From Jim Michalak's newsletter:

I recall in the old Small Boat Journal a reader writing to Phil Bolger, who had a regular boat design essay each issue, whether he could simply scale up a given design (of a dinghy in this case) to get something more suitable for his use. Phil said, "Yes, go right ahead." But added such a change would give a new design and that the reader, not Phil, would be the boat's designer.

If you feel up to the challenge, it can be a fun and satisfying experience. I did that with Bolger's Singlehanded Schooner a few years back because the original design wouldn't fit in my garage.

However, if you are new to boatbuilding, scaling someone else's design can become a real disaster. In that case, the best method is to look for a different design that actually fits your needs. If there isn't one, then hiring an experienced designer would be your best bet. Their hourly rates may look scary at first glance, but if you compare that to the cost of materials to build a ruined boat, it can actually be a bargain.

The folks at CLC do custom designs, either from scratch (Faering Cruiser) or modifying an existing design (Skerry Raid). If neither option fits your needs, they can also help you pick an existing design. Use the "Contact Us" link above to talk to them about what boat is right for you.

Have fun,

Laszlo

I think a large factor is how much do you want to increase/decrease the length.  Beam is a completely different animal altogether.

For example, I recently fell in love with the Oppikat design by Dudley Dix but was a little concerned about how my 200 plus pounds would work while teaching my growing son how to sail a catamaran.  The original plans are $55 for a payload capable of a captain and first mate.

Before actually pulling the trigger to buy the plans, I shot Mr. Dix an email asking how much it would be to scale the Hobie Wave sized catamaran up to something where we could either take the admiral or some camping gear.  He promptly replied that this was a common question and that it would cost about $300 for a larger verson that entailed adjusting the waterline and beam to keep it from looking awkward.

I've always wished I had pursued the esoteric knowledge and become a naval architect, but alas, that was not meant to be.  Until then, I'll stick with the pros as $300 is a whole lot less than building a large wooden experiment only to find out that it is only worthy of a very expensive viking funeral...

Any particular Stitch & Glue plan you're thinking of?

Remember you're dealing with 3D stuff when altering an existing design. The larger the structure, when you change one dimension you're affecting any number of others & not by the same scaling factor for all either.

With a traditional design built on frames you have opportunities to 'trial & error' alterations by eyeballing how changes to frame dimensions affect how strake patterns follow the new shapes. With S&G those opportunities aren't available: the planks either come together correctly or they don't.

If that's not daunting enough bear in mind a boat's shape has a profound effect on its seaworthiness and safety of everyone aboard. Altering an available design in ways uncalled for by the designer might render even a successfully completed craft unmanageable under way or a hazard to those aboard.