Problems in Downscaling Sailboatd from 1:1 to Models - RC Groups
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May 12, 2017, 01:21 PM
Registered User
22KV's Avatar

Problems in Downscaling Sailboatd from 1:1 to Models


I am building a one-metre model of a full-size racing keelboat. I want my model to be able to race against other one-metre boats. I understand that there are differences between the way that air, water, bouyancy, mass, etc interact at the 1:20 scale I am working at, and that I cannot just make everything 1/20th of the size of the real boat, but I don't know which of these are things that I have to worry about.

Obviously the size of sails, length of mast and boom, mass of the boat, length of keel fin, mass of keel bulb, etc. in the big boat are all very carefully calculated to achieve the best possible balance. What adjustments must I make at the 1:20 scale to still end up with a nicely-balanced boat that sails well?

I appreciate your responses.


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May 12, 2017, 01:51 PM
Registered User
Basically, you will have to increase the righting moment. How to do this and still maintain the appearance of the full-size boat is more art than science and there are all kinds of options. What full-size design are you working from?


May 12, 2017, 01:55 PM
Registered User
If you want to race against other one meter boats, you need to look at the applicable class rules. For example, if you are talking about an IOM, the sailplan, overall depth and weight are strictly controlled. It is unlikely with those restrictions your scaled down boat would be effective. If you want to build a one meter race boat better to study what works in the rc class not try to copy a full sized design
May 12, 2017, 06:28 PM
Thomas Armstrong
Most important issue is righting moment. For a 4000gr displacement on a 1m long sailboat, I'll say you'll need a 2500gr ballast located about 45cm below waterline. Think long & thing keel with a lead bulb. Look for RG65, IOM and Marblehead classes for examples so you can start thinking about it.

Also if you want to be competitive against 1m RC sailboats, don't scale down, but design fron scratch. Winning boats are much thinned that you would thing at first!
May 12, 2017, 06:50 PM
Registered User
Dick L.'s Avatar
I did a C&C 35 at 1 meter, and while it looked ok as a scale build, and it sailed ok, it was too much beam for a 1 meter boat even though scaled in size. It was a crusier, and probably twice as wide as a typical 1 meter. Looked good though - like a "real" sailboat.

If you pursue and want to race, pick a design, build some hulls for local friends.....and enjoy your one design racing.
May 12, 2017, 09:04 PM
Registered User
Hi Bruce.

You are thinking about the square/cube law were the sails are area and the hull is volume. You can either run much less sail or do tricks to increase the righting moment with more weight down lower.

The other problem is authority on the rudder as a scale rudder will be too small.

They may be toy boats but there is a lot on engineering in the sense that you can have a plan for a wooden bridge that works. If you simply try to build to the plan with steel the bridge is going to weigh too much and cost to much because of inefficient use of material. If you make a concrete bridge from wood bridge plans it might not work at all.

Personally I would look at a successful one meter design and then modify the deck furnature to look more like the target.

Have fun and take pictures!

Do you have access to the shipmate class?
Last edited by pwallace; May 12, 2017 at 09:14 PM.
May 13, 2017, 10:14 AM
The wind is free, go sailing!
Scratchy101's Avatar
This might be better posted in the Scale Sailboats forum.
Have a look at this post.
May 16, 2017, 06:03 AM
Registered User
22KV's Avatar
Thanks guys for your valued input which is appreciated.

@Earl - Thanks for that heads-up. Increasing the length of the keel fin and adding lead to the keel bulb as necessary to adjust the righting moment are both easy fixes. I would rather have the topsides and rig looking the same as the 1:1 boat and compensate below the waterline where no-one sees what's happening anyway.

@Tom C - I am not building an IOM, I am building a one-metre replica of a 1;1 keelboat, so I don't have to worry about any class rules. I will race it as an open class boat, but against other one-meter class boats.

@Tarmstro - Thanks, your figures give me a good starting point.

@Dick L - I am hoping that the scale model will look good enough on the water and will be quick enough in good wind that it will be exciting to watch.

@PWallace - Thanks, I hadn't thought about the rudder, but as per the above, I can increase that size and it won't matter because no-one will see it. The Shipmate class was popular here a while back but no longer. We have a very popular Seawind class at our club (20+ boats) and I compete in that class, but I want something that I can build myself and which will challenge my building skills (genoa, canting keel, twin rudders, etc.) and which will hopefully go like stink downwind!

@Scratchy - I hadn't seen that forum so will post there in future too.
May 16, 2017, 06:09 PM
Registered User
It's not a linear equation, the problem is that if you add lead to the bulb you also have to change the boat displacement (i.e. add volume), and if you want to keep the scale look that may be a little complicated.
May 17, 2017, 04:10 AM
Registered User
When scaling, it is good to remember that the conditions being sailed in are scaled, too. Just that they scale the wrong way for comfort. A nice 10mph breeze for you is like a 45mph gale for the boat at 1:20 scale.
May 21, 2017, 06:05 PM
Registered User
SirHarrisTweed's Avatar
I've built two boats from full size plans, scaled from one foot to one inch. I built them exactly as the full size plans dictated (but with wood keels), then added long drop keels with ballast bulbs.

The only part that involves guesswork is that bulb, but it's not that hard, either. Just put weight in the boat until she's at waterline to determine the needed weight. Then make the drop keel out of 1/8" aluminum bar. Start long, then test it out. If it's too stiff or runs aground too easily, use a chopsaw to take an inch off, then try that.

I have build logs in the scale forum, one for Archimedes and the other for Hagar. Incidently, I had no experience with RC, I literally just converted feet to inches and built. Both boats sail nicely.
Last edited by SirHarrisTweed; May 21, 2017 at 06:07 PM. Reason: Clarity
May 22, 2017, 03:16 AM
Registered User
22KV's Avatar
Thanks Sir Harry - nice answer. I figured I would go the same route with ballasting down to the waterline and then moving the bulb in both planes to fine tune it.
May 22, 2017, 11:10 AM
Registered User
SirHarrisTweed's Avatar
The lesson I learned was to make the hull as light as possible, concentrating the weight in the bulb. The more weight in it, the shallower the false keel can be. It's also much easier to transport if the boat is light.

The biggest challenge I had with keel placement was the fore and aft adjustment. The first boat I built I waited until I was done, then had to turn the hull upside down to cut a slot for the false keel to fit into. The second model I made the slot much more deliberately and at the very beginning. That required guessing where to place it. I went with the geometric center of the ballast in the plans and it worked fine.

I think the normal method is to make the bulb just a bit lighter than needed, then achieve your fore and aft trim with ballast inside the hull (I used lead shot).

Again, though, the only calculation you need to make is converting the lengths.

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