There are one or two people on here who like to dabble with designing boats.
Some like to sculpt directly with the plywood, some prefer good old paper and pencils, others try various bits of software;
Over Christmas I had a play with Freeship. The best thing that can be said about it is its free, but it does also do most of the things you want to once you've got used to its little idiosyncrasies.
This is a 14ft cuddy I sketched up to see how the software works.
this is the lines plan, (of a slightly different version)
and these are the developed planks.
The software will import and export txt. & DXF files.
I do just wonder about stability under sail. I guess that it is intended to be lightly built and without ballast in which case it seems to have a farily 'soft' turn to the bilge. That is not necessarily wrong, it will reduce drag and the boat will be easier to row and glide along well in light breezes but it will heel over in stronger winds unless well reefed or unless the crew use their weight to balance. I could be wrong about this, its just my first impression, looking at the end on views. Perhaps think what the hull would look like if the chines were smoothed out, I think it would be more keel-boat like than dinghy-like, but presumably without the weight of ballast that keel boats carry to balance their sails.
I see you have a flat bottom panel, like the dinghy I made many years ago. I do find that a nice feature - it dries out level and stable, doesnt tip from side to side when you are on board dried out at night. Also simplifies the supports on a road trailer. You may like to think about having some shallow 'bilge keels', not for lateral resistance sailing but just to help to keep the bottom panel clear of stones when dried out. Having said that, Alastairs boat has a flat bottom panel without the keels and he is happy for it to dry out almost anywhere with a good thick layer of grp to protect the plywood bottom.
On 29 Mar 2017 at 13:30, John P [via UK HBBR Forum] wrote:
> Having said that, Alastairs boat has a flat
> bottom panel without the keels and he is happy for it to dry out
> almost anywhere with a good thick layer of grp to protect the plywood
Remember also that my bottom ply is 18mm thick. The grp is 4mm thick.
Sail when you can, row when you must, motor only
when you have to be at work in the morning.
Free!ship seems to have stopped development, and the original website has died.
Ther are various versions around it seems. SourceForge, which seems to be its current "home" has versions up to 2.6 (if you look in the approriate directory). However I have also found links to a version 3.43 (although that may be Free!ship Plus - which seems to be a spin-off of uncertain parentage).
Does anyone know if either are still currently supported?
Thanks for your comments, I agree with your view that she's liable to be a bit tippy and it got me puzzling and trying to remember how I actually arrived at those images.
The purpose of the exercise was to have a play with the software and see how easy it was to use and what it could do. I wasn't really looking to design a viable boat at this stage.
The demo examples provided are all of ocean going tugs and the like and far too complicated for our needs so I started off with some offsets I had for a little 11ft dinghy I'd drawn a few years ago.
I put these into a txt file and imported them into Freeship.
So it looked like this
Amongst other things I had a play with the scaling factor and stretched the length out to 14ft.
Stretching the length seems to work OK and retains the same amount of rocker etc just stretched over a longer length.
I don't think I stretched the beam as the original boat was quite tubby, but I did stretch the Z factor to increase the free-board and I think this is where I went wrong.
For comparison the 14 ft version looked like this;
My Theory goes something like this, and as an engineer I'm sure you'll put me right if I'm off course. Increasing the Z (heights) by a factor means that the higher the original value the greater the increase.
So for example for a factor of 2 ;
The flat bottom at 0 doesn't increase
The first chine at 2 becomes 4, an increase of 2
The second chine at 6 becomes 12 an increase of 6
The third chine at 12 becomes 24 an increase of 12.
So if the beam isn't scaled up the effect is to soften the bilges and make the hull more rounded.
It would have been better to either increase the beam or add another plank.
An object lesson in not doing things just because the software lets you. Thanks for that.
As a 3D drawing package Freeship is pretty clunky, but it does the good things like developing planks.
As for all the fiddly bits like thwarts and bilge runners it would probably be better to export the lines to Autocad or whatever and add them in there.
I hear a cry from darkest East Sussex "what about using paper and pencil".
Scaling in Z alone will increase the "moment of inertia" of the boat - meaning that when it is rotating (rolling) it will require more force to stop it rotating. That equals less stability.
Moment of inertia is not linear, it is proportional to the weight multiplied by the square of the distance from the rotational axis. So doubling in Z could require 4 times the force to keep the hull stable.
That is the pure physics, but in reality things will be more fuzzy and variable....but I hope you get the idea.
Think of swinging a sledge hammer compared to a lump hammer of the same weight, using one hand. The sledge hammer is significantly more difficult to swing; if the handle is 4 times longer the "moment of inertia" is 16 times greater than the hammer. Blah blah blah....