Looking for advise on Lifting Keel Support

Discussion in 'Boat Design' started by Ooks, Feb 1, 2016.

  1. Ooks
    Joined: Oct 2013
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    Location: Brisbane, QLD, Australia

    Ooks Ian

    I am working on launching what is easiest to describe as an oversized trailer sailer. It is a custom build and design - I have spoken to the designer, but he walked away from yachts some 7 years ago when he finished the build of this for health reasons and has no records of it. She has been through a few sets of hands before mine so it is a bit of a risk in terms of the design etc. I knew all this when I bought her, and so far I am quite pleased with everything but I have some reservations about the keel and would appreciate some advice.

    It has a weighted retractable keel of 800kg - profile below waterline and metal scaffold above. So firstly how would I estimate the maximum load this will create when the hull slams down? is there a rule of thumb - 2-3G??

    the keel box is a straight box section so if whatever I use to support the keel gives way there is nothing stopping it save the seabed - so I need a system to hold it in place. I am alternating between
    Aluminium I beams through the keelbox and adding a support ring beam around the box to take the load or
    using straps or dyneema rope to transfer the load to the deck and allow the entire keel box structure to carry load.

    in either case I am planning on being fairly conservative - probably 4 supports each sized to take 100% max load. Was originally leaning towards the beams through keelbox option, but am hesitant to cut holes in it.

    So before I get to any more detailed questions - has anyone got an informed opinion if either of those options is preferential.

    A few people I mentioned this too got worried looks when I mentioned the dyneema option, but I am not sure this is just because it is different rather than a bad idea. The breaking strain on 10mm Dyneema is huge ~10,000 kg (and I will use 4 of them.
    The keelbox is 1800mm high and 1100mm.
     
  2. Ooks
    Joined: Oct 2013
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    Ooks Ian

    I did a bit on online searching and found a research paper around slamming of Open 60 ocean racers and the impact on canting keels. It was a bit more info that I could digest, however they performed some data logging and decided that peak loads are generated by a 4.5G deceleration during slamming in a vertical position.

    link here if anyone is interested:
    http://www.gurit.com/files/documents/Measurement_of_accelerations_and_keel_loads_on_canting_keel_race_yachts_V3.pdf

    my boat, being much smaller, slower and bouncing off smaller waves should realistically not see these loads, but if I use this as a starting point I want something that will support 4.5 x 9.8 x 800kg = 35280N. (or 3600kg).

    There seems to be a reasonable number of options to support this load to the deck (ropes or slings etc). To me this aspect of it seems reasonable. The supports are quite visible, easy and fairly cheap to maintain/replace on regular basis and if I stick with my 4 x layers of redundancy should be quite robust. Also there is a winch and 10mm dyneema rope system for lifting that would be the last line of defence.

    Supporting this at deck level I am not so sure on. There are various through deck fittings rated to that loading (pad eyes - hard and soft), or I have a large stainless steel frame that sits around the keel box opening on deck level I could utilise. The deck material can support the compression loads of the SS frame , but I would need ~ 10cm diam backing plate for pad eyes.

    Transferring the load from a vertical force to a lateral load (pad eye option) will create a crushing effect at the top of the keel box. Not sure if I could reinforce to account for this or transfer the load somewhere else (have an appealing looking bulkhead) or if it is just not feasible.

    Will keep posting my thoughts here - appreciate any input.
     
  3. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    You can't convert a vertical force into a horizontal force. You may transfer a load to points to the sides, but the vertical component will remain the same. The total force will be increased, because a horizontal component will be added. The total structure will likely have to be stronger and heavier.
     
  4. Ooks
    Joined: Oct 2013
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    Ooks Ian

    Gonzo, thanks for input for starters.

    OK - agreed the vertical force will remain - it will be carried by the vertical deck and walls of the keelbox as the support material passes over it but will leave a force pulling intowards the keelbox opening - equal to half of the total load (2 sides) less whatever friction does for me where it passes over the top of the keel box.

    Hmm thin dyneema would create a lot of pressure at that point so I would need to reinforce the top of the keelbox for that also. (ie it would essentially cut down through the deck material)
     
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    This isn't something you can just "whip up" on the fly, nor is it something that I believe you can do without professional support. Simply put, you seem to be looking for answers to questions that can't be answered without considerably more information, than provided.

    I'm not sure what you are truly concerned about, as keels or ballast boxes don't just generally fall off. Though some racers, engineered to be as light as possible and pushed as hard as possible have lost them, it wouldn't be typical and the margins used on this type of boat, wouldn't be commonly employed on anything, but an all out racer.

    There are some rules of thumb, for some of the loads you're looking to address, but also as a rule, the engineering needs to be justified.

    What type of boat is this, length, beam, displacement, lines drawings and other general dimensions, the types of service you expect it was designed to tolerate, etc. If you can provide these, at least we'll be on the same page and can offer some advice.
     
  6. Ooks
    Joined: Oct 2013
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    Ooks Ian

    Par - thanks for taking the time to look and comment. Maybe a little about myself and the boat for background will help. Myself first, because that is the easier of the two.

    I am an engineer with about 20yrs experience - but a process engineer specialised in water treatment. I have an appreciation for the applicability of rules of thumb and good comprehension of the level of detail and expertise that is needed to progress beyond that within a field. I am capable of doing simple structural and mechanical design calcs and have colleagues/friends with more detailed knowledge than myself if I need to access them. I would prefer not to outsource this if I am capable of doing it myself - but if the need arises I will. I am hoping that this forum will guide me in the right direction.

    I don't think I am attempting to whip this up on the fly, but I am considering options and trying to evaluate if they will work.

    The Boat - as I said it is a custom design and build. She is based heavily on a Mumm 36. Construction is from Nomex honeycomb and empty weight without keel is 600kg - so very light for a 36ft boat. I do not have any drawings save some sketches I have done myself, but that is mainly for deck and interior layouts. general photo attached. I believe she was built as a competitive coastal racer - intention that she would be road transported to events up and down the coast for races. Road rules have changed now and she will be awkward to road transport due to her beam. But the shallow draft is a pretty useful aspect for pleasure sailing around here.

    The boat was built around 2007, but has never been launched. The builder was a keen and active sailer/competitive racer who had designed and built boats professionally before - but they were smaller less complicated boats. I do not think he had any professional qualifications. At the time of building the boat he had a relatively successful business building caravans out of nomex honeycomb and translated these skills to the boat.

    I have no plans to race her aggressively or at all really - probably twilight social races if anything - mainly will use her to cruise in Moreton Bay (Qld Australia) and some coastal sailing. Fairly protected waters but you can get quite short sharp waves within the bay if the wind picks up.

    The previous owner (not builder) had the keel manufactured and intended to pin it with 2 x stainless steel bolts. I have since seen/discussed trailer sailer keel holding mechanisms and this seems to be a bit on the casual side, since failure will be pretty catastrophic. Currently there is no method of securing the keel and I want to consider options. I will use the boat mostly to take my young family out on the water - so I want to be confidant the keel is secure if I get stuck bashing into weather and waves with them on board. I will also need a mechanism to prevent the keel moving up, but I am less concerned with this as it will take much less load.

    Securing via the deck/top of keel box appeals to me as I don't have to cut through the keel box and everything is visible and easily maintained.
    The keel box is currently marine ply encapsulated in fibreglass and epoxy. solid timber posts at each end and SS C section for channels.
    If I cut though it to put a beam/pin to support the keel it will be very difficult to get a good reseal.

    Second photo shows the original setup with the SS top plate of the keel box. I disliked this setup as the winch was impossible to operate so planned to abandon it and have a more simple support for some sheaves running to an elecric winch below deck. Looking at it again it looks like probably the best option to use that to support any strap/rope carrying the keel.

    Basic Details of Boat
    Length : 11m
    Beam : 3.4m
    Keel Ballast 800kg
    keel depth below hull : 1.8m
    Keel depth below waterline ; ~ 2.4m
    Keel box (interior) 1mx0.1mx 1.8m

    please ask if any other specific info will help.
     

    Attached Files:

  7. tom28571
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    tom28571 Senior Member

    My experience with this kind of set up is of a daggerboard of a bit less than half the weight of yours. I would assume that the exit hole in the hull is closely fitted to the DB foil and that the upperworks cannot fall through the opening and this is very strongly built and is the main means of keeping the DB in the boat.

    Manually lifting the DB for transport or other needs by multipart sheaves will require a lot of manpower based on lifting my smaller one. Adding more parts for a higher ratio tended to be lost in more friction in the system. Really good high load sheaves are needed to avoid friction losses but I'm sure you know this. I wonder if a hydraulic piston operating a 2:1 or more ratio might not be a better solution than multi part sheaves of high ratio.

    Perhaps the simplest solution might be a ratchet jack such as those used to lift cars or tension wires or fences. These have been used on some other large DB lifting boat systems. At least it would not depend on any power source other than your arm. I agree that as much possible all loads should be borne by the DB box itself. While we here are merely kicking ideas around, it is likely that this problem has been solved successfully many times before.
     
  8. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    A safety lanyard might offer some peace of mind, if you feel it necessary. Enough to keep the dagger from exiting, in the event of a catastrophic failure. It appears the guides will do their job, so making the board goes only as far as you want is the key. A mechanical lock (bolt or whatever) might introduce problems in the event of a bottom strike. I'd consider a simple "T" stop at the top of the board armature would be cheap insurance.

    I too have concerns about simple tackle on that board. That looks to be a self clutching winch, which is the way, it needs to be high enough to use. Turning block to move the winch to a more advantageous location is an option, though these will need to be stout and stoutly mounted, which on the cabintop may be tenuous at best. Maybe a turning block on the cabin top plate and mounting the winch on the board case, inside the cabin is the way to go after all. Do you have inside photos?
     
  9. Stephen Ditmore
    Joined: Jun 2001
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    Stephen Ditmore Senior Member

    Though I've not designed such a mechanism, I've given the problem a little thought. I generally prefer a worm-grear type linear drive to anything hydraulic, and wonder whether one coud adapt parts from an Edson worm-grear steering system.
    I like the scissor-lift type of auto jack with a horizontal worm gear. Might something of that configuration work?
    http://www.sears.com/proformance-1....0950101000P?prdNo=34&blockNo=34&blockType=G34

    Anyone know how it works on a Hobie 33?
     
  10. tom28571
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    tom28571 Senior Member

    Any cable system directly supporting the DB has a potential for disaster if it is not fail safe with a brake. I do wonder just why the set up shown cannot be made to work if it is mounted high enough to allow a handle to turn and the ratio is high enough. Clearly the open middle of the upper frame is to allow the lifting mechanism to be there.

    I have seen a temporary lifting jack used on top of the trunk but don't remember what boat it was on, maybe a Hobie 33. Much depends on how often the OP needs to lift the DB and whether it needs to be other than full up or full down.
     

  11. Ooks
    Joined: Oct 2013
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    Ooks Ian

    All - I did not like the lifting mechanism and found a simple one that I have spoken to the designer of. It is simple, effective and proven and I will probably stick with that. I will want to lift the keel regularly so I am utilising a 2000kg rated electric winch and high load sheaves.


    My main concern atm is not the lifting mechanism, as this is really secondary to losing the keel. To be clear to Tom's initial comment I have no plate as described to prevent the keel dropping through. Keel box is a continuous opening - there is currently no method of preventing the keel dropping. I am asking these questions to build the primary method of stopping the keel.

    Many trailer sailers have a metal plate bolted through the hull with a profile as described by Tom. As the boat was not built with this I am trying to identify other options. With a plate method the keel load is supported by a number of bolts through the boat hull structure.

    My thinking is the failure mechanisms for that are:
    • bolt failure
    • plate failure
    • hull structure failure

    In my proposed method the keel is supported straps with a plate at the top - failure could be from:
    • straps failure
    • plate failure
    • deck structure failure (damage to keel box opening - probably not catastrophic)
    • keel box failure (unlikely - is essentially a very short 1.8m deep plywood/fibreglass reinforced I beam on either side)
    • hull structure failure (load on keel box essentially punching keel box out of boat. I suspect the hull area carrying the load in this solution is actually greater than the first)


    There are proven engineering solutions to the first set up and I don't see huge differences in the overall systems. It would be much easier to reinforce for the second options than try to retrofit a plate to the system.

    I understand there is hesitancy to rely on straps to support the load but I am not clear on why other than it is not commonly done. Similar ropes support much higher loads on rigs where failure is equivalently catastrophic.

    Thoughts? can anyone think of another failure mechansim I am missing.

    alternate option with pin/beam not ruled out if anyone has other opinions on that.
     
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