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-   -   Optimum Dagger Board Use (https://www.cruisersforum.com/forums/f48/optimum-dagger-board-use-21794.html)

solarbri 11-12-2008 14:15

Optimum Dagger Board Use
 
So, the wife, kids ,some friends and I are heading to the BVI's for the holidays.
We'll be chartering a Catana 471.
Having never sailed on a cat with Dagger Boards, what is the optimum positions for the boards to be in under different points of sail.
I imagine for downwind work; both boards up?
And then when working upwind; having the windward board down and the leeward one up?
Is there something I'm missing?
Any links to threads, books or articles would be greatly appreciated...

44'cruisingcat 11-12-2008 16:53

Bob Oram provided me with a "polar diagram" about dagger board use.

The basics are:

0-15 knots TWS:
angles less than 60', both boards down 100%
60'-110' boards down 50%
110'-180' boards up. (applies to all windspeeds)

15-25 kts TWS:
<60' boards down 75%
60'-110 boards down 50%

25-30 kts TWS
<60' boards down 50%
60'-90' boards down 33%

Factor 12-12-2008 05:54

Or as Gavin Le seuer would say - Winds up - Boards up

(Gee I wish my boat had boards)

SCCatSailor 12-12-2008 15:10

The general depths for the boards cited by 44' Cruising Cat seem good, but Charles Kanter (in his book Cruising in Catamarans) says you should never use both boards at once. Rather, only the leeward board should be used. He explains that daggerboards are actually foils and create lift. Obviously, lift on the leeward side will start to overcome the slight heel of a cat, making the boat more level and faster. Having the windward board down just creates more drag and does not improve pointing ability.

I've followed this advice on my Dolphin, but have to admit I haven't done any performance comparisons of one board vs. two. But Kanter's advice seems logical.

thinwater 12-12-2008 16:29

Boards aid autopilot stearing.
 
The autopilot works harder without boards as the steering is generally less positive, so you may want a little more board than when hand steering. I often find I need at least 1/2 board even with no sails if I am trying to motor into the wind, as the waves try to move the bow one way and then the other.

You will also want boards in marinas in order to get good control. With twin screws you can turn well without them, but blowing sideways is still an issue.

Under sail is different, and you got good advice. Cheers!

solarbri 12-12-2008 19:05

Thanks for all the advice.
I'll recheck Charles Kanter's book again.

cchesley 12-12-2008 19:21

Generally, (and because I'm lazy) I use one board down all the way, the other half way. I definitely found that both full down was slower. Off the wind for a while, I keep em both lowered halfway. In the Bahamas, I always sailed with half boards. There was a little loss to windward but nothing I couldn't live with. I've never bothered with keeping only the windward or leeward one down. Phil bolger is known to have said something like 'water doesn't care about assymetry'. Windward, leeward, really won't affect lift. The infinitesimal difference in draft when a cat is 'heeled' 2-5 degrees really can't be detected or at least can be insignificant compared to good vs bad helming.

Rough water, storm management does have some additional concerns but even there, it's more theoretical than practical. Fixed keel cats don't seem to 'trip' any more than any others but I'll stay away from that topic.

Evan 12-12-2008 22:37

I don't understand Charles Kanter's advice. It doesn't make sense if it applies to typical symmetric boards. Both boards are at the same angle of attack to the water and both generate the same amount of lift and drag! I agree the leeward board will contribute a bit of restoring moment, but it's insignificant if you are only heeling 5 degrees. (Hint: the sin (85) = 0.996)



In light winds and low boat speed, most boats need both boards to get adequate sideforce when beating.

SCCatSailor 13-12-2008 10:22

[quote=thinwater;232764]You will also want boards in marinas in order to get good control. With twin screws you can turn well without them, but blowing sideways is still an issue.

Thinwater brings up another interesting issue concerning board use around the docks. When I'm getting blown off the dock, I definitely want the boards down to help stop the sideways movement, but when I then try to "catwalk" the boat back to the dock using the engines I don't want the boards fighting the engines. I haven't really figured out which is more important, so I usually just leave the boards where they last were when sailing.

Another good use of boards I've found is as "curb feelers" in shallow water. In the ICW, I'll leave the boards down far enough so that they're below the rudders. Then, when (not "if") I touch the bottom, if anything is going to get damaged it's the boards. Also, when stuck in mud, simply lifting the boards will usually set me free. My theory is that board repairs are easier than rudder repairs.

Blue Stocking 13-12-2008 13:29

While probably not the same type of sailing Solarbri will be doing, I did sail Tornado competitively for a number of years in the 70s and 80s. We never sailed directly downwind because this point of sail created the greatest wetted area. We tacked downwind at about 150* off the wind.
This: A. Took us off the jibe
B. Brought the apparent wind a little further forward to the quarter
C. Allowed the hull to windward to lift slightly to decrease wetted area.
We used about half boards down to maintain maximum steerage control.
Still say that sailing a fast cat is the most fun you can have in a boat with your clothes on. :devil: :D

MidLandOne 13-12-2008 15:21

Regarding using one board or two I am just throwing this in the pot from a consideration of the foils point of view - there may be other things going on.

The drag from a foil increases as the square of the lift from the foil so one is (remember, just considering the foils) often better off to spread the lift over two foils rather than just one. That is two foils giving the same lift as one have half the drag, all else being equal.

Hence, for mono hulls, why it is good to spread the lift between a spade rudder and the fin keel, and is a reason why there have been a lot of goes at trying multiple foils under monos eg as has been tried on IACC boats. That last is an example of other things going on because other performance advantages (eg manoeuvrability) have been lost that out weigh any decrease in drag. Monos generally (not always) have the disadvantage that the foils are in line so to give the full advantage they have to be far enough apart for the foils to be outside of the turbulence of the forwards ones (not possible), a disadvantage cats do not have with their dagger boards.

So, just with that limited view I would have thought that in the case that one fully dropped dagger board gave the lift needed then one is better off to use the two boards only part dropped enough to give the same total lift but much less drag.

I wonder if anyone can comment as to how much that fits real life and what else might be going on?

MidLandOne 15-12-2008 21:26

Quote:

Originally Posted by MidLandOne (Post 233025)
...

...So, just with that limited view I would have thought that in the case that one fully dropped dagger board gave the lift needed then one is better off to use the two boards only part dropped enough to give the same total lift but much less drag.

I wonder if anyone can comment as to how much that fits real life and what else might be going on?

Mmmm, apparantly no one can comment or maybe your cats have got your tongues :).

Limpet 02-03-2013 07:36

Re: Optimum Dagger Board Use
 
Do any of the Cat Manufacturers such as Catana, offer a guide to the use of the dagger boards. There is so little information on the optimal use and even in this thread there are conflicting opinions.

1paulg 02-03-2013 15:27

Re: Optimum Dagger Board Use
 
I would have thought that the shape and angle of the boards would be a factor in determining the best board use for a particular boat - this would vary from design to design...

Dave852 02-03-2013 17:00

I am new to dagger board use having just sailed the boat once so far (though it was a 900 mile sail). One interesting thing about raising your boards all the way if sailing more than 110 degrees is the slippage will cause you to point up increasing your apparent wind which can drastically increase your speed.


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