Canoe Update

[catsittingstill]

So our regular readers will recall that I had the stations all drawn out and sawn out and sanded back to the lines for Binturong, and was getting ready to mount them to the strongback.

The strongback still has about 3/16ths extra rocker (the ends are 3/16ths lower than the middle.)  I decided that Binturong has so little rocker that 3/16ths extra won't do any harm, so I ignored that.  However when I set up the front end I discovered that the front station is so slim compared to the station next to it that the wood strips would have to take on a slightly concave curve there.  In other words, if they just ran straight from the second station back to the bow in front, they wouldn't touch the first station.

Concave curves are frowned upon in canoe building.  I'm not quite clear on whether this is because they change something about the handling of the finished boat, or whether it's just because snugging the strips right in to the station is hard to do consistently.  But I'm pretty sure I don't want a concave curve.

There are a few things I could do. 

I could leave everything as is.  None of the parameters of the boat would change.  On the bright side, I would find out just how hard it is to build and fair a concave curve.  On the down side, I would find out just how hard it is to build and fair a concave curve.  There are probably good reasons why previous canoe designers / builders have avoided them.

I could go back to Bearboat, and change the design so it doesn't have that concave curve.  One problem with this option is that it will be hard to see when I've just gotten rid of it--I don't want to make the nose fatter than it has to be, but the birdseye view doesn't have any way of generating tangential lines to compare it to, or to zoom in on a particular section.  Also the other problem with this option is it means a whole new set of offsets, a whole new drawing, and another sheet of plywood and sawing out and sanding back, and so on.  Realistically it would set me back at least two weeks.

I could leave out the end station.  This would give me a run of 27.5 inches from the next to last station to the end.  This is a long stretch of strips to leave unsupported--some designs only use stations every 18 inches or even every 2 feet, but unsupported strips can sag or twist or do other unauthorized, undesirable things.

Or... I could draw a new station that is wider.  I think I could just lay out the old station and use a compass to add about 3/8 inch on both sides at the gunwales.  Then I could use the french curve to get a smoothly varying line that would run from 3/8 inch proud of the old station line at the gunwale to flush with the station line at the tip of the station.  The down side of this is that I will be increasing the size of the boat under the water at this point, which will move the center of buoyancy forward (which I can probably counteract by putting my pack a little forward of my feet, which I would do anyway.)  This will also widen the boat at the waterline, 33 cm back from the bow, by about 1/4 inch (it would be 3/4 inch at the gunwales, but of course if the gunwales are submerged 33 cm back from the bow, you have bigger problems than how fast your boat is) which will tend to make the boat a little slower.  But realistically, probably only a little slower.  But it will mean I don't know where the center of buoyancy is anymore, and that makes me itch.

I think I am going to go with the drawing a new station option.  I don't really want to go back to the drawing board, I have things to do.  And as Kip points out repeatedly, if the boat is a dog I'll just unload it on some newb--I mean, some individual who places a high value on the aesthetic beauty of a wooden boat, and for whom handling characteristics are secondary.  Plenty of people thrash around on the lake in boats that are not designed for the purpose; I see them all the time, and they seem to be having fun regardless.

I tell you, you find some things out about your design when you go to actually build it that will probably inform the next design.  Like, don't get too fancy with the sheer, either.

Comments

[keris]

"if the gunwales are submerged 33 cm back from the bow, you have bigger problems than how fast your boat is"

Er, yes, that would slow the boat down rather drastically. Or perhaps 'down' is the operative word...

[boywizard.livejournal.com]

If the concavity is above the waterline it's not going to affect the handling or speed in any way. If below, I suspect it might have some effect on boat speed, depending on the amount of curvature. I can't see any likely effect on handling, which seems influenced more by the midship shape and the rocker. Esthetically, concavity can look a bit odd, especially if it is severe.

I think your hot-glue technique would be quite capable of 'snugging' the strips to the form; fishing line, straps, tape, etc. not so much. Of course, lots of people use staples (or nails, I suppose, in the old days), so I doubt that construction difficulties are the reason for the antipathy toward concavity. I'm just guessing, of course.

I think making a slightly wider station will work just fine, and as you say, if the boat is a dog (somehow that doesn't seem to be an appropriate term when referring to a boat), it will still be pretty. The prettiest kayak I ever saw would have been an awful performer (mostly made of ebony).

[catsittingstill.livejournal.com]

I agree regarding above/below waterline concavity and handling characteristics--and the concavity is greater above the waterline than below, so that's something. By the same token, the adjustments required to remove the concavity are greater above the waterline (where they will make no difference to handling except possibly how the boat behaves in wind) than below.

An ebony kayak! Beautiful, but *heavy.* I have seen pictures of a mahogany canoe, which was gorgeous, but would still be heavy--and I think ebony is denser than mahogany.

I have also seen pictures of a kayak under construction in what looked to be southern yellow pine. Definitely not my first choice as it is both heavy and stiff. But its strongly marked grain made for a beautiful boat.

[randwolf.livejournal.com]

I think you've made a good decision. I would worry about below-the-waterline drag at a concave area. I don't know much about fluid dynamics, but I can think of no small watercraft that use concave underwater curves, though perhaps that's a matter of convention rather than boat handling.

[catsittingstill.livejournal.com]

Well I think that speedboats and such do. I'm just not sure that the issues that apply to them apply to a canoe.

[randwolf.livejournal.com]

You're right, I think. In cross section, concave curves are pretty common. Viewed from the top of the boat, maybe not so common. I wonder if anyone has done hydrodynamic studies of canoes.

[catsittingstill.livejournal.com]

I believe some hydrodynamic calculations have been done. Actual drag tests are expensive and thus not very common.