How determine the exact measurements of a curve?

[Dakotaly]

Hello,
I am in the process of making an 18' aluminum mod-v boat. It seems like a silly question but how would I get the exact curvature on to a sheet of aluminum to cut out without a CNC machine?
Referencing the pictures below how would you all draw curves on a sheet of plywood or aluminum?

 

[gonzo]

Is that the developed shape or a perspective of it? The easy way of taking the lines off, is to make a model of stiff cardboard. Lay it flat on graph paper to scale it, and loft the shape on the aluminum.

 

[seasquirt]

Hi Dakotaly, if you already have the curvatures in numbers, like radius or diameter, and know where you want the tangents or starts or joins, flats, etc., marking it out full size can be as simple as a pencil drawing an arc from the end of a measured long piece of string, could be many or dozens of feet long, held in the right place, just like a compass on paper; if you have your geometry laid out correctly and to scale. Straight edges, rulers, sticks, string, whatever you can pivot, and clamp or hold a pencil to. You may need a stake in the ground 15 feet away or more to get your shallow curves. Not so simple with compound curves though. Make a cardboard model.

 

[rwatson]

For an 18 footer, the accuracy doesnt have to be mm perfect. Its different if its a scale model.

Seasquirt had some good hints.

Here is a pictorial using some of SeaSquirts methods, just showing the first say half dozen points, using a Compass to draw Circles. I didn't use a specific base measuring unit for the drawing, just imagine they are Centimetres for this exercise.

Have you heard of Tick or Ticking Sticks ?

I have used a variation of that which may give you some ideas.
I have used the Triangle "tick stick" method.
I prefer this instead of just drawing Right Angle lines on the Plywood, and measuring from the centreline to the edges, because trying to keep parallel lines accurate on a large sheet of plywood, is just a pain.

Once you have established the major points by geometric calculation, cut a bit of Plywood in say a Triangle, place the base of the Triangle (Red 4.00 long) on the arbitrary straight line at a planned distance ( Purple Cross) at say 43.7 from Point 3, and mark the adjoining interactions (blue line 3.64 and green line 3.87 ) , of the desired side.

Create a table of measured intersections using the 3 coloured Triangle, plotting points all along the arbitrary main line.

eg. One line of your table will be

For the next point, you can move the Triangle to the end of the straight keel line, but when you are doing curves, you can move the triangle just a few centimetres from the last plot to get closer measurements.

Hopefully you get some more ideas from this diagram.

There are plenty of videos about how to use more traditional Tick Sticks which are a straight version of this Triangle method.

 

[Dakotaly]

Thank you all for the answers! The tick stick is a very interesting though that I will likely use in some form.
Below I have outlined the shape a little more and I attached the Rhino files for those interested. The blue and red line when unfolded I know that I want them to be straight so that I have something to go off of. The yellow line is the one that I will have to do some form of tick sticking or just doing perpendicular lines as rwatson mentioned he didn't like to do. Along with doing some type of scaling. I have been messing with the model and hand cutting some paper to see what it would look like I just feel as if there has to be a way to mathematically find the curve even if I have to change the curve to match the arc of a circle which is what I might have to do to get a perfect curve on a large scale that isn't based on the detail work of scaling or ticking.
The only confusing part from your explanation rwatson/seasquirt is the circles I don't see the roll they play?
In a perfect world I would make the design work where the chine when unfolded is a straight line on the blue and the yellow line. The only true curve in the cut would be the yellow line on the wall.

 

[rwatson]

"The blue and red line when unfolded I know that I want them to be straight so that I have something to go off of"

No, no , no. You DONT need a straight line on the developed flat surface. You draw your OWN Line, (that's what ARBITRARY means.) Like this
GREEN line,

Very few boats have a straight edge on developed plates, and you shouldn't try to make them like that.

Now, you know the BOW point is X distance from the STERN point.

ALL other positions are RELATIVE to the arbitrary (GREEN) line.

Could you save your DESIGN Rhino file as Version 6, and I will use it for further exercises, to explain the next steps.
I don't think you have Unrolled your sides at all. This looks like a side view of the whole boat.

Watch THIS Video first

Then this one

 

[Dakotaly]

Here is the file. It is imported from Fusion 360 to Rhino. I made the Boat in Fusion before I realized that it doesn't really have unroll capabilities.
This boat is a river boat (jon boat) Bass boat hybrid. It doesn't need a deep-v or a complex shape since it wont be on anything but lakes. So my goal is keep it super simple for my first go at it. I was trying to keep straight lines where I can.

 

[rwatson]

Ok, that worked.
First, the lines on the hull are all UNFAIR, and so some Panels are not Developable.
If you try to UNROLL some of the existing Surfaces, you get an error message
But, you can do the Unroll exercise on problem Panels with these steps illustrated below. I would definitely NOT use the unrolled sections for actual cutting if you cannot Unroll the whole section.
This exercise is just a way to play with your drawing as it currently is.

I've attached the modified hull file for you to play with.

You are going to have to design your hull using more precise steps. Those Video's will help a lot.
You DO NOT need to keep any of the lines straight in order to be able to make panels.
When it comes time to cut the Plywood, you take the measurements off the Unrolled surface in your drawing, using the techniques explained, and use those measurements to transfer to your plywood panels.

 

[Barry]

You DO NOT need to keep any of the lines straight in order to be able to make panels.

While you do not need to keep any of the lines straight, the plates for the sides and bottom were parallel and straight on our hulls for the first 2/3rds prox and only began to curve
from that transition point to the bow.
We built our boats off male jigs/frames and did not use any CAD program as at the time we did not have a plasma table.
The boats were generally 12 degree constant deadrise for say 2/3rds of the hull and constant chine width up to this point.
Once the jig was made, we created sheet metal patterns to be able to mark out the aluminum sheets and cut them with a circular saw.

It would take 2 guys about 3 hours to cut out the 4 pieces.

 

[rwatson]

While you do not need to keep any of the lines straight, the plates for the sides and bottom were parallel and straight on our hulls for the first 2/3rds prox and only began to curve
from that transition point to the bow.
We built our boats off male jigs/frames and did not use any CAD program as at the time we did not have a plasma table.
The boats were generally 12 degree constant deadrise for say 2/3rds of the hull and constant chine width up to this point.
Once the jig was made, we created sheet metal patterns to be able to mark out the aluminum sheets and cut them with a circular saw.

It would take 2 guys about 3 hours to cut out the 4 pieces.

So, why are you guys using CAD at all?

Why dont you just do small scale models out of cardboard, and be done with it ?

 

[Barry]

There are a myriad of aluminum boat kits and plans in the marketplace.
For around $400 you can get a cad file that will allow to have the plates cut from a proven design ( ie pieces that fit without expensive aluminum wastage) as you try to figure out what works
or not. (Metalboatkits has many even if you only use their side, bottom and transom and finish above the gunwale yourself)
The savings that you will get from the program that will auto nest the various pieces will pay for itself in efficient use of the available sheet material and can reduce the cost of the actual
plasma, router or water jet costs.
For strong welds, the edges, keel to keel, chine to side, etc the aluminum sheets must touch each other before welding. Filling 1/4 inch gaps with multiple weld passes is not feasible.