Ama Daggerboard Location

[olsurfer]

What effects on boat handling would be created by having the ama daggerboards located farther forward than the mast on the main hull which has no centerboard?

 

[Stumble]

It depends on the sail plan, but moving the CLR that far forward is going to result in massive amount of weather helm assuming a sloop rig. My guess is that the weather helm would be so severe that at much above idle speeds no reasonable rudder could counter it.

In other words the boat will be uncontrollable and always try to point into the wind.

 

[bjn]

Why?

The force in the sail is perpendicular to the apparent wind direction.

On a beamy trimaran, it would not be impossible for the force to cross the ama forward of the mast. A board at the intersection point would give neutral helm, correct?

And the drag of the board might even create leeward helm, correct?

 

[oldsailor7]

It would depend entirely where the centre of pressure of the whole sail area is.
Usually it is at the same place or a small amount forward or behind, to give a very slight weather helm.

 

[olsurfer]

I am going to look at the pivot line on the rudder to see if there is about 15% of the rudder in front of the pivot line to help balance the helm. Also going to look at the center of pressure of the sailplan to see it's relationship in distance back from the foils. I have sailed the trimaran in question and I have never felt such a heavy helm, so much so that I'm not sure there is an auto-pilot strong enough to properly steer the boat. It also went into irons on me fairly easily, but got out of it by pumping the rudder. On the brightside, she is wicked fast and has done well in some races.

 

[oldsailor7]

Olsurfer, you are quite right. The rudder should be balanced in it's self and play no part in balancing the whole boat.

 

[Stumble]

You want the CLR behind the CE in order to give the boat a touch of weather helm. Moving the daggerboard forward of the mast is the exact opposite of this.

If the daggerboard is behind the rig, then you can always increase rake or prebend to pull the CE back to balance it out and reduce weather helm, but there is no good way to rake the mast forward past verticle to reduce it.

 

[PAR]

If you move lateral area forward, you can compensate with additional area aft, say a skeg addition or enlargement. Additionally you can play with rudder area to "load it up" a bit too, accepting some of the lateral area needs. Adding area aft will also help with the heavy helm, though this should be offset with the amount of incidence necessary, without adding a bunch of drag.

 

[Doug Lord]

Shared Lift

Arc 21 and shared lift concept by Bill Roberts: http://aquarius-sail.com/catamarans/arc21/index.html

 

[bjn]

This tri has the daggerboards well forward of the mast.
http://catri.catrigroup.com/catri-25-study-plans/

I actually asked a guy who sailed it about weather helm. He said it was "beautifully balanced".

 

[Doug Halsey]

Why?

The force in the sail is perpendicular to the apparent wind direction.

On a beamy trimaran, it would not be impossible for the force to cross the ama forward of the mast. A board at the intersection point would give neutral helm, correct?

And the drag of the board might even create leeward helm, correct?

The force on the sail is only perpendicular to the apparent wind direction if the drag is zero (i.e. never).
The lift is defined to be the perpendicular component. When you include the drag component, the total force is angled more aft.

 

[bjn]

That is true!
I didn't think about that.
So with an L/D between 5:1 and 10:1 the intersection will be roughly 5-10 degrees further aft.

 

[olsurfer]

This tri has the daggerboards well forward of the mast.
http://catri.catrigroup.com/catri-25-study-plans/

I actually asked a guy who sailed it about weather helm. He said it was "beautifully balanced".

I'll have to look closely at that Catri as the daggerboard position is the same as the tri in question. The Arc 21 and the shared lift idea is along the same line of thought as well. Thanks to all who have shared their wisdom with me about this! If these boats can balance out, maybe there's hope.

 

[Tom.151]

I am going to look at the pivot line on the rudder to see if there is about 15% of the rudder in front of the pivot line to help balance the helm. Also going to look at the center of pressure of the sailplan to see it's relationship in distance back from the foils. I have sailed the trimaran in question and I have never felt such a heavy helm, so much so that I'm not sure there is an auto-pilot strong enough to properly steer the boat. It also went into irons on me fairly easily, but got out of it by pumping the rudder. On the brightside, she is wicked fast and has done well in some races.

Pictures !

 

[olsurfer]

Would flying a chute move the sailplan CE forward enough to close the distance between the ce and the clr? Additionally making sure there is no rake in the mast might help bring the ce and clr closer together. Probably not a good idea to post pics as I am on thin ice as it is!