Daggerboard catamarans

[yeloya]

Originally Posted by Stumble

So it's may be that the daggerboard may only be 12% faster SOG, but cuts sailing time by 25%.

Thx Greg for posting. That's very close to what I would have thought..

Cheers

Yeloya

[Factor]

Originally Posted by yeloya

That's relative.. Let's remember that daggerboard cats are in any case slimmer, lighter hence better performing boats. In other words,if we add daggerboards to L 440 or 450 for example, how much increase in their upwind capability should we expect ?

Cheers

Yeloya

The terms lipstick and pig come to mind. Not a criticism of Lagoon, simply - why would you? No point taking a boat that is not aimed at performance and just shoving a dagger board on it.

[arsenelupiga]

Originally Posted by yeloya

-When beating, the book says that after 8-9 kts of speed both daggerboards should be fully retracted. Is this true ?


Cheers

Yeloya

my studies of foils suggests that at 8 kn upwind or more leeway on fixed keel cats is small. I think that is where catana manual is coming from. This I have confirmed somewhat when doing such speeds and higher upwind although currents play big role in our area so it is difficult to say precisely over longer distances. Any hobbyhorsing/nasty waves will induce additional leeway. That is probably where draggerboards really come into play.

[yeloya]

I believe the cats with daggerboard are reaching sufficient directional stability at these speeds and don't need anymore daggerboards to avoid the leeway. At this point, the extra wetted surface and drag probably outweight the benefits of the daggerboards.


Cheers


Yeloya

[rom]

Originally Posted by yeloya

Thx Greg for posting. That's very close to what I would have thought..

Cheers

Yeloya

So, if that is commonly accepted, then Uncivilized is right, that is enormous !! So how easy would it be to put daggerboards on my L440 ? just kidding please do not answer !

[CAELESTIS]

I think you guys just sold me on a Catana for my next boat.

[Stumble]

Originally Posted by yeloya

I believe the cats with daggerboard are reaching sufficient directional stability at these speeds and don't need anymore daggerboards to avoid the leeway. At this point, the extra wetted surface and drag probably outweight the benefits of the daggerboards.


Cheers


Yeloya

I really don't see why. Hulls are litterly the worst shape possible for generating lift. They are the absolute lowest aspect ratio you could possibly design within the length of the boat. So no, I don't think most Catamaran hulls generate any appreciable lift.

A boat built with daggerboard may need less board down in high speed conditions, but pulling them completely, let alone at a relatively low speed like 8kn I really doubt.

On my cat when the boat is going over 14kn upwind I may pull th boards up a bit to reduce the loads on them, but it still appreciably increases leeway angles.

[Cavalier MK2]

Originally Posted by yeloya

I believe the cats with daggerboard are reaching sufficient directional stability at these speeds and don't need anymore daggerboards to avoid the leeway. At this point, the extra wetted surface and drag probably outweight the benefits of the daggerboards.


Cheers


Yeloya

A thing to remember here is what happens to the water surface as the wind builds. You get waves but the top few feet also aerate and starting slipping with the wind. Your boat may not have additional leeway but if the surface is drifting you make more leeway that the boat with the dagger penetrating into the solid water.

[thinwater]

Originally Posted by Cavalier MK2

A thing to remember here is what happens to the water surface as the wind builds. You get waves but the top few feet also aerate and starting slipping with the wind. Your boat may not have additional leeway but if the surface is drifting you make more leeway that the boat with the dagger penetrating into the solid water.

^^ This.

[yeloya]

Below the video that I've taken when sailing south of Micanos at 35 kts from 90 degrees. I had 2nd reef on the main and the genoa half furled, making 14 occasionally 15 kts. She could easily make a bit more but didn't want to push her too much as I was new with this boat. Both daggerbords were half way down and the sea about 3-4 meters from the beam. You can see on the chart plotter that there are some leeway that could possibly be avoided by better use of daggerboards. Or not ??

https://www.youtube.com/watch?v=oGGFx-raXdw

Cheers


Yeloya

[valhalla360]

My experience is with a Gemini so not a dagger board but similar operation:

Docking, a total pig with both boards up. Slides sideways around turns. Drop a board even half way and it produces an easily controlled boat. Really doesn't matter much which board you drop.

Going up wind, you really want a board. The first time out, we forgot to deploy the boards and did 3 or 4 tacks making zero ground upwind. Dropped a board and suddenly she goes to windward. At high speeds, it's probably going to be more a question of hull design. If the hulls are designed such that they create a bit of lift, it might be reasonable that at high speeds, you can retract the boards and still make ground upwind though I'm betting you still lose a lot to leaway. Might make sense to only partially deploy the board.

Which and how many boards to use:
- If just messing about, we would use one board fully (didn't matter which) or both boards half way. It gave up performance but you didn't have to mess with them.
- If optimizing performance, the downwind board only. It's deeper in the water as the downwind hull is pressed down.
- If in storm conditions, the upwind board only. If you do get a gust that manages to lift the hull, the board breaks free and the boat can slide downwind bleeding off some of the excess overturning forces.

[UNCIVILIZED]

It's going to be a rare set of circumstances where having your boards down will hurt even half as much as having them up, or partially deployed. What are your concerns about putting them down?
Just think of them as retractable keels, that to some degree can be used for navigation by braille in questionable (thin) waters. As that's what they are... keels. One's with a lot of bonuses & perks that fixed keels simply don't, & can't have. Be said keels on mono's or multi's.

[catsketcher]

There are a few sides to daggerboard practice - performance and structural.

From a performance angle the daggerboards should reduce leeway and therefore reduce total drag. Hulls have lowest drag when they have no leeway - eg a square run. Any leeway increases the drag of the hulls. The daggerboards have drag from being immersed. How much? Well not much.

I just finished doing an F 18 cat regatta. We had, and the fast cats also had, full daggers down in both the strong and light upwind legs. No pulling up for tacks and pulling up on the downwinds legs was optional - you did it if you had time. But no racer pulled the symmetrical boards up when sailing to windward.

You can check the drag of boards - pull them up in a calm and go flat out under power. Then put them down and repeat. My 38ft cat goes from 9.0 to 8.9 knots. So they don't create huge drag even when fully down. The amount of drag from, say, 5 degrees leeway I can't measure easily but I would expect it to be huge compared. The amount of drag of my nice 0009 section boards - 80mm thick - is the same as the drag from a 2.6mm round section dowel. It is not much so pulling boards way up is not worth the increase in other drags - induced, leeway - much of the time.

The aero drag from the hull goes up by the square as wind doubles and yet we often sail slower as the wind builds up. The daggerboards produce lift based on their area, angle of attack and speed. In 30 knots, you will probably go to windward slower than in 15 knots. The aero drag of the rig and hull will increase by 4 and the only way reduced board area can cope is by increasing angle of attack. If you increase the loading too much the board will stall. So you want BIGGER foils in high winds, not smaller.

I think the reason Catana may specify reduced board area is because of engineering constraints. The shock loading, by waves, onto the board is greatly increased in large waves. Also the increase in rig drag and wind loads increases the loads in the boards. You could design them to handle these loads but the boards would be very heavy so designers go for the middle ground - pull them up in heavy winds.

The designer of my cat told me to pull the boards up a bit in big winds. The boards are under huge loads and reducing the span of a cantilever (the board is an example of this type of beam) by 2 metres to 1.4 metres reduces the reaction moment by 50%. If you are getting whacked by nasty waves it pays to have the boards up and accept extra leeway.

As always I would ask the boat what it wants. Daggerboards and rudders are the way our boats transfer a sail load that is often mostly sideways (when sailing to windward) and reduce the motion in an unwanted direction. As all designs have different aero drag, speed through the water and board size, it is impossible to give a one size fits all approach. A few tips from my end.

If the helm gets heavy, if the boat responds sluggishly, if you see vortexes off the rudders, if you see leeway at the wakes - put more board down. Also - if the board is hard to put it down - put it down - it shows the board is under load and doing something - creating lift and pushing hard in the case.

Put them down whenever you want more grip on the road and pull them up whenever you worry about high loads. But like Uncivilised said - why not put them down? You have them with you for a reason.

cheers

Phil

[Gordon]

Congratulations Yeloya, looks like the 47 we sail on, it's six years old now.
Yes a very good performance boat but from recent experience not the most comfortable for those less enthusiastic about living aboard a 'wet' and in rough conditions, unsafe when navigating around from cockpit to cabin etc. The rather exposed helm was a challenging experience when the conditions deteriorated,and this over a rather long period.
Well constructed and giving you the feeling of being quite 'bulletproof 'as they say.
Enjoy your new boat, am sure you will.

Regards Gordon.

[Barra]

Originally Posted by Gordon

Congratulations Yeloya, looks like the 47 we sail on, it's six years old now.
Yes a very good performance boat but from recent experience not the most comfortable for those less enthusiastic about living aboard a 'wet' and in rough conditions, unsafe when navigating around from cockpit to cabin etc. The rather exposed helm was a challenging experience when the conditions deteriorated,and this over a rather long period.
Well constructed and giving you the feeling of being quite 'bulletproof 'as they say.
Enjoy your new boat, am sure you will.

Regards Gordon.

Is that what they call a double edged compliment I wonder???