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Oct 28, 2018, 01:38 PM
Registered User

Scaling down Irene to the size of Emma

Hi folks.
I find Irene to be a bit large and heavy for my use. Emma, which I have completed, seems about right for me.
I would like to scale down the plans of Irene by a factor which I can choose on printing. The question is, if I want to have an Emma size boat
from Irene's plans, what should that factor be?
I have compared the two boats and here are some figures, Emma first and Irene second, with the ratio Emma/Irene.
Total disp. 6.1, 11.4 kg, Ratio=0.54
Keel wt. 3.4 , 6.4 kg, Ratio=0.53
Sail area 3393, 5484 cm2, Ratio = 0.62
Total length 100, 147 cm Ratio=0.68
Waterline length 100, 100 cm Ratio=1.0
Length scales proportional to the linear factor. Sail area scales as a square of the factor, and displacement scales
as the cube of the linear factor. Obviously there will be a compromise needed.
If I choose to scale so that I have the same displacement as Emma, then based on the displacement ratio 0.54,
the cube root of 0.54 =0.81, or 81%.
This gives a total length of 0.81x147cm=119cm, The waterline length would be 0.81x100cm=80cm
Sail area will be 0.81x0.81x5484 cm2=3598 cm2 or just 5% larger than that of Emma.
The total displacement will be 0.81x0.81x0.81x11.4kg=6.1kg or the same as that of Emma.
The keel weight will be the same, 3.4kg
I have a feeling this scaling might work out OK since a 20% difference is a relatively small scale-down.
This scaling would give me an Irene, just slightly longer than Emma, the same weight,
with two 20% shorter masts and slightly larger sail area.
But will it work? I certainly would be happy to have your comments, especially, of course, from Gary.
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Oct 28, 2018, 03:43 PM
Mad on modding
robcrusoe's Avatar
That is an impressive set of calculations.

All I can say, and from my own experience, are two things.

1: Sail size is not going to make a great deal of difference as to whether it sails Ok, or does it very well. Err to the larger area if you must.

2: Only Gary can really give you an experienced answer. He has directed some of my experimental notions to a very successful application, and by explaining the fundamentals of why he offers a particular solution, or just how he came about it for his plans.
He has never just came right out and said "forget it!" but just continues to get his own personal satisfaction of seeing what other do with his basic designs.
More often it is me just reporting "didn't work.."

My money is on what he suggests for the keel and bulb.

If it were me I'd just plan to go with what you have determined, but see what he thinks before you start cutting.

Irene is certainly an arm full when trying to launch of lakeside unstable rock lining.
Particularly when the wind is up and gusty (my preferred conditions)
Oct 28, 2018, 08:43 PM
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Gary Webb's Avatar

Scaling down Irene

Reply to irandar -
Cheers for your comprehension of Linear/Square/Cube factors which affect scaling of Boats.
You are right to acknowledge that scaling very far could lead to disappointment.
As you note, scaling Irene to 81% will give you a schooner with displacement about equal to Emma (about 1/2 of the original Irene).
Short answer, I think her performance will be satisfactory - the math suggests so - but only trial will tell for sure.
I suggest if you build the 81% boat (think light), then fashion the lead bulb to bring her total weight to the desired approx. 13-1/2 pounds.
Lastly, have No Fear, and let me and everyone else know how it works out!
Cheers, Gary
Oct 29, 2018, 02:33 AM
Registered User
Hi Gary,

I'm thinking about scaling Irene up to a 30 footer. If I run internal ballast, how much weight will I have to put on to keep from capsizing? Keep in mind that my doctor wants me to lose weight. Sailing is more important than waist line.


Oct 29, 2018, 12:14 PM
Registered User

Scaling down Emma to half displacement

Or how about building a smaller Emma with a displacement of 1/2 that of the original Emma?
Then the total weight would be about 50% less or about 3kg instead of 6kg.
The calculations are the same as proposed before going from the Irene size to Emma size.
So with displacement ratio of 0.5, the cubed relationship, gives a 0.79 as the
linear factor, or a reduction of 20% for the linear dimensions.
The new length is 0.79m, the new mast height is 0.79m.
The new sail area is 0.79x0.79x3393cm2=0.63x3393cm2=2127cm2
I could imagine that the keel and rudder dimensions could also be reduced by 20%
and also the bulb weight reduced by 50%. My smaller Emma would have a much reduced hull weight,
which is hard to calculate exactly because some things, such as plywood and controls would be the same. It could be that with a keel
of 1.7kg maybe the total boat weight would be only 3.1kg. Now wouldn't that be a cute little RC sailor?
Nov 02, 2018, 10:18 AM
Registered User

Irene for sale
Nov 04, 2018, 03:54 PM
Mad on modding
robcrusoe's Avatar

Stainless steel wire sourcing

You can get these anywhere, egg whisks.

They are usually stainless steel , but not always, although it is hard to tell. A small magnet will sometimes even be slightly attracted to some stainless steel (different compositions, I guess) but they are chrome coated and therefore durable.

Use the wire, which straightens easily and bends nicely, but with enough rigidity to make mast and other fittings for your boat. But not suitable for pelican hooks, unfortunately.

And super cheap. you'll find them in a $2 (or dime? ) store but also on any eBay. Two sizes, the thinner about 1.4mm, the larger around 1.6mm.
Nov 05, 2018, 04:19 PM
Registered User

Mold for Irene's lead keel bulb

Hi folks,
Do we have any guidance for making a mold for Irene's lead bulb? Did anyone try to make a plaster mold? I would like to get the shape as perfect as possible to avoid having to shape the lead afterwards. I am thinking of making a half bulb from styrofoam and covering with tape and casting in plaster. I hope the final weight can be estimated by displacing the positive form with water. I guess two plaster molds would be needed since the lead might destroy the mold. I have read that a two week drying time is needed for the plaster. Thanks for any tips.
Nov 05, 2018, 05:19 PM
Mad on modding
robcrusoe's Avatar
I have made a plaster mould and had limited success with it. Balsa is an easy and effective plug making material.

All I can do is point out some of the cons of doing it that way.

The slightest amount of moisture trapped within turns into violent steam and either fractures the cast or creates surface blemishes. I don't think there can be a set time to leave it dry right out as everything depends on on the ambient atmospheric conditions.
Getting the plug out of the cast is also problematic, you may do better than I did. Plaster, by its very nature, permeates any airspace, like behind tape and impairs the surface finish.

But plaster is cheap, so why not give it a try. If the plaster is thick enough to take the sudden rise in temperature as you pour the lead, then it should be good for unlimited pours.
Getting the cast out of the mould should, in theory, be simple. I suspended some thin wire into the mould cavity to aid release with use of pliers.

But cleaning up and smoothing off the castings is really not that difficult, just aim for minimal dross. I remove such as best I can then use progressive applications of car body filler, to get close to the desired finish (after mounting the two halves to the foot of the keel)then finish off with a few light coats of spray putty.

I work out the necessary amount of lead to weight before casting. Unless your half plug is perfectly symmetrical in profile you will need two to finish up with the matching halves to form the complete bulb.

I prefer Magic sand as it is reusable, highly shape forming and easy to adjust. The photos are just a quick demo, not as filled as when precasting.

Here's a few pics of my results. As you can see, each is very different, like snowflakes, and people.

But this is just how I do it, not necessarily the easiest nor best, but it works for me and Crossbones.
Nov 06, 2018, 12:19 AM
Registered User
Thanks Robcrusoe for all the info on making a mold. Is there any particular type or make of magic sand that is best? Do you try to figure out the finished weight from the balsa plug?

I ran across this nice pdf on lead keel making using foundry sand
Regards, Irving
Nov 06, 2018, 12:22 AM
Registered User
Hello all,
I need some tips on making a simple mount for my Mobius camera.
Thanks, irving
Nov 06, 2018, 01:17 AM
Mad on modding
robcrusoe's Avatar
Originally Posted by irandar
. Is there any particular type or make of magic sand that is best? Do you try to figure out the finished weight from the balsa plug?

I expect all magic sand is made up more or less to the same recipe and method. I've only used two different brands and can't say there seems any real difference between them.
That is a great article, and very technical, which none of my articles ever are.

If you want to pour the bulb in one casting then it seems to me a lot of of exacting work to set up and carry out it although it would probably be a great experience.
As to your question on how to get the lead volume to fit the specs I have to admit I do it the simple SIAS method. i.e,. Suck it and See.
I do it the other way by first weighing up the exact 50% of the finished bulb weight. I make the balsa plug but pine is cheaper and not much harder to shape. you could use any material since it wont be subject to heat, just magic sand or plaster.
I then level off the tray of Magic sand and work the plug, on its side, down until it is about 60% indented. The extra 10% of so is just for convenience when pouring. The molten lead is then poured in at a steady flow until discharged.
If it fits in the mould to the 50% mark you are home and hosed. If it doesn't, i.e. noticeably over or under, you will need to either modify the plug or, as now do, just carefully sculpt the indent in the sand, remelt the lead (do it soon after while it is hot and it will melt in minutes ) and then do another, until you are happy..
Don't pour two identical halves as they need to be each from one side of the plug, or the other. Hard to explain but only mention it as you'd find out anyway.
Magic sand can be reused over and over (our main box has seen umpteen castings) but if you cast into hot sand it will stick very slightly to the lead. with my last casting job I just scooped out the hot sand and filled it in with cold, and pressed the plug in again and cast away. Probably take 30 minutes to cool off enough.

I'm probably just a bit lay back about this, but I know I can always add some sheet lead between the two halves to increase the total weight, or if it is over then just drill some shallow large holes from the inside edge to lighten it.

I'd suggest you just do a tentative casting or two until you are happy with your chosen technique.

Whatever and however you do it, do so with all safety precautions and outdoors.
Last edited by robcrusoe; Nov 06, 2018 at 02:26 AM.
Nov 06, 2018, 08:45 AM
Registered User
Blue foam is easy to sand to shape. Cover with grease to aid release. I used a corkscrew to pull foam free of plaster mold. Easiest making bulb in 2 halves.
Nov 06, 2018, 10:40 AM
Registered User
Zbip57's Avatar
Originally Posted by irandar
Do we have any guidance for making a mold for Irene's lead bulb? Did anyone try to make a plaster mold? I would like to get the shape as perfect as possible to avoid having to shape the lead afterwards.
Shaping the lead bulb was actually the easiest part of the whole process. Lead is very soft and the rasp we used cuts through it no problem at all, peeling off strips like a cheese grater. I was most worried about how to go about safely melting and pouring the lead. I had an experienced friend help me with that part and it turns out it's not difficult either. Just make certain the mould is dry! Any moisture will instantly vaporize and spatter molten lead.

We used a wooden half-mould made from five boards. The boards are cut out using Gary's planned profile of the keel shape, the centre board with a square cut, and the next boards cut at 45 angles. The outer boards are just flat. Then the whole assembly was clamped together using threaded rods.

With a bit of math, you can figure out the volume of lead required to give you the desired weight. To check, I filled the mould with sugar, then measured that volume of sugar using a kitchen measuring cup. Density of Lead: 11.36 g/cm3, 0.410 lb/in3

At first we sealed the half-mould, capping it with another flat board, and then poured the molten lead through a sprue hole into the mould. But that was awkward as we couldn't really tell if the mould was completely filled, and then we didn't know how long to wait for the lead to harden before it was safe to remove the cap. We eventually found it much easier to simply leave the mould open and laid flat. That way we could see exactly how much lead to pour in until the mould was full, and how long it took for the lead to harden (not long at all).

See this post for photos:
Nov 09, 2018, 11:21 PM
Recent Convert
Originally Posted by irandar
Hi folks.
I find Irene to be a bit large and heavy for my use. Emma, which I have completed, seems about right for me.
I would like to scale down the plans of Irene by a factor which I can choose on printing. The question is, if I want to have an Emma size boat
from Irene's plans, what should that factor be?
Hi irandar

I built Irene to a Scale of 75% of the Plans so she is 3'8' long from stern to tip of bowsprit. I did not adjust anything just printed the plans to 75 % of their original size.( actually got my partner to Scale the PDF down and then got that printed , there are advantages to knowing a Graphic designer) i Then Scaled all the measurements on the Mast plans as well . i made up an excell spreadsheet to do these calculations as i also needed to convert to metric as well.

Having just taken Ketty Jay for her Maiden voyage I can report that everything worked out fine Probably the only thing i would do differently is the winch system as i am a bit tight to the Keel pins. obviously while the externals are scaled down the winches, servos and batteries all stay the same so some Playing around needs to be done with those.

As Gary said you want to make her as light as possible so you can maximize the Ballast to give a High Righting moment, Mines a bit heavy so only has 2.5 kilos of lead , got the rails underwater a couple of times today but she is still dry inside so no biggie, could fix it with a linger keel but then might end up grounding in the shallow end of the lake .

Her are some Pics of Maiden voyage, As you can see i also decided to make it slightly more challenging by adding a flying Jib, its controlled off a second winch and seperate Radio Channel.

Enjoy the Build

Last edited by Shaggy_From_NZ; Nov 10, 2018 at 12:36 AM.

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