So with this "fix" done I continued to crank away on the clamps. I was getting really close to having the rudder closed when I heard the dreaded cracking sound. So I quickly undid everything and pulled the rudder off the jig. Turns out that the the goop bead had started to separate at the leading edge at one end of the rudder.
I think my genius move with the clamps came back to bite me. I think the tang of the clamp must have acted like a lever to pull the plywood off the goop. That said the jig should have been holding this together. I think there is something about the geometry that is pulling the rudder up and away from the jig as I crank down on the clamps. My theory is that because the part of the wedge that slides on the angle is longer (in the vertical) than the part that slides on the rudder, I'm getting a little turning moment. Its working on both sides and bit by bit wants to walk my rudder out of its jig. I haven't done any actual calculations (I would have to make so many assumptions that any answer I could get on this would be meaningless). But I am going to fiddle with the wedge shape a bit before getting back to this. As far as R3 goes I epoxied (no filler) the crack and will try to see if I can still get R3 to close. I will also be sure to post before and after pictures of the cross section the wedges.
Hi Esteban
ReplyDeleteI've been following your rudder blade issues with great sympathy. It is entirely possible the angle of your jig needs closing up a bit, compared to what's on the plans. The KD 650 did, which I borrowed Rodney's jig to make my rudder for the Duo 480, and he forgot to mention he had to put a packing strip along the top of one side of the jig. So my first attempt, which fortunately was a practise run minus the CF, broke. Once i learned about closing up the angle on the jig, I was ok. Why dont you try a practise section , say a couple of feet long, minus the cf, with a packing strip along the top of one side of the jig about 10 mm thick. I know your stringer inside is virtually the same thickess as mine, but if you were to tell me the width of your rudder blade panels, and if they were the same as mine , i could tell you the exact correct angle for the jig, because once i got it right I measured it and cut some angles on my drop saw till it was dead on.
Also, when you are making up your sections, you should measure and mark on both sides where the stringer goes. This is before you glue the two sides in the jig. Drill and test fit one panel onto the stringer. Drill the same spaced holes through the plywood on the other side (but offset so the screws from each panel dont collide). Glue the two panels in the jig. When fully cured, dry fit the stringer to one side. Close the assembly up far enough that you can reach though the pre drilled holes on the opposite side and drill for screws. When you do your final assembly , BOTH sides of the rudder blade get screwed to the stringer, which should be fairly easy if the angle at the leading edge is correct.
A couple more point regarding assembling the blade, ( and this was something pointed out to me by Rodney). Dont pre glue the stringer on one side of your rudder blade in advance. Why? Because it stiffens up that side a lot , makes the panel harder to bend, and also makes the final shape of the rudder blade slightly asymmetric.
So the sequence is:
1: mark the location of the stringer on both blade components. Dry fit the stringer onto one side with screws at about 300 mm ( one foot roughly in your language). Pre drill the holes through the opposing ply side the same spacing.
2:remove the stringer
3: glue the two panels into the jig
4: dry assemble the stringer back onto the side that you dry fitted in 1 above. Close the assembly up and drill through the other side into the stringer. Then holding the panels together, dry fit the screws into that side also. When you let it go the it should stay there quite happily, with the front part of the blade shaped as it will when finished, and the back part more or less parallel apart by the thickness of the stringer. ( I made my blade a bit over length so i could temporarily attach a tape covered batten on each trailing (rear) edge , thus bracing the rear edge during assembly.
5: remove the screws and take the stringer out
6: first coat of epoxy to the inner sides of the panels, glue and screw the stringer into position in one side of the assembly. Wait long enough so you can apply subsequent inside coats , at least one more coat including the stringer. Glue and screw up the stringer on the opposing side, as you did when you did your dry run. You can have the assembly lightly clamped at the back now, but still about at the spacing thickness of the stringer. now you can get your CF into place, apply glue to the two trailing edges, and clamp the whole thing up.
Sorry for the mass of detail, but it works, and I was steered in the right direction by Rodney . Note it is essential that you get the angle for the jig correct, before you go any further, which is why I suggest some test panels
regards
Bryan
Hi Bryan,
ReplyDeleteWow, it must have been killing you to hold all that inside. In future just let it out. It will help me and it will bring more experience and know-how to the build. I'm ok with considering even positions I disagree with without taking offense. That said I agree with most of what you posted. Although, I would rather not start drilling holes in my rudder. I agree that gluing the spar onto one side is sub-optimal. One thing you didn't mention is how are you compressing the rudder? Right now I'm thinking my wedges are good for holding a compressed rudder in place but not so good for accomplishing the compression in the first place. If I'm reading you right the screws used as you outline are keepers, not used to compress the rudder in the first place.
BTW: the rudders are 22 deep, I would consider backing off on the angle a last resort since that is a design change. Recall I already managed to build one rudder to the existing angle (somehow).
Also: nice move leaving the spars too long to have something to grab in subsequent steps.
If I have to go to an R4, I'm sure you will see your input applied. At this point I'm still hopeful it wont come to that.
-Esteban
Hi Esteban
ReplyDeleteno, actually, I had to sit down and think it all through again, because I could see you were having problems. I had problems. Rodney had problems. Rodney solved it and passed on the information to me ( I did make a test panel which failed before i checked in with him) . The point is ,there are always different ways of doing things, which suit different builders, so we all have to find those ways ourselves. I also have spent a massive amount of time trying to explain details of my own boat construction to Mikael in Finland , and other potential builders of the Duo 480 for no gain to myself, other than to help, because others did the same for me. Also I would hate to see you get discouraged about the boat building process, I want to see your boat get built. Obviously , you're a pretty smart guy so you'll get this small obstacle behind you.
I understand why you dont want to put screw holes in your rudder. I was the same. On my test panel I initially had one screw at each end, with the intention of trimming off the ends of the rudder afterwards. But when you close up the assembly, even if it is all looking good, the non attached side of the stringer does not go down firmly against the stringer, as you found. The plans definitely say to use them when doing the final assembly , at least the Duo 480 Jigsaw ones do. In any event the screws are removed afterwards and holes are easily filled, at the same time as you fair and fill the edges of the rudder blade.
Have a look at my blog Aug 14. The taped battens are fixed with small screws from the insides right near the outer ends. That is in addition to the larger screws that are visible on the outsides , which were tightened up , just as the clamping starts. If the angle of the jig is near enough correct, then holding onto these battens with one hand , squeezing them in far enough to get the second side of the inside stringer screwed up, allows you to let go, and the whole thing just sits there , spaced out by the inner stringer width. Then you slide your wetted out CF into position. Then holding the two edges of the battens on the trailing edge , you can wind in the larger screws at the ends. Then you can let go and apply the clamps.
By having those taped battens pre attached flush with the trailing edges of the rudder blade, they dont slip around and make life difficult whilst trying to tighten them. Then of course, when it is cured, the ends can be trimmed off, because you made them a good inch or more over length in the first place, and the taped battens will just easily tap off the assembly.
You should not have to use any wedges etc for the assembly. I must admit , I was a little curious following your steps in that direction. But it is because you are having to squeeze in the panels which are at too great an angle.
BTW your blades are the same: 220 mm deep, 4 mm ply. Same as the KD650 also. My inner stringer is 19 mm square. The epoxy line in the leading edge is at 13 mm. Looking again at my blog photos Aug 14 you can also see the extra spacing strips I added (both sides I realise now) at Rodney's advice. I have got the exact angle the jig ends up at so I can supply it to you, if you wish. I still urge you to do test panels, including one with the packing strips temporarily attached near the tops of your jig.
hope this helps
best regards
Bryan
Bryan,
ReplyDeleteI was just messing around, you just went from short encouraging posts to a very detailed one that's all. I looked at your blog again and am trying to understand what you mean by "packing strips". If they are the strips of plywood material screwed (lengthwise) to the inside top of the jig, then I think you reduced your rudder angle significantly. I've got more bad news on R3 that I will put in a new post. BTW: thanks for your interest, it is good to hear that I'm not the only one that has struggled with this.
-Esteban
Hi Esteban
ReplyDeleteyes the angle was reduced significantly. The missing information as far as my jig goes is that it was Rodney's jig, which he gave me after he finished his KD 650. So I dont know the exact proportions of it, but yes, those strips are what was added to get a good result, on his reccomendation. On my blog you will see some little wedges with green tape on. They are the right shape for the inside of the blade. I took a couple of photos just now of one of them with the degrees angle marked, but something's amiss with our uploading program. I will get it into my blog photos tonight all going well when my wife, the computer expert , gets home. You may well get away with a bit less angle than what I ended up with , but you will see , that once you get it right the glued up panels come together easy enough. BTW on my rudder the stringer centre is at 66 mm in from the leading edge; in other words it starts about 56 mm in from leading edge, so i assume that is similar to yours.
Dont worry, I'm not intending to give you constant annoying attention on your build. It was just that the rudder blade aspect was a PIA to others of us too.
regards
Bryan
Bryan,
ReplyDeleteI'm not worried about too much attention. I like to hear from others with the interest and experience in what I'm trying to do. Gives me alternative ideas to think about. I'm not yet sold on the screwing through the rudder approach. Perhaps I just need to feel a little more pain before I give in. Not to get too meta on you, but the experience is somewhat like sailing. You can prepare, discuss, read, and provision, but when the time comes you have to decide how to trim your sails and where to steer all on your own.
-Esteban