Actually, the other way around. The longer chord will be thicker.
Well, this keel is going into a boat one way or another.
I’m trying to simplify it as much as possible, and accomplish the same, or similar sailing qualities to this good keel setup.
This is not a replacement or alternative for the “OEM keel” the viper in the picture will stay with the OEM keel because it is a known good setup. Your vote won… I’m staying with the narrower foil on ONE of the boats.
I only have one of these narrow foil keels, so I can’t sail both boats with this type keel.
This “aftermarket” keel is the alternative. I wish I did have two of these “OEM” keels, because the boat sails just great with it.
I have a second boat, without a keel, and it is going into that boat. For the sake of conversation I tried to leave that boat out of it, as to not confuse people. Being that this "aftermarket keel is going into a Viper it is esentially going into the same boat.
If the aspect ratio is actually an issue of concern I have the ability to leave it longer. But, I am severely restricted in my pond’s depth.
all I really want is a alternative keel that will provide the closest possible boat performace to factory. So, I thought cutting the keel to the same lenght (which I haven’t yet) and eliminating all factors besides the only difference of chord length would be a simple way to think about where “aftermarket” keel should be mounted.
Depending on which keel you have I may consider swapping keels. I ordered 3 keels with my viper, so I have an extra.
I am very, very apreciative of the thought. Even if it only develops to a thought. Obviously It has to work for both of us.
What I have for keels are:
-A “OEM” keel pictured that we’ve been talking about that is perfect for the boat, and perfect for my pond. 16 inches long.
-A second “OEM” keel that is longer, and straight down (no aft rake like the other). It came with the boat and was the alternative keel for the Viper. It is fitted to the boat. It’s a 19" fin and 7lb lead.
-A 20" keel that is too long for my pond. It is also too light for the boat at 5.49lbs. It is believed to be designed by Mark Mason, but really isn’t anything special. I don’t know what he was trying to accomplish, but this keel I really don’t have a use for. I’m going to scrap the keel and keep the bulb. The bulb on this is the same as the “OEM” bulb, but has had some weight removed.
-The keel in question that I’m trying to retro-fit to my Viper. it is made by Sailsetc, and was supposed to go into a Bantock design, but I went another direction. I also have a sailsetc bulb for it.
This bulb is overweight, but is raw from the factory.
To be honest, the best option for me is to retrofit the sailsetc keel that we’ve been discussing because I also have a keelbox, and tradiitonal/swing rig mast tubes that go along with it. Instead of eating those parts, or shipping them out in trade I can just glue them into my Viper if I can get the retrofit to be accurate.
The bulb I’d drop onto it would have been the one that is on Mark Mason’s keel. The shape is the same, and I could get it back to shape & wieght as the “OEM” very easily.
I was thinking about the CLR issue. If you locate the CLR as Alan says, or as Claudio does with cardboard, would it not then just be a matter of ensuring the increased profile is centered around the CLR? Perhaps not, as I think a 2.5 cm addition well in front of the CLR is diffeent than a 2.5 cm increase right at the CLR. Typically on the designs I have seen The CLR tends to be near the back of the foil. I would be guessing that you would need to add less to the leading edge of the foil, and more on the trailing edge. What the proportions are, I do not know, but I suspect thay are small, and the lining up of the MAX cord it probably your best bet.
Grav.
Hey keel sale on !.. strike a chord 
Now that, sir is starting to sound like a logical explanation.
If it is actually correct, well that’s for someone to confirm that knows more than I do.
But, if the max width of the foil is lined up with the position of the max width on the other foil, the lift should be created at this point and chord length really somewhat irrelevant except for the factors of increased drag, and lateral stall points. no? (but not the actual CLR
The forward and aft position of the “aftermarket” fin would then be determined by this point, and the overhangs would be X.
If the fin was lined up 50% overhang forward, and 50% overhang aft, the max chord width would not be aligned.
I set up two keels for my viper, both are the viper + length, one has the a IOM bulb at 2.5kg and the other is a 7 lb kit bulb. The IOM bulb keel is great for the light wind summer days. then I swap to the big bulb for heavy air days.
Why not just cut down the 20" keel to 16" and put a new bulb on it? Seems easier than fitting a whole new keel and keel box. I am pretty sure that bob used to put the keel boxes in while the hull was in the mold, so they end up straight and square.
I’m no expert, but I am having fun thinking about your question. The CLR, in my opinion is only about lateral resistance. I am really unsure about this “Lift” you speak of. I like to think of the shape of the foil allowing for a nice clean movement of water over the foil. The lift perhaps keept the boat from wanting to turn? The profile of the foil (side) helps the boat from slipping sideways, and you want that ballanced with the CE of the sails. I don’t think I am too concerned with the max cord alingment, but it seems a convenient thing to do… more like an educated guess.
It would be interesting to do the cardboard cutout thing twice. Once with each fin and see where the CLR Lines up. Another thing you could do is to find the CLR as Alan suggests, in the water wit no rigging. Then cut a profile of your new fin in plastic and tape it on the side of you fin and try the test again. All you are looking at here is a change in the surface area that yuor sideways moving boat sees. That should tell you if you are close.
Grav
I may cut down the 20" keel in the future, but it is not a priority. Reason being that the second Viper hull does not have a keel box installed in it.
I would like to use the Sailsetc keel, because I have a keel, keel-box, mast step, and swing rig step all available and ready to be glued in if I can get the placement right.
If I used any of the spare Viper keels, I would have to make a keel box for it, one step I can logically and easily skip If I don’t have to do it. The keel-box on the viper keels are also much smaller, and more difficult to align properly. The Sailsetc one is very long and I am very confident I can get it straight.
If I cut down the 20" keel I’d either scrap it,or use it, but either way it would be assigned to the “OEM” boat. because the Keelboxes are different.
Again, details I’ve omitted because they specific to my situation, but are irrelevant to the end product.
My mistake, I though both hulls had keel boxes in them. BTW I have a spare stern keel box to match my extra keel.
Ugh, I could have used that a couple of months ago before I cut a hole for the sailsetc keel in the hulll. Not your fault.
I thought I was confident in the placement, but then realized I was working with “art” instead of science.
In that previous thought process I would have been working under the “just mount it, and move it multiple times if necessary” philosophy.
At that time, I didn’t have the 2nd finished viper either, so I didn’t have ANY OEM measurements to work off of, just the best I could do. I tried finding key data points, including having Stan Goodwin put an inquiry into AMYA, but didnt’ get any significant data to work off of specific to the Viper. Here’s some of my efforts: http://www.rcsailing.net/forum1/showthread.php?5996-Sterne-Viper-Keel-Placement
So, the hole that’s in the boat is innacurate, but not oversize and an issue in the least bit if the Sailsetc keel/keelbox goes in. Just need to find out where to put this keel based on previous questions.
Then, I found the white Viper for sale for very short money, 3 rigs, 3 keels.
The keel hole in the boat is a non-issue if I mount the sailsetc fin/fin-box, Infact I would need to enlarge it quite a bit.
If I used the Sterne Keel box the hole is too big, since the keelbox is so small.
I could scab in a Sterne keel & Keelbox, but it’s not my first choice.
I’d have a more difficult time getting it straight, and also have to fill in some of the keel hole in the hull.
Had I known about that a few months ago, I’d be ALL over it.]
But, the important thing is, that If I can find where to mount this sailsetc keel so that the CLR is the same as a known placement on an OEM keel… Then all this back story and process I’ve had to go through to get a keel into this boat is irrelevant, and the boat is sailing the way it should.
My vote goes for lining up the thickest part of the cord. You will find that that is in the range that I was talking about earlier(25-50%). It’s either the right point or the closest you’re going to get. It’s narrowing down a wild-assed guess to a calcutated risk. Finding the thickest part of the cord is probably going to be a bit of a guess(+or- a 1/16th) but its time to break out the glue.
Don
You are absolutely right, Even if I find the exact Atom that I need to mount the keel at, it won’t come out exactly precise in terms of human error when installing.
But, all I’m hoping for is to narrow down that wild-ass guess to the calculated risk. The rest I can iron out the details in balance with the rig.
Matching the thickest part of the chord seems like a more realistic calculation than putting a 50% overhang on each side…
Obviously my best path from here is to get this keel installed as close as I possibly can on a calculated risk, rather than a wild guess.
I’m kicking myself hard that Slotracer has an extra viper keel and keelbox, which would have been the ultimate solution, but I can’t dwell on it because the project is at the point it’s at… and installing a “aftermarket” keel into the boat will not be a fatal flaw.
Any help is appreciated. Do others think that the max chord width should be aligned?
Yes, I would match the max chord thickness of the two fins. They are probably in the 25-35% range, but may be a little further back.
In the past couple of days since we left this off, I decided I would verify the thought that maximum chord width should be aligned between the two keels.
In order to verify that, I thought I would go with Claudio’s cardboard cutout method to find the CLR of the boat. I’ve made a cutout (not including the rudder) and used the dropper-line to find an intersecting line at the CLR.
I left the rudder completely out, since it’s not important, It will be identical between boats.
http://www.rcsailing.net/forum1/showthread.php?5335-ETNZ-IACC120-Dual-build-thread&p=53202#post53202
Not surprisingly, it was pretty much dead-center in the fin, which is why the M is so fun to sail. It feels like it could do a 360 within it’s own boat length.
Given that I now have a cutout, and a known CLR with the “OEM” fin, how can I now match a different fin to achieve the same CLR?
Based on my understanding, the only thing that is important is the CLR position in relation to a vertical line that runs perpiducular to the water-line… and shouldn’t move fore & Aft in order to achieve the same balance.
I have trouble following you … my fault not yours, however, I submitt Claudio’s illustrations on the MAIN CENTRES for reference on the whole picture that is more than just CLR.
Perhaps you should cut off the fin, and cut out and tape on a new fin, and see where the line ends up. Tape it in a few different points to see the effect?
Experiment!
Grav
According to that photo I found the CAD, using the yellow cardboard cut-out technique.
I have 3 more sheets of cardboard to work with… so while experimentation would be fun to see what moving the keel fore&aft would be fun, in practicality the whole purpose of this is not to experiment and explore and see what the end result may happen to be, but rather to get a similar result so the boat has the same balance.
I imagine I’ll blow through those 3 sheets just trying to get the positioning correct while trying to accomplish a known goal.
Not that cardboard is a major expense, but I’d rather use the resources trying accomplish the goal rather than experimenting.
So I apreciate the input, but I’d rather have some guidance on how to get the efforts to line up between alternate keels and have the boat sail as closely as possible to the original.