Lifting Keel

Discussion in 'Boat Design' started by JHuberman, Aug 20, 2002.

  1. JHuberman
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    JHuberman Junior Member

    I am designing a lifting keel of about 8,000 lbs. for a 40' sailboat, and I want to put my ideas out for the criticism of this group.

    ** The keel should be able to take a grounding load either horizontally or vertically without serious damage.
    ** It should hold fast when the boat is heeled past 90 degrees
    ** If the keel is under load because the boat is aground I should be able to raise it, and it should not jam.
    ** All the components should be field serviceable.
    ** The keel should have 5 feet of vertical travel.

    To accomplish these goals I am proposing surrounding the keel with a trunk that runs from the hull to the deck. There are two Delrin slides running up each side of the trunk and Delrin slides molded to the shape of the keel on the front and back of the trunk. The front and back slides are backed with a structural backing to keep the Delrin from cracking and an adequate amount of resilient material to take the shock loads from a forward or reverse grounding.

    To keep the keel in position during use or capsize, and still allow it to move under extraordinary loads without damaging the boat, and to allow it to be released slowly under load, I propose having compression panels on either side of the keel just above the waterline. By screwing in these panels, with a wheel through the trunk, to a proper pressure I effectively clamp the keel in place for normal loads but allow it to move under extraordinary loads. This would also keep the keel from rocking back and forth in the trunk.

    To raise and lower the keel, a line is attached to the top and led around a block to a hydraulic ram which supplies the lifting power. A pin is provided at the top and bottom positions, to hold the keel when servicing it or when there is no chance of going aground.

    This is a brief description of the anticipated design, and I look forward to criticism of the concepts and references to anyone that may have tried a similar design. I am also interested in other designs that meet the above criteria.

    Thanks for your time and consideration,
    Joseph
     

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  2. james_r
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    james_r Junior Member

    I too would be interested in other's comments, especially from the designers and engineers that frequent this forum, but since no one else has answered you I'll throw in my two cents worth.

    The one weak link I see in your design is the lifting mechanism. That cable makes far too many turns, increasing friction and the probability of parts failure. A better solution would be to mount the hyraulic cylinder horizontally along the deck. That way the cable would make only one turn and you'd have only one sheave to worry about.

    I'm uncomfortable, however, with using a cable to lift the keel in the first place. Stainless steel cable is prone to corrosion and sudden failure. I've lost a mast and had a couple of steel halyards fail in my day. The halyard failures were an inconvenience, the loss of the mast wasn't as dangerous as it could have been only because of the fast reaction of an experienced crew and a solid dose of good luck. I can't help but think about what would happen if the cable were to fail with the keel in the raised position.

    It would be better to have that hydraulic cylinder inside the keel itself with one end attached to the deck and the other deep inside the keel. Better yet, use two cylinders for redundancy. Another method others have used is to use recirculating ball bearings and a shaft which is turned by an electric winch motor. The shaft could run just behind or in front of the keel but inside the case.

    Look through http://www.iconsailing.com/ and http://www.turneryachts.com/ for a couple of examples.
     
  3. james_r
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    james_r Junior Member

    Another observation:

    The top portion of the keel which always stays in the trunk need not be foil-shaped (in your drawing it is unclear whether or not it is). In fact it would be an advantage if both that part of the keel and the trunk itself were rectangular. This would make fabrication easier and provide more surface area to dissipate grounding forces at the back of the keel.
     
  4. tom28571
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    tom28571 Senior Member

    Hey Joe,

    You gonna put this thing in the Windmill?

    I agree with James about the cable and sheave setup. A direct acting hydraulic ram would be the first thing I'd look at. I sail an S2 7.9M with a lifting keel of only 600lbs but still worry about what would happen in the event of a lifting line failure. Even with this relatively light load, the loss in force due to friction is amazing. One thing that the S2 has that would be a good choice is a tapered socket arrangement to provide full contact over all of the internal part of the keel when it is all the way down. Holds the keel solidly in place with no possibility of binding when you want to raise it.

    I've also had the boat to make sudden stops when encountering solid underwater objects like rocks. The keel trunk must be designed to take that punishment with (for me) a huge safety factor. To keep the keel down in the event of a roll over, an automatic locking paul that would have to be manually disengaged would be my choice. This is the most riskey of all keel setups and needs to be engineered really well.

    I expect that the total lift may make the direct ram idea unworkable. In that case, look at making the cable run over as few sheaves as possible and look at a redundant parallel cable setup with each one able to take the load.

    Where do you plan to sail the boat?
     
  5. Portager
    Joined: May 2002
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    Portager Senior Member

    Joseph;
    I am a mechanical engineer, although I have no experience with a similar system, therefore I feel only partially qualified to comment.

    First I agree with James that it would be better to have the hydraulic ram inside the keel and eliminate the cable. Cables need constant inspection and replacement at the first sign of wear because inner filaments breakage is difficult to detect. The fact is people do not inspect or replace them as often as the need to.

    I suspect that you didn't put the ram inside the keel because you want to allow it to retract if it runs aground. You could accomplish the same objective by providing a pressure relieve port in the hydraulic cylinder.

    I think you need an interlock to prevent someone from trying to raise or lower the keel when the clamps are on. Since you are using hydraulics to raise and lower the keel, why not use hydraulic pressure to unlock the clamps so when you actuate the cylinder the clamps unlock automatically. When you release the pressure, a bleed path would relieve the pressure and a spring re-applies the clamps.

    I am concerned with the bending moments side to side but especially fore and aft and the durability and longevity of the Delriv. What prevents the keel from cocking in the trunk? I would prefer to see guide bars and rollers with enough wheel base to convert the bending moments into side forces. I'll attack a plate to the keel and mount rollers to roll up and down the guide bars. I'd drill holes in the guide plates for the guide bars to go through as a backup for the rollers. Last thing, provide access panels at the top above the waterline to service/replace the wheels and design the guide bars to be removed and replaced from the top is necessary.

    Cheers;
    Mike Schooley
     

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  6. JHuberman
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    JHuberman Junior Member

    Thanks for all your replies.

    Regarding the cable, I was planning on using synthetic line. 14mm Zylon has a breaking strength of 34,000 lbs, Vectran and Spectra 16mm are at 30,000 lbs. The 8,000 lb keel plus 10% friction produces a total working load of 8,800 lbs. The hydraulic pressure could be set to not exceed 10,000 lbs of lift. (20,000 lbs push at the cylinder.)

    Regarding safety in the event of line failure, my current thoughts are to have a duplicate cylinder & line that is spring loaded to maintain tension on its line. It would have a relief valve set to bleed fluid at a rate that would safely lower the keel, but faster than the customary lowering of the keel on the working cylinder. The advantage of this safety system is that in the event of failure of the working hydraulic cylinder a re-routing of the hydraulic fluid would allow this cylinder to act as a working cylinder until repairs could be made to the out-of-service cylinder.

    Mounting the cylinder on deck is a good idea. It would also eliminate the hole that would penetrate the cabin top for the lifting line (that could potentially leak). I planned for it in the cabin so that it would be out of the weather, easy to inspect, and convenient to service in foul weather.

    The square top section (that always stays in the trunk) may be an improvement. I initially rejected it because I wanted as much contact as possible to minimize movement in the trunk and spread the load to as much of the resilient material as possible. I want as tight a hole as possible at the hull to minimize turbulence. If the only bearing area was the square section at the top I would need to make the hole longer to allow for movement of the keel at impact if the keel was only partially down, or plan to take the load at hull level. I will need to give this more consideration.

    The wheels and guides appear to be a smooth-running system that would not jam when it was working properly, but the added complexity and point loading at the wheels looks to me like it would have a higher probability of failure, and possibly require more maintenance. I will certainly consider it more carefully.

    Putting the cylinder in the keel creates several problems that this design tried to address.
    ** First it would need to be a "pulling" rather than a "pushing" cylinder. This requires a larger diameter (or higher pressure) cylinder because the rod takes up some of the bearing area.
    ** Tom is correct, I could not get as much lift as I want with the in-keel cylinders.
    ** The next problem is dealing with a vertical shock load. The relief valves would need to be large enough and the rod strong enough to handle these very large shock loads. This takes up more real estate on the piston or requires hoses that would also need to be set into the keel. The thickness of the keel would limit the size of the cylinder, possibly requiring two cylinders.
    ** Servicing the cylinders would be extremely difficult, requiring that the pin through the keel that secured the end of the cylinder to the keel be removed so the cylinder could be lifted out. Inspecting the cylinder for leaks or corrosion would be equally difficult.
    ** Finally the cylinders would need to function in a salt water environment because realistically the holes in the keel would fill with water. This would require specialized cylinders. Marine grade cylinders of this type would be about $5000 each while industrial grade (less expensive ram type) cylinders that would need to be protected in the cabin or a sealed deck box are about $500 each.

    A recirculating ball bearing screw lift is something I have never heard of. Do you know of a company that manufactures a product capable of vertically lifting these loads?

    I appreciate your thoughts on this matter and am interested in any further comments.
     
  7. tom28571
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    tom28571 Senior Member

    Joe,

    You have clearly put a lot of thought into this problem. The screw lift is an attractive idea. I have such a system on the chair lift elevator that I put into my house for my mother some years ago. A beauty of this method is that it is self braking and cannot fall as long as the end attachment fittings survive. It's far simpler than the hydraulic system but not completely fool proof as evidenced by the recent air crashes caused by screw failure in the tail sections of commercial aircraft. The screw would need to work inside the keel (there could be two screws) but if made of the right stuff and inspected regularly, should be as safe as it's possible to get.

    Shock loads would be taken by the screw(s) unless some provision is made to lock the keel down by other means, like a paul. My keel has a rectangular top section with delrin rubbing blocks and has endured for 20 years. I had a failure of one block but it was a failure of the method of attachment and not the block itself. These blocks should be mounted in compression and not in shear like mine was. No matter what you do, the loading of a grounding is going to be point loading so the full length support is not going to help that. The rollers would amplify the point loading though and I'd prefer the delrin blocks.
     
  8. james_r
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    james_r Junior Member

    My primary concern with using a cable or line to lift the keel is that one does not have positive control over the keel at all times. Once you release the compression pads you're at the mercy of the elements. Imagine for a moment that you've just hit bottom and are being pushed into shallower water. You release the compression pads to lift the keel. At the same time, waves are lifting your boat off the bottom and as you fall into the trough the keel smashes into the bottom and goes up into the trunk. As the next wave lifts the boat and the keel drops the shock load on the cable, hydraulic cylinder, and its attachements will be far in excess of the 8000 lb weight of the keel.

    Hydraulic cylinders or a ball screw drive, both attached directly to the keel, would allow you to have positive control of the keel at all times. Whichever method is chosen I would still use some method of locking the keel in place when in the fully raised or lowered positions. The use of compression pads or Tom's suggestion of a paul would both be workable.

    Several yachts with lifting keels use ball screw drives. The Perry designed Icon was supposed to use this system but, I believe, was outfitted with a hydraulic system. Take a look at Ball Screw Drive Image for an example. You can also check out INA or NSK or search for "linear actuators" or "ball screw drive" on the Web.
     
  9. james_r
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    james_r Junior Member

    This is a larger image of Bill Tripp's lifting keel design featured in the Turner Yachts link in an earlier post. You can click on the image to watch an animation of the keel moving up and down.
     
  10. JHuberman
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    JHuberman Junior Member

    James, your point is well taken, but your conditions are rather extreme. You have me crossing a bar with 6 foot seas in only 3 feet of water (between waves). We've already made some grave errors. How would any boat handle being lifted and dropped on a sandbar in 6 foot seas.

    But with a little luck, the tide is a foot higher, the storm is coming, and we decide we have to try and get over the bar. Of course in the excitement I forget to raise the keel. We're surfing in and wham the keel hits. The boat lurches (and there are no sickening cracking sounds). The wave we were surfing lifts us up and then smash it sets us down on the bar.

    The friction grippers can't hold the load and the keel slides into its trunk. The next wave lifts us up, but the keel is still held in position, and the rope is loose. We duck below and turn on the keel lifter. The hydraulics or ball screw starts to take up the slack. The next two waves jerk the keel up a bit farther, but finally the rope is tight and the lifting device changes pitch because it can't pull up with the keel gripped. When we hear that sound change we ease the gripper and the keel moves up. (Hopefully there was someone else on deck taking care of lifting the rudder and other problems there while we were screwing around with the keel.)

    I talked with an engineer at the Joyce Dayton company and he estimated I would need a 10 ton ball jack $1500, and a 3hp motor $300, to raise the keel in one minute. The sheave would need to be supported by a track, and the load would need to be axially centered.

    Bill Tripp's hydraulic keel looks very clean, but I am very interested to know how they deal with the problems we have been discussing. How do you service them, how fast you can sail into a ledge or sandbar before it fails, are there relief valves, are they double acting cylinders, how much do they cost?
     
  11. WPLANE
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    WPLANE Junior Member

    Hi there,

    Been following this thread with interest. I have engineered several of these systems and in one case followed the ball-screw idea through to completed mechanism- but, it never went on the boat. The boat was ICON (I noticed their website mentioned in an earlier reply www.iconsailing.com.) and we worked with Boeing engineers on the mechanism, drawing from their obvious experience in screw-type lifting mechanisms. There are some good photos of the "final solution" on that site.

    There are advantages and disadvantages to the ball-screw system. On the one hand the ball screw system, if properly designed, offers exact control of keel location -up/down position, and it requires relatively little energy to raise the keel. Another advantage is hydrodynamic- Hydraulic cylinders inside a hollow keel section are the simplest, most reliable and proven way of lifting a keel. However, the cylinder's diameter is directly related to its capacity, and clearance to the side skins is required as well. In the case of aggressive fins where significant ballast must be supported, this often drives the c/t ratio of the fin undesirably high. The ball screw system fares better in this respect, because the screw diameter/assuming two screws (synchronized) is far less. This also relates to grounding loads-the screws must be sized to take the sizable compressive load which occurs at grounding. This load can be transmitted tthrough the screws to a shock-absorbing base plate at the top of the trunk. This was thought to be an excellent solution to the grounding problem.

    But the ball-screw system falls short. The completed mechanism winds up being complex and heavy. While it is possible to obtain stainless ball screws at reasonable cost, there is still the issue that these screws will spend their lives in stagnant salt water. The sytem we designed used sealed bearings located at the top of the fin (which was forged 17-4-ph stainless) and the screws were surrounded by lubricant which would displace any seawater. Without absurd drive systems, a ball screw system will also be relatively slow to lift the keel. The system was scrapped mid-stream in favor of a more novel approach to the hydraulic system.
    At the time we scrapped the ball-screw concept, we had already gone to great expense to have the resevoir cylinders bored into the 17-4 fin. It was then a (relatively) simple matter to have these holes honed and made suitable for use as hydraulic cylinders themselves. Thus the fin itself became the hydraulic cylinder, elliminating the undesirable characteristics of a typical hydraulic system in regard to sectional properties, etc.
    Regarding your "clamping" concept. On Icon we used an interesting approach for this -our client's idea- "flat jacks". Essentially, two sheets of thin-guage stainless, bent to the sectional shape of the fin and seam welded together around the perimeter. A hydraulic fitting is installed on the outboard side and a tight-tolerance recess is provided at the base of the trunk. A relatively small amount of fluid will produce enormous pressure and is capable of holding the fin in place even in the event of grounding-the pressure can be set based on calculated values to hold or not to hold. That is the question.
    In any case the grounding issue becomes a real brain tickler. Just how burly do you want to get, at what point along the throw of the fin do you expect a grounding to occur, etc.?

    Notwithstanding the comments others have made, I would make the following general suggestions: 1. A grounding will produce a large forward load at the top of the fin and the trunk must be designed to withstand this. Extending the forawrd portion of the top of the fin as high as possible will help distribute this load. 2.The aft edge of the fin is a wedge and will tend to split your boat apart. Consideration to bearing at the trailing edge area is a good idea. 3. Make sure you are aware of the water absorbtion rate of the bearing material you use for guides(delrin, etc) and adjust tolerances accordingly.
     
  12. JHuberman
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    JHuberman Junior Member

    I was trying to explain the problem we are discussing to my 7th grade daughter, and when I explained how a centerboard solved many of the problems but lacked the bulb she suggested a centerboard with a bulb. While trying to explain why that wouldn't work I came up with this idea...

    A bulb is mated to the centerboard so it can pivot as shown in end view on the left. The bulb is thicker at the front so it can be set back a bit to allow a smooth front that can pass over an underwater line without catching. The centerboard is pinned into the trunk so it can rotate to lift up. The lever arm above the pin is about half the length below the pin, so the forces we have been discussing are still appropriate. (Except now the line and sheave carry the doubled load.)

    There are clearly problems to work out:
    ** The bulb must rotate smoothly against the centerboard and not get sand and junk jammed in the joint.
    ** The first sheave is in the trunk and under water, so field serviceability is a problem.
    ** There is no backup for the main pivot pin, and it can not be inspected or serviced without being hauled.

    Has anyone seen this done before? Any comments on some unforeseen problems.
     

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  13. ErikG
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    ErikG Senior Member

    Just a comment...

    I've seen a lot of new "non.extreme" yachts uses what looks a lot like a sheel keel, ie. a larger width at the bottom but o bulb as such. Could that be used or maybe a nonoptimal bulb ( a short one) on a swing-keel system.

    Swing keels have a couple of advantages, less interior space used to handle the keel system, pretty easy to set up so that it folds back in the case of a grounding. Unfortunately it has a number of drawbacks too, seviceability is not to good (needs to be hauled), and I'm not to sure about the drag issue at the hull keel joint.

    Comments?

    Erik
     
  14. JHuberman
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    JHuberman Junior Member

    Using a thickened centerboard instead of an articulated bulb is certainly a simpler solution, and thus has appeal, but when I use my limited knowledge and attempt to calculate the additional weight in the CB to compensate for the redistribution of the mass from the bulb, it requires almost 2000 additional lbs to maintain the equivalent heeling moment. In addition, if I eliminated the bulb, reduced the CB weight to 6000 lbs, evenly distributed the lead across the whole CB, and didn't allow for any foil shape in the CB it would have a length 8 ft, width 2.5 ft, and thickness 5.1 in. (8.45 cu-ft of lead / 6000 lbs).

    Hopefully my rough calculations are seriously flawed. :)

    Joseph
     

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