Zebedee's rudder

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  • 11 Dec 2015 01:35
    Reply # 3692816 on 3666132
    Anonymous

    Many thanks for that. It's a fascinating topic. The race will be really interesting.

  • 04 Dec 2015 18:23
    Reply # 3675230 on 3668618
    Deleted user
    Alan B Martienssen wrote:

    Many thanks for the plan!  I may, initially, simply thicken the existing rudder, doing an aft extension on a plywood boat is not the same as welding something onto steel. I'm sure it can be done, but it's a much bigger job. The effect of my ssimpler job  will be measurable by sailing with La Chica.


    This could be done fairly easily by laminating a number of layers of timber onto the back of the existing stern structure, much as rudder would be laminated up. Ideally there would also be several layers of double bias fiberglass cloth brought back over the existing hull structure to further tie everything together. The project would not be technically difficult, just involve some work with epoxy and fiberglass. If I was doing the conversion I would not do the extension aft from the keel to support the rudder, because really this will not do a lot to strengthen things, cantilevered so far out and aft of the keel. I would go for a stern hung rudder with balance area forward below the waterline, or build in a shallow skeg with the stern modification so as to get the lower rudder pintle further down so as to provide more support for the rudder. 
    Last modified: 06 Dec 2015 01:58 | Deleted user
  • 03 Dec 2015 19:21
    Reply # 3673194 on 3673173
    Paul Thompson wrote: Zebedee almost certainly will beat LC unless it's a predominantly windward course as it was last year. Then I'm under no illusions, LC will loose as we cannot win against Zebedee in a drag race, she has more sail area and a longer waterline.

    And neither of us can hope to beat Tystie, no matter what the wind direction. So what would LC like? Lots of windward work and light winds (6-10kts) :-)



    The race almost always has a huge amount of windward work, unless the wind shifts against the forecast.  Zebedee has the same waterline as LC, I believe and with her long keel, and hard chine has a fair bit of drag, too.  I thought LC had about 600 sq ft on her sails! 

    Don't be too sure about Tystie, either: David's just built yet another new rig and will still be tuning it.  I expect it will be a great joust between the three of you.  Make the best of it: you'll have SibLim to worry about next time!

  • 03 Dec 2015 19:14
    Reply # 3673173 on 3671825
    Annie Hill wrote:Looks like Zebedee is determined to beat La Chica at this year's Tall Ships!
    Zebedee almost certainly will beat LC unless it's a predominantly windward course as it was last year. Then I'm under no illusions, LC will loose as we cannot win against Zebedee in a drag race, she has more sail area and a longer waterline.

    And neither of us can hope to beat Tystie, no matter what the wind direction. So what would LC like? Lots of windward work and light winds (6-10kts) :-)



    Last modified: 03 Dec 2015 19:15 | Anonymous member
  • 03 Dec 2015 19:03
    Reply # 3673149 on 3672286
    Arne Kverneland wrote:

    Paul,
    I am not with you here. Flat plate keels, sails or whatever, surely produce lift. They just produce much less lift for a given area. La Chica’s new rudder, with her good hydrofoil section, surely must be very powerful in its own right, but the added distance from LC’s turning point (somewhere around the middle of the wl?) also means that the rudder has an easier job in producing the required turning moment. The end result is that you get away with lower rudder angle, which means reduced drag.

    You say that you not only go faster to windward, but also point higher. Remember then, if speed goes up with 10%, the “square rule” means that the lift of the keel, rudder etc. goes up with 21%, or in other words, you get the needed lift (of the keel + rudder) with less leeway.

    Remember, leeway is the keel’s angle of attack. No lift without leeway.

    Arne

    PS: Dirty keels or rudder: Yes, definitely! My Johanna simply died to windward with only a tiny bit growth on her fin keel. No surprise then that glider pilots clean and polish the wings before competitions.

     


    Arne, sure no disagreement here and all factors you mention do come into play, I was speaking in relative terms. Obviously the keel is developing lift, even a flat plate will do that but in comparison to what a correctly formulated and built foil will do it is relatively small. For the next improvement for LC, I'm going to see what I can do to improve the keels entry, some work there may reduce drag and improve lift... I will however stop short of replacing the entire keel :-).
    Last modified: 03 Dec 2015 19:04 | Anonymous member
  • 03 Dec 2015 08:44
    Reply # 3672286 on 3666132
    Anonymous member (Administrator)

    Paul,
    I am not with you here. Flat plate keels, sails or whatever, surely produce lift. They just produce much less lift for a given area. La Chica’s new rudder, with her good hydrofoil section, surely must be very powerful in its own right, but the added distance from LC’s turning point (somewhere around the middle of the wl?) also means that the rudder has an easier job in producing the required turning moment. The end result is that you get away with lower rudder angle, which means reduced drag.

    You say that you not only go faster to windward, but also point higher. Remember then, if speed goes up with 10%, the “square rule” means that the lift of the keel, rudder etc. goes up with 21%, or in other words, you get the needed lift (of the keel + rudder) with less leeway.

    Remember, leeway is the keel’s angle of attack. No lift without leeway.

    Arne

    PS: Dirty keels or rudder: Yes, definitely! My Johanna simply died to windward with only a tiny bit growth on her fin keel. No surprise then that glider pilots clean and polish the wings before competitions.

     

    Last modified: 03 Dec 2015 08:45 | Anonymous member (Administrator)
  • 03 Dec 2015 02:29
    Reply # 3671825 on 3666132
    Looks like Zebedee is determined to beat La Chica at this year's Tall Ships!
  • 01 Dec 2015 19:01
    Reply # 3669569 on 3666132

    Arne, yes I do believe that cleaning up the flow has something to do with the increased performance of LC's rudder that's also why I kept a reasonable gap between the keel and the leading edge on the rudder. Also it is critically important to maintain the required foil section, even small deviations from the correct shape can have relatively large effects on the lift generated.

    For example, if the rudder becomes dirty with just a light amount of fouling, I notice it immediately and you can see the flow is visibly disturbed. You also notice that she needs slightly more helm.

    I suspect that as the only efficient foil on the hull (I doubt that the keel develops a great deal of lift)  the new rudder may not only be more efficient at steering the boat but is also helping to lift it to the windward.

    What is notable when Zedebee and LC beat to the windward together is that LC is not only sailing faster, she is also pointing higher. This was not the case before I made the rudder changes, then Zedebee had the edge.

    Presumably LC is sailing faster because with the rudder mostly nearly centred there is a lot less drag but the fact that she is also pointing higher seems to imply that it's helping to lift the boat to the windward as well.

  • 01 Dec 2015 09:04
    Reply # 3668714 on 3666132
    Anonymous member (Administrator)

    I have begun to wonder if there may be another factor behind the improved steering and balance of La Chica, with her new rudder.

    The keyword is boundary layer: I once poked a stick vertically into the water behind the stern of my Johanna, while sailing at full speed. I then noticed that I could poke it 10 – 20cm down until the water really grabbed it. This was my first practical experience with the boundary layer, which until then only had been a foggy theory to me.

    The rudder sitting right behind a long keel must also work in such a slowed down water flow and thus be less efficient. The new La Chica rudder has now been moved so far away from the hull that the surrounding water most probably has brought the water flow behind the keel back to nearly full speed. Since hydrodynamic lift varies with the square of the speed, even a moderate rise in water flow will result in a significant rise in lift.

    Just a thought.

    Cheers, Arne

     

  • 01 Dec 2015 07:29
    Reply # 3668618 on 3666132
    Anonymous

    Many thanks for the plan!  I may, initially, simply thicken the existing rudder, doing an aft extension on a plywood boat is not the same as welding something onto steel. I'm sure it can be done, but it's a much bigger job. The effect of my ssimpler job  will be measurable by sailing with La Chica.

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