Scaling down a design to build a Mini / Micro Sailboat

Hello Boat Fanatics

I like tinkering in the garage and built a few strip plank, marine ply en glass boats over the years, mainly under the 18Ft size, due to the constraints of my Garage Size / length.

I have for some time to had the crazy idea to build a micro boat, similar to the Minuet, Mini 203 or other boats I have seen. BUT I am no designer and this is purely a fun project.

I would like to build a scaled down version of a local boat. My main question to all is if I down scale the line from 30Ft to 9Ft, is there a different ratio you need to scale the keel and sail area down to the hull?

Know this is a very "DIY for Idiots" approach, but I like to tinker around in the garage.

 

If you design or scale a design and build a sailboat you are indeed an amateur boat designer and builder which is enough to build anything even a 36 friendship catboat

You can make a small replica of minitransat or any boat just using a 3d model in sketchup for example and scaling it to your size, and then tweaking height and beam with same proportion tool

When you have the boat built then sail and tweak what you want sail height, leeboard, rudder, pivot centreboard, etc until you find the proportions that suit you

 

If it is a question of sailing with the new boat, it will not be as simple as changing the scale of a 3D model. The thickness of the hull planks, for example, will not follow this law and as a consequence the weight of the boat will not follow this law, nor will the draft, the freeboard, nor some other things (sail area...) either.
I would tell the OP not to trust in easy solutions (there are usually none) and to carefully analyze all the proposals that are made to him, especially the magic solutions.

 

You probably will like this sailboat, its an excellent coastal cruiser



Also the Colson 550 is amazing



Also Cox bay skimmer

 

Just use the square-cube rule when scaling; that won't cover everything, but it can help keep you within your weight budget.

Square–cube law - Wikipedia https://en.wikipedia.org/wiki/Square%E2%80%93cube_law

This is *not* an easy solution; it requires you to recalculate the dimensions of every structural part of the boat, such as ribs and stringers. I suspect it might be different for sails. Rope and line are fairly straightforward, just find a seller and tell them what kind of load they need to withstand. Note that 'kind' is not the same as *magnitude*; some things must be allowed to stretch and some are forbidden.

 

You probably already worked it out but one big problem is that the displacement/length ratio and therefore hull volume must normally increase drastically to support a human body on such a short hull.

The Minuet, Mini 203 and others look like fun but I wonder whether a more classic-style hull would work better. It would be longer but thinner and deeper to allow the human ballast to sit lower, and at low speeds stern drag is critical. The extra length could allow for a lower-drag stern.

The 2.4 Metres are much bigger and heavier boats, but upwind in light airs those things are surprisingly quick. A smaller version which just uses the sailor as ballast but has the same Metre-boat lines could be a lot of fun in light winds.

 

Search: Model Making, and scaling, and similar words / phrases on this site, and on others. I have a collection of snippets of info, and equations from here (bd.n), and from elsewhere, but I won't re-publish them because I didn't record, and so can't state the original authors.
Froude's law of similarity; Weight goes up with the cube of the length; are a couple of basic ones. Water viscosity doesn't scale the same. Model making forums are a wealth of info, and enthusiastic contributors. The Illusion, 2.4m, Minuet, and others don't look quite right once someone gets in the boat. My view is that 12 to 14 feet long / 4+m, works for a single occupant, as far as small dinghies (models) go; 3m / 10 feet long is too short without a motor. I liked looking at old movies, (pre-digital), where a model ship was involved in heavy weather, and the water's action was too 'thick' to be convincing, despite the size, and beauty, and accuracy of the models used.

 

Thank you for all the responses.

Yes, my thoughts too, the Minuet and Mini 230 is just to short and beamy. I am looking at a skinny hull and would go up to 11ft probably.

 

LDL hulls are lightweight and efficient but if a monomastil sloop is added they require 50% keel ballast/Displacement ratio

Maltese falcon, one of the biggest sailboats is a really slim boat if you scale it is like a slim kayak and it uses schooner three mast rig with a 50% aprox of mast height/LOA ratio when current sloops use 120% ratio or more like pogo with 150% aprox, just excess everywhere

 

You could build a smaller version of sharpie Egret



The same effort you are going to put can be used for a moderate larger boat

 

Scamp boat, a sort of cat lug rig round Sharpie or pram sailboat



https://sailboatdata.com/sailboat/scamp/

 

Ridiculous. Many modern boats do not have 50% ballast ratios. The Pogo 30 you pictured has just a 34% ballast ratio.

How can you claim to know enough to tell people what to do when don't even know how the boat YOU pictured compares under the subject YOU were writing about?

The tall rigs allow modern boats to go fast with smaller sails and minimal fuss. If you like working hard to go slow that's fine but to call performance rigs "excessive" is ridiculous.

As an analogy, it's as if you ride this;


instead of this



Either choice is fine, but calling the design of the fast gear an "excess" because you don't know how to handle it or don't want it is ridiculous. If you can't handle fast gear easily it's your own fault and nothing to do with "excess".

 

Smaller lighter boats react to shifting crew weight much more than boats above 30'. This could affect your ballast choices. Typical dinghy designs don't even use ballast. However, many sailing model boats have a much higher ballast to displacement and sail area ratio.

-Will

 

If you want to go to 11ft and float one person, it's going to be a heavy boat in terms of displacement/length ratio. The Van de Stadt is a fantastic shape but as you know it's extremely light so will probably be not much use as inspiration.

There was a very highly developed skinny 11ft one person boat - the Moth before it started to foil and before design tilted towards the high speed range that was possible with ultralight hulls, big rigs and wings. There's a great bunch of early Moth designs at Earwe. blog. The later pre-foil Moths went to a very narrow hull with less wavemaking resistance because they were going so fast with their wings, light weight and efficient rigs. The Axeman (top) is a classic early skinny hull - later boats developed narrower sterns like the Hungry Beaver, below.





These are really too narrow to be sailed by anyone but those with heaps of practise and require very light rigs with carbon masts if they are too stay upright at all, but they can give you an idea.

Perhaps the best inspiration would be an '80s style "fat skiff" Moth. With their wider hulls they had more wavemaking drag than the skinny skiffs so were much slower in a good breeze, but in dead light stuff they were at least as fast - and they are much more stable. Again, development went towards a narrower stern because the wide-sterned ones were pigs to steer - if they took up a heel to windward they would bear away no matter what you did as I know from painful experience!

There's a good piece on Magnum "fat skiff" development here;

When Flares Were in Fashion - the story of the Magnum design of International Moth https://www.yachtsandyachting.com/news/237652/When-Flares-Were-in-Fashion

complete with an orthogonal (?) set of lines.

Hope this could be of assistance.

 

Hi again smilicus, that yacht pic you posted, is that the 'local boat' you want to copy in miniature ? Looking at the relative sizing, at 11 feet long, or thereabouts, even if you sit in the wet bilge, the boom will be probably taking off your head. Yes it's a very nice yacht, but will you fit under the rigging at your desired reduction ratio ? Maybe get some graph paper, and scale yourself in a sitting position, as in a canoe, then scale the boat and rig around your mini-me outline, and see the clearance and width you get for your body. I was looking at doing a similar thing, and had some ideas: Cutty Sark, or the 1970's CVA 01 aircraft carrier, or a submarine (which was surface only), and I found that by the time I had a ratio I could fit inside, it was going to be a huge project. Cutty Sark with very complicated and fiddly rigging and sails would be too much work for a fun project, and be too big to trailer, to fit me in it, and be very slow and difficult to sail. The British CVA 01 which was never built, had runways both sides of the bridge block, but to fit me in, it would have to be about 40 feet long; but would have been so cool, with solar panels and electric power. And the same with a sub, too big by the time I fitted inside the conning tower. I gave up on making a model, and ruined a small dinghy instead, making it look older than it was, and costing thousands to do it; but at least I can use it like a small dinghy.
Another consideration is - what will you do with it in practicality, once you have it; what can you do in it, where can you go, and will you get bored being limited by where you can use it ? I also considered making a sailing canoe, which at least has a few uses, and can be more versatile. With a very nice model, you may be too precious with it - scratches and breakages could be 'devastating' if you use it like a 'normal boat'.
Not to discourage you, but to clarify your end uses, and expectations once the project is completed, and you have spent untold hours and money on it, just to keep it home so as not to hurt it, or panic if someone touches it. Building stuff is great, but to what end ? A mini mini-TransAt could look good and be almost useful, was another of my considerations. Can you take someone for a ride; use it for fishing; use it for cruising; or will it be as useful as a beautiful ship in a bottle ? Do whatever you like, as long as you will still like it in the end.