Scaling vs. Re-Spacing

Clearly, if the crew is not also scaled, its relative weight increases. But, probably, what you mean is that will influence the heeling moment.
If we think deeply, to solve this problem was invented the trick : small boats can carry less crew.
The OP already seems quite confused with the discrepancies between us. Needless to increase his confusion.

 

Easy, only take jockeys as crew. Or put your sailing friends on a weight-loss program.

 

Re-Scaled Scow

It has been done with relatively good success.(Of course the only crew was a Golden Retriever)

 

Scaling has been discussed many times and it boils down to simple physics, as Ad Hoc (and others) have pointed out. A small percentage can be tolerated, particularly if you have some building expertise, as the scantlings can be adjusted a bit for the new size, but once you venture over say 15% (some say as much as 25%, but the math doesn't agree) proportional enlargement/reduction, you pretty much have to start considering a more size/scale appropriate design. A stretch on the other hand is performed fairly often, from small to quite large vessels, though 40% is taking this a little too far as well.

Your scaling question is what a 40% change - well this is outside the range of a realistic approach, for the design of a new size of the previous boat. You have to ask yourself what you want about the larger design, implemented into the 25' design. With this in hand (an SOR) you can work through a design, employing the features and attributes the 39' boat has, just in the small size. Some things will transfer, while other features will need to be "adjusted", as this scale of reduction simply has some available volume issues to contend with. Most of the things you'll "lose" in this reduction are scale related, because people don't change size, even though the boat has. Headroom, berth sizes, furniture dimensions, etc. all get chewed up, when this happens, but again it's stuff you can work around, if you're starting with a reasonable design approach. A 40% reduction isn't reasonable.

Develop what you like and need from the 39' design, then search for a 23' - 27' design that can accommodate much of it and with additional toying, can more closly fit your needs.

 

and about RC boats, check the mass and length of the keels on 1m rc yachts. they are realization of this very topic.

 

I want the boat in that picture. Ironically, the builder designed it from the same design I have scaled from. Unfortunately, he passed away a few years ago. His name was Bryce Muir.
http://www.brycemuir.com/dispatches/datedispatches/B108log30Oct99.htm If anyone here can draw me that boat with the required information to build in plywood let me know, I would be more than happy to compensate. Feel free to send a pm.

 

I doubt you have a realistic idea what custom plans are going to cost you. Its not something someone can just draw quickly.

 

A 25' gaff sloop, with a scow hull isn't that difficult a project. There are a few around, though it's not a common type, never has been. The phrase "sails like a garbage scow" comes from the golden age of sail, where cat and sloop scows would drag things out to sea to be dumped, such as dredging spoil. They stunk something awful and sailed similarly. Is there a reason you'd like a gaff scow?