increasing boat plans

[dale]

I've found a boat whose lines I really love but it is a bit small. Here's the question: Can you buy plans for a boat, increase all offsets and come out with a bigger boat? Not just stretching a plan, but increasing beam as well.

Here's the one I like:

http://www.duckworksbbs.com/plans/jw/danielsboat/index.htm

The boat is 3.2m or about 10' couple inches. (the 12'4" is incorrect per JW)

I'd like to build it about 12', a 15% increase in size.

[Frank Hagan]

Somewhere, maybe Devlin's Boat Building, I read where you can increase a boat plan by 10 to 15% without too much trouble using the method you describe.  I can't remember at what point the author said you get into all sorts of difficulties in scaling the design up, but I think it might have been at the 25% + range.

You have to consider the load on things like stringers, etc.  A stringer's cross section may have to be increased when increasing the size too much, or additional frames may be required if the boat is stretched too far and the distance between them gets too great.  But I doubt you'll run into those kinds of considerations with a small increase in the boat size.

[PAR]

You can safely increase length only up to about 15% by increasing station mold spacing. This doesn't affect beam nor draft which is a good thing.

When you start attempting proportional changes, you run into a couple of physical laws that cause most designers to go prematurely bald. These are the laws of similitude and relativity. One of the most common things a designer may be asked is to make something a little larger or a little smaller, but this can be much harder then it seems at first blush.

In a nut shell if you double the size of a boat (for example) then the surface area increases to a factor of 4, it's volume to a factor of 8 and it's stability 16 times that of the original. The exact same thing occurs in reverse, so a 20' boat, reduced to a 10' version has 16 times less stability and weighs 8 times less. This is why smaller boats seem to have a proportionately larger beam to length ratio then bigger boats. If you look at pond yachts, patterned after real yachts, you'll see this most dramatically. They'll have fin keels that rival the sail plan in depth, with big hunks of weight, just to hold them upright in a gentle breeze.

Simply put, you'd be safe to respace the station molds out for the length you need on that Welsford design, though it appears to be the length you need (12' 4" LOD, 4' 4" beam) already. It would be a wise idea to drop John a line or give him a call (he's around 12 hours a head of us) and ask about you ideas. The reason being, the dry hull weight alone on "Daniel's" boat would not go from 145 to 167 (15% increase) as you'd think, but somewhere in the 230 pound range. Did I mention to you, these rules suck at times.

[dale]

Walter Simmons gives a bit of lattitude in the length of his designs by simply spacing the stations differently.  I was hoping to proportionately increase it though. 

And Daniel's Boat is 3.2m which comes out to about 10 and 1/2', not 12'4".  Again, this is per John.

[PAR]

Okay, there's a miss print on the DuckWorks site for his plans, which states 12' 4" for it's length. Talk to John, maybe you can make a wider garboard. Then again maybe Janette would be a better choice (12' 1") without all the fuss.

Daniel

Janette

[dale]

I informed John that both Duckworks and his own site have misprints.  Both sites have 3 measurements for the length: 10', 12'4" and 3.2m.  Hopefully they will change it.

I have looked at Janette.  Right length but a bit different.  Here are John's words regarding the two, "Janette and Daniels boats are quite different, DB is side decked, wider inproportion, has more buoyancy and more stowage. She was designed for a 12 yearold to go cruising in and is intended to be more capable and more able to copewith cruising needs; Janette is a daysailer, longer, lighter and simpler."

I could go with Janette but I'm still looking at resizing DB.