model yacht
 

Hi there, can anyone tell me if i want to scale up some plans is it just a matter of increasing dimentions by 20 or 30 % for example? Thanks in advance.

That's where you start.
You then have to figure out how thick to make the frames, what to make the frames out of(wood, FG or other), how much ballast to use, size of the rudder, etc. All of this comes into play when you start building. I'm assuming you're building a sail yacht so you are going to have to figure out how heavy the structure needs to be to handle the stress of the mast(s) and rigging under sail and base everything else from that.

Almost right. If making a larger one, you need to consider the increased stresses involved, if going smaller you need to consider increasing the fin depth over what simple linear scaling suggests. Real life forces do scale, but linearly as far as the model is concerned. Sailboats are more scientific than first looks suggest.

I like it, we're on the same page on this one. All to often, the information given by multiple people trying to answer a question is contradictory, to put it mildly. I've seen what can only be referred to as "arguments" break out over the most mundane question as well as a complex one.

Could have started there - I should have been fully awake when I wrote -
Should have been NOT linearly.
It's having to use roots and squares on numbers that throws a lot of people off.

Ok, so what you're saying is that i can do this but need to take into account stresses. The boat i wish to enlarge is 780 mm in length, the beam is 220 mm across, the mast is 1200 mm in height and the keel is 180 from bottom of hull to the bottom of the lead bulb. Just say i wanted to lengthen it to 1200 mm, what would all the other diamentions need to be? Approximately?

Do you have plans for this boat that we could look at? You're looking at almost doubling the size, using your 1200 length so we're going to need more to go on than just overall dimensions

Don't laugh, but i used plans from an old readers digest kids craft book. So i don't think i can upload them. It was not r.c. origionally but used a vane type steering control.

It is amazing what you can do with a scanner, or, these days, a camera on a phone.
A vintage plan intended for a vane boat will need some extra rethinking. Bear in mind that a self steering boat holds to a setting on the wind. The rudder is a device to keep it on course. A radio boat uses the rudder to overcome this, it will usually need more rudder. The balance of the sail plan might need some looking at as well so that constant crrection is not needed.
A self steerer will probably set itself to run at a fairly minimal heel angle, a radio boat has a driver who will probably not do that. While a vane boat has to make do with what it has been set for and respond to the conditions, a radio boat is being handled by someone who might not be as sympathetic at keeping out of trouble.
You need access to the insides of the boat for radio - water resistant access that probably does not appear on the original plan.
While the Readers Digest often had good reading, there is no guarrantee that a plan that they published was a good boat to sail back then. Unless the name of the design is known, it is hard to say whether doing it is a good idea. What was good for its intended purpose back then might have been very adequate back the, It might be now, I don't know.