The Offshore Voyaging Reference Site

Selecting The Right Hull Form

Don’t get the length of this article put you off, a lot of that is made up of photos and captions that are fascinating.

Also, many of you know Matt as an engineer, but what you may not know is that he studied naval architecture as part of his training.

Experienced old salts, those who have put a few tens of thousands of miles under the keels of a few dozen different boats, have their own words and ways to relate how a boat behaves at sea to how it’s built and shaped.

Naval architects too, have their own language and their own math to guide their decisions.

How then can the rest of us look at a yacht design and understand how it’s going to perform?

The good news is that with a good standardized way to represent the shapes it’s actually not too difficult to relate lines on a page to the feel and performance of the boat when it’s underway:

Cutting Up The Hull

To begin with, we need a way to represent the three-dimensional shape of the hull on a two-dimensional paper or screen. For almost as long as yachts have existed the standard way to do this has been the lines plan.

We start with a 3D hull. For today, to avoid any hint of bias towards a particular style of boat we’ll use one of the best known generic hull shapes in the design education world: the Freeship Default Starting Model, at the top of the article.

Stations

We’re going to take our boat to the bakery and put it in the bread slicing machine. First, we’ll cut it up transversely, like a regular loaf of bread. This gives us the equally spaced station lines, in brown.

Waterlines

Next, we’re going to cut it into layers parallel to the water’s surface. This gives us the equally spaced waterlines, in blue.

Buttock Lines

Now we turn our slicer vertically, and align it to the fore/aft axis. This gives us the equally spaced buttock lines, in green.

Diagonals

Those three, combined, give us all we need to make a standard 2D representation of the hull. Many designers, though, will now turn the slicer to an angle (usually, but not always, 45°) to also give us the diagonal lines, in pink.

Put it Together

When we lay all of those out flat, we get the lines plan: a clean, intuitive way to accurately represent the shape of the hull.

The Payoff

Okay, but how do I go from “what the hull looks like” to “how the boat will behave”?

Good question. An old salt1 will talk about “hard bilges” and “stiff motion” and so on—language that someone new to sailing might have a hard time parsing.

A naval architect will go on about “metacentric height” and “area moments of inertia”—equally impenetrable to someone new to the field.

Let’s look at each element of the lines plan, and its effect on the boat, in plain language and then relate that to photos of boats:

More Articles From Online Book: How To Buy a Cruising Boat:

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  2. Is It a Need or a Want?
  3. Buying a Boat—A Different Way To Think About Price
  4. Buying a Cruising Boat—Five Tips for The Half-Assed Option
  5. Are Refits Worth It?
  6. Buying a Boat—Never Say Never
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  10. Easily Driven Boats Are Better
  11. 12 Tips To Avoid Ruining Our Easily Driven Sailboat
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  32. Buying a Fibreglass Boat—Hiring a Surveyor and Managing the Survey
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