Kurt Hughes Daycharter 36

Discussion in 'Boatbuilding' started by Charly, Mar 10, 2010.

  1. groper
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    groper Senior Member

    If the rudder top is clear of the water, then the free surface of the water acts as an end plate via gravity- the flow cannot go above the waterline unless it works against gravity... So in this case chopping a wedge shaped hole for the rudder doesn't matter...

    I'm not sure what you mean by quadrant? I think you mean what we call them a tiller head?
     
  2. Charly
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    Charly Senior Member

    Sorry for the confusion. I have probably misused the term. In the plans the quadrant is that metal piece that pins to the top of each rudder post. It connects to the cross-arm with a pin.

    What I have been calling the "tiller quadrant" is actually called the "tiller arm" in the plans. It is an aluminum piece that is welded at a right angle to the tiller post. It juts out from underneath the crossbeam where the tiller post exits. It then is connected to two spectra lines that each go out to a turning block, one on each side at the ends of a Harken track that runs along the bottom edge of the crossbeam. From there they come back and each connect to a traveler car in the middle of the track. The traveler car is the connection point for the crossarms, which each run straight to the "quadrants" that are fastened to the rudder posts. Hope that makes sense.

    But back to the cassettes. They are connected to the rudder post with long bushings that are glassed around the rudder posts and back. The cassettes remain fixed fore and aft...they don't turn, it is the rudder post that turns inside them. Then, below that, the rudder blade is fixed permanently to the rudder post. The gap between the cassette and the top of the blade is minimal, but since the rudder would turn and the cassette does not, water could still flow over the top of the rudder blade.

    So the " wedge cut", if used, as I understand it, the rudder blade must exit the top of the water for the thing to be most efficient. This means that it would be "all or nothing"... and the wedge must be wide enough to accommodate any rudder angle, because the rudder could not go up under the boat. It would be like any other transom hung rudder. Sheesh, Im sorry to bore everybody, but I still have more to say...:D

    So, Im still squeamish about having the possibility of the rudder blade binding up under the boat, and it seems to me that the only way to have a really effective kickup is to have something that will allow the rudder to rise up regardless of the turning angle. I have another idea, but I would like to hear from others more experienced before I bend everybody's ear with it.
     
    Last edited: Nov 26, 2013
  3. Charly
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    Charly Senior Member

    Good question. In the "normal" fashion, with quadrant facing fwd., the connector arm runs straight to the tiller arm. It goes through a round, or oblong hole that is cut out of the hull side in the inboard side of the sugar scoop. This hole may also act to keep the crossarm from jumping up, so the quadrant could not flip. I have no idea, as my experience with this kind of arrangement is nil. But with the reversed quadrant, the connector arm would run over the top. There would be no need to bore a hole in the side of the boat. But it might be free to flop around.
     

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    Last edited: Nov 26, 2013
  4. SamSam
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    SamSam Senior Member

    If you reversed the 'quadrant' and were in effect 'pushing' on it, anytime the pivot pin was below a line between the two connection points of the crossarm, that is, at the 'quadrant and at the traveling car, the 'quadrant' will want to flip up.

    Looking at this picture, I can't see how the rudder would be allowed to kick up to begin with. It seems the crossarm will not allow it, either binding up in the hole cut for it or having to bend around the transom. Of course, as pictured, it wouldn't turn anyways as it is locked in the slot.

    [​IMG]

    BTW, I'm thinking the thing on the rudder is a 'rudder arm' and that a quadrant is a thing that is part of a circle.
     
  5. Charly
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    Charly Senior Member

    The hole has to be big enough to allow the crossarm enough latitude to move back and forth when rudder kicks up. I may have to bend the crossarm a bit to make it work out. The designer has suggested that.

    The rudder would turn just fine. The cassette is the thing that is locked in the slot. It is a big part of what gives strength to the whole assembly against the side load pressure. It is like a big gudgeon that has the ability to pivot fore and aft. (photos)
     

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  6. groper
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    groper Senior Member

    I'm not sure why Kurt designs them that way... but yes I can't see how they can be made to kick up whilst anything but straight... the idea of using a cassette to do the steering and the rudder slides within it means non of the steering mechanism needs to change geometry as it kicks up. If it were my boat I'd have hung a cassette off the step and run a horn off the cassette which leads to a hydraulic cylinder inside the steps. The rudder would simply slide up and down like a dagger board... but that doesn't help you does it!

    I don't think you have a choice but to ensure your rudders are straight before you beach it... which shouldn't be a big deal as Kurt said previously.
     
  7. Charly
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    Charly Senior Member

    Well I am going to go ahead and build the rudders this weekend, and when I fit them into the trunks that will tell the tale, I guess, on how much side clearance there is. The top clearance can be fine tuned with bog... but I am thinking why not put a strip of divinycell there on top of the blade to absorb impact. It could act as a kind of "crash block"... maybe even make it so it would come off on impact and allow the rudder to go ahead and kick up anyway even if hard over. They would be easy to get to and replace with rudder kicked up out of the water.

    While I'm at it, I think I will fill the unused fwd portion of the trunk with pour in foam to act as crash block if I ever ground out while going astern (it happens)

    Has anyone else done these things?
     
  8. Charly
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    Charly Senior Member

    still fiddling

    Here is the first rudder in position. It is a good snug fit in the trunk, and the kick-up function works (so far) as designed.

    Before I build up the top part of the rudder blade so that it fits nicely up against the keel, I have to fix the height of the post. This means deciding what to use for a washer/bearing directly underneath the "quadrant" piece. Whatever thickness I use will affect the post's height. The quadrant is pinned to the post, which is a schedule 80 pipe, that is reinforced inside with a plug, and outside with a sleeve. The outside sleeve is what will bear against the glass bushing of the cassette. (see photo) This sleeve is only 1/4 inch or so thick, so I guess I need to make up a snug fitting metal washer for it to ride on, then spread the load out with another plastic washer that I cut out from my trusty cutting board. Of course I have to do some more glass work atop the cassette, to create a nice flat bearing point.
    So, quadrant sleeve bears on metal washer, which bears on plastic washer, which bears on glass cassette.

    Any comments appreciated
     

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  9. Charly
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    Charly Senior Member

    the wedge

    With rudder in position, more or less, it is now easy to see how it would bump against the bottom if it had to kick up while turning. The lines drawn there denote a 30deg. wedge cutout on each side. Because of the relationship of the rudder post to the cassette trunk, it looks like I would not have to cut out any part of the trunk itself, so there shouldn't be any sacrifice of structure for a side load.

    The dwl comes up to the very end of the keel to the intersection of the little transom.
    So, if the top of the rudder blade aft of the post, was built up higher, so that it extended out of the water, the turn angle would be limited by the width of the cutout wedge~ 30deg each side. The top of the blade fwd of the rudder post could be faired like normal up against the keel without interfering with anything on kickup. Is 30 degrees enough? Am I missing anything?

    The designer has indicated that it is cool to modify like this, without endorsing anything specific (understandably). So I am officially "off the reservation". It's lonely out here:D
     

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  10. Charly
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    Charly Senior Member

    Well shucks. Now that I have cut out this much, I am able to play around with it and see things better. But it is now obvious that I will have to cut out a good deal more, if I want to have 90 degrees. The picture here shows barely 60.

    Each time I take away more material, I must be affecting other variables that all affect performance. Opinions and advice please!

    Questions:
    does the wedge create more drag the wider it is?

    should the transition below the waterline from vertical to horizontal at the cutout be a sharp 90 or a rounded corner? The water will flow across it at about 45degrees.

    If the top of the rudder blade is left at a height that can still go clear under the hull maybe I could have a kind of "hybrid" that kicks up freely between zero and, say 30 degrees, but would bind at any angle of turn wider than that?

    How is the waterline length of the boat affected by this, if at all?

    Thanks
     

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  11. groper
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    groper Senior Member

    The wedge cut out increase in drag will not be noticeable... very small.

    The transition should be a sharp 90deg, not rounded... although again you wouldn't notice the difference.

    Wouldn't it be unlikely that you'd have the rudder hard over when beaching? Though if it did happen you'd do damage I suppose...
     
  12. catsketcher
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    catsketcher Senior Member

    I have commented on thew other thread but I may have not nailed one point.

    The wedge will not make on iota of difference to speed, waterline length or anything else as you will build your cassette to fit the void. If the slot was parallel then the cassette would be too. As your slot is tapered your cassette will be too.

    If you really want to you can just cut out as much boat as you want and put it on your cassette. I think there will be some other problems you may encounter though that are reasons for going down the normal road.

    Let's say your rudder flicks up after hitting a log or crab pot rope (mine has about 4 times). Usually only one rudder will pop. If you are reaching in some wind you may be going along at 10 knots. Most of the blade will still be in the water but if you keep on making the wedge wider and wider you will have no support for the rotated rudder and it may snap off the mount. The pivot will be under huge torque as the rudder CLR will now be way aft and the only thing stopping it from twisting off is the pivot. Better to leave lots of cassette in the slot to help keep the rudder straight until you can come along and push it back down.

    There is another idea that you always hit the ground straight on under way. Well my rudders often haven't. I don't have the typical pivot after my boat went up on a sandbar sideways in a gale. The rudder couldn't rotate and it ended up breaking the pivot (thankfully instead of the rudder) and jumping up whereby I could remove it.

    My advice is not to fixate on turning and rudder rotation. Ensure you have stability when raised and even then you may find the rudder will not come up when your boat drifts stern first onto a shoal (that happened to us when Kankama was on her own just off a mangrove island and a squall ripped roofs off nearby takeaway shops. When we got back I could see the rudders had raised because she had pulled the anchor about 10 metres back onto the sandbar near the island, but she was fine by then.)

    cheers

    Phil
     
  13. Charly
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    Charly Senior Member

    Thanks guys, for all the feedback. Catsketcher, due to your post I now understand what Kurt Hughes was trying to explain to me over the phone about side loads. It makes perfect sense now:cool:

    So now I am leaning towards a "hybrid" version, since I have done the cutout already anyway... the top of the rudder blade in that case then would not pierce the surface of the water, but the small wedge that I have already cut out would remain open. It wouldn't be that much different from the original design which has the rectangular slot. All I have to do is make sure that the cassette is supported well when half way kicked up.

    I run aground a lot. The estuary area that I live in, where the boat will be used primarily, is a maze of winding skinny creeks with nasty sometimes lateral currents and opaque silty water. Getting out to the "big water" often means scooting across sandbars that shift often.
     
  14. caiman
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    caiman Junior Member

    I've only just noticed this thread otherwise I would have replied sooner.I've got a 26foot KH Tri and had the same concerns with 'kickupability' as you are having.I also sail in shallow waters and keep the boat on a drying mooring.I got caught out approaching the mooring on a falling tide with the rudder 'askew',ie stuck on the seabed and the blade turned outside the cassette slot and therefore jammed.Only myself on board.I had to get hold of a passing sailor in his dinghy and get him to stand on the bow while I sorted the problem out,in the water alongside the boat.I decided then that full kickupability was essential.I scoured the net and found a pic of a KH Tri that used the same cassette system,but achieved full kick up by extending the bottom of the cassette sufficiently for the rudder blade to clear the cassette slot,with the blade at any angle.This is the solution I adopted.It works well,having turned the original tiller arm 180' and making up a hinged tiller arm that slots over the original,and so makes the tiller fitting, and position,the same as previously.I made the cassette extension in the same foil shape as the blade.I added the 'lost' blade area to the bottom of the new blade as a 'sacrificial' piece.
    Regarding unintentionel kick up,on my cassette there is a pulley inset into front edge.There is an eye screwed onto the hull quite low down in the leading face of the cassette slot.A line extends from the eye,through the inset block and then through a turning block at the deck/slot 90' angle,and then to a jam cleat.This arrangement is OK ish in shallow water.When sailing in deep water,on my boat there are two tubes glassed in horizontally either side of the cassette slot.When the rudder is lowered,a piece of wooden dowel is slid into one tube,passes aft of the cassette,and enters the other tube.On more than one occasion I have had the dowel snap when hitting large jellyfish.The rudder is then held down by the other arrangement until the dowel is replaced.I carry loads of spare dowels.:D
    Hope this helps.
    Cheers
     

  15. SamSam
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    SamSam Senior Member

    If there is a big concern about rudder forces being lost off the top of the uncovered rudder, a wedge shape that would fill in the wedge shape cut from the hull could be attached to the cassette. Unlike the rudder, it would be solidly attached and non turning, but would rise up with the cassette if the rudder hits something and kicks up.
     
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