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.


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.


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.


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:

Login to continue reading (scroll down)

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

  1. The Right Way to Buy a Boat…And The Wrong Way
  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
  7. Selecting The Right Hull Form
  8. Five Ways That Bad Boats Happen
  9. How Weight Affects Boat Performance and Motion Comfort
  10. Easily Driven Boats Are Better
  11. 12 Tips To Avoid Ruining Our Easily Driven Sailboat
  12. Learn From The Designers
  13. You May Need a Bigger Boat Than You Think
  14. Sail Area: Overlap, Multihulls, And Racing Rules
  15. 8 Tips For a Great Cruising Boat Interior Arrangement
  16. Of Cockpits, Wheelhouses And Engine Rooms
  17. Offshore Sailboat Keel Types
  18. Cockpits—Part 1, Safe and Seamanlike
  19. Cockpits—Part 2, Visibility and Ergonomics
  20. Offshore Sailboat Winches, Selection and Positioning
  21. Choosing a Cruising Boat—Shelter
  22. Choosing A Cruising Boat—Shade and Ventilation
  23. Pitfalls to Avoid When Buying a New Voyaging Boat
  24. Cyclical Loading: Why Offshore Sailing Is So Hard On A Boat
  25. Cycle Loading—8 Tips for Boat and Gear Purchases
  26. Characteristics of Boat Building Materials
  27. Impact Resistance—How Hull Materials Respond to Impacts
  28. Impact Resistance—Two Collision Scenarios
  29. Hull Materials, Which Is Best?
  30. The Five Things We Need to Check When Buying a Boat
  31. Six Warnings About Buying Fibreglass Boats
  32. Buying a Fibreglass Boat—Hiring a Surveyor and Managing the Survey
  33. What We Need to Know About Moisture Meters and Wet Fibreglass Laminate
  34. US$30,000 Starter Cruiser—Part 2, The Boat We Bought
  35. Q&A, What’s the Maximum Sailboat Size For a Couple?
  36. At What Age should You Stop Sailing And Buy a Motorboat?
  37. A Motorsailer For Offshore Voyaging?
  38. The Two Biggest Lies Yacht Brokers Tell
Inline Feedbacks
View all comments
Iain Dell

Fascinating article which reads very well together with an earlier one by Eric Klem concerning weight distribution. Both have improved my understanding of why my boat behaves as she does and I now understand much better what I should (or should not!) do to improve it. Many thanks to both authors as well as John for such clear explanations.

Michele Del monaco

Great article, clear concept, very clearly written. Well done!

John Tully

Agree Agree, This is a great explanation of hull design and how it effects a boats behavior at see . Look around the current offerings from notable sea going boat manufacturers and you know they are no longer building good sea boats but great dock condos and weekend cruisers , I guess thats the majority of the new boat buyers out there nowadays. We have a Bristol 45.5 great offshore boat but some people don’t like the center board.

John Tully

Yes we actually love our center board . Ted Hood designed a good system that has worked for decades with out trouble, but everything has it limits . You still need to keep an eye on anything that’s 40 years old. Cables and sheaves need to be replace at some time .
The only issues I have with the current yacht manufactures is they are calling there boats “blue water” boats . There really should be some sort of grading program for a yachts usage . Just to help keep people safe . Its not easy to choose the proper boat for the job if you haven’t made the investment in time to understand what makes a good “blue water” boat.
On a different note , strange how there were so few sailboats at the Toronto boat show , I walked by the one Beneteau’s bow you took a picture of. Next year there will probably no keel boats there.

Your article is a must read for anyone trying to figure out what they need , not everyone wants to go offshore/off-grid.

Are you a yacht designer Matt ?

Karl Westman

Great article. Concise and well written. I have volumes on the subject in my sailing library. This summation tops them all.

Frank Hertel

Thank you, excellent article.

I want to offer one trick to get a feeling what weather helm might look like, especially in light of a autopilot helming the boat. Draw a waterline at 30deg heel and evaluate the distribution of area of that plan. I would also suggest “Preliminary Design of Boats and Ships” by Cyrus Hamlin.

Charles Roberts

Thanks Matt… great overview!

I was wondering if there are examples of boats with a close to plumb bow (longer waterline) but still relatively v-shaped forebody stations. I see many with plum bow and u-shaped… and many with a decent overhang and v-shaped. Do they have to be correlated this way?

Also, how would you characterize the scow bow seen on some new racing boats? Is this essentially the u-shaped forebody taken to an extreme for downwind / surfing?

Charles Roberts

Thanks for the detailed response. Interesting to think of the scow bow as a truncated normal one. Appreciate you going into the tradeoffs here.

Peter Dunbar

The radical scow bow (Revolution 29) is certainly eye catching.

It’s sad that such craft are faster than 8 meter yachts while being almost as ugly as the 8m is beautiful

Of course, comparing usable interior volumes…

Choices, choices !

Terence Thatcher

Fine summary. Well done.

Colin Speedie

Top stuff, Matt! Excellently and understandably argued.
Best wishes

Jim Schulz

Thanks for the great article Matt, and great choice of images to go along with it!

Rob Gill

Thanks for the article Matt, cool to be learning new stuff on this wonderful site. Some clarification please.

I am trying to understand diagonals as they apply to a Beneteau 473, LOA 14.5m, beam 4.3m, approximate beam aft at deck level ~ 3.8m (excludes the sugar scoop), but at the waterline is ~ 2.0m aft (~ 1.25m at the sugar scoop). A single spade rudder.

The hull is also reasonably rounded forward and aft with no chines, so she pounds if sailed hard upwind into big steep seas. The flip side is she will surf off the wind at 12->15 knots with immaculate manners and “two-finger” steering.

Yesterday we sailed close-hauled for two hours into TWS 15 knots, gusting 25 knots, 15 degree wind shifts, and accompanying waves. No one touched the steering wheel and no autopilot engaged – beautifully balanced upwind with the boat slowly luffing in the gusts and coming back to course in the lulls. Downwind…not so much.

Running (but not surfing) in steep following waves especially offshore (2m+) we are thrown off course easily and she needs very active steering. We have tried goose-winged and twin headsails, but nothing really helps.

So we don’t run in big waves, rather we tack down wind when the steering is much easier, our VMG (negative) is significantly higher (30%+), the ride much more comfortable. When we surf the steering goes super-light, the boat comes more upright and it’s like we are on rails.

I always thought the issue with running, was short steep waves/swells lifting and plunging alternate overhangs aft, causing us to roll and then veer off course, much like a displacement kayak in waves. Is this because we are alternately unbalanced as we roll, or we are unbalanced so we roll and veer off course?

Are we just better balanced pressed over to one side on a broad reach (say 10 degrees), bringing our waterline form closer to a more stable equilibrium fore and aft?

Rob Gill

Thanks for the detailed reply Matt, understanding adds to knowledge…!

Yes, the 473 was Groupe Finot designed and I was told, a shortened version of their successful Whitbread 60 hull. The standard masthead rig provides ample power, although a taller mast was an option.

Our 473 came from the factory with the one double guest aft cabin to port and a single crew berth and utility storage area to starboard, plus an extended galley and wet weather gear storage.

We wouldn’t venture south of NZ in our 473 (apart from likely facing mutiny), although these Oceanis boats have thick hulls (last of the hand-laid Beneteau boats) and massive U beam floors with big flanges securing the keel.

Many Northern Hemisphere 473s arrive in NZ every year and the boats seem to stand up to this well, talking to the crews we meet. It is trade-wind sailing I suspect they were optimised for, and in which they really excel – 200 nautical mile days being readily achieved. Our record is currently 220 nautical miles in 24 hours, and I suspect 250nm would be quite possible.


Jesse Falsone

Perhaps this is beyond the scope of your article on hull design, but I would suggest that pitch moment of inertia (also expressed as pitch gyradius) plays into motion comfort considerably. You have only alluded to it with the mention of long overhangs being more sensitive to the effects of added weight. While low pitch moment of inertia is generally desirable for performance, it may not always yield the best pitch accelerations for comfort. Of course, it cuts both ways, and higher pitch inertia can lead to pendulation that can stop of heavy boat in its tracks as well as be terribly uncomfortable. One other point concerning LWL and it’s effect on displacement mode performance. A long waterline without sufficient submersed volume may not be as effective at high Froude numbers. In other words, not all LWLs are created equal. A bow profile with large overhangs but a deep forefoot will have very effective LWL whereas a plumb bow with shallow forefoot will be far less effective. I take LWL with a grain of salt because it doesn’t tell the whole story where “hull speed” is concerned. Thanks for the anecdotal piece.

Pete Running Bear

Good stuff. What the heck is going on at the stern of that swan 47 Gimme Shelter? It looks like a hall of mirrors down there. That’s definitely not normal.

John Harries