Hybrid Construction

Constant chine radius (CCR) designs have been attracting my attention recently. I have seen it used for sailboats, for example. These images show a 12' solo canoe (lost pond style) that I have designed with ply bottom and sheer planks, and strip-built CCR bilges. The object is to simplify construction as far as possible.

The laminated ribs are bent about a dozen at a time over a ladder mold - all the same profile. lamination is used in preference to steaming because of consistent and predictable stringback, ensuring accurate profiles. The ribs are glued on 8" centers to the ply bottom plank, which is attached to a strongback (a 2 x 4 for this small hull) - eliminating the need for a building mold. The stems are handled in a similar manner.

Because of its constant radius the bilge strips can have constant angle beveled edges, all identical, and are not twisted along their length. Stripping stops at the top of the curved bilge and the ply sheers are added, using the portions of ply cutoff from either side of the bottom plank.

The short ribs near the stems are added before the gunnel is completed; they are cut down from standard ribs. The ribs provide the strips with cross-grain reinforcement avoiding the need for glass.

If she gets built, I plan to call her Rose Lee, after Gypsy Rose Lee, a famous stripper who also didn't go all the way. Any comments or suggestions?

 

To keep the same bevel on the stringers, they have to be cut with a curve. Loft the curve and you'll see.

 

Gonzo, the strips are cut straight but are only 3/4 wide. They would have to be curved if they were much wider, agreed, as the first strips have to bend across their width. I tested a sample strip and it handled the bend OK; the bend radius is more than 200 x width. The ribs can be clamped to the ribs over the curved portion to keep them from twisting; from the last full rib to the stem is straight which should be easy.

The original design was done in Carene08 which has a constant chine/constant plank width option, and the offsets were then imported into FreeShip for adjustment to final dimension and draft computation. FreeShip calculated the drag and stability which are similar to my earlier canoe (Dora) shown elsewhere and also to Rushton's Wee Lassie. This design has a better swamp angle than W/L due to reduced draft/more freeboard.

I did what I could on the theory to ensure she will be a handy and safe little boat. I am looking for opinions and previous experience from guys like yourself with hands-on knowhow than I have on the construction technique, before I build her.

 

I see little difference. If you have to edge set the stringers, adding some twist does not create much more work.

 

There should be a big difference in time. No molds to loft and cut out and no strong back to set up. The above boat uses 28 strips total plus the 3 ply planks; if it was to be built entirely of strips it would need approximately 48 - plus glass. Finally, only wood needs to be sanded which is faster and more pleasant than sanding epoxy. But I'm not trying to convert anyone ...

Gonzo: I haven't done a stripper yet, what is a typical build time, launch ready? I have heard 200 hours several times. That is a big reason why I am interested in finding an alternative method.

 

It's a shame Daniel Skira isn't here any more, you should drop him an email Terry as he patented the constant camber building method and would likely be a good source for your answers. I know we discussed it some time back.

Not all stripped methods require 'glass. Strip times can vary. I can strip up a canoe like that in a weekend, but it wouldn't be my first picnic with any of the techniques. How will you handle the plywood/strip interface?

 

Thanks PAR, it is good to confirm that I can avoid glassing. I did the math and the cantilever strength and stiffness of the ribs are greater than the center veneer of a ply-skinned boat would provide, and that is adequate.

The interface between the ply bottom and the first strip will need special attention although pre-soaking should take care of it. There is also the option of battens but it would be time consuming even though they would all be identical and could be pre-cut on a chop saw.

The joint between the last strip and the ply sheer plank can have an external rubbing strip which, I think, will also improve appearance; otherwise the change of material is going to look odd.

 

We are making cedar strip canoes of about 15', clear finished in about 90 hours. That's me paddling one on Christmas morning.

 

Thanks Gonzo: if 90 hours is the norm for professionals then the 200 hour figure I read should be a reasonable target for an amateur working alone producing a single boat.

The Blue heron site describes a strip built Wee Lassie (stretched to 13.5') that took 170 hours to complete, unfinished. I think that was a pro build but first time for the design.

If I can get back to some semblance of good health, it will be interesting to see how I do with this method. With all-plywood boats of about the same size, I have turned out finished boats in about 40 hours working at a fairly leisurely rate.

 

With a plywood bottom and sides, you should be able to shave twenty hours or so.

 

FWIW Terry,

I did a 16' daysailer and each strip took about 1.5 hours. Epoxy glued, hand nailed and my first try at it. I never seemed to get any faster during the project either. 90 strips and the most I ever got done was 6 strips in eight hours. My next stripper is going to involve compressed air and mechanical advantage.

Nice little project. Nice utilization of material.

I understand your desire to skip the glass and goo. Do you think the ribs will save time and effort in the regard? Most likely it will save bucks.

 

Ouch! That does explain the horror stories I have read. I’m not sure my attention span is good enough to finish a boat at that rate. this is a smaller boat with deliberately easy lines so I was hoping to average a pair per hour at 10 minutes each then cut the next pair of strips while the glue sets; all done in a day. I can almost hear the evil chortling of the more experienced guys; maybe I should shop around for a used bridge instead ...

True about saving cash. I wish I knew more about what I am trying to do - I haven't done laminating or wood bending. I am planning to do some experiments first to establish the optimum thickness and number of laminations. I’ll know more after that. I have some cedar for the strips and I am hoping I can use it for the ribs rather than the more usual stuff like ash, so I can use the same wood for the ribs and strips, but it’s kiln-dried and a bit splintery. If it doesn't work then I will have to make a trip to a decent woodyard; it's about an hour away.

 

My 90 hours for a canoe include milling the lumber and routing the bead and cove. Strips usually go six or seven at a time.

 

Strip building is like any building and dependent on your approach. Most back yard builders aren't especially organized, don't do things all at once and spend a lot of time scratching their head.

I don't think your common radius framing system will save much time, certainly not any weight and given the shape limitation as well maybe not as good an approach as a multi chine or all strip build.

For example, if you develop the scantlings for a rib free stripped bilge turn, multi chine, you can skip the ribs, which will likely have their weight made up in the extra strip/plank thicknesses and this will possibly be stiff enough to go sheathless. If it was me, I'd do the scantlings for a full traditional strip, knock 1/3 of the thickness off the strips for the plywood portions, use clever bulkhead/thwart location/spacing to avoid the ribs. You eliminate the shape limitations of a constant camber build, don't need goo and 'glass and the tedium of bending in ribs is also eliminated.

In the past, I've used a stripped bilge turn to "soften" the chines. It was done on considerably large craft and helped solve some issues, such as using a fair amount of "roll" in the topside and bottom planking to get the shapes desired, without having huge panel edge distortion/tension issues and to make the basically conically developed hull form, appear round bilge. This is commonly preformed in metal hulls, though more for appearance then hull shell stiffness.

As far as build times, you'll do better then most, from what I know of you. Strip building is tedious to those that are less organized then you. If you preform your work in the logical stages the method provides, then times will drop. In short, I've seen those that can only muster 2 or 3 strips a day, while others can slam through a dozen or more in the same conditions on the same design. You'll be well below the "horror stories" averages.

 

Paul: you make a good point about the impact of organization: I do my fair share of head scratching but once it’s done I get on with it! So I must take that into account when figuring out what - if any - time is saved. Maybe a good way would be to compare the time per area of the strip-built and ply sections.

Your comment on the tedium of strip building is one of my reasons for pursuing this concept. Replacing significant areas of strip-built hull with ply planks seems a logical way to save time. What has become clear is, that must be weighed against the time to form the ribs. At present I lack the experience to evaluate that time.

I share your concern about the impact of the shape limitation. Although FreeShip predicts the design should compare with my previous hard chine canoes and a stretched Wee Lassie, there is nothing like a one-to-one paddling comparison with a similar-sized boat and GPS in one’s pocket.

About going ribless: to get all the required transverse stiffness from the strips alone they must be either 0.4" thick cedar or plywood. Currently the ribs are about 1/3 the weight of the strips so there is little to be gained by omitting them. The ply strips require a chine joint method that I haven’t developed yet - unless I glass both sides - and going ribless adds the time and cash for a full mold. The ply sheer and bottom planks do not need rib support so I could save a little weight by tapering the ribs, but it’s not much. I can also reduce the rib count and add cross-grain strength with thin, closely-spaced short ribs just behind the strips, but it would look odd, I think. I doubt I can achieve a reduction of weight compared with strip-building,it's more the potential time saving and glassfree construction that's of interest.

This thread has lead to an interesting discussion. I hope I am not merely being stubborn, but it seems a worthwhile thing to try.