down a bit. A lightweight fairing might also be added to this 'flat plate nacelle' so it appears outwardly much more esthetically pleasing, as well as more curvature to shed those peaky waves.

And how about the maintenance factor, particularly in remote cruising areas. No need to haul-out the vessel to repair kick-up CB problems, or even bottom painting problems. Everything, including the cables, bearings, and boards is all above the load waterline. The initial building cost should be less by eliminating the trunks in two hulls, and the watertight integrity is much better. The twin boards might have to be made a little bit longer as they operate with a 'free-surface' end, but then they are asymmetric so they can be correspondingly shorter. I would further suggest that surplus helicopter blades are prime candidate sources for both CB blades and rudder blades....high tech, extremely strong carbon fiber fabrications that have a prescribed limited life span aboard aircraft, but are perfectly happy for our use.
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Old 16-06-2008, 18:41   #13
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There are two schools of thought on this. For performance keeping the leeward daggerboard down is usually prefered. For safety upwind in bad conditions the windward dagger down. This way the leeward hull can sideslip and the windward hull digs in, preventing the boat from tripping.

On a cruising cat, I would skip the assy. Daggers. It takes options away as far as possible setups. I doubt it makes a huge difference.



I'm looking at building a 40' cat and have been wondering about mini keels plus 1 conventional symmetric daggerboard. (A swinging version sounds too hard / too much loss of internal space)

Mini keels to enable easy beaching / protection for drive legs and rudders, board for additional pointing ability upwind when I want to make maximum ground [or race a mono ]. I would build the dagger into the guests side of the hull since mostly it will be just the 2 of us on board on passages and I hear they tend to rattle.

Reading the posts on this site it seems that its not too important which side the board(s) is down for upwind performance and presumably it would give me a few degrees of lift. Downwind I could raise the board and just have the usual mini keel drag. In bad weather where feasible I could make the board the windward side.

As a mono about to be cat sailor is there something I am missing or is this workable / make sense?

Twt
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Old 16-06-2008, 18:57   #14
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All input is welcome

Regards

Alan
There are a lot of Geminis out there.
Maybe some of them could tell you about asymmetrical boards and use during tacking etc.
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Old 17-06-2008, 02:36   #15
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Thanks guys,

I have decided to go with small fixed keels for grounding, rudder/prop protection, and 2 symmetric boards.

Toys with time: Your suggestion of stubbies and one symmetrical board sounds good, stay away from a centreboard, and put it like you suggest, on the guest side. A good rule is, If it rattles, its not working, so pull it up....


Regards

Alan
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