CNC Plans not Included

Started playing with the rudder and I can't help but think how great it would be to only have to build one (1) rudder/dagger and not have it in the hull. It would be depth adjustable, built on the other side of the main mast beam and have a fuse dowel to let it fold. It wouldn't be supported as far down compared to putting it into a hull so would have to be stronger. There would be more drag too but weight would probably be a bit less even with the extra beefing up. Spares would be easy to change out.

Have any cruisers done this without a nacelle?




Edit: Graingers opinion over keels vs daggers
Daggerboards vs. Fixed Keels https://www.graingerdesigns.net/the-lab/daggerboards-vs-fixed-keels/

One thing he doesn't consider is that raised daggers give you shallower draft for exploring shallows. In fact he puts it the other way saying that daggers have more draft, which is true when down but ... ))

He also puts a cutoff to size of boat for daggers and that is 50 foot at around 11 tons, otherwise he says too big.

 

I think you have misinterpreted graingers opinion on the 50ft cutoff point.

I interpreted it like this- the reason he gave a cutoff of 10 or 11 tonnes for a 50ft cat is because the boat will not have a high enough power to weight ratio to be deemed a performance sailing vessel and daggerboards are only preferred to minikeels for performance oriented boats.

The size of the boards or keels is dependant on the size of the sail plan, not the weight of the boat.

 

Yeah I would think that is what he was talking about as he offers dagger option on his larger boats as well

... Have you seen this

Happy Feet Unstayed biplane rig humming along nicely with the center foil and center rudder. That is impressive!!

Putting this for reference later

 

yep seen both of those... at the end of the day anything is possible, just depends on what you want and making it happen

 

Oh in case your wondering why Happy feet sails so well - its not because she has twin unstayed rigs or a single center dagger/rudder.

its because she has a high power to weight ratio - her OMR rating is about 1.1 if i remember correctly. Theres no magic in any of this, just simple power to weight and displacement to length ratios are by far the biggest determinant factor in how well a yacht sails. Small windage/frontal area is also very important for upwind performance. Upwind performance requires good lift/drag ratio or read that as power/drag ratio both in the foils and the rig.

 

Happy Feet. Around 1 min. mark, is the stbd hull twisting relative to the port hull?

 

I thought it was because she was partially foiling. The dagger is, as I understood, a foil that gives upward lift. But that is a small simple boat, I tried to do it on mine but the dagger would have to be really long and then it's going to interfere with my roof and push the boom up. However would this work with a rudder because that would simplify my steering greatly.

UOS ... It looked to me like the hull was twisting too.

 

Now that I think about it, the daggers will be in the way of the boom regardless. If I am using unstayed rig, it's going to prevent me from letting the boom swing free.

 

vertical daggerboards dont provide upward lift - well an extremely small amount if you want to be anal and consider the cosine of the heel angle...
you often read about the lift because that is what the dagger foil is doing with respect to the leeway angle of the yacht as it sails. The leeway angle is the Angle of attack of the dagger foil and it will generate a lift vector at approximate right angle to the leeway. This translates into an UPWIND vector of lift from the daggers not UPWARD lift.

With respect to the daggers fouling the boom - you will have an amount of height from deck level to boom level. This is probably 6 feet or so i would imagine? Therefore you can have 6 feet of vertical range in the dagger movement and you can keep a small portion of the daggers down from there meaning you could end up with something like 8 feet of vertical area once the boards are down. You could pull the stops out if you wanted to raise them all the way for beaching in which case your sails are stowed and boom travel is no longer an issue.

But by your own admission, you were not interested in racing or high performance so if this is the case, then you should really consider mini keels instead of dagger boards - its going to be much less hassle in the long run on a cruising boat... less effort to build, and cheaper to build etc most cruising cats use mini keels...

 

It's tempting to go the mini keel route but I am also concerned about draft. If I go daggers I think I would only go with 1 and keep it completely pulled except upwind. I don't like appendages on a boat. Im also thinking the top of the dagger should be foldable so it can be pulled completely up without removing from the cassette.

I was reading a thread where the guy said his daggerboard weighs 300 lbs ... I think I can lift that but then to balance it and lay it down, what a pain, all on a 24" wide gunell??!!!

 

I played around with the daggers some more. I am thinking a center symmetrical dagger with a flap. There are a few variations of it. This is the simplest and can be used with a similar smaller dagger aft. The flap on the main dagger can articulate either way to give you lift on both tacks. The stays connect at waterline to minimize weed pickup and can steady board both up and down.

The board shown is about 9.5 feet in the water, I think that is way too big but maybe not?? Another variation would be to make the main dagger only about 6 feet long and have an inner telescoping tube but it might be a lot of complications for very little gains.

The main dagger is held in place with a shock block or whatever you call it, if it releases it's easy to reset. I figure most the time it will be up anyway.

 

By far the most sensible way to fit a daggerboard in a cat.
Couple of things: The flap will be good fun to play with, but I am not sure it will be worth the effort of making the join smooth and the mechanism work. I'd be very interested in the results if you do.
If the chord is too small, it will stall coming out of a tack. Bigger chord will also give more support at the top. Most important, make sure the pivot point and the stay attachments are at the same level so the board is supported at all times.

 

A bending dagger might make the flap mechanism bind the way I have drawn it but I think that can be figured out.

I'm also wondering if two smaller high aspect ratio daggers would be better than one. There would be more drag but it would be much easier to make them stronger while splitting the load and making them lighter. They would both be connected to a bridle which runs to the catchblock. I think that would make the flap mechanism easier.

Is there a optimum ratio of cord to width and what is considered a good size for cord?

What do you think about slots in the wing as in this illustration?

 

Here is a view with 2 short daggers but if I understand right then the longer is better because you need a bigger chord? The short daggers have a 26 inch chord



Edit: was just reading more on leading edge slats. They double the lift over what the back flap gives you.

The leading edge slat allows greater angle of attack while keeping airflow from separating from the wing. I never even heard angle of attack mentioned with daggerboards and that is probably because it wouldn't work to well in a cassette.

Big airplanes use retractable ones but small planes use fixed slats. At cruise angles, apparently the drag is not that bad over retracted, especially when used with a slat on the trailing edge. Has to do with the Venturi effect.

Leading edge slats allow angles of attack up to 30° before separation occurs. I think Rob is right, on a catamaran the daggers should be on the bridgedeck and they should swing down like a centerboard so they can have a fixed angle of attack. You could make them rotate too but why? You only will use them going into the wind.

So if I understand correctly, you will not only point better but you will lose less ground to sliding and you might even slide into the wind )) that would be cool!

If I understand correctly #2 - This also means it would help prevent stalling on shorter chord boards.

But I think some rotation would be good to account for heavy or light winds. Going to work on this idea. Also, it seems, ...If you change angle of attack on daggers the boat is going to want to steer different. So aft rudders should actually compensate for which side of boat you want to deliver lift to and when you are headed down wind be neutral I guess. Actually scratch that thought, instead of steering away from the wind to point into the wind you would actually be able to or actually - have to - steer into the wind to keep boat from pointing directly into wind. I can see this working.

 

I drew a diagram of how I think an asymmetric dagger works. On the left a standard dagger provides windward lift to reduce leeway. On the right, the improved dagger is rotated to increases the angle of attack and create more lift reducing leeway. The red lines are where the boat points and where the boat actually ends up. I only show one board on one side but lets assume you would lower and raise a board with every tack.

The way I see it,

In regular to light winds -
The boat with the modified dagger (right) can either point off the wind and wind up at the same spot as the boat on left, only faster ... or it can point the same and wind up further upwind over the same time.

In higher winds or on a boat with a rotating mast, both boats will point higher increasing the AOA and lift of the daggers but the modified dagger with the leading and trailing edge slats is more tolerant of the angle of attack and enjoys less drag and is more resistant to stalling.

The modified dagger enters the water at the surface so it has extra drag that must be accounted for. Even if it's a wash, the benefits of having a board under the bridgedeck are huge. Just the safety of being able to kick up is enough.