Redbird 17' Sanity Check

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woodworkbythewater
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Redbird 17' Sanity Check

Post by woodworkbythewater » Thu May 30, 2019 6:41 pm

First canoe here.. Need some help from the experts as I prep to build a Redbird from the CanoeCraft Book. I laid out the station mold according to the table of offsets from the book, using hand-drawn graph paper and carbon paper to transfer the drawings to sheets of MDF, which were doubled up and cut out on a bandsaw.

My issue is there seems to be a large jump between station 4 and station 5. Where the hull would be "deeper" at station 5 than it is at station 4, essentially creating a "reverse rocker" of sorts. I'm not sure i made a drawing error, or if there's something else going on. All the station drawings were finished by connecting the highest points parallel to the baseline, then adding a 2 inch high "tab" to that line where it would connect to the strong-back. I did not pay attention to the Sheer line numbers on the table of offsets, as they weren't mentioned in the book.

I had issues attaching the photos and had to scale them down considerably, hopefully they are still readable. I want to take Ted's advice and build the most perfect mold I can before I begin. Any insight would be most welcome. Thank you.
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Cruiser
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Re: Redbird 17' Sanity Check

Post by Cruiser » Thu May 30, 2019 7:18 pm

Those tables are a great start, but you should check for typos/corrections for them, I only had one correction for my Freedom 17

Maybe contact BearMountain directly or maybe some one here can find a list of offset corrections .... make sure you know which version of CC you have, I believe there are different typos in each.

Brian

Stephen
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Re: Redbird 17' Sanity Check

Post by Stephen » Fri May 31, 2019 1:16 am

These are the corrections for the 2000 edition of Canoecraft. There are no corrections listed for the Redbird at all, so I don't think that is your problem. http://web.archive.org/web/201211081136 ... estems.htm

I don't have the 2015 edition but I know at least one error for the Rob Roy is still uncorrected. I suspect none have been corrected. Hopefully no new errors were added.

Looking at your chart there is definitely something off but it's hard to tell what. Canoecraft does not do a good job explaining how to turn the table of offsets into a canoe plan. You should check this thread. viewtopic.php?f=9&t=4658 These instructions are for a Freedom 17 so there will be some minor differences. Since it is an asymmetrical canoe every station is different, whereas your Redbird will have matching pairs of all but station 0.

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Re: Redbird 17' Sanity Check

Post by Cruiser » Fri May 31, 2019 8:15 am

Makes me glad I made that record of lofting, thank you for digging that up and posting it Stephen.

There isn't going to be any difference in the actual lofting for asym versus sym other than you don't need to loft as many stations.

Brian

Stephen
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Re: Redbird 17' Sanity Check

Post by Stephen » Fri May 31, 2019 3:21 pm

I'm not totally sure I know what's going on here but I have some ideas. There are three caveats:

1. I lofted the Redbird a LONG time ago in a graphics program but I never built it.
2. My copies of Canoecraft are MIA after a recent move so I'm relying on memory for methods and terminology.
3. Most of my knowledge is from studying other people's problems on this forum.

One thing I noticed in Brian's post on plotting is that his baseline is at the bottom of the page whereas yours is at the top. This isn't an error on your part, just a difference. I'm pretty sure I remember that the baseline for the Redbird is at the top of the page in Canoecraft.

The curves themselves that you have plotted look like they might be OK but it's hard to tell. I think your problem lies in not defining the sheer line and the strongback line properly.
I did not pay attention to the Sheer line numbers on the table of offsets, as they weren't mentioned in the book.
The sheer line is very important because it defines the gunwales of the canoe. The points you plot for each station define a line that starts at the centerline and arches toward the bottom of the paper. The sheer point for each station tells you how far toward the bottom of the paper the line goes. You should plot the height of the sheer on the end of each station curve. As an additional check on your plotting, you should be able to use a flexible batten to join all the sheer points for each station in a fair curve from station 0 back to the centerline. You can see that if you did that the way you have it plotted now you could not join the ends of the stations in a fair curve this way. Once the sheer points look right draw a horizontal line from each sheer point back to the centerline.

This brings us to the strongback line. You will see that each station plot resembles a mushroom. The hull shape forms the curve of the mushroom cap and the sheer line defines the flat bottom of the mushroom cap. You now need to add a stalk to each mushroom cap, but how long should they be? Because every sheer point is a different height, every mushroom stalk will need to be a different length. The bottom of each mushroom stalk defines the strongback line. You need to hold each mushroom cap above the strongback at the same relative height to each other as they have on your plan. (I'm not sure if this is making sense even to me. Sorry.) When you cut out your molds, you should be able to stack them from "0" to "7" on a table, then use a square or the factory corner of a piece of plywood to line up the bottom of each mushroom stalk (the strongback line) vertically with each other. Now looking down on the top of the pile you should see an exact replica of your paper plan.

Here's an object lesson in getting the strongback line wrong. viewtopic.php?f=1&t=3678 The builder here has added the same length of stalk to each sheer point. He has set out to build a canoe with a flat top like an aircraft carrier and a severely deformed hull.

When you plot your strongback line, the usual method is to find the highest point on the canoe, which is likely either the bow or the stern where it intersects the sheer line. While you could make your strongback line here, most people add 2" or so to prevent banging their knuckles on the strongback top later. You can add whatever you want as long as you add the same height to each station and to the stem molds. Be aware that if you add too much it will make the canoe pretty high off the floor to work on.

Looking at your paper plan I don't think it's quite tall enough to draw your mushroom stalks. Sorry about that. You might be able to tape a piece of paper to the bottom as long as accuracy didn't suffer. This is assuming the curves for each station are correct, which they may well be. If not you'll have to start from scratch anyway.

That's all I can think of for now. Please scream loudly if I made things worse :)

Stephen
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Re: Redbird 17' Sanity Check

Post by Stephen » Fri May 31, 2019 9:41 pm

You now need to add a stalk to each mushroom cap, but how long should they be? Because every sheer point is a different height, every mushroom stalk will need to be a different length. The bottom of each mushroom stalk defines the strongback line. You need to hold each mushroom cap above the strongback at the same relative height to each other as they have on your plan.
I still hate the way I explained this, the reason being that you shouldn't measure from the sheer points to get the strongback line at all. What you should do is measure from the highest point on the canoe (bow or stern), add 2" or so if you wish, then draw a single horizontal line at the bottom of the page to represent the strongback line, which is the same for all the molds. Then measure over from the centerline 1/2 of the width of whatever strongback you are building (6" for a 1' strongback) and draw a vertical line from the strongback line up to the station "0" sheer line to represent the sides of the mushroom stalks.

Now when you measure from the strongback line up to the individual sheer points for each station, you see that each mushroom will have a different height of stalk. This is what I was trying to explain so poorly before.

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Re: Redbird 17' Sanity Check

Post by Cruiser » Sat Jun 01, 2019 8:16 am

Stephen, I believe in that post series, I plot the forms both at the top and bottom and explain that there is no difference between the two views, it really is just a matter of convenience when using the area you setup for plotting.

WoodWorker, there is definitely something wrong with that station ...relative to the others. Stephen may be correct that your process to plot the lines may not be working correctly. I know it took me some head scratching to figure out how to get the lofting done ... I couldn't find it anywhere and that's why i did the detailed lofting post.

I know the method I posted works well, as I made the forms from lofting, then bought and checked them against the printed plans ... it was simply an exercise to learn how to loft and share the results. The post was designed for a person to learn the lofting process for a canoe, using the lofting tables ... there are a few areas where it is more than X Y plotting, it is important to get those points right as well.

I think most of what Stephen is stating is covered in that post and hopefully, if you review the method described there you can figure out where the two differ and that is likely where the issue is ... hopefully it is a simple fix and not a complete form "redo"

Please continue and keep us updated on what you find or don't find .... if you don't find the issue, I will dig deeper and plot up a set from CC and see what I get for the same table.


Brian

Stephen
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Re: Redbird 17' Sanity Check

Post by Stephen » Wed Jun 05, 2019 10:40 pm

Hello woodworkbythewater,

I have finally located my copies of Canoecraft and it would appear you are using a 1983 edition of the book. This means that some of the terminology is different and also the method of plotting the sheer is slightly different than what Brian has posted.

These are the values I have in my book for the Redbird sheer:

Chart A
St Sheer
0. 13
1. 13 1/16
2. 13 1/4
3. 13 1/2
4. 13 7/8
5. 14 3/8+
6. 15 3/8
7. 17 13/16
8. 21 1/4

It's probably safer if you could confirm which edition of the book you are using and whether these sheer values match yours before I offer further advice. Does Chart B in your book have sheer values listed?

I hope we can help you build a beautiful canoe.

Stephen

chrisd7306
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Re: Redbird 17' Sanity Check

Post by chrisd7306 » Wed Sep 11, 2019 7:12 pm

Hello,
I have the 1983 copy of Canoecraft which has the Chart B sheer values missing.
Chart A has all the values as displayed in the previous reply from Stephen
Is there anywhere on the forum that I can get a copy of these missing values?
I have tried a search of the forum and internet but it drew a blank.

Thanks for any help.

Chris

Stephen
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Re: Redbird 17' Sanity Check

Post by Stephen » Wed Sep 11, 2019 9:55 pm

Hello Chris
Does Chart B in your book have sheer values listed?
My 1983 copy of Canoecraft doesn't have sheer values in Chart B either. The book only ever had sheer values in Chart A. Sorry if my question in the previous post misled you.

I'm unaware of any online information with respect to the 1983 Canoecraft. There are corrections on the Bear Mountain site for the 2000 edition but the offset tables in that book are substantially different from the old book and wouldn't apply.

Stephen

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