Which Daggerboard Down in Heavy Weather

[Steve Strand]

Hi All
We are pretty experienced monohull sailors building our first mulltihull, a 48' Mumby-designed aluminum catamaran. My question has to do with which daggerboard should be left down in heavy weather. It seems that there is conflicting information, with one group saying leave the leeward board down and others saying that the windward board should be left down. Are there real data or tests out there that answer this very basic question?
Thanks
Steve

[estarzinger]

It depends on what your objective is.

If you have the wind forward:

- And if you are aiming to maximize sailing performance then lee dagger down - it will give you the most draft and grip.

- But if you are aiming to minimize capsize possibility then in fact both boards should be up, so you will slide sideways rather than trip on the board.

- Having the windward board down is a way to split the difference, giving you some draft/grip when you are flat, but pulling the board out of the water when your hull lifts reducing the trip possibility.

If you have the wind aft, then both boards should be up, unless the boards help your steering/tracking (which they usually don't).

[44'cruisingcat]

In heavy weather I'd have no board down.

[Steve Strand]

thanks Evans & 44. What I would really like to know is if there has been any tank testing or modeling to come up with optimal strategies or if we all just keep repeating what we have heard that seems reasonable. I am quite intreagued by the idea of just slipping away from big waves, I think it would/will make me nervous.

[tsmwebb]

Originally Posted by estarzinger

- And if you are aiming to maximize sailing performance then lee dagger down - it will give you the most draft and grip.

A bit beside the point but for best upwind sailing performance I find that both boards down is the way to go. This is probably boat dependent. In racing different classes seem to have different preferences.

The motivation for using the windward board only is that as the board comes clear of the water the boat can't trip over it. It's plausible to me but I haven't seen it happen in practice.

I have occasionally set just the weather board on my boat and it hasn't caused any problems... But, again, this may be somewhat boat dependent. The CE of the weather board is aft of the CE of the leeward board wrt to the CE of the rig upwind. So one might expect a change in helm depending on the combination of boards used. To complicate matters the CE of the hull may well change as the leeway angle and trim change... The balance of all of these things will vary and is is possible that some boats may not behave well with just the weather board down. In practice I haven't heard of any serious problems (eg. lee helm) and it works okay on my boat. But, I'd test it before things get gnarly if I were you.

Tom.

[boatman61]

Heavy weather tactics for me would be boards up.... hate tripping...
Bad one in '76 when the local buses turned into 'Flesh Eating Dragons'....

[tsmwebb]

Originally Posted by Steve Strand

thanks Evans & 44. What I would really like to know is if there has been any tank testing or modeling to come up with optimal strategies or if we all just keep repeating what we have heard that seems reasonable. I am quite intreagued by the idea of just slipping away from big waves, I think it would/will make me nervous.

This is an interesting read: http://www.wumtia.soton.ac.uk/papers/CSYS2001BD.pdf.

Tom.

[estarzinger]

Originally Posted by Steve Strand

thanks Evans & 44. What I would really like to know is if there has been any tank testing .

Yes, google the Wolfson Unit. They have multihull tank tests on line - there is one that is quite interesting and relevant - it supports the ' all boards up' as the lowest capsize probability. I don't think they tested windward board down because that's usually not normally considered, but you can read between the lines and anticipate what the test results would have been.

[ozskipper]

I would be talking to the designer. Then why not take the boat out in a stiff breeze and try both techniques and see how she performs. Try it close hauled, close reaching etc and note your pointing ability, heeling moment and handling characteristics. It may also be worth trying 50 then 75% of both boards down to see what happens.

[Joli]

It would depend on whether or not the boards were assy or sym.

[tsmwebb]

Originally Posted by tsmwebb

This is an interesting read: http://www.wumtia.soton.ac.uk/papers/CSYS2001BD.pdf.

Tom.

Originally Posted by estarzinger

Yes, google the Wolfson Unit. They have multihull tank tests on line - there is one that is quite interesting and relevant - it supports the ' all boards up' as the lowest capsize probability. ...

Op. cit.

Here is the Wolfson index: Wolfson Unit MTIA - Papers and Articles

Tom.

[Curmudgeon]

Originally Posted by tsmwebb

This is an interesting read: http://www.wumtia.soton.ac.uk/papers/CSYS2001BD.pdf.

Tom.

With all due respect, I did not find this to be interesting at all. Academic gobbledygook, with conclusions that seemed, well, completely inconclusive.

[tsmwebb]

Originally Posted by Curmudgeon

With all due respect, I did not find this to be interesting at all. Academic gobbledygook, with conclusions that seemed, well, completely inconclusive.

Sorry you didn't enjoy it. There isn't much published on this subject either in terms of casualty statistics or test data. I think the paper is pretty good particularly given the state of play and relevant to the discussion. Even though it is a limited study it touches on the factors involved in wind induced capsize and capsize in breaking waves based on model testing and does a quick survey of casualty data for larger multihulls. There certainly is a lot of data in there. They do reach some conclusions eg this on breaking wave capsize:

... There is little rotation in the wave
to turn the vessel and it is pushed ahead of the crest by
the broken water. Any rotation of the vessel is brought
about by its resistance to movement through the water.
Typically these models heeled with the wave slope,
then heeled further as a result of the wave impact on the
windward hull or float which was reacted by the
resistance of the leeward hull or float in the unbroken
water. On some occasions the breaking crest
subsequently struck the leeward hull, rotating the model back to windward.

And, they provide a nice collection of test results that you can work with if you wish. All in all, not bad for a freebee, no?

Tom.

[dana-tenacity]

I've done two long deliveries in Catanas, both happened to have the "factory manual" aboard - one I picked up from the factory. Both said to raise both boards if boat speed was over 8kn, said they were not necessary and might be prone to damage, confirmed by the Catana people doing the handover.
To help the boat in really bad stuff, both up definitely.

[Steve Strand]

Thanks everyone, this has been helpful, I was even able to glean a bit from the Wolfson stuff, tho. it is a bit hard going. Dana-tenacity, would the 8 knots and boards up apply to working to windward? Seems like there might be a lot of leeway & 8 knots is not all that fast (I think).
thanks
Steve