I am in the process of plotting plans for a Prospector project. I have asked for some asistance on a number of other items and have everything figured out except the few following things. I would really appeciate some help.
1.I have cut out the station molds in cardboard and they look great. But, nowwhen I look at them, I notice there is a problem. I followed the directions in the old Canoe Craft that mentions going up two inches from the sheer line of each station to make the strong back line. Now that I look at them though, it seems like they should probably all run up to the base line. Am I right about this?
2. My other stumbling block now is the stem mold. Here are my questions. The last plotted point is more than 10 inches from the centerline. Do I just follow the natural curve to the centerline to square the stem mold? Or does it end st the last plotted point?
3. How do I determine the shear line of the stem mold?
Plotting Station and Stem Molds- Please Help
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jpowers123
- Posts: 42
- Joined: Sun Jun 28, 2009 1:46 pm
I've been busy on other projects and really have not had the time to see what's going on in this forum. I see that you haven't had any replies yet. I'll try to pull out my copy of Canoecraft and see if I can remember how we plotted the forms. I went from the book only and I don't recall having much difficulty.
I've got the book in front of me turned to the page for the Prospector so I can relate directly to your questions.
1. I would hazzard a guess that the addition of 2" from the sheer line to make the strongback line is merely to raise the molds up away from the strongback since this canoe is less than 16 feet. If your strongback uses the design in the book the top will be 12" wide and 16 feet long. At the stem the sheer line for the stem will be just above the strongback unless you raise it up 2". When you drape the cloth on the canoe when you glass it there would be almost no space between the wood strips and the strongback. By raising it 2" you give yourself some working space. Naturally, if you add 2" to the stations at the end, you need to add 2" to the other stations.
2. The last point in the table at WL 22" is 10" + 5/16" from station 6. That's the key. The last point ends at station 6 which has a profile of 1' 11" + 1/16". Technically, this is supposed to be the centre of the mold at station 6. If your stations are made of 1/2" plywood this point is 1/4" off but on these curves that's not enough error to worry about IMHO.
Note that if you are going to have an inside stem, you will need to plot the curve then remove the stem thickness (typically 3/4" but you may have thiner/thicker stems). I did this by plotting the curve from the book then measuring 3/4" inside this at several points and used a batten to mark the new curve. Once you do that you can cut the station mold, bend and glue the inside stem to the shape of this mold, mount the stem mold to the strongback, and add the glued stem to the mold. now you can match the height of the stem to station 6 exactly when you shave it to add the rolling bevel needed. This was a long winded way of saying don't sweat it, you can correct the very slight plotting error later.
3. The easiest answer as far as plotting the sheer on the stem is you don't need to IMHO. I just followed the natural curve down to the baseline of the mold below the 2" WL point. The sheer line at the stem can wait until you plank the boat, again IMHO. You can follow the natural curve using batten or strip starting at least from station 4. I built a redbird and I found that using a strip to follow the sheer was too severe so I used a batten, established a sheer point on one side at one end, measured this point and marked this at the other three locations (other side and both sides at other stem).
One extra thing that I would mention since it looks like you are only at the plotting stage. I would recommend that you follow the natural curve to the baseline for all of the stations. If you look at the photos in the book the stations look like a series of mushrooms fastened to the strongback. There is no technical requirement to cut the stations in at the sheer line. As long as these are clearly marked on the stations, you can follow the sheer when you plank it. We cut our stations this way early on then we wanted to follow a suggestion to add 1 strip extra to add more capacity (by adding more freeboard). We had to figure out a way to do this since there was no mold to support this strip. If we had followed the natural curve we would have had no problem.
If this hasn't answered your questions or you still have more, bring them on. I've finished a couple of projects that has kept me away so I have time to follow this forum more closely now.
1. I would hazzard a guess that the addition of 2" from the sheer line to make the strongback line is merely to raise the molds up away from the strongback since this canoe is less than 16 feet. If your strongback uses the design in the book the top will be 12" wide and 16 feet long. At the stem the sheer line for the stem will be just above the strongback unless you raise it up 2". When you drape the cloth on the canoe when you glass it there would be almost no space between the wood strips and the strongback. By raising it 2" you give yourself some working space. Naturally, if you add 2" to the stations at the end, you need to add 2" to the other stations.
2. The last point in the table at WL 22" is 10" + 5/16" from station 6. That's the key. The last point ends at station 6 which has a profile of 1' 11" + 1/16". Technically, this is supposed to be the centre of the mold at station 6. If your stations are made of 1/2" plywood this point is 1/4" off but on these curves that's not enough error to worry about IMHO.
Note that if you are going to have an inside stem, you will need to plot the curve then remove the stem thickness (typically 3/4" but you may have thiner/thicker stems). I did this by plotting the curve from the book then measuring 3/4" inside this at several points and used a batten to mark the new curve. Once you do that you can cut the station mold, bend and glue the inside stem to the shape of this mold, mount the stem mold to the strongback, and add the glued stem to the mold. now you can match the height of the stem to station 6 exactly when you shave it to add the rolling bevel needed. This was a long winded way of saying don't sweat it, you can correct the very slight plotting error later.
3. The easiest answer as far as plotting the sheer on the stem is you don't need to IMHO. I just followed the natural curve down to the baseline of the mold below the 2" WL point. The sheer line at the stem can wait until you plank the boat, again IMHO. You can follow the natural curve using batten or strip starting at least from station 4. I built a redbird and I found that using a strip to follow the sheer was too severe so I used a batten, established a sheer point on one side at one end, measured this point and marked this at the other three locations (other side and both sides at other stem).
One extra thing that I would mention since it looks like you are only at the plotting stage. I would recommend that you follow the natural curve to the baseline for all of the stations. If you look at the photos in the book the stations look like a series of mushrooms fastened to the strongback. There is no technical requirement to cut the stations in at the sheer line. As long as these are clearly marked on the stations, you can follow the sheer when you plank it. We cut our stations this way early on then we wanted to follow a suggestion to add 1 strip extra to add more capacity (by adding more freeboard). We had to figure out a way to do this since there was no mold to support this strip. If we had followed the natural curve we would have had no problem.
If this hasn't answered your questions or you still have more, bring them on. I've finished a couple of projects that has kept me away so I have time to follow this forum more closely now.
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Patricks Dad
- Posts: 1476
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- Location: Warrenville, Illinois
I'm not following the thread entirely (having never lofted from the book) but the above did raise a small question for me. It was my understanding that the true definition of the station position was suppose to be the edge of the mold toward the end of the craft (rather than center of the mold). As the strips lay across the molds they contact with the outer (toward the stems) edge only due to the curvature of the strips around the hull (except amidship where they basically lay perpendicular to the station mold). Station molds should be aligned with their outer edges on the station mold positions not centered. At least this is how the plans define placement I believe. Not sure how this changes (if at all) for lofted plans from the book. I agree that this probably isn't a big deal but just a detail of interest to me.2. The last point in the table at WL 22" is 10" + 5/16" from station 6. That's the key. The last point ends at station 6 which has a profile of 1' 11" + 1/16". Technically, this is supposed to be the centre of the mold at station 6. If your stations are made of 1/2" plywood this point is 1/4" off but on these curves that's not enough error to worry about IMHO.
Could you comment further?
That is a very good point that I must admit I did not notice. However, that just proves my point that it doesn't really matter. I put the molds based on the centres of the mold and not the edge and after the boat is done I can't say that made any difference at all. I stand corrected.
The bottom line is that these are not build to micrometer specifications.
The bottom line is that these are not build to micrometer specifications.
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jpowers123
- Posts: 42
- Joined: Sun Jun 28, 2009 1:46 pm
Thanks Frugal, you have definetly answered my questions. So you are recommending not making them look like mushrooms but instead cutting an angle from the sheer line to the strong back line? I have seen this in some other books and it seems like it would be a stronger station.
The only other question I do have is I have noticed some books/webpages have cut the station molds right to the sheer line and others have gone an inch or two past the sheer line and then marked the sheer line on the mold. Is there any benefit to either strategy. It seems lime squaring the first strip would be a lot easier if it were cut at the sheer line since you could actiually slide a square under the edge, move the strp down and it would be perfectly square. I suppose that ir you cut at an angle to the strongback that tis is defeated though, huh?
The only other question I do have is I have noticed some books/webpages have cut the station molds right to the sheer line and others have gone an inch or two past the sheer line and then marked the sheer line on the mold. Is there any benefit to either strategy. It seems lime squaring the first strip would be a lot easier if it were cut at the sheer line since you could actiually slide a square under the edge, move the strp down and it would be perfectly square. I suppose that ir you cut at an angle to the strongback that tis is defeated though, huh?
I wouldn't cut them at an angle per se but I would extrapolate the curve using a batten or CAD sw if I had that. That way if I wanted to add a strip below the sheer line (which will make the canoe higher above the water) I could. Anyway I intend to do this on the next build.
If you have a clear marking on the Mold it won''t be hard to follow. You could also cut it square as most people do and use the small square like you propose. It will work either way.
We never used paper forms when we build our Redbird. Instead we carefully marked the points directly on the plywood and used a batten to follow these points and traced a line between these points. Using this method it is relatively easy to extrapolate the next point. We usually did this for one and copied those measurements for the other side so that they were consistent.
If you have a clear marking on the Mold it won''t be hard to follow. You could also cut it square as most people do and use the small square like you propose. It will work either way.
We never used paper forms when we build our Redbird. Instead we carefully marked the points directly on the plywood and used a batten to follow these points and traced a line between these points. Using this method it is relatively easy to extrapolate the next point. We usually did this for one and copied those measurements for the other side so that they were consistent.