CAD program for redesign of existing keel design

Hello. Newbie here. I am planning to build a Rozenante (LF Herreshoff) but wish to redesign the keel a little bit. Is there a good CAD program that will allow me to put in the offset lines from the lofting plan and then tinker with that? I have VectorWorks now, but I can’t get it to make a fair image of what I want. Thanks

 

What improvements do you wish to have? That is a very refined and sophisticated design, so it will be hard to make it much better, but easy to make it worse.

 

Hi. Thanks. Well, first I have decided to lengthen all the linear dimensions by 7%, making the LOA 30’ rather than 28’ (bad idea?). That makes the displacement approximately 22.5% larger and the draft about 3” deeper. I wanted to flatten the extra 3” into a sort of Scheel Keel shape in order both to get the draft back to the original depth and also to give me a flatter bottom profile for resting on sandy bottoms. Is this a stupid idea? Is the scaling up idea (without changing the ballast keel shape) a reasonable idea? Or should I just go back to the original length (which, to me, is a great design but is a little too short for what I want). Thanks.

 

The volume increases by 22.5%, but not necessarily the displacement unless you increase the scantlings too. Are you looking for more headroom? The stability of the boat will increase a larger percentage than the sail area. Either you also modify the sail plan, or the ballast could be reduced.

 

Thank you for that. I think the sail plan modification would be easier to do. I am definitely looking for more headroom with out raising the cabin roof very much! I also plan to use silicone-bronze floor cleats (in the cabin) rather than the taller floor timbers that were originally designed, which would give me an inch or two more headroom. Pretty narrow floor but better than hitting my 6’2” high head quite so often. This is all just floating around in my head now. I’ve got the plans and have lifted the lines onto sheets of Masonite at the 107% size. I just want to work out the ballast keel shape and would like any advice I can get here (and elsewhere). Thanks again. Edwin

 

Just to aid understanding the concept a bit better,is the intention to increase the section spacing by 7% or to scale all dimensions by this amount?The latter case may require a bit more ballast to get the increased volume down to something lie the intended waterline.

You might find the evaluation version of Rhino suitable for your exercise and if it is,paying for the installation will leave you with a very good piece of software.It will easily scale in 1,2 or 3 dimensions.

 

Answering your specific question, there are many programs that will allow you to make a 3D model of your ship. But you should better specify what you need to do with it to be able to opt for one software or another. In addition to changing the profile of the keel you will have to fix many things. The changes that you propose suppose the need to make many calculations, practically to make a new project and, in addition, starting from an offset table, which is not the most comfortable way to start either. Are you in a position to make these calculations, both because of your knowledge and because of the software you have? Do you need any help or someone to do that project?

 

Thank you All for your input! So, is increasing the linear dimensions by 7%, the scantlings by the rules in Gerr’s book on Boat Strength, and the volume by 22.5% resulting in a waterline that is proportionately positioned between the shear and the garboards as the original was not a very straightforward adjustment? I am a carpenter and sailer who has experience repairing a wooden sailboat (a National One—William Crosby). I do not know much about the nuances of boat design but am not looking for a racer. I love the Rozenante and plan to follow the Doug Haylen (Redhead) version (a true yawl rather than Herreshoff’s ketch rig). I would just like to scale it up by 2 feet in length (and everything else in direct proportion to this 7% increase). I would love some help with this idea, especially from from someone who could explain what s/he is doing and why. Thank you All again. Edwin

 

Why change a perfectly good design……you will end up with a orphan boat that is worth pennies of what an unchanged boat is worth. Changing a design is a very involved process

 

Somewhere online there is a good illustration of a man sitting in a Rozinante and it looks rather "cosy".I doubt that scaling up by 7% will do much to reduce this feeling and post#9 makes a good point.You would also have to undertake quite a lot of calculating to establish that the definitive waterline is somewhere similar to it's intended location.Adding perhaps an inch to the sheerline and increasing the camber of the cabin beams might be an altogether simpler way to get more headroom.

 

Herreshoff claimed that headroom was over-rated. He believed that you don't walk around inside of boats, but sit or lay down. Unless they were larger boats, he kept spaces small. This also creates a safer boat in rough weather. I have sailed in modern "open concept" boats in storms, and they suck. I have had to make spiderwebs of lines to have something to hold on to.

 

Thanks everyone. I shall take your advice to heart! Edwin

 

Hi Edwin,

have you ever considered Image Segmentation to do so ? In blue are the bounded zone that have been detected. With that, you obtain automatically all the coordinates that you need to reconstruct an offset table or a 3D model. Basically, it is called "Vectorization".

Here is an example that I just made, from a simple screen capture of an image found on this site.

Rozinante - Professional BoatBuilder Magazine https://www.proboat.com/2022/03/sailingyacht-hullforms/rozinante/

 

Reading LFH's writings and his letters on the Mystic Seaport Museum site makes it appear that he wasn't actually all that experienced as a cruising sailor (or a racer, come to that). He seems to have spent comparatively little time cruising and I don't think he ever did a single major cruise or owned a single yacht.

Since he had fairly little experience, his thoughts on headroom are open to debate to say the least. I certainly walk around inside boats and I did when I lived on one for years.

 

I am not sure what writings and letters you refer to. However, he spent his life on boats; cruising, designing and building them. Perhaps reading "The Common Sense of Boat Design" may change your mind.