Proposal for catamaran rudders

This is a rudder proposal for a cruising cat with 2 engines.

I like high aspect rudders for efficiency and control. I also like rudders to kick up on grounding or to get close to shore.

It is a little difficult combining these features. This is what Bob Oram designs:

http://www.boboramdesign.com.au/

I also like the low cost and simplicity of transom hung rudders - the foils are easier to make (like a dagger board) and the rudder shaft does not have bending loads. The bearings are less expensive. The difficulty is in making the rudder balanced or near balanced. There are solutions for this but they don't appear completely satisfactory.

My proposal combines some features from both these approaches.

Firstly there is a carbon fibre space frame to take the load from the rudder to the hulls. It is kick up by pivoting on a horizontal axis.

Secondly the rudder blade bearing is at the side of the rudder, not at the front. The centreline of the rudder blade is at a slight angle compared to the axis of rotation of the rudder. This is so the drag on the blade does not generate any net rotation moment on the rudder.

The attached diagrams illustrate the proposal. It is the starboard hull. I have shown it for a hydraulic set up but other means of actuating would also work.

In summary the advantages are:

Kick up
High aspect ratio
No rudder shaft in bending
Cheap bearings
Foil is easier to make.

I would be interested in your feedback

Brian Rodwell

 

Brian, looks pretty groovy to me, I like the pintle arrangement- that looks like a clever "ballance" device - is there something scientiffic in the amount of rudder CL angle offset for speed range? or would a link bar arrangement kind of set port in balance against stb?or is it that the "line of pintle" kind of averages the immersed length of the foil? the pivot up mechanism looks smart too in separating the hinge points heaps for better loading- looks more elegant & have greater steering range than the "cassette style"- I worry about the "pinch" points potential for injury in the picture link. I think the carbon sounds cool & may be "the best" in regards to the foil itself but think the triangle/ tetrahedron pivoting frame may be an easier fab in metal for integral bearing surfaces & attachment of steering ram. Looks nice to meregards from Jeff

 

Brian, I like your design. This is very similar to what I plan to do on my 11.9m catamaran. How far back are you placing your bearing?
Cheers
Andrew

 

more details on catamaran rudder proposal

The way I have drawn it, it can move +-30 degrees. It would not be too hard to increase that but I can't see the point. Attached is a pdf drawing looking from above. I have put in a slight amount of Ackerman angle in the connection.

The angle of assymmetry is, as you suggested, simply to balance the areas (actually moments) either side of the rotation axis. This would not work in the case of routinely flying a hull. But I don't intend to do that.

Stainless steel for the space frame would probably be cheaper and less work. I don't have welding equipment. But more important for me, is that I would expect the carbon fibre solution to be half the weight.

I put the axis 20% from the leading edge. This value was recommended as a mximum by Gerr in The Nature of Boats.

Brian Rodwell

 

You can get balance other ways

Gday Brian,

The set up shown looks good but I don't see why you have the hinge off to the side. If you use don't want dagger rudders because yours kicks up then you can have a cutaway so that the rudder cuts under the hull.

Alternatively you can use a straight dagger rudder if you like and then kick it forward. This is an easy way to get balance.

The other point of concern is the lift loading that one side of the frame will have at maximum loading. The great thing about cassettes is that they are easy to make strong. If this starts getting complex then it will probably be easier to make cassettes.

cheers

Phil Thompson

 

Very interesting rudder system. Have you gotten as far as the building of a prototype?

Chris

 

info on how to make the space frame

AndrewK!

I have some ideas on how the make the space frame from carbon fibre. If you are going that route, contact me offline and I will give you my thoughts.

Brian Rodwell (b_rodwell@hotmail.com)

 

No I haven't built a protoype.

I am keen for this to be built but I don't have a suitable boat to test it on at the moment. If anyone has the interest to be involved in this experiment this calendar year, then I am prepared to do the following for the carbon fibre spaceframe, rudder, shafts, bearings and tiller:

1. Complete the detailed design, costing, drawings and building instructions for their particular implementation. The only functional requirement is a sloped transom. I would not recommend this design on a vertical transom.

2. Publish all the above on this forum, to get critical feedback and thereby reduce risk.

I would complete this design work part time. I could finish it within 2 months.

I would ask in return is photos of the build and reports on the results.

If anyone has an appetite for this, contact me at:

b_rodwell@hotmail.com

Brian Rodwell

 

Cat Rudders

Hi Im interested in your design challenge. Im building a 12.5 meter 4000Kg pescott cat and want retractable rudders for all the reasons you state. Mark designs a basic underhull shallow spade as standard.

I understand that having the rudders under the hull increases effectivness for a given area. I think this has been achieved on later generation Open 60 (Beamy Twin rudder) mono hulls.

Has anyone seen how they execute this in detail? My understanding is that they have a v cut in the hull and the rudder casette has horizontal element which moves with foil to fill v cut.

Im also interested in shape suggestions I'm reviewing many suggestions in foil design on this site.

Regards Joe blaney

 

want a rudder design for 12.5 meter 4Kkg cat

[ The great thing about cassettes is that they are easy to make strong. If this starts getting complex then it will probably be easier to make cassettes.

[/QUOTE]

Im looking for material to help me build a kick up or dagger style rudder system do you have and pointers where I could get good design or build suggestions

Like all points raised in your comments

 

A cleaver design but two issues come to mind: 1) it seems to me that with two rudders, the slight angle of attack the rudders have would increase the drag, since both rudders presumably would be toe-in or toe-out to counter act the other, the increase in induces drag with the toed rudders would cost you some drag to allow the side hinge. Unless I am misunderstanding something here with your design (correct me if I am wrong), you have to have the rudder at zero angle of attack to free stream flow minimize the drag.

2) although the transom attached rudder is simple and allows easy access to the hinge, the problem is that is is less efficient (effective) because you lose the end-plate effect of the bottom of the hull, so some energy is lost by the wave making at the surface. One way to solve is this put a plate on the rudder that is at the water surface, that moves with the rudder. Of course this adds complexity and cost.

 

Toe out is ignored on catamarans because it would create useless drag when the rudders are centered. Ackerman is for cars, not boats

 

Brian,

Personally, I think that you have the rudder planform backwards. The curved trailing edge and the way the tip is shaped will likely have better lift and less drag if you flipped it front to back. The straight edge, now the leading edge, would work better as the trailing edge. Here is an example of the rudders that I have been designing lately for some fast monohulls, this one on Bagatelle, and all with very favorable results.

Eric

 

This has nothing to do with Ackerman streeing. allow me to clarify my question, it appears with the off-set hinge axis there has to be some slight angle on the rudder blade to prevent sideways pressure on the tiller. What it looks like to me that brian is proposing is that the rudder blade would have to have either toe-in or toe-out to counter the off-set hinge line. This would cause the extra drag.

Ackerman steering is not related to toe-in, you can have either toe-out or toe-in alignment and still have Ackerman streeting on a car. Ackerman geometry has to do with different amount of angle change on the inside wheel than the outside wheel since the inside wheel has to make a smaller turning radius.

I suspect there actually would be some benefit in a catamaran since the hull on the inside of the turn makes a different turning radius than the outside hull. I have never seen anyone address this on a catamaran, it seems to me there should be some benefit. I have noticed how sluggish and unresponsive most smaller cats I have sailed are on tight turns as compared to both cars and monohulls. I can not help but think this might be contributing to it. With both rudders making the same angle turn in a tight tack for example, each would be trying to turn the hulls through the same angle, which means there would be a huge drag penalty. Small tiller angles would likely have little effect, but large tiller angles necessary on tight turns would likely have a substantial drag penalty.

I am planning on building a small wood catamaran next summer and I was going test it out. Build an Ackerman steering rudder set-up to see if it would reduce drag in a turn (as in a tack) and make it more responsive.

 

It has been addressed and the conclusion is that it confers no benefit to a catamaran. Ackerman angle and toe-out are not mutually exclusive or completely separate things. Toe-out can be used to (roughly) approximate Ackerman angles in a steering system that lacks a sufficiently rising deflection rate for the minor radius side. This type of setup works well when only small deviations from straight are normally used, and the small drag penalty can be tolerated or even confers some other benefit like increased straight line stability. High aspect ratio rudders operate through only small angular deflections, but the toe out adds no appreciable stability to a boat and the drag penalty can't be tolerated. Remember that rudders turn a boat by creating lift thereby deflecting the stern, not by 'tracking' a different line through the water the way a tire does on pavement.

As you've noted, offsetting the operating axis of a rudder pivot laterally will result in the rudder deflecting slightly if allowed to trail naturally while under way. A corollary of this is that if such a rudder is held in what appears to be a straight, undeflected position, it will in fact be producing a small amount of lift in one direction. Allowing both rudders to settle in to their natural 'trailing' angle will then result in the least drag and no lateral lift. The rudders will not appear to be undeflected, though hydro-dynamically, they in fact are. To balance all forces and ensure equal lift potential in either direction, the offsets should be avoided, if possible, or at least opposed.

But none of this is really 'toe-out'; that's done to approximate Ackerman angles over small steering deflections.

Jimbo