Keith's dead on about this, and his earlier idea of going with a thin profile. If you are starting from scratch on your keel, let us know the chord and height (length) that satisfies the box rule for the i550, and how much lead you're going to carry in your bulb/wing. I can give you a calculated section width to provide the beam strength you need to keep the bulb in place, avoid vibration, and so forth, and then give you offsets for a NACA profile that suits your target speeds. The NACA0012 carries its max section at 30% of the chord; NACA has others that carry it further back and I think we agree that would be better for you to maintain as much laminar flow as possible, and you're minimizing the L/D ratio.
Tim, in your afternoon job, do you have access to a good large plotter? I've just sold one of my 72" machines and the other isn't working at the moment, but if you can generate plots I could send you DXF files for the templates you want after they're generated. I've got some written discussion of the sharp trailing edge vs. squared trailing edge controversy--the side of the debate I tend to fall toward is that a sharp trailing edge eliminates the necessity to design the squared edge to suit a fairly narrow window of improvement--in other words, if you square off, you'll see a gain in a narrow band of speeds but deterioration in the rest, whereas a sharp edge gives you good all-around performance. I just can't locate it at the moment, and I'm late for an appointment. I know your carbon fiber guy can give you a good, incredibly stiff trailing edge; that's what I'd do. But if you want something different, I'm open-minded. Dave Shaddock -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Sneddon, Keith Sent: Wednesday, March 19, 2008 12:53 PM To: [email protected] Subject: RE: catalina27-talk: Keel Design-OT-OT-OT Keels fail at the top joint because: 1) that's where the maximum bending moment (the product of the load times the distance from where the load is applied) occurs and is reacted, and 2) The load generally has to go around some corners and through some joints, and this makes the load "pile up" and increase in local areas. The label that engineers put on this for fatigue (repeated loading and unloading causing accumulated damage) calculations is "stress concentration", but most of the truly geeky aerospace types will correct you if you use that term for a measure of static load multiplication. Better catch phrases are "shear lag" or geometric stress multiplier". Anyway, design problems like those details get way way harder in composites, and the ramification of getting it wrong is more catastrophic, as metals (and to some extent, wood) tend to permanently deform before they give up entirely, and will often redistribute the "offending" load to the point where you get to limp home. Composite failure is brittle by nature, so once it starts to go, it generally just explodes or falls off. Keith Sneddon -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of David Shugarts Sent: Wednesday, March 19, 2008 1:12 PM To: [email protected] Subject: Re: catalina27-talk: Keel Design-OT-OT-OT I realize I will be way out on a limb here, but it's my belief that you could go to 8 percent and not have any problem with strength, when you are talking carbon fiber. It's THAT strong. I think if you can find some examples of keel failures, they don't fail in the middle, but at the top where it attaches to the hull. This is a very humble opinion because I don't have anything at risk here except ridicule, which I have learned to survive -- you get plenty of it in any marriage. Demitri On 3/19/08 12:59 PM, "Sneddon, Keith" <[EMAIL PROTECTED]> wrote: > I would use the thinnest section that I could "show good" structurally > for the loads anticipated (with a reasonable safety factor) for the > performance I hope for out of this hot-rod. The Lift-to drag ratio at > the angles of attack you would expect to see is always better for the > thinner sections. > In terms of trailing edge, the limiting factor is probably the minimum > thickness you can expect to "hold" under the environments you sail in. A > bent or broken sharp trailing edge will likely be slower than a slightly > blunted one, and a crack that starts at a sharp corner may find its way > into the meat of your section under the vibration/acoustic environment > under your rocket ship. > > Keith Sneddon > #4760 "Are We There Yet" > -----Original Message----- > From: [email protected] > [mailto:[EMAIL PROTECTED] On Behalf Of tim ford > Sent: Wednesday, March 19, 2008 11:01 AM > To: [email protected] > Subject: Re: catalina27-talk: Keel Design-OT-OT-OT > > Dave Shaddock wrote: > NACA0012 profile, if he's carrying 150 lbs of bulb underneath a carbon > or glass skinned fin-a profile with 12% of its chord at max width will > withstand side loads > > > Well, actually, the NACA 0012 I spec'ed was for the rudder. > > At the risk of incurring the wrath of the listee's who are darned sick > of this thread, I would like to prolong it with > 2 more issues (where else is one going to find resources like both Dave > Sh's, Mark, Peter, Keith and Phil and anyone > else whom I may have inadvertently failed to include, on one internet > forum that is comparatively non-contentious, > friendly and not full of abrasive know-it-alls, I ask you?) > > - section for the keel...the 0012 I mentioned is for my rudder....so, > stick with the 0012 section for the keel? (I already have the templates > but it's a relatively minor job to make new ones) Go with a more robust > foil 0013? The bulb will end up being approx > 165-170 (I need a bit more RM because my crew is lighter)...tip to root > length, keel foil is approx 60 inches. Keel foil > is 13.5 root chord, 12 inches tip chord. Laminated 1.25 inch sections of > > white oak, skinned with e-glass. > > - ok, and this is where it gets really murky...trailing edge > shape/trailing edge width. There was a pretty long thread about this > on SA (I cant find the darn thread, though) on this topic. J24 > measurer's look at keel trailing edges VERY closely. As I understand it, > a squared off trailing edge is fast (as oppposed to one that comes to a > point), but only fast to the extent that the width of the edge > (measured perpendicularly to the keel chord) is the proper > dimension...some percentage of the max profile maybe? This is where > I cant find much info. > > Remember the design is a box rule and the keel and rudder design are > completely at the discretion of the builder/designer...which > makes it both fun and a bit frightening...the downside is, like the > first TP52's, the first boats are likely to be slower than the later > builds as the faster stuff shakes down and become part of the lore of > the class. > > Thanks again for contributing to a project that is not particularly C27 > related. > > For anyone who's undies are in a knot over this thread, I can make it a > sub-list and continue the discussion there-OR-take > it to boatdesign.com and suffer some abuse there. > > tf > > >
