Steps when drawing lines plan

Good thread about lofting- I am only getting started- how do you know how many of each lines to use?

 

You need to divide the hull up into stations (transversely) , water lines (horizontal), and buttocks (longitudinal). If you are doing manual calculations using Simpsons you need an odd number of stations starting with zero at the bow where the designed waterline hits the line of the bow (the forward perpendicular) and stopping at the aft end where the designed waterline meets the hull. (the aft perpendicular) Usually 11 or 13 will do. If you are doing this using a program like Delft you don't need to worry about how many stations there are. The same logic works for waterlines and buttocks but there will be fewer of them. Someone else here will point out any mistakes in the above, and yes I left out diagonals.

 

If visualization from the lines is a problem, it may help to take the buttocks plan and cut a slice of foam corresponding to each line. Now stack them up and sand smooth. Do the same with the waterlines or the sections (is that the right term) and you'll get similar shapes. How similar depends on your craftsmanship and whether the thickness of the foam (or wood) is right.

Another trick might be to take a model boat (or a salad bowl) and put it upside down on a table. Now set up a pencil holder that can be set for a particular height, that can slide on the table. Set the pencil at 1 inch up, and draw on the hull (or bowl) with it. Then two inches, and so on. Those are waterlines. Now attach the hull, on its side, to a milk crate or other right angled surface. The lines you draw that way are buttock lines. Now stand the boat up vertically against that right angle surface, bow or stern down, and draw the sections?

Model boat guys call the sections (minus planking thickness) the "shadows". They'll set up the shadows at even intervals, in a straight line, and plank them to get the hull shape. Or make slices and sand using the buttock lines as mentioned above. If you watch a traditional planked boat being built, modt of the frames are more or less equivalent to the sections. I don't know if this is helpful, but the following page shows the process:
Building a Wooden hull http://www.randpsystems.co.uk/IOM/BuildingAWoodenShell.htm

Another thought is that a set of waterlines is like a contour map. But I doubt if you can get the USGS to map your carved half-hull.

One more thought experiment, which you should do with a lines plan in front of you. If I've screwed up any of the terminology, I apologize. I'm better at remembering how things work than remembering their names. Let's look at the profile view, which I think is sometimes called a sheer view. At any point along a buttock line, you can measure the distance aft from the "forward perpendicular" and the distance above the base line. Those are two of three coordinates of a point. Each buttock line is drawn to represent parts of the hull at a constant distance from the center line. So there are your three coordinates. If you pick an intersection of a buttock line and a waterline, you can find that same intersection on the half breadth drawing, keeping in mind that the buttock lines are straight in the half breadth drawing. If the drawings are right, you can measure the distance of that point from the forward perpendicular and it will be the same as on the profile drawing. You can find that same intersection on the body plan, where both the buttocks and waterlines are straight. The distance from the centerline will be the same, as will the height above the base line. I hope this is helpful, but I'll admit I don't KNOW if it is. I've been looking at boat lines, occasionally, since my Dad used to bring home his boss's old copies of National Fisherman. Early on, I saw drawings of Rozinante, which may have been what set the hook. I've been visualizing stuff from other sorts of formal drawings for a long time, too, so it's hard to know what makes sense to someone who hasn't been doing that.

As far as actual boat lines, I don't know how well I could come up with them from scratch. I've been able to develop 3D models of a couple of boats by tracing images of lines drawings in a CAD system at least twice, though I haven't tried it with offsets yet. And my model of Raison's Magnum has a stubborn area that refuses to behave. Nor have I figured out how to imprint buttocks, sections, and waterlines on the 3D models I've made. Or, at least, not in any reasonable time. And sometimes the CAD system will only let me draw a certain number of them before it goes on strike. Sigh.

 

I haven't explored diagonals myself. Maybe someone can explain what makes them useful when you already have the other lines.

 

Simply put, this is just one more check and any check is helpful to get the perfect fairing. In some boats it is very useful to check the stern and stem areas.

 

"A picture is worth a thousand words". This is something I did in 1978 when I was starting on the Westlawn course. It has evolved a lot since but it shows most of the lines (no diagonals though) and I have added labels so you can see what each set of lines is and how they define the body shape. Diagonals are just an added way to define the submerged portion of the body. The latest iteration can be seen on my website at Boat Design | New Boatbuilders Home Page https://newboatbuilders.com/pages/design.html and Boat Design | Logo | New Boatbuilders Home Page https://newboatbuilders.com/pages/design_logo.html. The last shows the tables of offsets which are derived from the lines plans.

 

You're right. Somehow I overlooked that. Must be getting old. LOL.

 

Shouldn't some other view show the diagonals, too, so you know where they are?

 

yes. They should be in the body plan view. I'll have to look in my archives.

 

Somewhere I have my original paper blueprints, but I thought I had had them all scanned. I guess not. Anyway. Heres' a 25 ft runabout that shows one butt.

 

Yep. For sailing vessels you fair to the diagonals, for power vessels you fair to the buttocks. The reason is that these are the most important lines because they most closely represent the flow of the water past the hull; especially in the stern region.

 

I never would have thought that the fairing was done one way or another depending on the type of propulsion of the boat.

 

<shrug> Perhaps others don't, but I find that powered vessel rarely run with 10 degrees of heel, but sailing vessels often do. YMMV.

 

I was taught basically the same thing. On planing hulls you want pretty much straight buttock lines particularly in the aft 1/3 of the hull, but on sail boats you need smooth curves in the diagonals because that is basically the lines of water flow when the boat is heeled.