Calculating Bevels

[ajmoir]

Hi,

I'm currently investigating the idea of building a carvel hull completely out of plywood. Ribs, strakes etc all from plywood. The interesting thing to me is can I do all the work up front and have the plywood milled on a CNC machine. The hull should then be easy to assemble.

I've seen various clinker/lapstrake hulls built for small rowing skiffs that seem to implement the idea pretty well.

I've also played around with scale models I've designed and everything seems to work out.

The current problem is how to work out bevels for both the ribs and the strakes. Is there a formula for this? Ideally the more general the solution the better e.g. would work for a scale model as well as the full size.

[wdnboatbuilder]

the edge of the planks wont be 90 degrees to the rib. there will be an ever changing bevel there.

[cyclops]

Thats were a experienced wood person is home in bed asleep, while a crew of programers is going crazy and getting nowhere fast. It is far more accurate and less redoing, if 1 of a kind is done by hand and a good eye, ( A Cyclops? ).

[wdnboatbuilder]

thanks for the humor one eye.

[cyclops]

No joke . Build the proto of wood and screws. Take apart. Lay or support for a 3 axis scanner. Scale the program. Done. Then the auto milling turret can go like crazy. Talented people are needed.

[wdnboatbuilder]

next question: how are you going to fasten your planks. if you edge screw into the ply it will hold for a littler while but over time in motion they will work free in a short time. I would suggest to back your frames with somekind of wood..being fir, cedar, mahog.....give your screw a chance to do it's job

[PAR]

CNC cut clinker and carvel planking isn't that uncommon. The bevels can be picked up directly from the plans generated cut file.

Plywood ribs, used in the traditional sense of bent rib, wouldn't be an effective use of materials. Plywood structure is very commonly used, but its application or engineering into the structure, relies on different principles, then the traditional methods incorporating solid lumber stiffeners.

A homogeneous structure could be designed from all plywood and cut with a multi axis router. This is basically how kit boats are assembled. Some of these have temporary station molds, others some remain as bulkheads, others still, have all molds as the bulkheads or other structure, like furniture.

The key is an accurate set of plans that has been developed into a "debugged" cut file. Unless semi or full scale mass production is anticipated, the dollars for the extra engineering and code work aren't cost effective, particularly in a one off design. There are several software offerings that will do the job. How's your AutoCAD skills?

[cyclops]

The all plywood boat sounds cheap and doable. I am building a 20' 2 seat racer. 1930's with a vertical bow. I came to a fact of plywood frames, keels, engine stringers, full bulkheads and the transom. I have to edge all the contact points with lightweight 1.5" X 1.5" triangular gusset strips to prevent fractures of the end grain and the surface of the plywood skins at EVERY intersection of them. They will be almost full length and glued in place after a trial assy. of the covering skins. With a vertical bottom grid work on 12" X 12" squares I can pull a cuty of pre cutting loads of gussets that are 90 degrees. About 2/3 of her length. --------Still, there is a lot of gluing manhours on all the skins of a boat. Light, rigid and outrageously strong. To me the gusset manhours would, guessing, wipe out any advantage. It is way over designed, still gussets are a lot of dummy time required. I am using a auto gluer for the 2 edges of the gussets.

[Raggi_Thor]

Stringers, ribs and frames of plywood is wasting 50% of the wood fibres.

[CDBarry]

You can get frame bevels by calculating a station on both (fore and aft) sides of the frame, then connect them.

[ajmoir]

Originally Posted by CDBarry

You can get frame bevels by calculating a station on both (fore and aft) sides of the frame, then connect them.

could you elaborate or point me to a good link/book on the subject.

I understand that I need to calculate the bevel on the fore/aft faces of the frame. But you give me no info as to how to do this. Excuse me if I'm being dumb.

[ajmoir]

Originally Posted by PAR

CNC cut clinker and carvel planking isn't that uncommon. The bevels can be picked up directly from the plans generated cut file.

...

The key is an accurate set of plans that has been developed into a "debugged" cut file. Unless semi or full scale mass production is anticipated, the dollars for the extra engineering and code work aren't cost effective, particularly in a one off design. There are several software offerings that will do the job. How's your AutoCAD skills?

The cut file, is this just the g-code? If so, the bevels must be generated by the cad software. How? I don't see anyway that they could just appear as a consequence of lofting/framing/planking in CAD.

By de-bugged, I'm assuming the current cad software doesn't do a good job of calculating these bevels and that they must be adjusted manually. This is precisely what I don't want to do.

[ajmoir]

Originally Posted by wdnboatbuilder

next question: how are you going to fasten your planks. if you edge screw into the ply it will hold for a littler while but over time in motion they will work free in a short time. I would suggest to back your frames with somekind of wood..being fir, cedar, mahog.....give your screw a chance to do it's job

I was thinking of epoxy/epoxy fillets and/or through bolting with a backing washer. I was not thinking of screws.

[ajmoir]

Originally Posted by wdnboatbuilder

the edge of the planks wont be 90 degrees to the rib. there will be an ever changing bevel there.

Bingo, now what's the formula?

[cyclops]

To answer your anchoring problem. There must be more than the simple and time consuming gluing blocks of correct grain orientated wood at each attachment point?------ That maybe why so many custom and short runs use solid wood. Beats the anchoring problem as you attach the ribs/frames.-----The complete use of plywood is to gain the 31,000 PSI compressive strengths of the vertical elements when the boat re-enters the water from 10' to 15' high unexpectedly. The boat will survive. The driver will not.