Boat Size and Price are About Displacement and Righting Moment

Tons=$ € £

Question

  • Q1: What do the Adventure 40 and a Westsail 32 have in common?
  • Q2: What’s the biggest difference between the Adventure 40 and a Valiant 40?
  • Q3: What parameter governs the size and expense of the mast and rigging of a sailboat?
  • Q4: Of the common metrics used to describe a boat (LOA, LWL, Beam, Displacement, Draft), which one tells you the least about the boat and how much she will cost to build and maintain?

Answer

  • A1: They are both about the same size.
  • A2: The Valiant 40 is a bigger boat, almost 30% bigger.
  • A3: Maximum righting moment.
  • A4: LOA (Length overall).

The point of this little quiz is to show that boat design can be really counter-intuitive. And while we voyaging sailors certainly don’t need to become naval architects to select the right boat for our needs, the way that boats are generally described and talked about these days does set us up to fail if we don’t take the trouble to understand the fundamentals of how our boats work.

Size is About Displacement

For some reason the first metric that we always use when describing a boat and comparing it to others is length overall. But that is simply wrong. The right number to use is displacement, being the weight of the water that a boat displaces. Or to simplify further, the size of the hole in the water that the boat makes.

If we think for a moment, this truth becomes obvious. If two boats weigh about the same, like the Adventure 40 and Westsail 32, the volume of the hole in the water they make will be about the same too, and therefore they will be capable of holding about the same amount of gear and people.

Further, it will take about the same amount of material to line that hole in the water (the hull) and about the same amount of furniture and gear to fill it. And theretofore these two boats are about the same size, and will, assuming the same construction techniques, cost about the same to build.

Using the same logic, we can see that the Valiant 40 is a bigger boat than the Adventure 40, and will be much more expensive to build, even though they are about the same length, because the former weighs much more than the latter.

Rig Size is About Stability

Now let’s turn to the size and cost of rigging a sailboat, what determines that? The answer is a single number that has nothing to do with overall length: the maximum righting moment.

Imagine you attach a line to the top of the mast of a sailboat and use it to winch the boat over on her side. The strain on the rope will start off quite small, but as the boat is heeled and the keel is pulled away from the vertical, the pull needed to keep heeling the boat will increase until a maximum load is reached. That is the maximum righting moment. And, as we perform this experiment, the loads on the rigging will go up in exactly the same way.

The same thing happens when the heeling is a result of wind pressure on the sails. The loads on the sails and winches will again max out when the boat’s keel weight exerts the maximum force against them in trying to right the boat. And if we make the boat more beamy, or increase the weight of the keel, she will be more stable and the rig will cost more too because everything will need to be stronger and bigger.

By the way, don’t confuse maximum righting moment with safety from capsize or knockdown. It is perfectly possible, in fact likely, that a long thin boat (like the Adventure 40) of a given displacement will have a lower maximum righting moment and therefore a less expensive rig package than a short beamy boat, and still be safer in the ocean from capsize risk.

Don’t believe me? Think about a catamaran, which has a huge maximum righting moment due to her extreme beam—this is called form stability—but a quite low ultimate stability angle and almost no ability to return to upright from a capsize.

(I’m not beating up on cats here. I do know that they have advantages too. I’m simply using them to illustrate a basic point.)

Also, don’t make the mistake of assuming that a longer narrower boat with a lower maximum righting moment must heel more (be more tender) than a shorter fatter boat of the same weight. In fact, because the former hull form tends to be easily driven through the water, if designed properly, she can be sailed fast with less sail area and little or no more heel angle than her beamy competitors.

Why 40 Feet?

So we can now see that you could build the Adventure 40 at a lot of different lengths and still spend the same amount.

So why 40 feet? Well, first off, she may get a bit longer, or a bit shorter, as the designer optimizes the hull form. Displacement may go up a bit too, as we make sure that she is sea kindly. But a few initial sketches have confirmed that she will be between 39 and 41 feet overall, and comparatively narrow, but not excessively so. This combination will yield the best combination of speed and sea-kindliness.

Adventure 40 Price

[The paragraphs below are based on 2012 prices and so we must adjust them for inflation.]

The point of all this is that the Adventure 40’s target price of less than US$250,000 is a lot more credible when you understand that she is a smaller boat than many 40 footers and also, because she is narrower with a lower maximum righting moment, she will have a cheaper rig, sail, and winch package than many of her heavier and/or beamier competitors of the same length.

To see what this means in the real world of hard cold cash let’s compare the Adventure 40 to the Beneteau Oceanis 45, a 23,000 lb boat that sold for about US$225,000 in 2012. The Adventure 40 will be:

  • Substantially smaller.
  • Have a much smaller rig and lower maximum righting moment.
  • Have no dealer network.
  • Be tiller steered instead of having twin wheels. The savings here alone should pay for the massive strength that we plan to build into the Adventure 40.

It’s all starting to make sense, wouldn’t you say?

Comments

OK, I know I simplified in this post, maybe even oversimplified. For example I left much about form stability and everything about wetted surface, prismatic coefficient and water plane area out of the discussion.

Please don’t waste a lot of your time and mine nitpicking at those items, or any others that you can think of, that are not material to the overall point of this post. Keep in mind that I’m trying to explain, in the simplest and shortest way possible, the factors that effect boat size and cost to build.

On the other hand, if you have a question, or if I have missed something that is material to the core point of the post, I’m all ears. Please leave a comment.

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John was born and brought up in Bermuda and started sailing as a child, racing locally and offshore before turning to cruising. He has sailed over 100,000 miles, most of it on his McCurdy & Rhodes 56, Morgan's Cloud, including eight ocean races to Bermuda, culminating in winning his class twice in the Newport Bermuda Race. He has skippered a series of voyages in the North Atlantic, the majority of which have been to the high latitudes. John has been helping others go voyaging by sharing his experience for 25 years, first in yachting magazines and, for the last 20 years, as co-editor/publisher of AAC.

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