Using Hulls to build a hard chine hull

In the Footy forum Dick Lemke asked…

"Muzza - do you (or anyone else know) if either program will allow you to develop panels that would be “bent” to shape? I know that hard chine panels can be printed flat, but I am wondering about compound bending?

I am thinking (not to hijack the post) about a compound bent panel of thin ply for a set of multihull hulls. It is done in big boats, but can’t find any technical info - and just wondering if anyone using either of these programs has seen anything in the instructions on use?


I built a double-chine IOM to my own design using Hulls. The bends in the chines are in two dimensions and worked OK but not perfectly. I think this had more to do with issues associated with my printing the panels onto A4 sheets and then attempting the sheets of paper to get the full sized panel. As I built the boat by placing the panels over stations it may also have something to do with errors in my stations.

I scaled the plan down to just 12 inches long (Footy sized - but not Footy displacement). This was so that I could print the panels on single sheet paper to see whether the error was mine or the software. I glued the paper panels onto some card, and then made a mock-up hull with the card with tape. This time the panels were a perfect fit, but the hull came out slightly longer than the 12" I had intended. this may have been something to do with scaling issues between the software and the printer.

As you can generate DXF files from Hulls, it is easy to insert a scale. Then you would have an accurate check of the panels once printed.

Here is my double-chine IOM (printed from Hullform). In this case I did the initial design in Hulls, then imported the offsets to Hullform to check various things. I used Hulls to print the panels. You can get an idea of the way the panels curve from this image.

And here’s a construction pic showing the a panel pattern printed from Hulls and the balsa chine reading to put into place on the hull. you can see that there is some compound bending, especially toward the bow. The panels were mounted onto the stations from the bow first. They were held firmly there by clamps and pins before fitting the rest of the panels by moving progressively aft. I found that if I moved from the stern forward, it was too difficult to hold the compound bending at the bow.

Hi guys,

I just posted an example in the Footy forum of a hull I built using the FreeShip program’s “develop plates” feature. The FreeShip file and pics are posted there.

Bill H

Thanks - almost, but not quite. I will give you a full-size boat scenario to maybe further clarify.

Remember - this is for an 18 foot cat hull - so it may not work at our small scale…

  1. A panel is cut from 1/8" (3mm) ply, to “some dimensions”.
  2. The two panels are joined along the keel using tape/epoxy.
  3. At the stern, the panels lay fairly flat, but toward the bow they are supported with the hulls forming a “V” shape until the epoxy cures.
  4. The bow panels are brought together and glued to form a vertical and plumb bow.
  5. Then the remainder of the panels working back from the bow are carefully bent using large straps. These straps are gradually tightened, which curves the sides upwards. Because the hull was taped together at the joints, it forms a semi-circular curve.
  6. The hull side are continued to be lifted and squeezed together until they fit into an outline cut from plywood that reflects the shape of the deck looking downward.
  7. Once they are fit into this “deck profile” tension on straps is released and the hull sides are allowed to “spring back” until they are held in place from further spring back by the deck profile.
  8. At this point, bulkheads and stringers are inserted, and cross ties or mid-level decks installed to help hold the hull in it’s predetermined shape. Dagger board trunks are built and inserted. Blocks of wood added where chainplates and forestays will connect. Transom is fitted and glued into place. Strips are glued to the curved tops of the bulkhead sections to form a wide spot to adhere deck.
  9. Finally deck is glued down and strips of duct tape, and straps are added to force it to conform to the curved deck area and along the gunwales.
  10. When all cured, tape and straps are removed, and deck profile panel is lifted off leaving a very lightweight hull that has taken on a nice shape from the compound bending of the ply planels. 4 oz. glass is added for abrasion resistance, and fillets, fairing, etc. takes place.

I’ve done this three times (6 hulls) but I never asked how the preshaped panel was cut in the first place. I just laid out the panels according to provided dimensions, and the hull took on a nice shape - thin, plumb bows, gradually turning into semi-circular cross sections from about mid-point back. These hulls, were 18 feet in length (5.5 meters) and when completed, they weighed in at about 43-48 lbs. each!

Since then I have thought about this process and was wondering how the original panels were laid up in the first place, since the boat had no sharp chines on the hulls. I did find software for about $2800 (US) that will provide this (I think) but was hoping someone might have fooled with one of the free programs. I suppose I could always scale the 18 foot dimensions down to 4 feet, but …

Hope this adds more clarity to my question.

Thanks Muzza - as I did understand the issues of hard chines, having built one kayak that way. It was brutal trying to get the bow, deck, sides, bottom and stern pieces all to come together and actually “look” good.

Sorry Dick, I didn’t realize your post was in response to what you saw on the Footy page.

The FreeShip program may help you. It seems to allow for more than flat chine surfaces to be “developed” and it has a feature that shades the parts of your plan that are suspected of being bent beyond what is reasonable. It also indicates where edges needed to be stretched or compressed to make the shape unfold. The program is made for full-size boat planning, and has features that are helpful for multi-hull designs, too. They have a user forum at their site where you can get help from others, also.

Perhaps you could download their manual and read the section called “develop plates” starting on page 34, to see if it might meet your needs.


Thanks Bill -

Will do. I was looking for the easy way out - that someone had fooled with them. Look’s like “it be me!” :spin: Will report back.

Ok Dick - now I see what you mean.

I don’t have any answers for that.

Dick, here’s a file for you to play with.

It’s a FreeShip version of my Ode to Roger Footy. It’s a complex shape, and I didn’t take the time to get it perfect, but it’s close. Open it in FreeShip, click Tools, then Develop Plates. The unfolded surfaces magically appear!! Even though the shape is complex, it seems to have (with a minor exception) been able to unfold it successfully. You could try cutting it out of stiff paper and taping it together to verify that it works. I might try it myself, now that you’ve made me curious!

Have fun…Bill

OK, OK…I had to try it. The good news is that it came out closer than I thought it might. The bad news is that it didn’t come out right. It’s just too complex a shape to make out of primarily 4 big flat shapes. I think the program should tell me that. Based on this attempt, though, I’m guessing that it should work well for things like cat hulls that aren’t quite so banana shaped. It might even do well on banana shapes if divided up into more panels than I used. Maybe that’ll be my next experiment.

Happy Thanksgiving to all…Bill

Do a search for “stressed plywood” and “tortured plywood”
As you already know it is usefull for cat hulls and other long skinny low volume hulls (Int moth perhaps)

Hey Dick,

There is a pretty complete explanation of bent ply construction and how to do the panel layouts in a book, “The Gougeon Brothers on Boat Construction”. They use Tornado building and the development of an ama for one of their trimarans in illustration of the concept. In the book they use models in a trial and error method of getting the hull forms that they want. Unfortunately, there is no information about determining displacement in their process. The book is a good reference but I bought mine a dozen years ago, before computer drafting programs became common. The Gougeon Brothers may now have a program for generating bent ply panels or may be able to direct you to someone that does. Best of luck in your search - Niel

Thanks for suggestion Niel -

I also have their book (pre-computer drafting) and probably time for an update. Jan (Gougeon) presented me with the copy - so lots of sentimental value and freindship. (Used to live about 35 minutes from their shop and was on-site to see some of their creations - Rogue Wave, Golden Dazy, Slingshot and Adrenalin Formula 40 during her shake down cruises on Saginaw Bay. Tried to keep up with her on my big cat - but she just walked away like I was dragging anchor.

Will drop another name on you - John Lindahl from Fennville Michigan. His “A” Class / 18 Square Meter plans are still available that use the fold-up method of a single sheet per side - but it winds up with a semi-circle towards the rear as an underwater shape. John fooled around with 4 foot models, then he tried full size. After getting his shape, he cut along keel and measured to come up with repetitive shapes for his boats - refiing each generation as it was built. I have fooled with 4 foot deign, but balsa keeps slitting when I try the compound bend - do maybe cold-mould layup of very thin veneers might be possible instead. That still requires a plug to build over however.

Hey Dick,

I live in Brooklyn, NY and Pearl Paint here in NYC carries 1/32nd aviation ply sheets 12" wide by 48" in length. It is pretty flexible and not too heavy, and doesn’t split readily as thin balsa sheets tend to do. The cross-grain in the ply adds some impact resistance as well. I’ve played with it as per the model making suggestions in the Gougeon Brother’s book, but I’ve not produced a hull I cared to continue working on. In your 4’ models you probably used 1/16th of 1/8th marine ply which is more readily available.
Pearl is a national chain now but if you haven’t one in your vicinity I’m sure that a bit of hunting about you’ll be able to find 1/32nd ply from another source.
By the way, I’ve been an admirer of the Gougeon Brothers productions for as long as I care to remember, and of Dick Newick’s designs via the Phil Weld book, “Moxie”.

Ratz - closest is Chicago - but reminded me of Dick Blick which we have up here. I did check Pearl Paint’s drafting film (for sailmaking ?), and prices aren’t too bad ! Unfortunately their matte mylar thinnest is .003"

Here is polyester pricing/sizes. Has anyone used any of this weight for sails?

SKU # Item Description …List Price …Pearl Price
945233 18x24 .015 Duralar Polyester Drafting Film $4.89 …$3.91
199857 18x24 .020 Duralar Polyester Drafting Film $6.60 …$5.28
199855 24x36 .015 Duralar Polyester Drafting Film $8.56 …$6.85
199858 24x36 .020 Duralar Polyester Drafting Film $12.11 …$9.69
860825 24x40 .005 Duralar Polyester Drafting Film $2.30 …$1.84

Thanks for the tip.