Re-Sizing a Full-Size Template

Makes perfect sense to me.  The modified diagrams look like everything works.  A far cry from one of the local shop rats who once told me, "DUDE... if you go longer, you gotta go narrower" or some such crap when I asked him about enlarging a shape proportionally.

Copy template onto blueprint paper.  Take paper to local blueprint shop.  Have them enlarge the sheet by your desired percentage.

Move to San Diego. Borrow old school fish from friend /co worker or next door neighbor.... Make sure the fish you borrow is a top name brand. Rip a template...you're not the first guy to do it..... forget about the math...now you have a real template made from a top name brand.....ha ha......I own a REAL fish template...... ..........and................................................................................................................................................................................

Old news but worth repeating. Master shaper Greg Loehr taught me that for every inch in length adjusted you should adjust width by 1/8 inch. You otherwise end up with a poorly proportioned shape when you just enlarge or shrink based on percentage. You can usually adjust up to 6 inches either way using your base template. 

All the best

The '1/8" width for every 1" length' rule-of-thumb helps to maintain the same outline curve or "proportion". Notice the difference in outline curves when only width or length is change. The '1/8" rule' will give you a width target, when desired length is known, but it doesn't define the curve between the nose and the wide-point, or the tail and the wide-point. Neither answer is complete. In the end, it usually comes down to an approximation, derived from either a judicious application of an existing template, or a keen eye and a sharp pencil.

Another consideration when enlarging an existing shape is how to extrapolate rocker. Aku adds 1/32"-1/16" at both ends for every 1" of length added. Thickness flow or what some call "foil" is also to be considered. Aku will automatically change rocker, thickness, both or neither depending upon the designer's choice.  

When hand-shaping in the bay, I have a general idea of where I want the "adjusted" end-point rocker to fall as I increase length, and I just "eye-ball it" from there. Likewise, with thickness I have a 12" up-from-tail  and 12" down-from-nose number I want to match up with "thickest point" number. Using those number from the exisitng board works well enough, unless I decide to increase thickness. In that case depending upon the new boards length and the amount of thikness added (or subtracted) I might adjust the end numbers a little. Its a ballpark reference, and here again, how it "looks" determines the final number.

Yep. Well said. All approximation. In the end you have to please the eye. 

All the best

""If" you want to maintain the same rocker curve -- tangent angles relative to given points; curve slope (steepness/flatness) -- you have to multiply the rocker heights by the same proportion the the length changes.  Otherwise the rocker curve slope become steeper or flatter.  But, maintaining the same rocker curve shape may not be well suited for the new length."

Absolutely. 

All good.  Useful info and insights from all, lots of ways to manipulate shapes up, down, sideways, etc etc.  These are tools we should have in our computer, the one between our ears, to pull up when we need them.  Good thread.

When I worked as a ghostshaper, pre-CNC days, we all had multiple tempalates of each model.

1/4" of centre width per every 2" in length. So Greg's 1/8" rule is viable.

Since I handshape all my boards, I route the outlines.

Have several templates for each model.

Cut them on bandsaw and clean 'em up with my beltsander.

10-15 min. to make 'em perfect.

Same math works.

As the length increases the nose and tail widen as well. Just not the solid 1/4" at centre.

Same outline, just scaled up or down.

Works perfect.

That's how you end up with 200+ outline templates.

Barry, when you are making a new template (bigger/smaller) for the same model do you just use the template that you already have to connect the dots and create the curve of the new template?

No way you all sck. whet i dew is i git graf papera jest make bigger squates. so one little square equals a inch. i did my fis my 5.0 fish n skaled it up to a7.0fish. itendedup being7.0 bye 28 wide. itseuf jest fine for me.

...math works at a some point for this application; it is like BarrySnyder says.

-a fish should be less than 6; if not is not a fish, does not have that "fishness" in the design; would be only a big floaty board with a swallow tail. I remember a Rusty model long time ago; a dog.

Connect the dots

or do the math...

Eh,

I may joke

But in all seriousness

Just went thru this dilemma!

Stretching a template.

As sadly follows

Two build sheets and basic photos of the product.

Came down to experience and eyeball.

BTW, when I was a lad, it was numbers and battens.

Good worthy templates came at a price.

If it were simple, well just

Rejoice in the craft…

Scaling a board up or down is always going to be a compromise, there is no way that every feature gets scaled equally, otherwise fins and fin boxes would have to come in infinite gradations, that would be silly!  Rocker, plan shape, curves, fin placement, it all gets fudged to make it work.  Mathematical proportions are a starting point, and valuable.  But aesthetics, common sense, intuition, and experience all have to factor in too.  When I scale a board up or down, I just draw it up new from scratch, but its different if you had a customer who wanted "this specific board" but a different size.  Kudos to guys like Mattwho and Barry Snyder who work this kind of magic.

I see it is quite a while ago this thread was alive, but i was wondering if you could post the rest of the details, spesifically those about the rail and rocker outline, on Steve Lis' original scetch?