Enlarging an existing design

Some years ago I produced plans and a building manual for an eleven and a half foot Norwegian pram. Jacques, a Belgian woodworker, built one of these and liked it so much he wants to build a 5-meter (16 foot) version--about a 30% increase. I'm not sure how to advise him and thought there might be somebody out there with the necessary experience? Please keep your comments concise as I may forward them directly to Jacques.
Simon Watts

 

I'll leave this one to you Paul...

 

A 30% enlargement is just too much to expect from the laws of mechanical similitude and comparison (relativity). It would be kind it everything scaled up nice and uniform, but it's not the case and things quickly get out of hand. It would be your best advise to select a design of the size (volume) he actually needs, possibly one that needs just a little "tweaking", say less then 10% in length difference from the original, certainly no more then 15%.

http://www.boatdesign.net/forums/boat-design/scaling-down-america-junior-plans-28584.html

In this thread it's discuss it in a little more detail.

 

simon

do you mean straight forward scale up by 30%?...in naval architectural terms, a geosim?

If you keep everything the same, then scaling up by 30% is easy, you just multiply all your linear dimensions by 1.3.

BUT, your volume will increase by 1.3^3 = 2.2, this is a significantly larger boat, being more than twice the displacement. Unless of course that is what you want.

Perhaps better narrow down what you really mean by 30%, or just geosim up (scale) a bit, then as PAR noted above, just tweak it slightly, ie increase length uniformly etc etc

 

Scaling

Try this excel spreadsheet
hjs

 

If you designed the boat why is it so difficult to design another longer? Strech the station spacing by 30% , increase the freeboard a bit and re-fair the lines

 

As Gonzo points out, if you designed one and it worked well, it shouldn't be too complex a job to design something similar a bit bigger.

The Barkla scaling factors, applied to your original hull, should produce a shape with comparable performance in a somewhat larger (or smaller) size. Of course, the vessel's scantlings will have to be re-calculated for the greater loads encountered in a larger hull.