How to connect catamaran rudders

Discussion in 'Multihulls' started by MichaelRoberts, Dec 29, 2020.

  1. MichaelRoberts
    Joined: Sep 2015
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    MichaelRoberts Archimedes

    New years resolution: build the Archimedes 60 rudder and helm system.

    The twin wheels are on opposite sides of the bulkhead wall aft of the salon. Nicely sheltered from spray. The kick up rudders are transom hung.

    The plan is to Dyneema (new verb) from wheel hubs over sheaves to the tillers. Tillers are inboard, they pass through sealed slots in the transom.

    But how do I keep the two tillers parallel?

    Options are
    1. Hydraulic cylinders
    2. Dyneema from hull to hull through an existing conduit in aft beam
    3. A torsion rod through the side to side conduit. It could be rotated by the Dyneema from the wheels.
    4. A teleflex like cable ... the loads could be hundreds of kg
    5. Some sort of extension rods like on the IMOCA 60s

    Please I need some wisdom
     
  2. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    Option 3. Tie rod, set up as Ackermann linkage.
     
  3. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    Do you mean a parallelogram linkage? Ackermann linkage was originally designed for road vehicles to steer the front wheels on either side different amounts so that all wheels steer around a common point.
     
  4. Rumars
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    Rumars Senior Member

    As with road vehicles, catamarans benefit from having proper Ackermann linkage when turning. Most sport boats have one, because they can see the benefit. Cruisers seldom have one, but they are no tacking monsters. Justt google "Akermann linkage catamaran" you will get a lot of results, even from this forum.
     
    Last edited: Dec 29, 2020
  5. wet feet
    Joined: Nov 2004
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    wet feet Senior Member

    This may give you some ideas to work with.Basically a series of pull/push rods and idler arms with bell-cranks where you need to turn corners.If you have less curvature in the cross beams,you won't need as many sections.Your requirement for kick up rudders does complicate things a bit.IMOCA 60's use a central tiller with spherical bearings in the linkages and only use the leeward rudder for steering.Whatever you decide,there is much to be said for keeping things out in the open,both for ease of inspection and for ease of repair if necessary.
     

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    waikikin likes this.
  6. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    Gday Michael

    I went to your website to look at some pics and I saw two different versions of the rudders - one underhung and one on the transoms. I would worry about the engineering of the rudders hung on the small transom step, the loads will be very large. I would use a cassette here instead of mounting the rudder on the back of the transom. As well as being weaker it will also get in the way of the dinghy and swimmers. I also don't get why the boat has no aft platform and dinghy storage area. It looks like it has a very large box beam. I am not sure what the purpose of the large box is - inverted flotation maybe, I did this on my cat and then removed it - the chances of flipping are so low it is no use to have a void in a really useful spot.

    The reason I say this is because the tie system has to be part of the design of the back of the boat. I don't understand why your plans have a large number of steps up to the deck and back down to the cockpit. I would consider redesigning the cockpit area including the steering, aft beam, dinghy arrangement, swim platform and transom steps to better integrate steering and usability. I think the aft end of your cockpit/transoms needs a total reworking to make it much more usable. A targa for solar panels and the dinghy crane is a feature most cats have as well.

    An aft platform with a tie rod between tillers is the easiest arrangement. Installing an aft platform with a tie rod between tillers would be useful. When doing this I would also redesign the aft steps to better allow people to walk from one stern to the other when the dinghy is down, or for kayak storage.

    Don't think that my advice is to wreck your motivation, keep at it. I have built a few boats and I fervently wish that someone had taken me aside a few times during the builds and given unqualified advice. As it was I have had to modify my boats constantly and have spent many hours wishing I had done things differently the first time.

    cheers

    Phil
     
  7. MichaelRoberts
    Joined: Sep 2015
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    MichaelRoberts Archimedes

    Hi Phil
    Ooops the website is rubbish, no one is supposed to see it yet, haven't had time to tidy it up. I used one of those design team outfits and I got PURPLE, my least favourite colour. Oh well what can you expect for $400.

    Well yes, I understand all that stuff about transoms and dinghies and davits - but I want to be a bit tucked away from the ocean - I want the cockpit up and away from some rogue wave from behind. The salt water will rust my Zimmer frame.

    The transom steps are built - they lift up (with linear actuators) to reveal an engine room. If the top steps were to be eliminated then the gorgeous shower in front of the engine room would have no headroom.

    Anyway, it's too late. For better or worse all that stuff is built.

    The dinghy sits on a lifting platform which doubles as a jetty in the down position. I've got the hots for a Williams 3.85 jet tender because we will never be able to back this thing into a Mediterranean berth. It will have to anchor out in the bay.

    The transom rudders swing up in boxes with integrated tiller. There is a very strong carbon skeg supporting the lower hinge point and some serious lumps of wood bonding the 35 mm thick transom to the keelson and stringers. When It's installed I will test it for a lateral load of 1 tonne on the foil.
    Rudder boxes and tillers.jpg
    Yes there is a targa for a 3.5 kW array of solar panels - I just have found some 350 watt flexible panels 1 m x 1.05 m - inexpensive too. Why do I need so much solar? Tell you in next post.

    Phew is that enough
     
  8. catsketcher
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    catsketcher Senior Member

    Sounds cool. The closest arrangement to your I have seen close up is probably Fantasia - Andrew Stransky's fast 50ft cat. She also has transom hung rudders. She uses the Newick style system of a sideways mounted tiller that acts on a push rod that goes forward. This push rod could be attached to a track with spectra pulling it forward and back. So you would not need a tie bar.

    My first steering system used 11 pulleys and had no tie bar. It had a lot of friction and after the steering locked when surfing over a bar (about 50 metres off the Ballina breakwater) I went to a tie bar. But I feel a push rod could get the gear off to one side of the steps and inside your engine room easily. It also gets the pulleys and what not further forward so that the transom is cleaner.
     
  9. catsketcher
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    catsketcher Senior Member

    PS, as for Ackeman - my cats have ackerman but I sailed on a really nice fast cat with no Ackerman at all, but if you cut off your little tillers, you could get glue them back on with ackerman if you like.
     
  10. MichaelRoberts
    Joined: Sep 2015
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    MichaelRoberts Archimedes

    Rudder hydraulic system.JPG
    The autopilot hydraulics need to be incorporated
    This suggests a hydraulic linkage solution - with a separate Dyneema cable from each helm wheel to its respective tiller
    This simple hydraulic circuit using double acting cylinders might work
    Can't fit the type of equal displacement cylinder that has a rod out each end without using a bell crank
    The problem of the differing displacements on each side of the piston is solved by connecting like sides and having the cylinders pointing in opposite directions.
    Ackerman can be achieved by inclining the cylinders a bit forward - how much Ackerman?
     
  11. MichaelRoberts
    Joined: Sep 2015
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    MichaelRoberts Archimedes

    Completed hydraulic system for catamaran rudders.
    Few typo errors in above picture now corrected
    Peer review needed, any criticisms / opinions welcome
    What is not shown is the cable system - one cable from port wheel to port rudder, another from stb wheel to stb rudder. 6 mm Dyneema running over a few 75 mm black acetal sheaves. A tensioning device takes up the slight change in length (6 mm) as tiller swings from centre to 35 degrees.
    Thank you for your comments
    Complete hydraulic system for rudders.JPG
     
  12. Russell Brown
    Joined: Jul 2012
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    Russell Brown Senior Member

    Looking forward to the cable system drawing. I just did one for a power cat and had to use SK 99 dyneema as the lower grade had too much stretch and added friction.
     
  13. MichaelRoberts
    Joined: Sep 2015
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    MichaelRoberts Archimedes

    Thanks for all the advice
    Think I'm going all hydraulic for simplicity of installation and hopefully improved reliability
    Here is the hydraulic circuit at concept stage - a few check valves not shown
     

    Attached Files:


  14. tspeer
    Joined: Feb 2002
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    tspeer Senior Member

    Here is how the water rudders are rigged on a Dehavilland Beaver's floats. The bellcrank on the air rudder (upper right) acts as your wheels. What may not be immediately apparent is the steering cable forms a loop that runs from the air rudder forward under the fuselage, down to the float and aft to the water rudder tiller, then from the opposite side of the tiller forward and over to the other float's rudder, and finally from that tiller back to the tail.

    You can do this as one continuous cable, with the tillers clamping onto the cable. The cable would also run between and through the twin wheels. This has the advantage that you can tension the cable independently from the alignment of the rudders. Once the cable is tensioned, the rudders can be aligned and the tillers can be attached to the cable. The tillers can face forward, or they can be crosswise like the ones on the Beaver. In the case of crosswise tillers, the cable can run through one end of the tiller, along the tiller to the other end, and then continue on; with the clamp being in the middle. Or, for security and redundancy, clamps can be placed at both ends.

    If you use separate cables, then every time you tension one cable it affects the alignment of the rudders and you end up chasing the tension from cable to cable. The continuous approach avoids this. It also avoids having to use hydraulic systems, etc. An autopilot can attach to the part of the loop that connects the two rudders, or between the wheels, or anywhere, really.

    BeaverFloatRigging.png
     
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