Drawing Station Molds From Offsets?

[Scot T]

I have some offsets for a very nice design I’d like to build in the not too distant future. Can I use those numbers to plot on paper the shape of the stations for the mold? I realize that they are (in this case) to the inside of the planking and I have to subtract the thickness of the ribs, ribbands etc. Is there a method whereby I can subtract that thickness and arrive at the proper points for the station shape?

I read a few days ago (I cannot find the article now, of course!!) about a fellow expanding a design he particularly liked by multiplying the offset numbers by the difference between the two sizes. If I remember correctly, he had offsets for a 16’ and thought a 17’ would suit his need better. He figured that 17’ is 106.25% larger than the 16’ so he multiplied the offsets by 1.0625 (and set the stations further apart) and arrived at a canoe that was the same design but larger in length and width (and possibly deeper, I don’t remember). Therefore, if the above technique works one way (make bigger) it should work the opposite should it not, (make smaller) given the correct formula?

I’m in the process of building my first mold for a wood/canvas canoe and am working off a set of drawings for the mold stations which are intended for a cedar stripper. I marked them, cut them out and cleaned them up nicely and then with the use of a gramil (marking gage) subtracted the thickness of the ribs etc to arrive at the size needed for the w/c mold. Cut and cleaned up again. A bit of extra work it seems so that’s why I’m asking about a method to save time by drawing instead of cutting. Anyone have any hints?

 

[Blue Viking]

Unfortunately, I am one of those persons who cant count past ten with taking my shoes and socks off...BUT...as you described your intentions, I am seeing a canoe, if your making it smaller, as being a much narrower vessel!...I hope you receive some input on this. I have just made some stations in an attempt to reproduce the design of the 1974 STOWE from Vermont with a little more rise on the stems, and one inch deeper. The intention of this design is to get a 14' canoe that is wide, 40" and therefore more stable for the avid flyfisherman and enable him to stand and have better balance while flycasting in tight places...Its that balance thing and I feel that this type fo vessel would allow the position of the feet (wider apart) so as to give more balance....Too old to go back to school and wouldnt make a good student anyway GOOD LUCK

 

[Dan Miller]

Depending on the source of your offsets, you probably should loft the canoe full size before doing anything else. Not only will you discover the inevitable mistakes in the table of offsets, but you will ensure the hull is fair before beginning. You'll also be able to pick up a lot of other details useful for the construction of your canoe, including stem and gunwale bending patterns, deck shapes, thwart lengths, etc.

Once you've lofted, you'll have full size stations that you need to reduce. Assuming you've lofted on plywood, lay a piece of mylar on top of your stations and secure it in position. Draw your reference lines - centerline and baseline (equals location of strongback). Now simply measure your station reduction and mark it as a point on the mylar. I eyeball this perpendicular to the station mold at any given point; there are more precise ways to do this that are described in lofting books, but this is plenty accurate enough for a canoe sized project. Label each point as to its station number. Give yourself enough points to later spring a batten around. Once you've marked your points. Transfer them to your station stock with an awl, spring a batten around them and cut them out.

You could probably get by doing the same directly from your offsets, but lofting is such an integral part of the boatbuilding process and offset tables so often have errors, that I don't recommend skipping it.

BTW, it does appear that Chestnuts pleasure canoe line may have been designed by increasing or decreasing the size of one by a percentage overall. But this is not confirmed yet...

 

[Steve Lapey]

When I made my form for the cedar canvas canoe I reduced the templates that I had used to make a stripper canoe by the ammount of the total dimension of the planking, ribs and the stock that I would use to strip up the form plus 1/4" (the thickness of the stripper hull)
This procedure gave me the information I needed to cut out the new stations to make the new form. Two canoes later, it appears to have worked. The wood canvas canoes are the same dimentions as the stripper. Good luck with your project.

 

[Dan Lindberg]

Scot,

The answer to your question is yes, to revise an existing design "slightly" larger or smaller the preferred way is to multiply all the dimensions by the percentage desired.

With that said, it probably doesn't happen very often, except for maybe the designers. Within the stripper community, usually only the length is changed (by changing the station spacing slightly, and uniformly). If I remember correctly, when I asked this question of the designer of my plans, he said to limit the change to 5-10% max. This does result in a different shape then the original.

On my current project, I increased the size by just running that full size station drawings through a copy machine set at various enlarge sizes. If the machine works correctly, this results in all the dimensions increasing by the desired amount. (If you do this watch the copies, some machines "slip" in the length direct and don't copy exact in that direction.) I ended up at I think using the 5% larger lines.

And I agree with Dan, if your starting with offsets, you should loft the lines full size to assure they are fair. There are plenty of reports of offsets having errors and you'd rather find any before you cut wood.

BTW, I started with full size drawings that had been layed out and faired by the designer/supplier. As I changed more then just the size, I should have lofted it but being lazy I didn't, and I did end up with a couple "corners" that i didn't/don't like.

Dan

 

[Todd Bradshaw]

If you really want accurate stations, you also need to understand the methods of deducting planking thickness. On a canoe hull, you can often get away with just deducting a fixed amount everywhere from your hull's outside (like 1/4" or a touch less all around for a stripper built from 1/4" strips, for example) but it doesn't really produce a truly fair hull shape and the thicker your hull materials will be, the less fair it becomes. This is because planking intersects the stations at various angles in various places on the hull. On a dead straight run, a 1/4" plank really does add 1/4" of hull thickness to the dimensions of the stations. But up near the ends in particular - where the beam of the hull is narrowing and the canoe is coming to a point, some of the planking is crossing the stations at an angle and plank thickness is effectively increased, requiring a slightly larger plank thickness deduction at those points. See drawings below and notice that the red arrow showing plank thickness at the station is the same length in both drawings. To really get accurate stations and a fair hull, you would need to deduct slightly more for planking thickness in places where the planks meet the stations at an angle - otherwise your hull would tend to bulge slightly in those places.

To make things worse, on any single canoe station mold you may have some places on the boat's sides that are angled and in need of a slightly bigger thickness deduction and others (like on the bottom) where plank and station are meeting pretty squarely and no increase in thickness deduction is needed. The farther away from a square intersection between plank and station you get, the greater the deduction needs to be.

When dealing with small boats and thin planks there are obviously practical limits to striving for accuracy and chasing that perfectly fair hull shape, but it doesn't hurt to be aware of the concept and what's going on - even if in our typical building scale the measured differences tend to be small. A good lofting book will explain the most efficient methods for making accurate plank thickness deductions as angles change in different spots on the hull.

 

[peter osberg]

The math may hieghten the satisfaction for some, but hopefully will not dissuade anyone from playing with the lines of a canoe and building it by "rack of eye". Visualizing from a set of line drawings seems to work for some. After all, canoe designs predated the notions of drafted plans and offsets.

 

[Dan Miller]

Lots of butt-ugly and poorly performing canoes have been "built by eye". Consider that before foregoing the lofting process. You're investing a lot of time and materials, and it would be a shame to end up with a piece of junk...

 

[Scot T]

Thanks Guys,

Excellent answers. I was just being a lazy butt and trying to cut down on the work. It never makes things better to cut corners. At least that's what I tell my young chefs in the kitchen...nice, I don't even listen to my own words.

Todd, that's a good point you make concerning the thickness changing as the angle changes. I knew about it but as my last canoes were strippers I disregarded that info with them (not going to build those anymore, the epoxy hardener and I don't have a good working relationship. The stuff gives me a nasty rash). I can see that with a w/c form that there is a much greater thickness one has to deal with between the canoe planking and the form stations so there is more need to take that point into consideration. Very good point, I'm glad you reminded me.

Dan, I'll go over the "Lofting Demistified" articles again and I'll pick up that lofting book that keeps calling out to me every time I go into the local "used nautical books" store. It will be a valuable and interesting exercise to loft the next project full size. Get out a couple sheets of plywood on the shop floor and go to it. I might as well start over with the one I'm working on now as it is still early in process of the building of the form. "If you don't take the time to do it correct the first time, you sure as H... take the time to do it correct the second!".

That's interesting about the Chestnut pleasure canoes maybe being designed by increasing or decreasing the existing plan. I had wondered about that concerning some other companies with a large range of models and sizes. Seems like a logical thing to do.