Steering Wheel on a Cruising Catamaran

Discussion in 'Boat Design' started by b_rodwell, Feb 4, 2007.

  1. b_rodwell
    Joined: Apr 2002
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    b_rodwell Junior Member

    I am preparing a design for a large (>50') cruising catamaran. I would like to get feedback on having only one small steering wheel - about the size used on a small runabout. The proposed steering details for your comments follows.

    . High aspect ratio kick up rudders (low steering effort).
    . Emergency steering directly off the rudder shafts
    . Rudders linked by direct mechanical (push/pull/rotate) linkages (no cables yet)
    . 2(for redundancy) x autopilot motors connected to the direct mechanical linkage
    . Cable linkage to one small steering wheel in a forrard (a la Atlantic & to some extent Gunboat) cockpit. The wheel is used in close quarters manouvering, berthing, anchoring and picking up moorings.
    . Joystick and autopilot controls at an internal navigation station at the forrard end of the bridgedeck.

    There are no plans to have steering functions in the rear deck area (apart from the emergency tillers).

    The reasoning behind this proposal is:

    .The very small amount of time spent behind the wheel in a cruising situation.
    .The opportunities to improve the layout for people if you don't need to plan for a large diameter steering wheel.

    I look forward to your critical comments.

    Brian Rodwell
     
  2. Tad
    Joined: Mar 2002
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    Location: Flattop Islands

    Tad Boat Designer

    Brian,

    First I'll say that the autopilot is going to break, so your manual steering has to be up to the job.

    To calculate required steering force at the wheel requires calculating the torsional moment from the rudders, divided by the power of the steering gear, including the quadrant and wheel. The larger the wheel diameter the smaller the power required, the most one person can exert is their weight on the edge of the wheel, This is not easy with a tiny wheel.

    Their are various mechanical systems (see Whitlock) available to reduce required power. All also reduce feedback or steering "feel". Some sailors dislike this, others don't care.

    Force from the rudder is based on speed squared, rudder area, CE arm, and a lift coefficient. See Principals of Yacht Design for a method.

    All the best, Tad
     
  3. b_rodwell
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    b_rodwell Junior Member

    When harbour sailing or racing, I like having some feel in the steering.

    For cruising, I don't expect to be on the helm much of the time. I am planning to use high aspect ratio spade rudders and I can put the turning axis wherever I like. I understand that the centre of effort for this type of rudder is roughly at 30% of the chord length from the front.

    In theory, if I put the axis there, then there would be no turning arm and no force at the steering wheel from the forces on the rudder. There would only be the friction. My inclination is to put the axis at about 25% of the chord length rather than 30% to ensure that I don't have an unstable configuration where the forces tend to increase the rotation. The forces with the axis at 25% should still be quite small. Are there flaws in this approach?

    The big single handed trimarans are not hand steered much. Does anyone no how they set up the rudders as regards the axis of rotation?


    Brian Rodwell
     
  4. Gerard DeRoy
    Joined: Jan 2004
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    Gerard DeRoy Junior Member

    I agree with what has been said to this point.
    -Autopilot breaks. Yes but not much if properly used. Sails balance and proper steering forces calculation are the main points.
    -Balanced rudder I agree and I am using the same around 25 % area forward of the axis to make sure I feel the rudder.
    - I am designing a rudder system at this time for the Azilda(my galery for details)
    -The steering wheel is inside the pilot house.
    -I do not want the autopilot outside.
    I want to use the simpler tiller autopilot that I have been using for so many years without proplem.
    - Do not like the one connected to the steering wheel with belt.
    - Want to use a cable mechanical system.
    - one option I am studying is to use the rack and pinion system where I could connect the autopilot cylinder right on the rack( travelling transversal to the boat).
    What do you think?
     
  5. b_rodwell
    Joined: Apr 2002
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    b_rodwell Junior Member

    I just referred to The Nature of Boats by Dave Gerr. He says not to go further than 20% for the axis of rotation - so 20% it is.

    Brian Rodwell
     
  6. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    Be Careful

    Hello Brian,

    I would urge caution in your rudder design. You can reduce power required to turn the rudders by reducing chord and making the rudders higher aspect. If you go too far your rudders won't work as well at stall ( coming out of a tack or resisting a broach on a wave).

    I don't really get how you will have a wheel out the front. Where will your sheets go? For tacking and such the wheel and the sheets must be together. Also for safety on a multihull I would be very wary of not having the wheel where I could instantly get at it and the sheets when resting in the cockpit. Unsafe cockpit design has led to more than a few capsizes.

    I don't think an autopilot will be up to the job on big waves when conditions get really rough. Even if it does you will need a good wheel to run the NSW bars. I would urge you to consider other ways to increase amenity in the accommodations. Compromising the wheel will compromise your safety.

    On top of this, having a steer with a good system is fun. I spend much of my time offshore under autopilot but my best days run ever (Coffs to Southport 160 miles in 16 hours breakwater to breakwater) was done hand steering 80% of the way because it was an absolute hoot!

    cheers

    Phil
     
  7. b_rodwell
    Joined: Apr 2002
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    b_rodwell Junior Member

    I note your point about too high in the aspect ratio. I heard a story of a racing cat built in Sydney with a very high aspect ratio dagger board. It was fine in a straight line but stalled when they tried to tack to the point where you couldn't tack. Also the complexity of the build increases as the ratio goes up. I guess I am thinking of no more than 3.5 to 1 as a practical limit.

    As regards the sheets etc, I was thinking of leading them all to the forrard cockpit. I would get the advantage of having no sail controls in the rear cockpit. One thing I didn't mention is that I have a door to access the forrard cockpit from the bridgedeck cabin. Same as Chis White's Atlantic and the 2 Gunboats. So one disadvantage is that if you can't change the sail settings by using an electric winch, then you would need to go into the exposed forrard cockpit.

    Brian Rodwell
     
  8. catsketcher
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    catsketcher Senior Member

    Maybe a different tack

    Gday Brian,

    I sailed on a cat that had incredibly high aspect rudders and daggerboards. It was impossible to tack and needed new rudders and boards when delivered to Queenlsand. Low aspect works well at low speed so it is a good idea to stay around 3:1.

    Have you thought of a different tack altogether? Our 38 footer has an open bridgedeck so that you get a great view from the cockpit and you can close it up when you want to. The Seawind 1000 is a good example of this. I love it and would never have a normal bridgedeck cabin. I can see very well and still have protection. Even though the 1000 is a very popular boat few designers have gone this road. It means trying to squeeze a little more room into the hulls with chamfer panels and the like but you end up with very easy to sail and live in boat. It would not be hard to try an design large slide away panels for the aft end of the cabin so you can see and get lots of room.

    I understand the Chris White idea but I like being behind a cabin when working to windward offshore and you lose a fair bit of room up front. If you can forego the leather interior and have a verandah style cabin the open bridgedeck can work very well.

    cheers

    Phil
     
  9. FAST FRED
    Joined: Oct 2002
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    FAST FRED Senior Member

    Why a wheel?

    Many launches are steered with a vertical tiller that has great leverage and takes up almost no cockpit room.

    Best is the simple tiller AP will be happy to operate with low loading , easy access , and really fast simple replacement.

    For most cruisers either the AP is steering or your in port , where quick steering is a bonus.

    FF
     
  10. catsketcher
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    catsketcher Senior Member

    Don't do it!

    Hi Fast Fred,

    I did put a vertical tiller on my 38 ft cat and got rid of it after one year. It is not easy to use and highly loaded on a big boat as you can't gear it down unless it gets so big you have to run around with it. Stay with the wheel.

    cheers

    Phil
     

  11. b_rodwell
    Joined: Apr 2002
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    Location: Sydney, Australia

    b_rodwell Junior Member

    Thanks for your feedback. It was very helpful.

    Brian Rodwell
     
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