J/30 Class Association Forums J/30 Forum The Boat, Sails and Equipment Keel Fairing - Trailing Edge Location

I'm planning to fair my keel this off-season and would like to know how you determine the proper location/placement (fore/aft) of the keel. The keel specs say that the "corner of the transom to the trailing edge (of the keel) at hull is 3820mm-3840mm."
Where is/what is the corner of the transom? How do you determine it? Etc?? Any info would be appreciated. Thanks.

Steve Sullivan
Temptation, #311

The "corner of the transom" is where the vertical portion of the stern meets the bottom of the hull, in the center directly ahead of the rudder. In measuring boats over the years, this spec tends to be quite accurate as delivered. As a higher numbered hull (311) you are likely to have a short, fat chord length. In my opinion general fairing for smoothness will help. To get to templates, you'll be adding material to the leading edge, which has the added benefit of moving the center of effort forward. I'm out of my league on this both in terms of design theory and repair, but I'm told that is a good thing_

Anyone know why (or agree) with the general concensus that moving the keel (centroid) to the forward limit of the spec is generally thought to be good? I could argue the aft-biased sail plan needs the keel at the back limit to reduce helm in heavy air... or that light air performance is what needs help most and keel-forward helps pointing.

So if higher numbered boats tend to have short fat keels, do lower numbered boats have thin long keels? I've been fairing mine, but just as given. Haven't measured and haven't planed to "move it." I have hull #62.

I can only give you one data point: #270 is about 1" short on keel length, about 6mm too thick (per side/from CL, @ about 40% chord) and pretty close on chord length (maybe 3mm short). Haven't checked fore-aft position yet. Rudder is very close to true; maybe 1mm thick @70% chord.

Here is another twist on this question; how does one actually measure the proper slope of the trailing edge of the keel to the bottom aft corner of the keel? It is easy to measure the distance from the corner of the transom to the hull intersection of the keel, but another useful dimension would be the distance from the corner of the transom to the bottom aft corner of the keel.

Best to take that off the lines-and-offsets drawing (J-Boat would not give them to me...anyone have them?) Otherwise need to guess depth of keelson from WL. Specs say to measure angle (I use a template). I understand this not usually checked by measurers.

I believe the hydrodynamic reason for "moving" the keel forward is to move the CLR (center of lateral resistance) of the hull forward. J/30s are similar to J/24 in that they suffer from almost no feel in light air. You can do two things, move the CE (center of effort of the sail plan) aft or the CLR forward. Moving the sail plan is usually the easier of the two.