Best Drop Keel Design

Discussion in 'Boat Design' started by charliep, Sep 19, 2011.

  1. charliep
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    charliep Junior Member

    I am looking for the best designed lifting keel / drop keel boat in a 25' - 28' range. My boat is 26 feet long, 3600 pounds displacement of which 1600 pounds is the drop keel. Unfortunately the drop keel is not very easy to get up and down. I would like to re-engineer the entire contraption with something which can be more easily raised and lowered.

    In particular I would like to find a design which could have multiple heights. Right now the system only has two heights: up and down. I'd like to find a system with a height in the middle to allow sailing in shallow water or faster sailing in more moderate winds.

    I will do all the work myself but I am just wondering what good systems are out there for a drop keel of this weight. Thanks for your suggestions.
     
  2. charliep
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    charliep Junior Member

    Wow! The silence is deafening.

    Are there any boats out there where the lifting keel / drop keel is designed to be set at multiple positions? Basically I would like one position all the way up for trailering the boat, one position half way down for rivers and streams and one position all the way down for normal sailing. We did this all day long back in the day with the dinghy or 420. Now I want to do it with a 1,600 pound keel.

    The present design calls for a pin to go into the keel when it is all the way down. When the keel is all the way up the winch rope holds it up. The keel is designed in a box so that it is not exposed whether it is up or down.

    I have been reading a lot about keels falling off boats so I understand that the base for the keel must be very strong and that the rope holding up the keel should over compensate for the weight of the keel. I am just looking for a better existing system which is working with three positions right now.

    Perhaps this post can just become a list of boats with drop keels and with pictures of their mechanisms. Right now I have found the following boats with drop keels:

    Hobie 33
    Hunter 26
    Flying Tiger 7.5
     
  3. tom28571
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    tom28571 Senior Member

    As remember, the Hobie 33 could be difficult to get up and we only did so when the boat was stationary. My own Grand Slam 7.9M (26 ft) had a 600# daggerboard with 1050# inside ballast and could be raised when sailing but was not such an easy task. 1600# lifting keel sounds like another can of worms depending on whether it is a daggerboard or a forward pinned type like the Chrysler 26. The Grand Slam had a molded socket for the crowned upper part of the DB to fit in so it could not crash through the boat bottom if dropped. Never had that happen but it would have been traumatic even if no damage was done. 1600# is another level of force to deal with. I modified mine for a four part system with a deck winch and a 10" handle on that. Even with that power available, it took some effort to raise it.

    I have no idea what limits the height adjustment on yours but suspect that the weight is beyond "reasonable" human capability and a mechanical system is required. I have an elevator that works with a long screw that takes less power than most reasonable pulley or drum systems would need and provides automatic braking besides. Such a screw would be available from the wheel chair lift people.

    Good luck with the project.
     
  4. PAR
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    PAR Yacht Designer/Builder

    The problem Charlie, is you need some engineering preformed. Looking at different approaches is fine, but isn't going to tell you much other then the engineering approach, some of which will be more clever then others.

    Most of the drop keel production craft, have lousy stability curves with the keel raised and then there's a purchase issue in the partly raised position. It could be as simple a pin or a ratcheting lifting assembly, but again it an engineering issue that needs to be properly addressed, or you could have huge issues.

    With the posted generalities about your boat, there's little one can offer. Laminate schedules, design approaches, hoisting options, all need to address spicific loading situations.

    In short what boat do you want to make this modification on? Year, make and model, plus optional equipment, such as the tall rig or what ever? Do you have plans for it?
     
  5. fcfc
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    fcfc Senior Member

    Drop keels:
    http://www.sylvanayachts.com/
    http://www.simpsonmarine.com/en/new-yachts/new-yacht-details.aspx?id=64
    http://www.dixdesign.com/28didi.htm

    Beware, beneteau has stopped production of the 27.7.

    Seems that the drop keel is not a good solution. Current trend is the swing keel like http://www.pogostructures.com/?m=7&s=1&l=fr

    keel rotation axle is at the bottom level. And the keel box is fully closed, so structurally way better. And easier to hide in the interior layout.

    The keel wil lift itself in case of grouding. So reduced loads on the system.

    Ballast is somewhat lighter than with a drop keel, as you have no length for the bulb, but draft a bit higher to compensate for this, and for narrower chord.

    The narrow chord is needed to limit the up draft. BTW, when keel is in up position, keel CoG is still way lower than with a drop keel. So better stability.

    You do not need a lift AND a locking mechanism. The pogo only has an hydraulic ram with a pressure limiter. Manual pump, electric on option.

    The only problem with swing keel is that the additional draft is letal with most rating rules. High draft with only moderately efficient keels (high thickness in the lower end, to still have some volume for the ballast) is a safe way to NOT win races.

    Other drop keeler http://sailracewin.blogspot.com/2010/10/m34-recent-images-plus-video-of-dean.html
    http://www1.ft10class.info/boat/FT10INTERIOR.jpg
    But again, both with serious race intent.
     
  6. charliep
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    charliep Junior Member

    Par:

    I am an engineer and while not anywhere close to being a boat designer, I do understand the importance of a lot of the information you mention. The point is I would like to find out whether such a system exists today and how it functions. If it is impossible to design then there is not too much point in trying to engineer something which others have tried and failed.

    Do you know of any such system today?
     
  7. Petros
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    Petros Senior Member

    I think what you are doing is a great idea, keels can usually be improved quite a bit. However if you redesign your keel you will not be able to race it in club racing since it will no longer qualify under the current rating rules. If you do not care about that than you have a lot of options.

    One idea already noted is that you put ballast below the floor and use a lighter keel. Separating the ballast from the keel would make the keel much lighter and easier to lift, so you can use a much more simple way to lift it. I do not see why you can not place another hole in the side of the keel where the locking pin can be installed in the half way down position, giving you your intermediate keel position.

    IF you design a lighter but deeper keel with a bulb on the end you also can get by with much lighter keel. The extra depth and weight concentrated on the end would give you the same righting moment. It will also be faster since you will have less weight to carry around. I would also consider a new wing keel design, it will give you a much deeper effective keel without the depth, this would be useful in your partially deployed position for shallow draft. Both of these you should get professional guidance or you might create something unsailable.

    Another option is to redesign the keel with a fixed shoal draft keel, at the sacrifice of windward performance. IT would greatly simplify the design and reliability, less mechanism to maintain and you would not have to raise and lower it to get it on the trailer (this may take some altering of your trailer).

    I do not like the performance penalty associated with a swing keel design, I like the simplicity of a dagger board (though you must have a reliable means to keep it in the trunk or you could loose it). However one as heavy as yours means some mechanical way of lifting and stowing it. Does it use block and tackle to raise it now? How about converting to electric or crank screw drive? Or even a simple large lever arrangement?

    I am also not sure the perceived benefit of less risk to damage when the swing keel is struck or run aground. I have owned both dagger board boats and centerboard boats that have been run aground or struck objects. the centerboard with the swing keel designed suffered damage to the trunk, the daggarboard suffered only minor damage to the LE of the keel and easily repaired. These were on much smaller lighter boats, but the same risk applies to striking objects with either design I think.

    Whatever you do keep it simple! Most sailboats have become Rube Goldberg devices, way too complex and it takes all the fun out of recreational sailing. The few parts your redesign has the less cost and maintenance it will have in the long run.

    Good luck.
     
  8. WhaleBird
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    WhaleBird New Member

    I joined this forum to talk about this very subject, so thanks Charliep for starting it. I am hoping people visiting this thread will provide links to pictures of larger boats (e.g. bigger than dinghies) featuring moving keels (center boards and dagger boards). Swing keels (canting keels) and water ballasting is not of interest to me as they are too radical (e.g. dangerous).
    I think every dagger board I have ever touched was stuck in it’s well, swelling locked it tight. On the other hand centerboards always worked but these boards were not very heavy (~30 lbs). Some of my friends beached their Wayfarer’s on a shale beach for a picnic, later they could not lower their centerboards because small stones got into the well and jammed against the board. It was very challenging to get the boards down so they could sail home!
    Check out those IMOCA OPEN 60 Sailing boats ( http://yachtpals.com/foncia-michel-desjoyeaux ) with two carbon fiber dagger boards. I wonder if those are weighted or just provide lateral resistance. And I wonder how they are adjusted up and down I sure would love to take one of those for a spin!
    I am thinking two hydraulic pistons (self equalizing) to lift a weighted dagger board up and down with infinite adjustment. Oil does not compress so adjust the keel to whatever level then close a valve on circuit to effectively lock it. The mechanism should allows the dagger to lift several inches (perhaps a couple of feet) higher than the keel line to prevent debris from jamming the works. Oonce floating again, a quick flush with a water hose would help clean out the space before lowering the keel. The vessels CG with keel fully retracted would have to be calculated. The mechanism would have to be designed so as not to be damaged if the keep recieved a heavy load (grounding at speed)...
     
  9. WhaleBird
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    WhaleBird New Member

    Here are 6 pictures of the biggest centerboard I have found so far, the boat is around 50 ft LOA and I believe the boat is somehwere in France. It shows a decent cable and pulley arrangement that looks like it might be 1:4 reduction. The centerboard trunk takes up a fair bit of space because it extend all the way to the cabin top. The top is open giving very good access for cleaning/inspection. Maybe you can stow the fenders in there! The boat is steel or aluminum making this kind of project much easier to incorporate.
     

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  10. WhaleBird
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    WhaleBird New Member

    Here is another sail boat high and dry waiting for the next tide. Notice how the rudder is designed to take the bottom, strong skeg. Actually there's more going on than just a strong skeg, it looks like the rudder pivots but I cannot see exactly what's going on. Anyone have some idea's?
     

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  11. fcfc
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    fcfc Senior Member

    No. There is NOT.

    As I have said in my previous post, drop keels are nowdays only used for racing boats. Where performance without rating penalty is premium. The racing men will accept all other limitations and complexity of use for this.

    Cruisers that do want a practical system, easy to lift, low maintance, all use a swing keel.

    Sailing performance wise, drop keels and current performance swing keels (a la pogo) are similar, but swing keels do have a rating penalty due to the fact they need more draft than drop keels.
     
  12. tom28571
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    tom28571 Senior Member

    I agree that swing keels are far more practical than daggerboards on large boats. The lifting effort is far less and the mechanism is more reliable. Having had both systems and sailed several more different ones, I consider the daggerboard for high performance and the swing keel for practicality. Dick Carter designed some fairly large swing keel boats in Europe, French I think. I saw one of those in a boatyard and it looked very workable.

    I never have had the big problems with jamming daggerboards though. On smaller boats that was avoided by placing the DB a bit off center and a keel strip on center to prevent crap from plugging the slot. On the larger DB boat, the slot was only snug at the hull bottom so there was not much room for debris to get in and little surface for it to lodge on if it did.

    For many sailors in thin water, some sort of lifting keel, CB or DB is the only good choice unless you like shallow draft keels with wings, which I consider worse of all worlds for running aground.
     
  13. Eric Sponberg
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    Eric Sponberg Senior Member

    Charlie,

    I have designed a few lifting keel boats, Bagatelle, and Saint Barbara. Bagatelle had a block and tackle lifting crane that was situated above the slot in the top of the deck house. The owner later changed to a fixed keel for reasons other than the lifting feature. Saint Barbara's lifting keel was originally going to be a hydraulic and reverse tackle system. The hydraulic cylinder laid fore and aft on the berth next to the keel trunk, and it pulled on the multiplier end of the block and tackle, so 1' of travel of the hydraulic cylinder gave 4' travel on the lifting cable. The owner discarded that system because the hydraulics always leaked, regardless of the fact that he had the best aircraft hydraulic hardware installed. Now he uses a block and tackle system and crane that leads to a deck winch.

    On the Scandinavian Cruiser 20, which is based on the earlier SC 40 preliminary design I did for SC, the keel lift is by a special halyard attached about halfway up the mast. You set the keel either full up for trailering, or full down for sailing. Conceivably, you could pin it at just about any level if you designed it that way.

    Also, look at Hake Yachts' Seaward line of boat and their Smart Keel system. They build a 26'er, 32'er, and have just come out with a 46'er, for which I designed the hull lines. The company owner, Nick Hake, designed his keel lift system. It is an electric motor and cable system on a 2:1 wire tackle. This is about as simple as it gets.

    That should give you some ideas.

    Eric
     

  14. michael pierzga
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    michael pierzga Senior Member

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