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Building the Bay River Skiff 15 #152


Don Silsbe

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I just picked up my centerboard, for the first time since I bonded the lead to it.  Holy Moley!  The plans call for a 1/4" pivot, for the unweighted c/b.  Are any accommodations made to the pivot for the increased stress?  (i.e., increasing the bolt diameter to 3/8"?)  I will add an extra block, to up my cascade system to 16:1.  I will be more mindful of the terminal attachment of this system.  I am using 3/16" Amsteel for the first stage of the lift system.  What else?

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That's encouraging, Chick.  I'm starting with a 4:1 cascade system.  We'll see how that works out.  If I add another block (only about $10), I can up it to 16:1.  I'm not sure that I'll have enough "throw" for another cascade, though.  

 

In case you're wondering, this is what a 16:1 cascade system looks like.  The photo is of a boom vang system.  A 4:1 system takes one less block.  It's an inexpensive way to get a lot of mechanical advantage.  Double and triple block sets are priced out of this world!

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Hi Don,

 

I stared at that vang system for a while and I think maybe I understand it.   Tension on a line passing through the empty block at the top of the picture is doubled by each of the other blocks nearby.  There's something that worries me about this, though.  As you tighten things up, the empty block at the top moves toward the block at the bottom, and once it arrives at the bottom, I think you might be stuck. The 2nd block moves half as far and each block moves half as far as the one before it.  The top block doesn't move very far.   I think this system might have a pretty limited range of motion - which is fine for a vang, but not so good for a CB lifter that has to move the CB handle something like 2 feet.  I could be off in the weeds on this one.  What do you think?

 

Bob

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Well, that's what I get for taking something at face value.  According to Pinnell and Bax, it's a 16:1 system.  (http://www.pinbax.com/index.asp,  search word "kicker")  According to Ronstan, it's an 8:1.  ("Vang 10", http://www.ronstan.us/marine5/vang_systems.asp)  With a cascade system, the mechanical advantage of the movable blocks are multiplied, not added.  So, with three blocks it should be 2X2X2=8.  (It's darned difficult to find technical data on cascade systems, btw.)

 

On the subject of how far the blocks move, that term is "throw".  I know that I have enough throw for a two-block 4:1 system.  I might be over the limit for adding one more block.  I'm going to bench test everything, so we'll see.  

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This just occured to me Don. You said " I am using 3/16" Amsteel for the first stage of the lift system." Be sure that whatever you use inside the trunk to attach to the board is large enough that it can't slip between the board and the trunk wall. You should have about 1/8" clearance, so 3/16 outa be ok. Some folks like to use actual 7 x19 wire rope of about 1/8". I'd worry about that.

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That system looks like a 2:1 coarse adjust with an 8:1 final.  Definitely not a 16:1.  Cascade blocks are simple but do take more room to operate than multiple parallel blocks.  That is why that system has the 2:1 first pull (the block on the ring) to take up while the load is low and the 8:1 to finish it off under high load so it does not take up a lot of room.

 

Unless you have put in way too much lead, you don't need much power to lift the CB.  The lifting rig in the plans is adequate and works well on Lapwing.  You are apparently  forgetting about the natural buoyancy of the CB.

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My plans don't call for this type of system, Tom.  They call for the uphaul and downhaul system.  I went with the weighted c/b system, because it offers a clean lounging deck for my first mate.  I have about 16# of lead in the c/b.  (I forgot to weigh it, before attaching it!)  I guess I need to take a peek at Chick's plans, to see what you're talking about with that ring.  I do plan to test it all in the workshop, before finalizing anything on the boat.  I'd prefer to keep it to 4:1, but my bench test will tell me everything.

 

Chick-- they are offering emergency repair kits for broken standing rigging made of pure Amsteel and a couple of thimbles.  Every ocean sailor should have a coil of Amsteel on hand, in case he loses a wire shroud, and doesn't have a power swaging tool on board.  And wire rope for running rigging is ancient history.  Moderns use either Amsteel or a two-part high tech braid like FSE Robline's Dinghy Control Line.  That one is essentially a Dyneema core with a polyester cover.  Just sayin'.

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The properties required for a line functioning as a centerboard pendant differ significantly from those required of a line functioning as standing rigging.  Amsteels low elongation at high loads and high strength in small diameters is useful when used as standing rigging but perfectly useless as a centerboard pendant. As Chick points out the small diameter could be a huge liability. With the loads you are dealing with in your centerboard pendant I doubt  you would  be able to measure the difference in elongation under load between Amsteel and some larger diameter piece of Sta set or what ever. I am guessing you are only dealing with a load of a few hundred pounds. Ideal qualities of a pendant are resistance to abrasion, ease of splicing, ability to hold a knot, and resistance to jamming. Amsteel no matter how well it functions as a shroud is stressed when it comes to functioning as a pendant. 

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It is true that my loads are around 100#, by my calculations.  Using Amsteel or SS wire rope is overkill in either case.  The main concern should be jamming.  3/16" Amsteel is larger than 1/8" SS wire rope, so is less prone to jamming.  I could go with 1/4" Sta-Set for everything.  But Amsteel splices better than Sta-set, so I am going to splice up the first stage using it with thimbles at either end for better load distribution.  It will be replaceable; if it proves to be unsatisfactory, I can replace it with whatever.  But if I use wire rope, I will have to go to a rigger to get it swaged, and change a sheave and two cheek blocks to wire-compatible equivalents.  Photos will follow, of course.

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I'm moving right along on the small bits, as it's too cold to work on the boat outside.  I made a c/b mock-up, first using a dummy c/b section.  Then, I completed the pennant attachment to the real c/b.  Finally, I got to test the final lift on my bench mock-up.  Results:  the final lifting force is around 20#.  My Boca Grip's fish scale only goes up to 16#, and it was a bit beyond that.  Conclusion: 20# is a little high for the final lift, so I think I'm going to add a double block to get 6:1.  That'll get me down into a comfortable lifting effort range.  I don't think I'll have enough throw to simply add another cascade to the system.  I'm gonna try and figure it out, though, because adding another cascade would cost about $15, whereas adding a double block is close to $30.  

 

Some notes on my pennant:

  1. I wanted to have a replaceable pennant for the first stage.  I know that many simply go with a knot in a hole, and might even epoxy it all up.  I wanted to keep it serviceable, in case I (or somebody else) changes their minds later.  This is why there is so much hardware at the attachment point.  One of my design objectives on this build is to use as little hardware as possible, and keep it simple.  This is a departure from that objective, in order to make changes later, if desired.
  2. I wanted to make sure the eye bolt stayed put, so I went with an encapsulated nut.  I will epoxy this in for eternity in the next couple of days.  I don't think it's going anywhere.
  3. This pennant is my 5mm Amsteel.  It has a long tail, to allow for the final installation.  The rest is just clothesline, and will be replaced with Sta-set in the boat.

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Centerboard Cascade System.MOV

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Looks good. I doubt that you'll need a further reduction. It's surprising how much less the pull will be in the water. I like your eye bolt solution. I wonder how good that old work bench is gonna sail...

 

By the way, on your previous post: "Which of the two in the attached photos is more prone to jamming?" Another consideration is that wire rope would cut into the plastic sheave.

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That old work bench will sail as well as a Puddle Duck Racer!  LOL

 

I believe that the effects of buoyancy are negligible, especially when this lifting above the waterline.  The final 10-20 degrees of blade lift provide the highest resisting force.  This is a classic engineering problem-- the sum of the torques about the pivot must be zero, in order to overcome the gravitational pull on the blade.  As the blade goes horizontal, the resisting moment arm (the vertical distance between the center of gravity of the blade to the pivot) increases.  If you like, I can make you a diagram, but you need to know a little trigonometry.  (Yes, that is a threat!  LOL)  Graham and Alan will back me up on this one, I do believe.  But since I am cheap, I will probably add the final stage after I've struggled with the current set-up on the water.  

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