Single Dagger Board vs Double Boards

Discussion in 'Multihulls' started by CatBuilder, Apr 26, 2011.

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Alan.MJunior Member

You're talking about racing hobies though. A cruising boat is something totally different.

For instance- downwind we don't have any board down - the boat steers fine.

On other points of sail, sure there's assymetery, but in a sailing boat going anything but DDW there is ALWAYS some degree of assymetry. One hull will be more deeply immersed, there's weather helm etc etc. Then on a cruising boat there are different hull loadings etc etc..

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rayaldridgeSenior Member

John Shuttleworth doesn't seem to agree with Petros:

http://www.john-shuttleworth.com/designs/S70design.html

The situation Petros describes seems unusual, in that the air was extremely light, and the boat had no directional stability without boards downwind. I would think asymmetric drag due to a single board would only be a problem at very low speed, when skin friction is a significant component of drag.

Just a few days ago I hung a deep high aspect centerboard under the center deck of my tiny cartop cat, but that was mainly to keep the clutter of a case out of the hulls, which have little room for such a structure. And also, it was an opportunity to experiment with a central foil, something I've been curious about for a long time.

3. CatBuilderPrevious Member

I actually have the real numbers on drag on my boat from the boards. The drag is 70lbs (32kg) of drag with both boards down all the way at a given speed that I can't recall.

Assuming one board that is slightly larger, one might "guesstimate" that the drag of a larger, single board is 50 or 60 lbs (25-30kg).

I'm falling asleep at the keyboard here, but I was going to try and calculate the hydrodynamic drag of a 5 gallon bucket or something to compare.

Petros has a valid point. It might be interesting to see if that asymmetrical drag is significant enough to cause some helm correction on a larger boat or not.

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redreubenredreuben

Catbuilder, what design is your boat ?
RR

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DryfeetJunior Member

FWIW, Mainecat has built over 55 Mainecat 30's that only have a single board. My Mainecat 41 has two, of which, I only ever use one....

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rayaldridgeSenior Member

It's hard to imagine a situation other than the one Petros cited where asymmetry from an offset daggerboard would be a significant component of all the forces on the boat requiring rudder compensation. Downwind, most cruising boats don't need boards, except maybe in really heavy air, and at high speed, skin friction is a very small part of drag-- most of the drag comes from wave-making, or so I understand. Sailing higher, sail forces would require a lot more rudder compensation than such a minor source of drag, and even so, on one tack, the asymmetric drag from the board would be helpful rather than harmful, or so it seems. Of course, on the other tack it would increase the asymmetry of forces the rudder needs to offset, so it seems to me to be a wash.

Of course, I could be completely wrong, and I frequently am. However, I wonder why Shuttleworth's extensive tank testing showed the single daggerboard to be the most efficient solution.

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sabahcatSenior Member

I dont know of many proven offshore boats that will or have broken boards

I do know of many that are pissed off with the snap off tips that do so and leave a mess of glass and foam dragging and causing difficulty to pull up.

John Gross has the fastback range of catamaran
These also have single boards and the owners report no problems

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8. Joined: Jan 2009
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redreubenredreuben

I am neither one way or the other and loving the discussion ! But I do favour in theory at least the Kelsall arrangement of the pivoting board between the hulls. Somewhere else Kelsall has stated that the surface ventilation problem is simply a matter of adding some extra depth. The pluses are many,
(1) 1 board
(2) doesn't compromise hull integrity
(3) doesn't use up interior space
(4) could be pivoting/ swing
(5) easy access for antifoul/repairs
{6) takes to the hard
(7) easy to adjust for shallow water (in pivoting form)
(8) possibly use central pod for other services like fuel, gas, outboard, batteries.

Minuses,
(1) would require decent structure to handle loads
(2) surface ventilation ? (fence ?)

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rattusSeĆ±or Member

Hmm - last time I sailed a Maine Cat 30 (we've done it on 2 different boats for 2 weeks total), they had 2 boards. Was really handy (and simple) to adjust the boards to trim out any helm imbalance; kind of a fine tune versus the sails.

Mike

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Alan.MJunior Member

Also the case would be right in the middle of the saloon in most boats.

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rayaldridgeSenior Member

Alan, one advantage of a central swinging board is that a "case" as such is not really necessary. In my little cartop cat, the support is just a couple of ply brackets braced against lateral forces by being slotted over the beams that stiffen the center deck. All this is underneath the deck. In the interest of simplicity, I added a lever to my foil which projects above the deck, but this probably could have been avoided with control lines. Of course, this is a very small boat with very small forces, but if you were willing to use water stays to support the board laterally, the structure required could be quite modest even in larger boats, or so I would think.

I've seen pictures of the early Kelsall cats with the centralized arrangement, and these appeared to be open bridgedeck cats. He built what amounted to a very thin but deep central hull to support the board, rudder, and outboard.. This hull touched the water and I believe the claim was that this helped prevent ventilation of the foils. I don't know if this arrangement was prone to pounding, but Kelsall still likes the idea today, even though it's 40 years old.

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redreubenredreuben

central board

What he said,

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======================
The Hydroptere .ch guys obviously feel it is a good solution when conditions don't allow foiling-they must have very carefully assesed the potential of ventilation. And ,of course, this is a high performance boat......
Also, http://www.boatdesign.net/forums/multihulls/wave-splitter-catamarans-5741.html

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14. CatBuilderPrevious Member

The design is this:

Length overall: 44'-10" (13.67 m)
Beam: 25'-0" (7.62 m)
Draft: 1'-9"/6'-3" (0.53/1/90 m)
Weight: 11.900 lb (5.398 kg)
Displacement: 18.292 lb (8.315 kg)
Sails - main : 495 sqft (46 sq m)
-blade: 232 sqft (21.55 sq m)
-genoa: 368 sqft (34.2 sq m)
-spinn: 755 sqft (70.14 sq m)

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PetrosSenior Member

Shuttleworth did not actually test the configurations, but ran computer models of them. No way to know how accurate this is because the prediction is only as good as the assumptions built-in to the mathematical model. I know because I do this for a living. There is nothing as good as real world testing.

All of you that argue for one board in one hull are missing the point. It will sail just fine and you would never know the difference. But there is a drag penalty is some conditions due to the large asymmetry of the configuration. If you do not care about the drag asymmetry than it is not an issue.

I think I would go for one large center board:
1. It would not take up hull space in either hull.
2. result in a stronger hull that is less prone to leak around the trunk.
3. save building time
4. save weight
5. save cost.
6. It would lessen the work load on the crew to adjust just one board instead of two. 7. It has no asymmetry issues that a single board in one hull might cause.
8. I suspect that a single center board is less prone to damage than boards in the hulls.
9. A centrally located center board would be easiest to reach from all parts of the boat.

I like the idea of putting in pod in the center that can also be used for stowage below the center salon floor. Leaving it exposed however would allow easier access for cleaning and inspection as pointed out. You would not even have to raise the hull out of the water. Also, with an exposed center swing keel, you can still retract it even if badly damaged, unlike any type of hull mounted dagger board. This I think would be a big advantage on a cruising cat, you can land on any beach for repairs.

The only advantage to a single dagger board in one hull is that it would be more efficient since it would not be a surface piercing foil (as compared with a center mounted dagger board). This advantage I suspect would be lost from the additional trim drag due to the asymmetric configuration (which can be significant).

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