Daggerboard catamarans

[44'cruisingcat]

It's not really hard. Sailing loads are from the side, collision loads would be from the front. A glass or carbon I beam section down the middle of the board would resist sailing loads while giving way in a collision.


Also you can just tack on a sacrificial section, say 1/2 a metre of lightly glassed foam on the bottom of the board.

[Catmandu]

FWIW about 10 years ago we sailed in a Catana factory sponsored event from Martinique down the windward islands (st Lucia, st Vincent, beqway ...). We sailed with some very experienced Catana sailors who pushed boats hard as bragging rights are everything when everyone got together in the evenings for dinner.

Rule of thumbs that I was taught:
1.) boards 1/3 - 1/2 down when maneuvering in marina period.
2.) Boards 1/2 to 3/4 down close hauled and tacking
3.) If the boards are "thunking" back and forth in the cassettes, they are loading / unloading and doing little good - pull them up
4.) in certain conditions it is advantageous to pull them up to optimise AWA and slide sideways towards your destination esp. in light airs. Pinching to get 60 AWA w/boards up while sliding to Leeward is always faster than say 80-90 AWA directly to destination w/boards down.

5.) it's a bit of a black art and will vary from boat to boat. YMMV.

But they are fun...

[catsketcher]

I broke a board on my Twiggy decades ago. It was in a calm and I hit a rock very gently. The boat stopped but what broke the board was the gentle swell that pushed us to leeward just after we hit. It broke very easily sideways even though it stopped the whole boat just a second or two before.

Schionnings are designed with the idea 44C talks about. A big hardwood beam down the middle and foam front and back. A friend broke his Schionning board in nasty choppy conditions without hitting the bottom.

I also like the chop it off and put it back on again method. I chopped the bottom 300mm off my boards and only glued them back on. 17 years later they are still there.

To be fully clear about my own use. I have both boards fully down upwind up to about 15 knots. Then the boards come up to 3/4 because I don't want to break them. On a reach they are max half down. Broad reach only one about halfish (side depends on if the kayaks are on the deck). Squarer than a broad reach both all up. Manouvring - all down if possible. Especially in any wind. Seems to work on our boat.

I like the clunking tip - it is obvious and simple. Many boards won't clunk because of snug cases but it makes a good point.

[Catmandu]

Sorry forgot one:

6.) daggerboards do hit things ... sometimes at speed and sacrificial lower sections are a good thing.

[Cavalier MK2]

I think Dick Newick hit this one with his trunks. The lower 1/3rd+ angles back to leave room for a sacrificial "crush box" using foam etc.... It acts like a shock absorber and you carry a few spares. The top forward edge of the board that stays in the trunk is cut back and a sacrificial fiberglass ring spring goes on it. You carry a few spares. These things work together to keep you from having to replace sections of your daggerboard.

[Factor]

Originally Posted by Seaslug Caravan

French mini keels and motoring suddenly start to make sense.

Maybe to those with little real world experience

[arsenelupiga]

Originally Posted by Seaslug Caravan

Here U go....

Upwind Catamaran Sailing onboard ROAM — Sail Surf ROAM

Surf Sail Roam with a beautifully equipped 48 foot performance cat with daggers.

The theory pales into insignificance when you gotta nurse the boat to windward so as not to batter the crew around too badly. Averaging 8 knots and tacking through 110 to 115 degrees gives a VMG directly to windward of around 4.5 knots.

Good luck pushing one of those "jobbies" with flat bottoms too windward that hard.

French mini keels and motoring suddenly start to make sense.

bet, no love in this thread for you

Agree with you though. I came to same conclusion after numerous boat sail tests, including ones with draggerboards. They are just too demanding.

[44'cruisingcat]

Originally Posted by Seaslug Caravan

French mini keels and motoring suddenly start to make sense.

So you have a French motorsailer?

Not happy with it?

Not surprising you seem to be so unhappy, and to know so little about sailing.

For the information of us all, what is this boat that has dissapointed you so?

[Seaslug Caravan]

Personally I've always liked the Tenant style of dagger boards and rudders. Hollow glass with "back to back " glass "C" shapes making up the shear web.

Same profile top and bottom so can be end for ended if damaged.

[valhalla360]

Originally Posted by 44'cruisingcat

It's not really hard. Sailing loads are from the side, collision loads would be from the front. A glass or carbon I beam section down the middle of the board would resist sailing loads while giving way in a collision.


Also you can just tack on a sacrificial section, say 1/2 a metre of lightly glassed foam on the bottom of the board.

The problem is boards are typically 10-20 times as long as they are wide, so beam calculations make them much stronger fore & aft.

Using a soft core can work as it crushes the leading edge but adds it's on complication (if it's only a sacrificial part, what if you fully extend the board?) and as others have said, some designers create crash boxes, so the board just pivots back (anyone ever see if these work?)

Another thing to consider is the rudders. A damaged board and I can probably limp along as long as it doesn't breach the hull. Mess up a rudder and that can be a much bigger problem if it's not easy to disconnect the damaged rudder and get it pointed straight ahead. How many of these new performance oriented dagger board boats have kickup rudders?

[UNCIVILIZED]

Originally Posted by Seaslug Caravan

Here U go....

Upwind Catamaran Sailing onboard ROAM — Sail Surf ROAM

Surf Sail Roam with a beautifully equipped 48 foot performance cat with daggers.

The theory pales into insignificance when you gotta nurse the boat to windward so as not to batter the crew around too badly. Averaging 8 knots and tacking through 110 to 115 degrees gives a VMG directly to windward of around 4.5 knots.

It's numbers like that which make you realize why lead mines (mono's) still rock upwind, unless we're talking full on modern racers. As a 40', 40yr old dinosaur circa IOR does better. Albeit the ride's surely less pleasant.

Building board that can handle the full sideloads from flat out upwind sailing can be done. Though as Stumble's said, you'll need carbon. And the strength & stiffness of the board won't be being derived from the cores. So lumber/timber cores in such instances are normally useless extra weight. If in fact you can even find timber that's sufficiently strong & stiff enough to contribute anything to such boards. As the carbon will likely be providing this. And little enough timber can rival high density foam when it comes to physical properties on every axis.

Also, designing trunks, including replacable crash boxes/blocks to withstand solid hits on the boards isn't new, nor rocket science. But again, it comes down to money, & compexity. Much like with board design. And you can even build automatick kick-up mechanisims for daggerboards if you've a mind to. Which serve a purpose similar to those ffor centerboards. But without the huge trunk drag that goes along with CB's.

[44'cruisingcat]

Originally Posted by UNCIVILIZED

It's numbers like that which make you realize why lead mines (mono's) still rock upwind, unless we're talking full on modern racers. As a 40', 40yr old dinosaur circa IOR does better. Albeit the ride's surely less pleasant

I asked Mick why their upwind angle is so low. ( 38' is reaching for us, 45 apparent has the wind almost on our beam.)

He said they'd had their headsail cut very deep for reaching, so it doesn't point all that well.

[Factor]

Please stop bringing facts into the argument, spoils the whole thing :-)

[UNCIVILIZED]

Originally Posted by Factor

Please stop bringing facts into the argument, spoils the whole thing :-)

Yep. I got yelled at for doing exactly such a thing, almost a year ago to the day, in another thread on catamaran daggerboards. It just ruins the whole fun of things. Telling the truth & all

[rom]

Originally Posted by 44'cruisingcat

Our autopilot has a VMG optimiser function. It usually settles on 30 degrees apparent, but may go as far as 32' in very light wind.

We'll usually tack through 90 - 100 degrees.

Holy cow 30° ! I just read the other link about ROAM, very interesting ! I am especially intrigued now by outremer tacking 110 vs. GB tacking 90, while both were "probably" at the same AWA. I believe you have an AC42 AP right ? First time I hear about VMG optimizer. Could it decide to go outremer like or GB like while keeping the AWA (and thus sail trim which is where my interest is) you've set ?