Well I can tell you from that drawing that something dramatic will happen to the helm. Just as it will if you tilt it back the same amount. It doesn't show the relationship between the center of the pivot point and the amount of blade is in front or behind. I've spent enough time messing around with various rudders and castings on the boat trying to find nirvana. I remember something about the "Aussie" way of setting up the rudders on a N6.0. Kicked way under. If what you show doesn't change anything why is it done? Why does Robbie change his?

To go back a bit none of this matters if the sail plan isn't balanced with the center board. I set my mast rake to get the boat to sail to weather with the jib and main sheeted firm and the traveler out a few inches and the rudders up. When I travel in the rest of the way the boat heads up. I gauge how much load I put on the rudders by how far out on the traveler is when I'm sailing without rudders to weather. Then I adjust the rudders to get the feel I want. Is this right - wrong? It works for me.

For what it's worth the Nacra rudders are seven (in the water) inches shorter than the Mystere's. The boat steers like a sports car with power steering and only time I ever get lee helm is when I try to go upwind with the chute.

This is my point exactly. If the sail plan is balanced with the hull/foil combination, the rudders are only needed to add fine control...and of course major control in tactical situations. I think the better the balance, the smaller the rudder needs to be. Hence the NACRA "pin heads".

The problem is, down wind with a spinnaker, the sail plan is in a different balance configuration, so more rudder influence is needed to maintain control. In fact, with the spinnaker moving around so much under sheeting input, the balance centers are constantly changing. You need more rudder input to make course corrections. Y'all know what happens when the sail's steering power overcomes the rudder's ability to compensate!

Perhaps we need a computer and trim winglets for better off-wind control.

Adjusting rudder area ahead or behind the pivot affects "feel" on the helm. Too far back and it "feels" heavy, especially when the sail plan is not balanced. Too far forward and you go "over center" and it gets away from you.

<<<<<Controls are sometimes dynamically balanced to assist the pilot to move them. By having some of the control surface in front of the hinge, the air striking the forward portion helps to move the control surface in the required direction. The design also helps to counteract adverse yaw when used in aileron design.>>>>

Sail boats are just like airplanes in that the better balance you can achieve without control surface input, the lower the drag. Then the control surface is only needed for major corrections.

I don't have time for another sketch - but the center of lift does not move on the rudder (not significantly anyway) with the angle that it flows through the water. This is why raking the entire rudder system forward or back does not affect the helm. When you rake the rudder in relation to the pivot axis, then it affects the helm because you are moving the center of effort of the rudder in relation to the pivot.

However, the efficiency of the rudder (or wing) is greatly affected when you move the whole thing forward or rearward. For example, visualize the difference in the straight(ish) wing of an aeronautical heavy lift cargo plane and the swept back angle of a fighter jet's wing. If the rudder shape / draft line, is perpendicular to the water flow, the water flows around the foil as the foil was drawn / designed. If you angle the rudder forward or rearward in relation to the water flow, this has the effect of stretching out the designed foil shape. I can only imagine that the likes of Robbie change the angle of the gudgeon's to keep the foil shape flowing through the water at it's designed angle or to perhaps slightly elongate the foil profile....some pretty dramatic changes happened to the Tornado (I assume you have a boat that was built pre-spinnaker). Perhaps they found that those foils work better at a more swept angle (sweeping forward or rearward has a similar affect).

Hi all,

from a technical point of view, Jake is right about the sweep effect on feel (or correctly named: the hinge moment), if the axis is swept with the rudder blade.
Sweeping the whole rudder + axis affects the distribution of lateral area (smaller effect) and the flow over the blade. Sweeping backward is good for highly tapered foils in terms of drag but worse in terms of maximum lift and vice versae.

However there is one common misunderstanding in one of the posts: Aircraft have no minimum drag, if they are 'balanced' (e.g. without control surface applied). And in consequence the deflection of a rudder of a sail boat does not necessarily create more drag (as long as the deflection is reasonable small). That is because the rudder is in the wake of the daggerboards and if deflected in the right direction, there is an interaction, which creates a pushing (forward) force. Sorry, no pictures at the moment, because I have to go for sailing now.

Cheers,

Klaus

I think we should all agree with Jake's original assessment: that raking the hinge angle doesn't change feel on the helm as much as moving rudder area forward or behind the hinge centerline.

Thanks for all the advice!

I raked the mast a bit this weekend. The wind was very light, too light to get the feedback for a decision about grinding on the rudder blades. But that's the way I'll go if I do need to rake them.

I've attached a couple of photos. Not the best due to the smoke from all the wildfires, but they're all I have for now.

Maybe I'm missing something. When you say "grind on the rudder", what part of the rudder are you talking about?

Right now the rudders are fully raked, which means that the rudder is touching the casting. In order to get more rake I'd have to grind the rudder in that area.

Actually, because the rudders have been fully raked for a while, they've already started the process just by wearing against the casting in that area.

Rhino--I just drove over Donner Summit on Sunday and could not believe the smoke! It smelled like a BBQ outside and I could barely make out Donner Lake from I-80. I hope relief comes soon!

It smelled like BBQ in the Donner Pass? Did you stop for a snack?

I know, I know - eeewwwwwww! <img src="http://www.catsailor.com/forums/images/graemlins/tongue.gif" alt="" />

Yeah, always makes for some good travel conversation with the kids...

"So, kids, ya ever hear of the Donner Party...well the story goes like this..."

It hasn't been that bad on this side of the hill. The Chico guys have had a rough time of it, though. Jason (Nacra 5.8 with red bows) came racing last weekend after getting evacuated. Fortunately it looks like the firefighters held the line at the Feather River and his house is now safe.

Right, well as you rake forward you increase the load on that very spot. Like forward swept aircraft wings. Ever go canoeing? Ever sit up front and stick your paddle out front to bow steer? So does it make sense to remove material from that spot?

I suggest you experiment a little before you start grinding. Try spacing the top back and see what happens. It's easy to go back.

The way I see it there is three ways to to rake rudders forward. Grind the lower part of the casting or rudder, move the pivot point or space the top back in relation to the bottom. What's the easiest?

Rudder rake is a feel thing. It won't fix a lee helm problem.

Just wondering, does your jib overlap? Have you tried rolling up the jib with the chute up, if it does?

I'd be only grinding in the area where the rudder touches the casting - essentially cutting a slot in it. That part of the rudder should never be in the water, at least on a Nacra 5.8.



I understand that. I also understand that actual lee helm on a cat with a spin can't be fixed - you're putting a whole butt load of extra sail way up forward. What I want is for my boat to feel like the factory spin cats.

Changing the pivot axis relative to the boat but not relative to the rudder blade itself won't make any difference in the feel of the helm. I think Jake's diagram settled that question. If anything it would increase the load on the rudder slightly by moveing the center of lateral resistance slightly forward.



The boat has a self-tacker and a fully battened jib. So no roller furling.