Design flow for plywood hull?

Hi!

I am new to boat building. I thought as a first project I might build a kayak from plywood.
First I thought I will look up some free design on the net similar to my needs, feed the plans to the table saw, and glue it together.
I have realized that this is not as easy as that. I have find no plans as finely developed as that, and one possibly needs multiple design tools and an understanding of boat design which I do not yet...
I now have a design flow in my head. I realize that maybe there may be no design tool helping in every step, and some steps should be done by hand. I try to ask my questions along this design flow. Please help me fill in the leaks, and correct any false assumptions I made (there are probably many).

I will revolve around open source or at least free software tools, like hulls, freeship, qcad, etc.

1. Come out with a basic design in an electronic form. This part is relatively easy, "Hulls" program have many hull designs, and if I start a new project in freeship, it also comes up with a reasonable boat along the parameters given. I can also put pictures into background of freeship and recreate the design.

2. Make it developable. Hulls makes developable hulls afaik, and the designs coming with it are fine in this respect. Unfortunately it is very limited what I can do with hulls, because it keeps crashing in windows emulator. My questions here:
- I guess that having developable hull means that the edges corresponding to the same plate should have the same arc. Is it true?
- How can I convert an arbitrary design in freeship into a developable approximation?
- What other (open source) tools are there to plan a developable hull, or convert it to be developable?

3. Keep it developable. Maybe this question is silly, and maybe it is pertaining only to freeship. I started with a developable design. Then I did transformations which I thought keeps it developable. But they did not. The transformations are the following:
- remove a face
- add a new face using three points of a just removed face
- add a new face using the points of a station (wanted to add a beam/bulkhead). It made the plates bend, which I do not actually understand.
What could be the problem with that?
Also how can I add new elements in a developable manner?

4. Define structural parts, like bulkheads, beams and strakes
Sorry, here I not even know the terminology, so I try to put in as math.
I want to define vertical, horizontal, and along-the chine intersections of the hull.
Then I want to substract from these faces, to make them lighter and make room for things, like the leg of the kayaker.
There are more possible ways I can think of, but don't know how to do them:
- define these things in freeship. But how can I do that?
- export the hull into a 3d cad program, and do it there. I have brlcad handy, but I am too dumb to use it.
- export the hull into a 2d cad (qcad for example). It works with the stations, but I cannot see a way to export other intersections of the hull.

5. Convert the output for the table saw.
I did not even begin to figure out how it can be done, whether a table saw can tilt the saw, etc.

 

There is a lot of questions to answer here and a lot to each answer. I will try to answer some of these questions but not all at once so you will have to be patient.

Developable: to have a design be developable it must be able to be laid out onto flat sheets. It must also conform to certain physical constraints...it has to come from either cylindrical or conical projection...in other words the panel must be able to take a cylindrical or conical shape to come together. You cannot truly bend a sheet of plywood into a compound curve so it must either bend one way, the other way or roll into a cone shape...hence the above projections.

Keeping it developable...your first mistake is trying to develop a 3 sided face...Freeship doesn't like less than or more than 4 points to a face...although you can use more if you keep them in line...but to what point? The mesh that freeship uses is optimized for four point faces or combinations of faces containing 4 points.

Adding bulkheads and such...The best way to add a bulkhead is to create a separate surface and then place it within the hull. To do this...extrude an edge by .1, then extrude it again immediately by 2 or 3. This will give you 2 faces, a small one and a large one. Delete the lines connecting the original edge and the large face and you will have a separate surface. Make the separate surface a new layer and then export it as a .part (this will allow you to import it at will and use it for other designs). Now you will need to position this surface in the right place and shape it into your bulkhead. Switch between Shaded view and Wireframe to nudge what is sticking out beyond the skin to just disappear...you will get the feel for it as you go. To make a hole in a bulkhead you will have to add points to the interior edge then move them around to fit...keeping in mind that you are working with half a hull, that Freeship likes its points in sets of 4 and you are generally trying to keep things flat for a bulkhead...to keep it developable. If you keep in mind that to be developable it must bend in a cylinder or a cone ask yourself if when you modify a surface...will a sheet of ply be able to use one of the two to bend to that shape? A bit of red in an otherwise green (developable) panel does not necessarily preclude you being able to get plywood to take the shape...but a lot will and it depends on where in the panel the red is. If it is along one edge only then there is a decent possibility that it will torture into shape but if it is in the middle of the panel then there is a problem and your shape won't work.

Below is the internals I did for one of my doodles...it all started with one panel as described above, then extruding and shaping as I went until I had what I wanted. I then added another face and did the same thing over again for another portion. You use the move, rotate and sometimes scale commands available in the menus. It is better to stretch one extrusion to get the panel size and shapeyou want rather than to extrude more than once for a single panel...unless you specifically want to have separate portions to a single plane or panel. High deg sharp angles (the seat tops and footwell walls for example) are best done in separate extrusions, trying to bend a single panel would be difficult and would not enable you to develop the panels properly for sheet materials. Don't expect to put a hole in the middle of a panel or face...you would have to create it at the juncture of two faces by moving the edges of each face back and shaping the resultant hole. This is why the model doesn't show port lights...although I could have created a spot on one of the panels which I assigned to a new layer and then colored the resulting layer after shaping it with added points. I would have saved the unblemished model and done the other on a copy as the resulting panel would not be developable because of the added points...technically.

 

This would probably be better off in the Software Forum

(moderator note: thanks -- moved to the software forum)

 

Assuming you really want to build a kayak and not play with computers for a long time, I have some simple advice:

With freeship, besides the 4 pointed faces, it really really helps to use a minimum number of control points.
This helps with fairing, which the "free" freeship won't do for you. It is easy to make a CAD/freeship model that looks fair on screen, not neccessarily so when you build it full size. Try a model first! That way you can avoid "adjustments" later, or an ugly boat....
If you keep the shape simple and check the "developable" view and the panel edge errors in panel layout, you can make freeship modles that are developable without much trouble. I think what you were saying about the "arc" of the panels applies to the length of the edge -naturally along a chine, the two panel edges are the same length.
I forget the standard (+/-0.004??), but if the panels' edge error (shown in Plate developments window) is very small, and the panels look developable, they will be, and should fit. (but no "guarantees," ok )This is a simplification, not considering perfect fit or thick panels, but it should be close enought for stitch and glue with 3-4mm plywood.

Freeship keys are:
minimum number of control points
Start by making the "automatic" hull freeship gives you developable and transform it into what you want mainly by scaling and moving control point "stations" and chine lines.
Adjust control point stations and chines one point at a time, checking that it's still "developable."
Before you know it you will have a relatively fair and hopefully still developable model.
Freeship will let you do lots of things besides generate lines and panels, but for a kayak, that is really as far as I would take it.

I would just get the lines right, make sure freeship thinks it's "Developable", test it with a little model, then go build you boat: Make full size hull panels and assemble them. Make sure the panels fit and the boat is not twisted, then make bulkheads and parts to fit the real boat, not a computer model!

You may want to bend the panels around a "master" bulkhead or mold, it is easy to generate this from offsets taken from a station inserted on the computer model where you want the bulkhead to go. Then wrap/stitch the panels around it. If they don't fit, adjust the bulkhead, not the panels (this is why you may not want to cut out all the parts from the model before you assemble the hull, if everything is not "perfect," nothing will fit!).

If you are going to cut panels with a saw, not a CNC router, I wouldn't get too deep into the CAD/CAM wonderland. All you need, to build a kayak, is to get fair developable panels, loft them out onto plywood, cut them with any convenient tool, and stitch them together. Bulkheads, deck beams and other parts can then be developed in "real life" to fit the hull.
From my point of view, there would be little point modeling access hatches or lightening holes for a kayak in CAD - you could just use common sense and cut the holes where you need them!

If my simplistic approach does not suit you, of course, you can model it to the last gram and millimeter on computer.
Some people take a lot of pride and pleasure in that, for me, it's more just about getting a project done, and keeping it "flowing."
Nothing wrong with either approach.

Finally, the simpler you can keep it, the easier it will be to make a fair model and a boat whose parts fit together well.
Good luck.

 

Something to add...the "skin" that you see is infinitely thin...there is really no thickness. When and if you do a bulkhead...you must subtract the thickness of the real life skin from the dimensions of the bulkhead if you want it to fit properly. If you set the aftmost point at zero, then set your stations at whatever interval you need, you can add a vertical station to represent a bulkhead at whatever point you need. When you export the HULL offsets, you will find these dimensions at the specified station location in the offset file. You then loft the station full size and reduce the dimensions by the thickness of the outer skin to achieve the correct dimensions for the bulkhead. A little epoxy putty and some tape will finish the job. Epoxy does cover a multitude of sins.

 

This is good Lewis...never thought about hull skin thickness and bulkheads before...

I`m still having trouble with Pram shaped bows...

 

why plywood? It is done all the time, but it is more work and it cost more than skin-on-frame (the traditional way to build "native" kayaks). Skin on frame does not need plans, you can use any local wood, lashed frame, and either cotton canvas, nylon or polyester skin, with any oil based paint. It will cost less, weigh less and be faster and easier to build. I have build 8 kayaks this way, and one small sailboat. most cost less $100 US to build, and about 100 hours. A stitch and glue plywood kayak would cost about $600 to build, and about 300-400 hours. There are lots of sites and even free plans on the internet for building native type skin-on-frame kayaks.

If you still want to build in plywood, the site below has free kayak design software specifically for kayaks, that gives you the cut patterns very quick and easy.

http://www.washingtonwoodcraft.com/home.html

Good luck

 

Durability . Stiffness .

The way I read the post by magwas , I get the feeling that he wants to get started in boat building. I could be wrong.

( from your link ) .......Keyword being WOODEN here.

Nothing against skin on frame . Many benefits as you say. I will try it myself one day. Nice and light. Also can be quite beautiful. Art in fact.

Post some pics if you have some. I love looking at beautiful , simple , handcrafted boats .

Thanks for the link.

 

Thank for all the answers, all was very helpful. As freeship did not want to extrude an interior edge, I did the following:

Saved the state before adding bulkheads, keel, etc.
Defined the control points by adding a plane intersection where I wanted the bulkhead. Added a new face there, and assigned it to a new layer.
Saved this as a part, reverted to the saved state, and imported the part back.
This way the bulkheadbecame a separate entity, not deforming the hull.
I made the edges to be crease, mirrored it, and marked as not symmetrical (this way it will be plotted in one piece.

I did something similar with the keel, but developed it by extrusion, as it is a longish piece.

I did not play too much with holes and small details, as it is suggested.

I am also making a small model first, as suggested.

Yes, I want plywood because the kayak is meant as a first design, with which I learn ship building.

Thanks for all the answers again.

 

First, what about thisone?
http://www.nessmuking.com/articles/...ulti-chined-kayak-plans-for-plywood-building/

Second, why not pay a few dollars for a proven and tested design, easy to build and with predictable qualities? How does the cost of plans compare to you your total budget?

 

Sounds like you are on your way then...good luck with the project.

 

I just think that designing a kayak and building one is two different tasks, both can be very interesting, but if you pay for example 100USD or so fro a good set of plans, that is a bargain compared to the hours you need to design one yourself.

 

That just about sums it up.........

And ........" a good set of plans "is proven...no hidden design faults , relatively small and subtle differences in the lines can have a big impact on sensitive , light craft , like kayaks.

 

Sure for you, living in the US. Not for me.

 

Sure, but sometimes designing your own boat is (more than) half the fun.
Building your own design can be very rewarding. But I agree, it's not always cost effective.