Kempenfelt 12-1/2

LCF and Displaced Volume CG balance

Not to be overly rude, but I noticed in your model that it is not statically balanced. As shown on the attached picture the longitudinal center of flotations isn't aligned vertically with the center of gravity for the displaced volume of water. It means that there will be an equalizing force pushing the bow up and the stern down until these two centers are dead vertical above each other.

I went through this experience myself and want to extend my experience to others. If you apply approximately one degree transverse rotation to your model, the whole picture will be changed including wet area, resistance, KM, etc. etc. and you will have to re-think your model -- JMHO.

Kind regards.

 

Thanks for the heads-up, but the LCF is the center of the waterline area. It's the point about which the hull rotates when weight is shifted fore-and-aft.

The CoG of the displaced volume is the Center of Buoyancy, not the same thing, and is usually slightly forward of the LCF.

The boat's CoG is actually not shown in FreeShip. Essentially, FreeShip tells you where the CoG needs to be for the boat to float on its lines.

In practice of course for such a small, lightweight boat all these things will move around with the crew . . .
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In principle the vertical chine method employs ply planks laid athwartships instead of fore-and-aft. For most boat designs it is not necessary to cut the ply into separate planks, these can formed by clutting slits partway across the ply to allow the ply to be assembled to the boat in one piece, and simplifying alignment of the many planks.

Probably the best plan would be to bend the inwales to shape, cut the slits and then glue the ply sheet(s) to the inwales. In practice the slits will need to be wider at the keel but this can be shaped with a plane as the fingerlike planks are bent one at a time over the building form. Also for most hull designs the fingers can become wider toward the stem.

 

Thread Update

I have shelved my plans for the Kempenfelt 12-1/2 as I don't really have a use for a sailboat at this time. However I am still interested in the vertical chine construction method and I would like to try it out on a canoe. I don't think vertical chine is a good name for it, perhaps transverse planking would be better.

Unlike the previous post showing the ply sheet partially cut through to form fingers, I plan to cut the ply sheet into separate planks. Using the constant bilge radius design approach combined with a circular arc sheer plank yields plank outlines that are easy to develop and, interestingly, identical for much of the hull. That could lead to significant time saving during a build. The identical planks would cover the midsection, perhaps 50 to 75% of the hull's length, with the skin near the stems planked in a hopefully single piece of ply using a more-or-less conic shape to avoid excessive ply torturing.

I am looking at Walter Dean's classic canoe the Sunnyside Cruiser, it's a bigger boat than I usually build which will be easier to get in and out of than my solo boats: I'm getting stiff in my old age. Since this is a totally different design I'll start a new thread when I start the build, and post a link here once I start. Restarting boat building has been delayed by a recent protracted illness. I actually finished treatment early this year but it took a long time to rebuild my strength and get over the drug side effects, then tackle all the jobs around the house and garden that I put on hold.

 

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Glad you're getting better, Terry! Best of luck....

 

Mason Smith's Adirondack Good Boat design uses Constant Camber construction to build what could be described as a fat canoe with a transom, though that description might be misleading for a very nice boat.
http://www.adirondackgoodboat.com/goodboat.html
http://www.adirondackgoodboat.com/goodboat2.html
http://www.adirondackgoodboat.com/goodboat3.html

 

I have always like long skinny decked sailing canoes myself.

 

Wayne: the Sunnyside Cruiser is long, skinny and half-decked . . . close enough!

David: the chipmunk canoes on that site, while cold-molded, are similar to what I aim for in this next effort I am planning.

Doug: many thanks for the good wishes, I am looking forward to my next chapter as a boat builder . . .

 

David, I took another look at the Good Boat site. I noted that the Chipmunk canoes are built on the same mold as the rowing boats. recently I was working my way through some old issues of the Wooden Boat Magazine and in Issue #64 (May/June '85) I came across an article called "Bagging The Gull" which describes an similar process to that used for Good Boats.

The Gull has a molded wooden laminate hull that is vacuum-bagged on a Constant Camber half-mold that is used for both sides of the hull, which are trimmed and taped together with a transom in a manner similar to Kurt Hughes' cylinder mold method. Unlike KH-CMM the sides are slewed across the mold to achieve the desired shape of stem foot; the second side is slewed the opposite direction - clever! The same mold can be used for several designs, like the Good Boat mold.

It has gave me an idea to impove transverse plywood planking. As I was saying in post #34, I can use a constant bilge profile - which I now realise doesn't necessarily mean constant bilge radius - combined with a circular arc sheer plank to obtain identical plank developments. I propose to use that over the center part of the hull and employ larger ply panels for the hull at each stem, allowing them to take a natural shape as much as possible to define stem shape without too much torturing.

I'm still playing around with the idea and performing some experiments to prove - or disprove - the entire concept, establish building requirements from a strength perspective. I also want to see if the shape at the stems is predictable, so I can avoid the build it then draw it approach which is intellectually unsatisfactory.

 

I read through the thread once more - can't get this idea out of my mind. I never addressed my model-making efforts, but I tried it and it seems I have little talent in that area. Mainly it's a problem of getting the proper mterial, model shops do not carry what I need in the sizes I need for stuff like gunnels, the hardwood I have won't bend, the softwood I have is just too coarse-grained, also my workshop equipment does not have the precision I need for such small dimensions. So if it is to happen, it must happen full-size.