Kick up catamaran redesign

[unbusted67]

I'm looking at a daggerboard cat with fixed spade rudders and am wondering about the feasibility of redesigning/refitting with kick up rudders.

Has anyone ever toyed with the design of having rudders inboard of the two hulls? If you designed them deep enough couldn't that conceivably give you as much purchase/steerage as the spades?

This particular cat has outboards, what about positioning the rudder behind the outboards for better steerage under power?

[44'cruisingcat]

I did exactly this to a Schionning a few years back. It had been built with daggerboards, outboard motors, and fixed spade rudders.

Didn't make sense to the new owner to be able to lift everything but the rudders.

I cut out the middle part of the bottom transom steps and converted them to swing up cassettes.

I also enlarged the rudders, as they were too small.

The whole job including painting took 3 weeks.

[Mr B]

Your twin outboards would give you plenty of steerage under power,
There would be a lot of work involved getting the rudders under the boat.

[valhalla360]

Google the Gemini catamaran rudders. It's been done and it's a good system but takes a bit of engineering.

If you could get hold of a totaled Gemini, you could probably steal the critical parts.

As mentioned, if the outboards steer, no significant advantage in putting the rudders behind them.

[unbusted67]

Originally Posted by 44'cruisingcat

I did exactly this to a Schionning a few years back. It had been built with daggerboards, outboard motors, and fixed spade rudders.

Didn't make sense to the new owner to be able to lift everything but the rudders.

I cut out the middle part of the bottom transom steps and converted them to swing up cassettes.

I also enlarged the rudders, as they were too small.

The whole job including painting took 3 weeks.

I’ve seen this done elsewhere on this forum. I guess I’m confused by the term cassette. can you explain to me what that means? how does the rudder shaft
junction with the rudder when lowered?

Originally Posted by Mr B

Your twin outboards would give you plenty of steerage under power,
There would be a lot of work involved getting the rudders under the boat.

Why do they have to be under the boat? presumably the further aft, the smaller the turning radius.

[Scaramanga F25]

Cassette rudders. See Design by Chris White.

Or: www.buildacatamaran.com for how to for pics. Building Kick up Rudder Cassette,

[Scaramanga F25]

Cassette rudders. See Design by Chris White.

Or: www.buildacatamaran.com for how to for pics. Building Kick up Rudder Cassette,

[Mr B]

Rudders inboard of the two Hulls, I would assume thats under the hulls,

[44'cruisingcat]

Originally Posted by unbusted67

I’ve seen this done elsewhere on this forum. I guess I’m confused by the term cassette. can you explain to me what that means? how does the rudder shaft
junction with the rudder when lowered?

.

The cassette is a section of the hull that contains the rudder shaft and tube which can pivot up, lifting the rudder.

Here's a photo of mine while I was building it.



The advantage of a wide cassette like this is that it allows the rudder to be steered while partly lifted, or floating. It also allowd the rudder to kick up even when not in the straight ahead position.

Also due to the width I was able to enclose the tiller arm and steering ram inside the cassette.

[Fore and Aft]

I surveyed 44’ Cruising Cat and his rudders are a good system. No reason why you could not retrofit them to an exsisting cat if you have sugar scoops. Even if you do not have sugar scoops they are fairly easy to build and then do the rudders.
I have surveyed a 30 footish Sailing cat that had just one rudder in the middle. The owner was in the process of changing it to twin rudders.
Cheers

[valhalla360]

Originally Posted by Fore and Aft

I have surveyed a 30 footish Sailing cat that had just one rudder in the middle. The owner was in the process of changing it to twin rudders.
Cheers

Good point. I think this got missed in the discussion so far.

A rudder hung in he middle away from the hulls would not benefit from the end-plate effect. The same size rudders would not have the same turning capability.

Plus structurally they are harder to do since the lever arm is longer.

[unbusted67]

Originally Posted by Scaramanga F25

Cassette rudders. See Design by Chris White.

Or: www.buildacatamaran.com for how to for pics. Building Kick up Rudder Cassette,

Having no luck with that. Can you provide me a more specific url?

[Zach]

Wider the cassette the better. I ran a catamaran for charter that had kick up rudders, and the cassettes were narrow enough that you could not steer the boat and have the rudder kick up even a few degrees. That meant that in shallow water you couldn't readily sail the boat, and meant motoring with the rudders straight driving like a skid steer to maneuver. Turning with one ahead and one astern and vice versa. Works fine with slack tide and current, but falling tide in tight places is not the best if you can't use your rudders to steer.



Make the cassette deep enough from the hinge/pivot point that you can gain purchase and lift the rudder. Make the tillers on the rudder hinge, so that you can gain clearance to lift the rudders clear of the water to prevent fouling and remove them from the boat if need be. Having a rudder that won't clear the water, and a cassette pin close to the water line means that even though you have kick up rudders you need to haul to be serviceable if the rudder stock hits the stairs before you get horizontal enough to deal get under them, unless you use a lift bag.



Keep all lines away from the cassette. About the worst failure method I found, was that a painter could drop into the slot with the rudders kicked and if you touched reverse gear the rudder would try to lever downward and jam the line into the slot tight enough that it was a two man operation to clear it. During which time you had no steerage.



I thought a pair of outboard motor cranes on above the cassettes would be handy for self-rescue as you could lift them clear of the water and hold them up with a block and tackle. Getting a purchase on a cassette from any winch on the boat doesn't have a fair lead or enough vertical purchase to be helpful unless you have a design intent. I wanted something that would hold things clear, as it is a pinch point for finger tips.



Rod ends don't have quite enough rotational freedom to hinge forward with the tillers forward of the rudders, without being the bind point.


Day dreaming one day, a delrin block inside a 2x2 piece of box tube with a 3/4 inch bolt through it would give a tiller end the ability to rotate as needed as the end connection of the tiller could spin 360 degrees, and meet the eye of the tie rod. Swivel-hitch coupling would be something to google, as they use them on motorcycle and off road trailers.



Our cassettes were not heavy enough to hold back the pressure at 12-14 knots. Ended up bonding a pair of eye bolts into the stern deck, and using wooden dowel rods as a fuse. Kick up the rudder, it breaks the dowel rod, but the dowel had enough ass to keep the rudder down at speed. So consider adding some lead weight to the rudder blade if you are neutrally buoyant or buoyant now.



Cheers,

Zach

[valhalla360]

Originally Posted by Zach

Wider the cassette the better. I ran a catamaran for charter that had kick up rudders, and the cassettes were narrow enough that you could not steer the boat and have the rudder kick up even a few degrees. That meant that in shallow water you couldn't readily sail the boat, and meant motoring with the rudders straight driving like a skid steer to maneuver. Turning with one ahead and one astern and vice versa. Works fine with slack tide and current, but falling tide in tight places is not the best if you can't use your rudders to steer.

Depends on the design. The Gemini cassettes are only about 3" wide but you can steer with the rudder lifted by any amount.

Obviously, the best steering with them fully down but even fully up, you still have a reasonable amount of steering.

[Zach]

Originally Posted by valhalla360

Depends on the design. The Gemini cassettes are only about 3" wide but you can steer with the rudder lifted by any amount.

Obviously, the best steering with them fully down but even fully up, you still have a reasonable amount of steering.

Good comment.



The blades on the one I worked on, pivoted not very far from the bottom of the cassette. It looks like the gemini's by photos have a few inches of cassette below the body of the hull to let you steer.