Modifying/utilizing forms...


I have forms for a stripper.
How difficult would it be to use them to build a mould for a wood/canvas canoe?
The forms are for the 15' Ranger (a downsized Prospector).
Please and thank you.
Well, I'm no expert, but I'd imagine they'd be usable, just maybe have to be trimmed down to account for the thicker form-sheathing, ribs, and planking used in W/C construction. Cedar strips are usually, what? about 1/4" thick? Building a W/C mold, you'd have to factor in that the sheathing is usually 3/4" thick, ribs are 5/16" thick, and then planking is 5/32" (metal bands account for some thickness too, I suppose, though I doubt 20 guage strips count for much). So the difference in thickness would be... well... just over an inch, if I'm doing my math correctly. So in order to get the same exterior hull shape and dimensions as your strip built boat, you'd have to trim each station down by just over an inch before building them into a mold. Hope this helps, and if it doesn't, hopefully it starts a conversation. Best of luck
quite possible

I should think it is very possible. If you don't account for the difference in thicknesses betwenn stripper and rib/plank all you'd have is a different design. OR--you could make an open style form. See past Journal article by--I think Alex comb.
Actually, if you want the shape to be smooth and accurate you have to vary the deductions made, depending upon the angle at which a particular section of the boat's planking and the form's sheathing cross the stations. This includes small variations in the amount deducted from specific spots on individual forms. For example, look at a station that's about 25% of the boat's length from one end, like one you might find near the bow seat on a tandem canoe. In this spot, the sides of the hull are tapering toward the stems. Assuming that our station forms are square to the strongback or keel line, the side planking is crossing the plane of this station form at an angle. If our canoe's ribs, planking and the planking on the canoe mold add up to say a 1" deduction, it will only be accurate in spots where the planks cross the stations squarely. If they cross the station at an angle, as they do here, more has to be deducted from the station. This is because a 1" thick board is only 1" thick if measured straight through, perpendicular to its surface. Measure its thickness at an angle and you will get a larger measurement. The greater the angle, the thicker the measured amount becomes and the more you need to deduct from your station form in that spot.

On the boat's bottom at that same bow seat station, there may be very little deduction needed. The sides may be tapering toward the stem, but the bottom generally won't be tapering nearly as much unless the boat has a lot of rocker. So for a typical canoe shape, a proper station in the bow seat area would likely have the "normal" 1" deduction along the bottom of the station where the planks will cross the station pretty squarely, and might have an extra 1/4" or so deducted from the topsides where the hull's taper is causing the planks to cross the station at an angle. Each station form will thus need deductions made from its shape which depend on the angles at which the ribs and planks of the canoe (and in this case, the thickness of the planking on the canoe form) will cross it. Failure to do this tends to make boats with humpy or wavy sides.

Confusing as it may sound at first, this is pretty standard, old-school boatbuilding. Designers draw boat plans to the outside surface of the hull, not to the forms required to build that hull. Making the proper deductions for the thickness of the forms, frames, ribs and planks that will eventually be the hull is the job of the builder - and doing so properly is what eventually makes the boat come out as an accurate representation of the original design. We might not really notice a big difference if a canoe came out 1" bigger all around than the design called for, but it might make a substantial difference on a big boat, with that extra hull volume throwing off a whole slew of rather important performance and safety calculations. In both cases, it can result in a hull that has wavy sides that aren't fair.

The thicker the hull, the more important this deduction work becomes. On a 1/16" thick fiberglass canoe, even if the glass crosses the plane of the form at a pretty severe angle, the deduction is small enough that you could probably ignore it. On a strip canoe that's 1/4" thick, you can usually start to see the shape distort in places if you don't make the variable deductions. By the time we get to the combined thickness of ribs, planking and the planking of the form itself for a wood/canvas canoe, it could be a big problem.

Traditional boat builders use a home-made gizmo called a bevel board to help determine how much to deduct in various spots from their station forms. It's certainly possible to recycle some stripper forms and build a w/c building form, but check the library for old boatbuilding books first and most will have a section where they discuss deducting for plank thickness and they will explain the process in greater detail and probably show how to make and use a bevel board. This is, by the way, one of those tasks that modern computers do very well. With good boat design software, you can specify the hull thickness and the computer can spit out the dimensions for accurate building stations with all deductions made (and in some cases even print or cut them out). It is still possible though, to do them the old-fashioned way with little more than some scrap wood and a pencil.
Figured as much...

Thanks Todd. I kinda knew deep down that more was involved than just trimming off the thickness of the form strapping, and planking, and ribs. I think I'll do some homework for a while till I get it all straightened out, first in my head, then on some lumber. I may just let the idea die. Not sure, but I've got the winter to think about it.
Again, thanks to all for your input.
For the record, Howard Chapelle's book Boatbuilding says (subsection 'Deducting Plank Thickness in Body Plan Sections') that the method for reducing the size of offsets that are drawn to the outside of planking to make molds to fit the inside of the planking simply reduces the station sections by the uniform thickness of the planking. He does not indicate a need to correct for the bevel of the planking. Using a compass set at the thickness of the planking, draw a series of points inside the outside-of-plank section that now represent the inside-of-plank section. No correction for angle.

Total plank, rib and form thickness for a wood/canvas canoe is around 1.25". Assume, at a point near the bow, the angle of the planking to the centerline is 20 degrees. The trig says that the apparent total thickness of the plank/rib/form at that angle is 1.33" - or, slightly more than 1/16" difference. This is certainly negligible. Wood canvas canoes are not that precise to worry about 0.080".

Chapelle also, in his thorough Chapelle-esque way, discusses the construction and use of bevel boards, which are used primarily for measuring bevels for sawing out components like sawn frames or transoms, not for making the mold stations.

I built a stretched Ranger from the Ted Moores' Canoecraft offsets by adding an inch to the station spacing. It's a great boat. There is confusion in the text where page 42 says that the offsets in the plans are to the outside of the planking, but when you look at the offsets they actually appear to be to the inside of the planking - the Ranger has a stated beam of 35 1/4", but the half-breadth offset for the middle station at maximum beam yields a beam of only 34 5/8". So, my point is, be worried about precision but don't stop if things aren't just perfect.
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How about spring-back? A stripper does not have a lot of spring-back but a WC does. Would it be enough to consider while building the mold so you don't end up with a rounder bottom than you planed for?
I agree about spring back being present in WC canoes, but so far it hasn't bothered me. I am working on a mold now and building the lines a little flatter through the bottom than I hope to end up with. I have built boats, like the stretched Ranger I described, on a mold built to the designer's offsets for stripper and, yes, it is rounder in the cross sections than the design, but it is not a problem, at least for the boat I built. I did take the precaution to thin the ribs from 5/16 to 1/4" from the turn of the bilge to the inwale both to save weight and to help the ribs through the turn, inducing less retained tension and therefore tendency to arc through the bottom. The use of half-ribs will also give some resistance to arcing. I think the amount of springback also has to do with the cedar used, and how well it is steamed when bent. So - yes, plan for it if you want to take the time to loft the drawings.
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