Saturday, August 28, 2010
Sorry I have been away for so long. Last post was just before Memorial Day and here it is a week before Labor day. Where does the time go?
Lots of project work at the boatyard: a stern rebuild on a 33'Bertram, re-glaze the entire wheelhouse (9 large windows)on a Lyman-Morse 53, various and sundry break/fix projects, and regular day-to-day boatyard operations have kept me pretty busy. There is also the work that has been going on on Strider. A complete topsides strip was necessitated by the fact that one could wrap sandwiches in the sheets of paint that were peeling off. It was a bear of a project, but it is behind me now. She sits in the shed, drying out.
Before I get going on the Dark Secret stuff, I need to apologize for the misuse of the term "metacentric height" in a previous post. This will be clarified (I hope) below.
My sincere hope is that this post will forever quash the debate about the keel on Valgerda. I have primarily been using two sources for the data on stability calculations: Chapelle's Yacht Designing and Planning, and Skene's Elements of Yacht Design. Both books talk about stability as applied to larger vessels (closed systems in which items that will affect a hull's stability remain in fixed positions) as opposed to small open boats (open systems in which all moveable items will have some effect on stability). Chappelle talks about stability in a very conversational, "touchy-feely" kind of way. Skene's takes a much more direct and quantifiable approach. It is entirely math dependent.
The short version of the conclusion at which I have arrived is as follows:
1. The height of the keel has a very small effect on the lead casting's contribution to stability. In fact, the difference is negligible. The LCG (longitudinal center of gravity)of the casting, as designed, is approximately 14 1/2" below the DWL at a point about 1/3 of the way between stations 5 and 6. This point is marked on the lines plan by the termination of the arrow drawn from the noted casting dimensions into the casting in the profile view. The traditional keel I have fitted to Dark Secret puts the LCG at 11 3/4" below the DWL.
Here is where the math/geometry gets funky. Pay attention. The 'midship section when the boat is at maximum heel has to have the same immersed area as the section when the boat is at rest. More accurately, the volume of the boat does not change as it heels, so for all intents and purposes the area of any given immersed section will remain constant through changing heel angles. When this heeled section is calculated, you will see that the heeled waterline runs from a point just below the, let's say starboard, rub rail (it better be below the rail; there's no deck) to a point just outboard of the rabbet on the port side. By finding the center of area of the heeled section and squaring up from it until it crosses the at rest centerline, we determine the metacentric height, for whatever that's worth.
Now for the martini shot: by plotting the distance between the heeled centerline and the a line squared up to the heeled waterline from the keel's center of gravity, we are able to do a simple force x distance calculation to see what the lead does for stability. By comparing the shallow keel to the deep one, you will find that the difference is something on the order of 24 ft./lbs. As I said previously, sliding my 200 lb. ass 1.25" to windward will more than make up for any "lost" stability.
2. While we have quantified the the keel's contribution to stability, its primary reason for being is to bring the boat down to its lines. The traditional faerings had heavier scantlings, and were expected to earn their keep by hauling stuff from here to there. In the absence of both of these factors, the lead is required.
3. The very low aspect ratio of the keel as drawn is not likely to contribute in any significant way to windward performance. Period. Make of this what you will, but for me there just weren't enough pros to outweigh the cons when making the decision as to whether to retain the keel profile as designed. Mr. Atkin himself says that windward performance, even with his "improved" keel, will be limited.
By the time I was done at the drafting table the drawings I had were an absolute mess. I have posted a much simplified (and approximated) version of the above math exercise. There is a much clearer explanation of factors influencing stability located here: http://www.codecogs.com/reference/engineering/fluid_mechanics/ships/stability_and_metacentric_height.php
As far as the photos are concerned, all of the parts inboard of the rub rails are dry fitted. Inwales, mast bench, mast step and floor timber still need to be permanently installed. I laminated the mast bench to a slight curve. This is purely for looks. With the strong curves elsewhere throughout the boat, I was worried that a straight mast bench would appear to have a sag.
The mast is glued up, tapered, and eight-sided. The blanks for the remaining spars are milled.
I have been working on the short list of what boat to build next. I'll cover that next time.
And, finally, Brandon Ford has been building a Valgerda of his own out in Oregon. He recently launched her. Check out his blog here: http://valgerda.blogspot.com/
Congratulations, Brandon. Nice work.