Hard chine dinghy hull, flat patterns?

Well, theres been one or two made to this pattern over the years, all with 6mm plywood soles ... I think they are up to sail number 5400 now, although the last 2000 or so are mainly fibreglass. But theres a good 3500 or so plywood ones been built.

I guess you theoretically could use strip construction, but to the best of my knowledge, no one has to date. The finished weight of the hull is typically 70Kg for a light one, weight at the ends is especially to be avoided!

The class rules specify the chine points in Y and Z at the bow to within +- 10mm so there is no option of changing them very much.

I just cant see how a keel line that is supposed to bend up at the bow works out to those patterns .. I've printed them on card, joined them edge to edge with tape, and put the aprroximate angles into the sole, and it ends up with the keel dipping down at the bow, instead of up ...

Anyway, I'l post up the results of the above, and the files

 

Interesting ...

I opened the .fbm file in DelftSHIP free, and it creates different and much more correct looking flat patterns, with the curves exactly as I expected .. but of course, in DelftSHIP free, you cannot save the .dxf .. so it looks like there is a bug or maybe intentional error in FreeShip's flatt pattern generation .. if I get a spare few hours, I'll pull the source from Sourceforge/github and have a fiddle

To be fair, DelftSHIP is cheap, just 150 euro, but really doesnt give me anything extra that FreeShip has .. apart from correct flat patterns ...

 

FS was the developing software for DS, There may still have been a bug or two in it. I know DS has gone through a few upgrades and fixes so that may have been ironed out.

 

Thats fair enough, and, programmers have to eat too! If I had made something like that, I would certainly want to make some money from it. If I hadn't been out and bought (yet another) boat today, I might have even paid for it I don't think my wife will allow me to spend more money on boats this week, so it will have to wait.

 

I did look at them side by side and there is a definite difference. Sent you the .dxf from the DS development.

 

Hi,

Have you seen Freeship Plus/Hydronship?

http://hydronship.net/index.php?lang=en

It has (forgive the pun) boatloads of reports that neither the 2.6 ver of Freeship nor the current Delftship have. Plus, FSPlus/Hydronship features include a few import/export formats that might quite useful if you find you must use them (keeping in mind that importing/exporting could introduce issues...).

Should you choose to download it, it seems to be easier to use the Download button that is in the upper center of the page, near the rudder. The one by the right hand side takes longer...

 

Yes, I have seen it. It is no improvement, is has exactly the same bug copied from FreeShip.

The plates developed will never make the hull as seen on the screen.

The bug even exists still in DELFTship, I have a .fbm file of my hull that looks perfect on screen, but the plates in DELFTship are correct ... and another .fbm that looks almost identical, but the plates are completely incorrect.

At the moment, I have no confidence that any version of this series of programmes can reliably develop plates that I could cut out and be certain they would make the hull ... I am not happy to risk some very expensive wood on patterns, until this bug is fixed.

 

Question: when drawing your plates, do you have the plates/surfaces at a Low, Med, High, or Highest setting?

 

In FreeSHIP and its derivatives you do not draw the plates, you draw the hull, using control points, and then program develops the plates from the model.

I have tried low, medium, high, highes .. the result is the same. incorrect p[late development.

If you imagine the cenrte line of the boat, the keel .. the sole plate should have a convex line, because the keel is convex, and the plate is at an angle.

The sole plate as developed, has a concave line, completely incorrect, resulting in a keel that slopes *down*, instead of *up* at the bow.

 

Maybe solved?

So, I think i have an answer

I tried emailing the DELFT developer, but got no answer after some days, so I began to look for a solution, as I lost confidence that FREE/DELFT ship are doing this right.

I took my model ... and I created flat plates in freeShip .. they look incorrect.

I turned the model 180 degrees about the vertical axis, in free ship, they still look incorrect.

I did the same in delft ship. the patterns look different depending on which way the boat is rotated (in one rotation they appear the same as FREEship, in the othr rotation, they appear different)

I saved .dxf files from FreeSHIP, and I got someone with a paid for version of DELFT to save .dxfs from DELFT with the model rotated 180.

I also saved a .IGES version of the model, and opened it in rhino.

In Rhino, I unrolled the hull surfaces, I checked that in Rhino, the unrolled surface remains the same shape, even when the hull is rotated 180 degrees in the vertical plane.

Then I imported the two versions of the .dxfs from Free/Delft ship.

I overlayed them.

here is the result. DELFT/FREE are in red and blue, Rhino is in black.



In my opinion, Delft/FREE ship is adding or subtracting some amount depending on which way the hull is rotated, Rhino is always the same and lies in the middle.

My solution will be to fair the hull and check the stations in FREE ship, export to IGES and unroll the plates in Rhino3D, using one of my 25 "saves" to store the completed patterns. (note: the Rhino demo allows you to save 25 times for free, so be careful and keep those saves for outputting completed .dxfs!)

Second note: FREE!ship has an export as .IGES ... DELFT does not seem to have this feature, so develop your hull in FREEship. I think DELFT can open FREE .fbm files, but not the other way around as DELFT saves in a later version of the .fbm format.

 

It still looks like the Rhino one has a hook to it, whereas the one in green looks like it would be the correct one.

 

I tend to agree with you, but the way DELFT/FREE ship seems to vary depending on hull rotation is a worry.

It's almost tempting to machine up a solid on the CNC mill and lay up the various patterns to get a better ide of which is right.

If I get some spare time, I'll hunt down some other software that is capable of unfolding surfaces from IGES models. I seem to remember some fairly well respected open source libraries, in particular one from an American government projext that may shed some light on which one is closest.

 

I have two suggestions:

1) Before cutting any expensive plywood, buy some $10 panels of subflooring material and do a test run on your pattern you think is right. Wire it together using station molds created from the offsets defining the required hullform. if your panels lay into the molds properly, then you should be good to go. If they don't, they should give you an idea of where to make changes.

2) Set up your station molds in a building strongback and wrap them with the subfloor material. Mark the panel edges at the molds and use these as flat patterns for either the actual panels or for the numeric file for the CNC cut.

 

Yes, I thought about the cheap plywood, I hope to be able to validate the patterns electronically, but I suspect I will check one panel in cheap ply.

I won't be using the second method, as although possible, is not really my idea of how to desing things. I've built probably hundreds of tonnes of steel structures directly from CAD models, I will find some way to perfect the electronic techniques for wooden dinghies